Influence of recent Asian SO 2 and Asian NO. emissions change (2001 to 2010)
on particulate matter: shifts in Asian sulfate enhancement over US surface,
major production pathway, and lifetime
by
MASSACHUSE TTS RNM"79E
OF TEC HNOLOGY
Flora Min
6
B.S., Biological Environmental Engineering
Cornell University, 2010
LIBR) ARIES
M.Eng, Biological Environmental Engineering
Cornell University, 2011
Submitted to the Department of Earth, Atmospheric and Planetary Sciences in
Partial Fulfillment of the Requirements for the Degree of
Master of'Science in Atmospheric Science
at the
Massachusetts Institute of Technology
September 2014
D 2014 Massachusetts Institute of Technology. All rights reserved
Signature redacted
.--------------------------Signature of A uthor .................... r --------------------------Department of Earth, Atmospheric and Planetary Sciences
Signature redacted
...........
Certified by ..... ...
Susan Solomon
Professor of Earth, Atmospheric and Planetary Sciences
Thesis Supervisor
Signature redacted
Accepted by..........
20
...........................
Robert D. van der Hilst
Schlumberger Professor of Earth Sciences
Head, Department of Earth, Atmospheric and Planetary Sciences
2
Influence of recent Asian SO 2 and Asian NO. emissions change (2001 to 2010) on
particulate matter: shifts in Asian sulfate enhancement over US surface, major
production pathway, and lifetime
by
Flora Min
Submitted to the Department of Earth, Atmospheric and Planetary Sciences
on August 27, 2014 in Partial Fulfillment of the Requirements for the Degree of
Master of Science in Earth, Atmospheric and Planetary Sciences
Abstract
A 3-D chemical transport model with coupled oxidant-aerosol chemistry (GEOSChem) is used to analyze the influence of recent (2001 to 2010) growth in Asian NOx and
Asian SO 2 emission on transpacific transport of Asian particulate matter, especially sulfate,
by quantifying and analyzing the details of chemistry and its sequential influence on
transpacific transport during spring and summer. From 2001 to 2010, the spring averaged
Asian sulfate enhancement in the western US model surface layer increases by 0.01-0.03
pg M-3 (about 4% of the surface sulfate concentration over the US at 2010 emission level).
The net chemical production rate increases in the troposphere over Asia but decreases over
the Pacific Ocean. The sulfate production pathway shifts toward gas phase production by
OH (7-8% increase-China; 2-3% increase-Upper atmosphere Pacific Ocean) with
corresponding reductions in aqueous phase production by H 2 0 2 ; resulting in a switch of the
most important production pathway from aqueous production by H 2 0 2 to gas phase
production by OH. Almost doubling the Asian NO. emission during the period has a
negligible influence on Asian sulfate enhancement over the US; this change in Asian NOx
emission causes the net chemical production rate to increase in the Asian boundary layer,
but to decrease in the upper Asian atmosphere, and in the atmosphere over the Pacific
Ocean and the US. This is because the Asian NOx emission growth strengthens the oxidizing
power in the Asian boundary layer to more actively form sulfate (mostly removed by wet
scavenging), reducing available SO 2 for further production away from the source. The
Asian NOx emission growth is the major driver in changing the sulfate production pathways
toward the gas phase. The 20% increase in Asian SO 2 emission during the period is the
primary driver in increasing Asian sulfate enhancement over the US; this change accelerates
net sulfate production rate, and makes a minor contribution to shifting toward gas phase
production by OH over the course of the transpacific transport. The calculated sulfate
column burden shows a notable increase as a response to the changes in Asian emissions.
However, the shifts in sulfate lifetime over China due to changes in Asian SO 2 and/or
Asian NO, emission are almost negligible (generally 1-2%), and lifetimes over the upper
level Pacific Ocean are generally reduced by 3-4%.
Thesis Supervisor: Susan Solomon
Title: Ellen Swallow Richards Professor of Atmospheric Chemistry & Climate Science
3
Acknowledgements
First and foremost, I would like to express my sincere gratitude to my advisor Professor
Susan Solomon, who is my esteemed promoter, thanks for accepting me as a graduate
student, your warm encouragement, thoughtful guidance, critical comments, and correction
of the thesis.
I would also like to thank the rest of my thesis committee: Professor Noelle Selin and
Professor Colette Heald for their encouragements and insightful comments that allowed me
the opportunity to develop deeper thoughts and understanding of this research.
Lastly, I would like to express gratitude toward my family. My father, Euoo Sung Min, for his
wisdom, bottomless support, and warm encouragements, which seeded the motivations for
me to dream of being a better person. My mother, Soon Uk Chung, for her warm heart,
measureless care and supports, which in all nurture me and give me the strength to take big
steps forward. My older sister, Yoonyoung Min, for her ardent enthusiasm as a scholar as
well as for being a loving and caring sister, and a lighthouse to shine and guide.
4
Introduction
Aerosols influence diverse aspects of the atmosphere as well as human well-being.
Some influences on our atmosphere include direct and indirect radiation effects [IPCC,
2013] and stratospheric ozone depletion [Solomon, 1999]. Sub-micro aerosols in the free
troposphere are primarily composed of sulfate and organic materials [Murphy et al., 2006],
which are major components of particulate matter. Particulate matter is a complex mixture
of extremely small aerosols (particles and liquid droplets), which is composed of various
chemical species, including acids (nitrates and sulfates), organics, metals, and dust particles
[Seinfeld and Pandis, 1998]. In remote national parks, particulate matter may cause
visibility impairment while in cities, particles are linked to severe health issues including
cardiovascular mortality, inflammatory lung injury, stroke mortality, and pulmonary
inflammation [Pope, 2000; Pope and Dockery, 2006].
In the past, the United States faced severe particulate matter pollution. This sparked
active mitigation on domestic particulate matter precursor emissions through legislative
actions, such as implementation of National Ambient Air Quality Standards (NAAQS) under
the Clean Air Act. Since then, emissions of precursors for particulate matter, including sulfur
dioxide (SO 2), have been successfully decreased and current particulate matter exceedances
of health related National Ambient Air Quality Standards (NAAQS) have decreased
dramatically (with remaining problems in regions over California and the eastern US).
However, reductions in health related issues didn't necessarily transit into reductions in
visibility impairment issues in wilderness areas and national parks. In order to solve these
problems, US Environmental Protection Agency (EPA) implemented the Regional Haze Rule,
which mandates reductions in domestic anthropogenic pollutant emissions over the period
of 2004 through 2018, with a goal of achieving "natural visibility conditions" by 2064 [US
EPA, 2003]. In the US, visibility problems are mostly due to aerosols including
carbonaceous, sulfate, nitrate, and soil dust [US EPA, 2003]. Domestic emission control will
be the primary concern, but prior studies have shown that transboundary influences from
foreign countries may hinder the goal of achieving "natural visibility conditions" [Park et al.,
2004; Park et al., 2006]. Also, prior studies showed that Asian sulfate affects visibility
problems, especially over the western US [Park et al., 2004; Park et al., 2006; Heald et al.,
2006].
In contrast to the successful reduction of particulate matter precursor emissions in
the US, China experienced a steep growth in particulate matter precursor emissions,
including sulfur dioxide (SO 2) and nitrogen oxides (NO, = NO + NO 2) over the last decade
[Granier et al., 2011; Zhao et al., 2013a, 2013b; Lu et al., 2011; Zhang et al., 2009; Streets,
2003; Lin et al., 2010,2013; Gu et al., 2013; Mijling et al., 2013], as an undesirable byproduct
of rapid economic advancement. This growth in emissions raised concerns in large cities of
China, as it could cause visibility impairment issues, as well as severe health problems such
as an increase in premature deaths and respiratory illness. Also, growth in Chinese
emissions can expand its influences on an intercontinental scale from China to the US and
Canada through long-range transport of Asian pollution going across the Pacific. Thus, there
have been a number of studies that analyze enhancements of particulate matter levels over
North America attributable to Asian influence [Park et al., 2004; Park et al., 2006; Heald et
al., 2006; Dunlea et al., 2009; Liu et al., 2008].
The episodic transpacific transport of Asian pollution plumes monitored through
satellite data [Heald et al., 2006; Zhang et al., 2008; Lin et al., 2012], showed a signature of
Asian plumes that are lofted from the Asian boundary layer and reach the US. Aircraft
measurements provided detailed snapshots of transpacific transports of Asian pollution
through chemical composition analysis on plumes that were encountered while crossing the
Pacific Ocean over to the US, focusing especially on air masses with distinctive
5
characteristics attributable to Asia. Often, plumes with notably high sulfur content are
found to originate from Asia [Liang et al., 2007; Lin et al., 2012; Dunlea et al., 2009;
Donkelaar et al., 2008; Andreae et al., 1988]. Also, high contents of chemical tracers, such as
methyl tertiary butyl ether (MTBE) and Halon H-1211, are used as indicators of Asian origin
[Dunlea et al., 2009]. Plumes with large amounts of MTBE, which is used as a gasoline
additive in Asia, signal that the air masses are from Asia because North America no longer
uses MTBE as gasoline additives [West et al., 2007]. Plumes with high Halon H-1211 level
may similarly be identified to be from Asia, since this Halon is largely linked to emissions
from developing countries like China [Barletta et al., 2006].
These Asian plumes are most often identified within the free troposphere over the
Pacific Ocean, and they can be traced over land, including the western US [Zhang et al.,
2008; Heald et al., 2006] and Canada [Donkelaar et al., 2008]. Aerosols in the free
troposphere over the Pacific are largely from Asian origin [Andreae et al., 1988; Donkelaar
et al., 2008], especially from East Asia [Donkelaar et al., 2008], and are estimated to account
for more than 50% of sulfate column burden between 500-900hPa in the free troposphere
over British Columbia [Donkelaar et al., 2008]. The lower free troposphere (600-900 hPa) is
a main pathway for transpacific transport of aerosols [Heald et al., 2006] including sulfate
[Donkelaar et al., 2008], because the marine boundary layer has active wet scavenging
[Heald et al., 2006]. Also, during lifting, condensation and wet deposition efficiently
removes pollutants in the boundary layer [Brock et al., 2004; Donkelaar et al., 2008]. In the
free troposphere, an increasing trend of ozone during the period under higher Asian
influence has been recognized [Cooper et al., 2010], which further supports active transport
of Asian pollution.
The plumes that enter the US boundary layer are less readily identifiable at the
surface, presumably due to dilution of plumes while descending into the boundary layer
[Jaffe et al., 2005; Price et al., 2004; Zhang et al., 2008; Lin et al., 2012]. Thus, the influence
of Asian pollution on the surface is manifested as an increase in background pollution levels
[Zhang et al., 2008], rather than as distinguishable plume air masses. This increase may be
associated with simultaneous large-scale enhancement of pollutants such as sulfate and
ozone over the western US in the surface layer, which has been attributed to increases in
Asian emissions [Zhang et al., 2008; Cooper et al., 2011; Zhang et al., 2011]. Modeling
studies indicate a significant Asian sulfate enhancement in the surface layer, with Asian
sulfate accounting for about 30% of total sulfate background levels over the western US
(pollutant emissions held at 2001 level) [Park et al., 2004; Heald et al., 2006], in contrast to
Asian nitrate and Asian ammonium, which was found to have almost negligible influence
[Park et al., 2004].
A number of prior studies based on observational data give further evidence of
Asian sulfate influences over the US. Episodes of elevated sulfate concentrations with
signals of Asian origin have been observed [Jaffe et al., 1999, 2001, 2003a, Bertschi et al.,
2004], and the sulfate concentration from surface data averaged only for selected days with.
maximum simulated Asian influence is found to be higher than the seasonal average [Heald
et al., 2006]. A high spatial correlation with no significant time lag on Asian aerosol levels
among an ensemble of northwestern US sites with geographic proximity were found in a
modeling study [Heald et al., 2006], in concordance with observed large scale influences of
Asian sulfate over the northwestern US. Often Asian dust and sulfate have been found
together, with high correlations between Asian sulfate contributions predicted from a
model and observed dust concentrations from surface measurement sites in the US [Heald
et al., 2006]. Studies on other chemical species, like ozone, further support an Asian
influence on the US surface layer. Asian ozone is considered to be more efficiently
transported than Asian sulfate, because the wet removal efficiency of particulate matter is
6
higher than that of ozone [Park et al., 2004; Dunlea et al., 2009; Brock et al., 2004]. Increases
in surface background ozone over the US have also been attributed to rising Asian
emissions, [Zhang et al., 2008; Cooper et al., 2011; Zhang et al., 2011; Fiore et al., 2002]
which is supported by the finding that 11 widely scattered surface ozone measurement sites
in the western US showed similar inter-annual correlations (sometimes with almost
simultaneous enhancement in nearly all sites for certain times), which suggest a large scale
process operating across those regions when Asian influences are relatively large [Jaffe et
al., 2010; Jaffe & Ray, 2007].
Mechanisms for lofting pollution from the Asian boundary layer, and the subsequent
transport across the Pacific have been investigated [Liang et al., 2004; Liu et al., 2003;
Dickerson et al., 2007; Luan et al., 2012]. Pollution in the Asian planetary boundary layer is
typically lofted into the free troposphere through frontal lifting (associated with warm
conveyor belts within midlatitude cyclones), deep convection, orographic forcing, and
mountain-chimney effects [Cooper et al., 2004; Liang et al., 2004; Liu et al., 2003; Dickerson
et al., 2007]. Warm conveyor belt lifting has been identified as the more important transport
pathway, and a few transport events associated with warm conveyor belt lifting have been
shown to cause notable effects over the Pacific and the US [Cooper et al., 2004; Lin et al.,
2012; Dunlea et al., 2009]. These warm conveyor belts are particularly active during spring,
and work as the major mechanism throughout the spring season [Liang et al., 2005; Luan et
al., 2012]. In general, the maximum Asian pollution outflow occurs during spring [Jacob et
al., 2003; Jaffe et al., 2010; Zhang et al., 2008, 2011; Lin et al., 2012]. In addition to warm
conveyor belt lifting, convective processes can be also important in summer for a longrange transport [Thompson et al., 1994; Li et al., 2005; Luan et al., 2012]. While ascending
to the free troposphere, air masses often experience cloud formation and precipitation
processes. Consequently, soluble gas species as well as hygroscopic aerosols are lost
through wet scavenging. Once in the free troposphere, lifted Asian pollutants are efficiently
transported across the Pacific to the west coast of the US by prevailing free-tropospheric
westerlies, while experiencing chemical transformation on a time scale of about 1-2 weeks
[Jaffe et al., 2001; Stohl et al., 2002].
A sequence of chemical transformations affecting Asian aerosols and aerosol
precursors has been mapped out: from ascending in the Asian boundary layer, crossing the
Pacific, to descending over the North America, based on findings from aircraft campaigns,
which examined the particle size distribution and composition measurements of plumes
with Asian signatures [Brock et al., 2004; Dunlea et al., 2009]. The efficiency of transpacific
transport of Asian pollution in relation to chemistry depends on sequential linkages among
precursor emissions and corresponding chemistry: in the planetary boundary layer over the
source region, wet scavenging removal of soluble gas species and hygroscopic particles
during lifting to the free troposphere, and interactions of species available for further
chemical transformation in the free troposphere over the Pacific during the transport
[Brock et al., 2004; Dunlea et al., 2009]. Even though two prior studies by Brock et al. and
Dunlea et al. paint a general picture of the chemistry involved in transpacific transport,
much of the topic remains to be studied as was pointed out in Dunlea et al., 2009. Two
studies focussed on measurements of individual plume events encountered during aircraft
campaigns; thus, they are limited to providing insights on the mean temporal and spatial
chemistry as well as detailed chemistry (such as identification of major chemical production
pathways, the responses to the recent changes in precursor emissions in Asia, and
quantification of the responses in chemistry and physics for transpacific transport); these
objectives all can be achieved with simulation studies using global chemistry circulation
models. Other studies analyzed overall changes in the Asian sulfate influence over the US
[Liu et al., 2008] and shifts in the chemical production pathway over the source region (East
7
Asia/Southeast China) [Manktelow et al., 2007; Barth & Church, 1999], with sensitivity
testing on sulfur dioxide emissions in East Asia/Southeast China for the years 1985 and
2000. However, not enough attention has been given to the response to concurrent shifts in
oxidizing agents or NO, emissions in the region, despite dramatic changes that have
occurred - e.g., almost doubling of the Chinese NO, emissions from 2000 to 2010. The
chemical coupling between ozone precursors (including NO) and sulfate [Unger et al., 2006;
Meng et al., 1997] has been analyzed for regional chemistry over China, but the chemical
relationships also occur during long-range transport, which is further complicated by the
coupling between chemistry and physics during the transpacific transport of Asian pollution.
The objective of this study is to the broaden the knowledge of the influence of recent
(2001 to 2010) Asian emissions changes on transpacific influences on particulate matter,
especially on sulfate, by quantifying and analyzing the details of chemistry, and its
sequential influence on a transport by (1) comparing the simulation result on Asian
particulate matter influences at the US surface against selected prior modeling studies as a
model validation step, (2) studying the alterations in Asian sulfate influence over the US
with various emission inventory scenarios, (3) identifying the current major chemical
production pathway of sulfate and understanding changes in chemistry over China as well
as over the Pacific Ocean with various emission scenarios, (4) analyzing the reasons for
responses over the Pacific, and (5) considering the possible influence of shifts in major
production pathway on sulfate column burdens and lifetimes of sulfate over China and the
Pacific Ocean.
Background Chemistry
&
From the planetary boundary layer over China, freshly emitted pollutants
experience chemical transformations and form secondary pollutants, such as nitrate, sulfate,
and ozone through oxidation. Ozone is mainly formed from oxidation of carbon monoxide
(CO), methane (CH 4 ), or nonmethane volatile organic compounds (NMVOCs) in the presence
of nitrogen oxides (NOx = NO + NO 2 ). The main source for nitrate aerosol formation is
oxidation of NO. to form nitric acid (HNO 3), which is subsequently neutralized with
ammonia (NH 3). Sulfate is formed from oxidation of SO 2 to sulfuric acid (H 2SO 4) through
three major chemical production pathways, which are gas phase production by hydroxyl
radical (OH), aqueous phase production by hydrogen peroxide (H 2 0 2 ), and aqueous
production by ozone in a presence of metal catalyst. Sulfuric acid can exist in the gas phase
but is extremely soluble (vapor pressure of H 2 SO 4 is much lower against H 2 SO 4 -H 2 0), so
sulfuric acid is mostly in aqueous (aerosol) phase [Seinfeld and Pandis, 1998]. Gas phase
production by OH and aqueous production by H 20 2 are relatively more significant than
aqueous production by 03 globally, as well as over China [Manktelow et al., 2007, Barth
Church, 1999; Unger et al., 2006].
Sulfate and nitrate are both neutralized by ammonia to form ammonium sulfate
((NH 4 ) 2 SO 4 ) and ammonium nitrate (NH 4NO 3), respectively. However, neutralization of
sulfate proceeds, and then neutralization of nitrate follows next, only if excess ammonia is
available after neutralization of sulfate. Ammonium sulfate is predominantly in condensed
(aqueous or solid) phase because ammonium sulfate does not exist in equilibrium with the
gas phase. In contrast, ammonium nitrate is semi-volatile and exists in equilibrium with the
gas phase as nitric acid and ammonia. Therefore, under low ammonia condition, ammonium
nitrate can dissociate to release gas phase ammonia, which can subsequently experience
irreversible reaction with sulfuric acid to form condensed ammonium sulfate [Kuhns et al.,
2003]. This rather simple thermodynamics of H 2SO 4-HNO 3-NH 3 provides an accurate
representation [Sienfeld and Pandis, 1988].
8
,
The chemical transformations Asian pollutions experience during a lift from the
boundary layer over Asia to the free troposphere are well outlined in Brock et al., 2004 and
Dunlea et al., 2009, which are based on aircraft measurements of particle size distributions
as well as particles and gas species composition analysis. In summary, while Asian air
plumes in the Asian boundary layer are lifted to the free troposphere, highly soluble gas
phase species such as HNO 3, H 2SO 4, NH 3, and hygroscopic particle including sulfateammonium-nitrate-organic composites are incorporated into clouds, and then are removed
through precipitation. The insoluble species such as 03 and peroxyacetyl nitrates (PANs)
remain high in concentration without much influence from the wet removal process. A
substantial amount of semi-soluble species such as SO 2 may survive the lifting process;
however, efficiency depends on a number of conditions that controls the oxidizing rate of
SO 2 , such as PH of the cloud droplets aerosols are being incorporated, and availability of
oxidizing agents including hydrogen peroxide, ozone, and metal catalysts. A number of
numerical studies as well as observation measurements support that a significant amount
of SO 2 survives after a rigorous wet scavenging process in convective clouds [Brock et al.,
2004; Dunlea et al., 2009; Barth, 1994; Crutzen and Lawrence, 2000; Kreidenweis et al.,
1997]. Thus in the free troposphere over the Pacific, a modest concentration of SO 2
significant amount of PANs [Miyazaki et al., 2003] and ozone [Zhang et al., 2011], and
relatively much lower concentrations of soluble gas species (HNO 3, H 2 SO 4 , and NH 3) and
hygroscopic particles (including sulfate-ammonium-nitrate-organic composites) are
expected to exist [Brock et al., 2004; Dunlea et al., 2009]. Across the Pacific, oxidation of
species including SO 2 is expected to continue [Dunlea et al., 2009; Brock et al., 2004],
leading to the overall particle growth in these plumes. Subsequently when air masses
descend over the Pacific or the US, PAN will thermally decompose to release NO., which will
be eventually oxidized to HNO 3 on a timescale of about a day at typical tropospheric OH
concentrations [Seinfeld and Pandis, 1998].
Model description and simulation set up
The GEOS-Chem (version 9-01-03, http://acmg.seas.harvard.edu/geos/) global
chemical transport model (CTM) with coupled oxidant-aerosol chemistry [Park et al., 2004]
is used to simulate the sulfate-ammonium-nitrate inorganic aerosol system for springs and
summers of 1985 and 2010 with varying emission scenarios. The year 2010 is simulated
under fixed meteorological conditions at 2010, but with several emission scenarios in order
to analyze a genuine influence shift in Asian emissions may have on chemical relationships.
The year 1985 is simulated to consider a condition before the accelerated emission growth
in 2000s over China, and to compare with prior studies, which used different models and
emission inventories. The GEOS-Chem model is driven by assimilated meteorological data
from the Goddard Earth Observing System (GEOS-5) at the NASA Global Modeling and
Assimilation Office (GMAO) with 10 latitude by 1' longitude horizontal resolution and with
native vertical grid of 72 hybrid eta levels, extending from the surface up to 0.1 hPa. For
computational efficiency, horizontal resolution is degraded to 20 x 2.50 and reduced vertical
grid is used (total of 47 vertical eta levels). The top 37 vertical levels from -80hPa to
0.01hPa are lumped into 11 levels to reduce the vertical grids. The first 31 vertical levels
from the surface are pure sigma levels and levels beyond are at fixed pressure levels. The
temporal resolution of the GEOS-5 meteorology is 6 hours with shorter time frame of 3
hours for surface variables and mixing depths. For simulations of year 1985, Modern-Era
Retrospective Analysis for Research and Applications (MERRA) from Modeling and
Assimilation Data and Information Service Center (MDISC) is used because GEOS-5 is not
offered extending back to1985.
9
The coupling between ozone- NO. -hydrocarbon-aerosol chemical mechanism and
the H 2SO 4-HNO 3-NH 3 aerosol thermodynamics provided in GEOS-Chem [Park et al., 2004;
Park et al., 2006] is an essential modeling tool used in this study. The details of this scheme
are provided in Park et al., 2004. In short, tropospheric oxidant chemistry is driven by a
detailed ozone- NOx -hydrocarbon chemical mechanism that involves about 80 species with
about 300 reactions [Park et al., 2004], and is coupled to the aerosol scheme through
aerosol formation, heterogeneous reactions, and aerosol's influence on photolysis rates
[Martin et al., 2003]. GEOS-Chem aerosol scheme incorporates the sulfate-ammoniumnitrate system [Park et al., 2004], as well as carbonaceous aerosols [Park et al., 2003; Liao et
al., 2007], mineral dust [Duncan Fairlie et al., 2007], and sea-salt [Jaegle' et al., 2011]
systems. An external mixture of aerosols with lognormal size distributions is assumed.
Thus, the GEOS-Chem aerosol scheme does not provide aerosol micro-physics and it does
not model internal mixtures of aerosols. From this aspect, GEOS-Chem may be a less
favorable model to be used for studying the radiative influence of aerosols, which is
sensitive to internal and external mixtures of aerosols. However, GEOS-Chem offers
information on mass concentration of aerosols and it should be sufficient for the purpose of
this study.
GEOS-Chem uses a standard resistance-in-series dry deposition scheme for gas
species and aerosol species including sulfate that depends on surface type and
meteorological condition [Wesely, 1989; Wang et al., 1998]. Dust and sea salt dry
depositions follow the size dependent scheme by Zhang et al., (2001). Wet deposition is
implemented for both soluble gas species [Park et al., 2004] as well as hydrophilic aerosols
[Liu et al., 2001]. The aerosol wet deposition scheme implements convective updraft
scavenging, rainout and washout from large-scale precipitation, as well as return of aerosols
to the atmosphere following evaporation. Wet scavenging of soluble gas species depends on
various factors including their effective Henry's law partitioning [Mari et al., 2000; Park et
al., 2004], and scavenging of SO 2 is influenced by the local availability of H 20 2 as a fast
aqueous-phase oxidant [Park et al., 2004].
Five different sensitivity simulations with linear scaling on Asian NOx and/or Asian
SO 2 emissions are conducted for spring and summer of 2010: (1) the default standard
simulation with all emissions at 2010 level (hereafter referred to as "2010_base") and four
sensitivity simulations with linear scaling on (2) Asian NOx emissions, scaled to correspond
to the 2001 emission level ("2010BNOX01"), (3) Asian SO 2 emissions scaled to correspond
to the 2001 emission level ("2010B S0201"), (4) both Asian NOx and Asian SO 2 emissions
scaled to the 2001 emission level ("2001_base"), and (5) Asian NOx and Asian SO 2 emissions
turned off ("2010B zero"). For all five simulations, all pollution precursor emissions (except
Asian NOx and/or Asian SO 2) are held at the 2010 emission level, which are automatically
obtained from a base year emission inventory to 2010 following a default GEOS-Chem
scaling method. The "1985_base" simulation uses the monthly Asian emission at base year
2004 that is scaled down to the emission level at 1985 using the automatic GEOS-Chem
scaling method. For every simulation, a spin up time of at least six months are allowed,
generally starting the simulation from September of the prior year. The analysis is taken for
the following spring and summer. The summary of the simulations and corresponding
emission scenarios are outlined in Table 1. The results from these simulations are used to
analyze a number of different concepts presented in this study. Please refer to Table 2 for
detailed explanations of the calculation method, goals, and abbreviations used for cases that
are addressed in the following analysis.
The standard global emission inventory for GEOS-Chem is GEIA [Benkovitz et al.,
1996], which is replaced by regional inventories such as NE199
(http://www.epa.gov/ttn/chief/net/1999inventory.html) for the US, BRAVO [Kuhns et al.,
10
,
2005] for Mexico, EMEP emissions for Europe (http://www.emep.int/), and Zhang et al.,
(2009) for East Asia. The base year (2006) emission inventory for Asia [Zhang et al., 2009]
was developed in order to support the Intercontinental Chemical Transport ExperimentPhase B (INTEX-B) campaign during April and May of 2006. The relative emission trends
used for automatic GEOS-Chem emission scaling over Asia are from the Regional Emission
inventory in ASia (REAS) [Ohara et al., 2007]. REAS provides gridded emission inventories,
which include better spatial detail than national scale emission inventories. The linear
scaling on Asian NOx and Asian SO 2 emissions for sensitivity runs is implemented by
multiplying the base year emissions (at 2006) with a scalar number as an estimate. The
down scaling to 2001 emission level is conducted by dividing the default base year's 2006
emission by China's estimated anthropogenic emissions growth rate during 2001 to 2006
from Zhang et al. (2009). For example, the base year (at 2006) Asian anthropogenic SO 2
emission inventory (47 Tg) used in the model is divided by 1.36 (from the estimated 36%
increase of Chinese SO 2 emissions from 2001 to 2006 in Zhang et al., 2009) to represent the
Asian SO 2 emission level at 2001. Considering the steep rise in Chinese emissions, scaling of
the whole Asian emission inventory by Chinese emission growth rate might give an
exaggerated perspective of the shift in Asian emission inventory. However, bearing in mind
that the research focuses on transpacific transport of Asian pollution, which is
predominantly comprised by East Asian emissions [Donkelaar et al., 2008] with the largest
contribution from China, the scaling method used above should be sufficient. Below, the
validity of this scaling method is discussed by comparing with prior studies. The percentage
shift of Asian NOx and Asian SO 2 emissions from 2006 to 2010 is estimated based on
reviews of several emission inventories [Granier et al., 2011; Lu et al., 2011; Zhao et al.,
2013a], technology and economy based bottom up inventory analysis [Zhao et al., 2013a,
2013b; Lin et al., 2013; Lu et al., 2011], and satellite observation estimates [Lamsal et al.,
2011; Lin et al., 2013; Li et al., 2010; Lin et al., 2010; Gu et al., 2013; Mijling et al., 2013; Lu
et al., 2011]. The linear scaling on Asian NO, and Asian SO 2 to the 2010 level is conducted by
multiplying the base emission level (2006) by an estimate of about a 10% reduction in SO 2
and about 30% increase in NOx from 2006 to 2010 [Granier et al., 2011; Zhao et al., 2013a,
Zhao et al., 2013b; Lu et al., 2011; Lin et al., 2010, 2013; Li et al., 2010; Gu et al., 2013;
Mijling et al., 2013; Lamsal et al., 2011]. Monthly average Asian SO 2 and Asian NOx emissions
used in the simulations are 1.76 Tg S/month (spring) and 1.16 Tg N/month (summer) for
2010 and 1.43 Tg S/month (spring) and 0.58 Tg N/month (summer) for 2001, respectively.
In short, Asian SO 2 emissions increased by about 22% and NOx emissions increased by
about 100% from 2001 to 2010.
The year 2010 was chosen for this study because meteorological conditions during
the spring of 2010 were particularly favorable for Asian inflow into the US. The winter of
2009-2010 was under a strong El-Nino condition, and the transport of Asian pollution
during the following spring may be enhanced [Jaffe, 2010] due to elongated eastward
extension of the subtropical jets [Koumoutsaris et al., 2008]. Moreover, the amplification of
the upper level pattern of a weak ridge over the northeastern Pacific Ocean, a trough over
California and a ridge over the central US in 2010 with a surface anticyclone over the midlatitude northeastern Pacific Ocean, enhanced subsidence and northwesterly surface winds
over California, allowed more efficient Asian pollution transport into the US boundary layer
[Cooper et al., 2011]. The two seasons, spring and summer, are analyzed in this study
because spring is the season with the strongest transpacific transport of Asian pollution
[Jacob et al., 2003; Jaffe et al., 2010; Zhang et al., 2008, 2011; Lin et al., 2012], and summer
has stronger convective lifting in the Asian boundary layer [Thompson et al., 1994; Li et al.,
2005; Luan et al., 2012], which may alter relative significance amongst transport pathways
11
and also change the upper level chemistry due to the enhancement in wet scavenging of
aerosols and soluble gas species.
Results
Model validity from prior studies
Asian enhancements of the aerosol concentration ("2001Asian" and "201OAsian";
Table2) are defined as a contribution of Asian emissions on augmenting pollution levels,
which are calculated by subtracting the aerosol concentration simulated with Asian
emissions turned off ("201OB_zero") from that of a chosen sensitivity simulation
("2001_base" and "2010_base", respectively). Park et al., (2004) and Heald et al., (2006)
provided excellent analyses of the validity of GEOS-Chem (version 5.03 & 7.01.02,
respectively) simulation on particulate matter concentration in the US model surface layer
by comparing the simulated results with measurements from surface observational
networks, such as Interagency Monitoring for Protected Visual Environments (IMPROVE)
and Clean Air Status and Trends Network (CASTNET). These studies also examined the
validity of GEOS-Chem simulations on Asian sulfate enhancements by comparing the
simulation results with aircraft measurements [Park et al., 2004] and IMPROVE data [Heald
et al., 2006].
Park et al., 2004 conclude that the model does not overestimate transpacific outflow
of sulfate from Asia over the northwest Pacific between February and April of 2001 by
comparing the observed mean vertical profiles of non-sea-salt sulfate concentrations from
GEOS-Chem to measurements from Transport and Chemical evolution over the Pacific
(TRACE-P) aircraft mission. In fact, Park et al., 2004 suggest that the simulated column
burden of non-sea-salt sulfate in the lower atmosphere (0-5km), where active Asian outflow
is observed from aircraft missions, is underestimated in the model by up to a factor of two.
Heald et al., 2006 conclude that GEOS-Chem can simulate the magnitude of Asian
sulfate enhancement in the model surface layer over US with reasonable accuracy by taking
an average of IMPROVE data selected for specific days for each grid box that are under
maximum local Asian influence and comparing this against an average of simulated Asian
sulfate enhancement during the corresponding days. The study shows that the seasonal
average of surface sulfate concentration during maximum Asian influence (1.04 pg m-3 ) was
higher than the seasonal mean (0.69 ptg M- 3 ) over the northwestern US (where Asian
influence was highest in the study) by a magnitude that is comparable to the simulated
Asian enhancement in the model (0.44 gg M-3 ) averaged during the selected days. Also, the
simulated Asian aerosol concentrations among ensembles of IMPROVE sites with spatial
proximity in the northwestern US were found to have a high spatial correlation (95%) with
no significant time lag. This Asian sulfate influences on the western US surface sites were
also found in prior literature based on observational data [Jaffe et al., 2001, 2003a; Bertschi
et al., 2004], supporting the large scale plume events over the region.
Heald et al., 2006 estimate 50% uncertainty on GEOS-Chem's ability to simulate
Asian sulfate enhancement through statistical analysis of bias and correlation with
northwestern IMPROVE sites under days with large Asian influence. The study found that
when the Asian influence is strengthened (by increasing Asian emissions), the correlation
and the bias (compared to IMPROVE) are both increased; and when the Asian influence is
zeroed out (by turning off Asian emissions), the correlation and bias are both lowered. An
underestimate in Asian SOx (SO 2 + SO 4 -) outflow during the TRACE-P campaign in spring of
2001 was suggested [Park et al., 2004], and an error in the SO 2 oxidation rate was suggested
from the conclusion that simulated SO 4 concentrations were higher in comparison to
12
measurements during the ACEAsia campaign [Heald et al., 2005] as well as during the
INTEX-B campaign [Donkelaar et al., 2008].
Asian Aerosol influence over the US at 2001 emission level: comparison with
prior studies
Prior studies [Park et al., 2004; Heald et al., 2006] and this study report similar
Asian particulate matter concentrations in the model US surface layer at the 2001 emission
level (Figure 1), confirming the validity of the rather simple linear scaling used for
estimating Asian emissions in this study. The springtime mean Asian sulfate enhancement
in the US model surface layer simulated with emission levels at 2001 and meteorology of
year 2010 ("2001Asian" case in Table 2) is highest over the southwestern US, reaching
nearly 0.14 pg M-3 and decreasing toward the northwestern US (to 0.06-0.07 pg M- 3 ) and the
eastern US (Figure 1-a & Figure 2-a). Relatively prominent northwesterly transport of Asian
sulfate (Figure 1-a) towards the southwestern US signals the influence from sustained
surface anticyclone over the northeastern Pacific Ocean during the spring of 2010. The
mean Asian sulfate enhancement in the model surface layer estimated in this study closely
matches the results from prior studies. The estimates of prior studies reported 0.1 Vg m-3 as
an annual average [Park et al., 2004] and 0.16 jg M-3 as a spring time average [Heald et al.,
2006] over the northwestern US, which is the location of maximum Asian influence driven
with the meteorology of 2001. The locations of maximum Asian sulfate influence are
probably different between this study (southwestern US) and prior studies (northwestern
US), because this study used the meteorology of 2010 while the prior studies used
meteorology of 2001 for their simulations.
The percentage of Asian sulfate concentration to the total surface sulfate
concentration at 2001 emission level is calculated by dividing Asian sulfate enhancement
("2001Asian") by the surface sulfate concentration ("2001_base"), hereafter referred to as
"2001Asian/2001" case (Table 2). The percentage is highest over the northwestern US and
has a rapid fall toward the central and eastern US (Figure 1-b; Figure 2-b). The high
percentage of Asian sulfate concentration (more than 20%) in the northwestern US can be
attributed to a large Asian sulfate concentration over the region, as well as a low total
surface sulfate concentration over the region. The total surface sulfate concentrations over
the northwestern US are low because the domestic emissions of sulfate precursors over the
region are relatively small. In comparison, the low percentage of Asian sulfate over the
eastern US are primarily attributable to high domestic sulfate precursor emissions that
augment the surface sulfate concentration significantly.
In contrast to Asian sulfate influences, enhancements from Asian nitrate and Asian
ammonium are much less pronounced at the surface layer (Figure 1-c&e), despite observed
Asian outflow from flight measurements over Asian marine boundary layer. One of the
reasons can be attributed to an active deposition loss of aerosols in the boundary layer,
which was discussed as a major difference between CO transport pathway and aerosol
transport pathway [Heald et al., 2006]. Despite high NOx emissions in the source region and
successful Asian NOx transport resulting in non-negligible Asian contributions in
augmenting surface ozone levels over the US [Lin et al., 2012; Cooper et al., 2011; Zhang et
al., 2008, 2011; Fiore et al., 2002], Asian nitrate concentration ("2001Asian") is negative in
the model US surface layer over most of US (Figure 1-c). This indicates that the total nitrate
concentration over the US decreases when Asian NOx and Asian SO 2 emissions are turned on,
which is also illustrated in Park et al., 2004. Although the negative contribution could be
considered counter-intuitive, this result can be explained from the thermodynamic
relationship between sulfate and nitrate that was discussed in the background chemistry
section. The probable reason for the negative contribution on nitrate concentration by
13
Asian pollution is that the amount of available ammonium to neutralize nitrate decreases
when Asian sulfur dioxide is turned on. The ammonium available for neutralization of
nitrate is reduced because the enhancement in sulfate concentration due to augmentation
from Asian sources leads to more consumption of ammonium by sulfate for neutralization,
which proceeds ahead of neutralization of nitrate. This explanation is further supported
with our finding that in contrast to the "2NOX" case (Table 2), which has a large increase in
nitrate concentration over most of the atmosphere, with notable enhancements in nitrate
concentration reaching the upper atmosphere over the US (more than 30%), the "1.2 S02"
case illustrates a significant reduction in nitrate concentration over the whole atmosphere
(with more than 40% reduction) and extends beyond the eastern US boundary (Table 2;
Supplemental Figure 1). This indicates that the increase in Asian NO, emissions alone from
2001 to 2010 enhances the Asian nitrate concentration over the US, while increases in Asian
SO 2 emission alone from 2001 to 2010 leads to a significant reduction in nitrate
concentration over the US. Another interesting finding on Asian nitrate is that Asian nitrate
represents about 20-50% of the total nitrate concentration over the Arctic region at 2001
emission levels (Supplemental Figure 2).
The enhancement in ammonium concentration from Asian sources in the US surface
layer is not substantial, but is non-negligible; ranging from approximately 0.05 pg m-3 in the
southwestern US and decreasing to 0.02 pg M-3 in the central US (Figure 1-e). However, the
contribution in terms of concentration is clearly not as prominent as that from Asian sulfate.
Park et al., (2004) stated that Asian ammonium enhancement is negligible over the US by
illustrating that Asian ammonium concentration ranges between 0-0.1 jIg M- 3 over the
whole US. This range is in broad agreement with the results of our study; however, further
analysis in this study indicates that this small contribution (ranging around 0.03-0.05 jg m3) may have significance in terms of the percent of Asian ammonium over the total
ammonium concentration. In the western US, the percent of Asian ammonium relative to
the total ammonium concentration is about 10-20%. The result should be interpreted with
caution, since ammonia emissions are highly uncertain and the significance of this finding is
unclear. Thus, for the following sections Asian sulfate is the focus of this study since it plays
a dominant role in enhancing the pollution level over the US, in contrast to the negative
nitrate enhancement and uncertain Asian ammonium influence.
Shifts in Asian sulfate influence over the US from 2001 to 2010 approached
with various emission scenarios
The response of Asian sulfate enhancement over the US to the rapid growth of Asian
NO, and SO 2 emission from 2001 to 2010 is studied by implementing various emission
scenarios to modeling simulations, as they are summarized in Table 1. Please refer to Table
2 for the various "cases" used for analysis in the following. These various emission scenarios
break down the historical shift in Asian sulfate enhancement from 2001 to 2010 into
contributions from individual shifts in Asian NO. or Asian SO 2, and test whether
simultaneous enhancement of Asian NO. and Asian SO 2 together has a constructive or
destructive relationship on Asian sulfate enhancement.
Asian sulfate enhancement in the model surface layer over the US is calculated for
various emission scenarios (Figure 2; Table 2). The increase of Asian sulfate concentration
from 2001 to 2010 in the US surface layer due to the simultaneous increase in Asian NO. as
well as Asian SO 2 is calculated by subtracting surface sulfate concentration of "2001_base"
from "2010_base" ("2001to2010 case" in Table 2; Figure 2-c). From 2001 to 2010, Asian
sulfate enhancement increases by about 0.03 jg M- 3 in the southwestern US to 0.01 jg M-3 in
the intermountain region. The eastern US (more than 0.02 jig M- 3 ) has a greater
enhancement than the intermountain region (0.01-0.02 pg M- 3 ). The increases of Asian
14
sulfate concentration corresponds to about 3-4% of the total sulfate concentration at 2010
emission level over a broad region in the western US ("2001to2010/2010" in Table 2;
Figure 2-d). The percentage of increase in Asian sulfate concentration from 2001 to 2010
over the total concentration of sulfate at 2001 condition ("2001to2010/2001", not shown)
is about 5%. The response of the Asian sulfate enhancement over the US to doubling of
Asian NO. emissions ("2NOX" case) is almost negligible (Figure 2-e,f). In contrast, "1.2 S02
case" dominates the shift in Asian sulfate from "2001to2010" case, and is slightly smaller
than when both Asian NOx and Asian SO 2 emissions are elevated, especially over the
northeastern Pacific Ocean (Figure 2-g,h). Thus, the strengthening in Asian sulfate
enhancement is predominantly attributable to about 22% rise in SO 2 emissions over Asia.
The enhancement caused by increase of Asian NOx emission increase is much less
pronounced and the simultaneous enhancement in Asian NOx and Asian SO 2 together has a
small constructive relationship.
Recent shift in Asian emissions and responses of chemistry during transpacific
transport.
&
The alterations in chemistry along the path of transpacific transport (from the
Chinese boundary layer to the upper levels over the Pacific Ocean) are analyzed to
understand possible reasons behind different Asian sulfate enhancements over the US as a
response to implementation of various emission scenarios. Prior studies analyzed the
sulfate chemistry over the Southeast Chinese/East Asian boundary layer for 1985 [Barth
Church, 1999; Manktelow et al., 2007] and 2000 [Manktelow et al., 2007]. By conducting
sensitivity analysis on SO 2 emissions over the source region, two studies investigated shifts
in sulfate chemical production pathways and the resulting alterations in sulfate burden and
lifetime over Southeast China/East Asia. In this section, analysis from the two prior studies
are further extended geographically, including Chinese boundary layer and the upper level
atmosphere over the Pacific Ocean (major pathway for Asian outflow) to consider the
transpacific transport. These findings are also advanced provisionally to consider recent
shifts of chemistry from 2000 to 2010, which involve the rapid doubling of NOx emissions
during the period and about 22% increase in SO 2 emissions. This analysis involving the
influence of doubling of NOx on sulfate chemistry leads to a further investigation on the
influence of oxidants on sulfate production pathway, which is a novel approach taken in
studies of transpacific transport. The analysis also includes the shifts in chemical
production pathways and their sequential influences on sulfate burden, lifetime, and
efficiency of transport from China to the Pacific.
Major chemical production pathways and the responses to the recent shift in
Asian emissions
The production rate of secondary pollutants, such as ozone and sulfate, depends on
the availability of oxidants, which relies on ozone precursor emissions, such as NOx and
Volatile Organic Carbons (VOCs), near the source. A small number of prior studies analyzed
chemical couplings between ozone precursors (including NO) and sulfate [Unger et al.,
2006; Meng et al., 1997], and found that changing oxidant concentrations can lead to about
20% increases in the sulfate burden in the surface layer over China [Unger et al., 2006].
However, the implications of the relationship have not been fully understood and remain to
be explored. Generally, an elevation in sulfate levels may have an influence on ozone
concentrations by slowing down the rapid chain production of ozone through enhancing
heterogeneous conversion of NOx to nitric acid (HNO 3), which is a reservoir species [Unger
et al., 2006]. However, with the recent change in China's steep growth of NOx emissions, it is
more important to consider influences that ozone precursors may have on sulfate by
15
changing the total available oxidant budget and by shifting the partitioning of
concentrations amongst major oxidizing agents.
Nitrogen oxides are important precursors for ozone, and ozone is a source for the
production of the hydroxyl (OH) radical. Hydroxyl radical is a precursor for hydrogen
peroxide (H 2 0 2 ) indirectly; thus, increases in OH levels from a rise in NO, concentrations
may lead to enhancement in H 2 0 2 levels. However, the relationship is more complicated
because formation of H 2 0 2 is less favored when the NOx concentration is high. The reason is
because the Hydro-peroxy radical (HO 2 ) self-reaction, which forms Hydrogen peroxide
(H 20 2) as the termination step of a chain mechanism for ozone production, is suppressed
under high NO condition. Under high NOx conditions, Hydro-peroxy radicals react with NO
to form OH and NO 2 rather than reacting with itself to form Hydrogen peroxide. This
reaction with NO produces OH and NO 2 back into the system and allows continuation of the
chain mechanism for ozone production. Hydro-peroxy radicals are byproducts from
reaction between OH and VOCs. Thus, a rapid increase of NOx emissions in China during the
past decade may not only have enhanced ozone and total available oxidants levels, but it
also may have caused a shift in partitioning between the two major oxidizing agents for
sulfate production: OH and H 2 0 2 . Hydroxyl radical concentrations may have increased more
rapidly than H 2 0 2 concentrations, since H0 2 has higher possibility to react with NO to form
OH, rather than to form H 20 2 by self-reaction.
The change in partitioning of concentrations amongst major oxidizing agents (OH,
H 2 0 2 , and 03) leads to shifts in partitioning of sulfate production pathways: gas phase
production by OH and aqueous production by H 2 0 2 , 03, or sea-salt. The partitioning in each
chemical production pathway is defined as a percent of sulfate column burden produced
from each chemical production pathway. For example, the gas phase partitioning
percentage over China is calculated by dividing sulfate column burden produced from gas
phase chemistry (OH) by the total sulfate column burden produced by all four chemical
production pathways represented in GEOS-Chem (gas phase (OH), aqueous phases (H 2 0 2
and 03), and sea-salt (03)) over China. Although the enhancement in ozone levels may be
large, competition between the gas phase production by OH and aqueous production by
H 2 0 2 is the focus of this study, since these two production mechanisms are generally about
ten times more important than sulfate production by ozone over China as well as over the
globe [Manktelow et al., 2007; Barth & Church, 1999].
Comparison on sulfate production pathways with prior literature on 1985
conditions over Southeast China/East Asia
Two prior studies calculated the relative significance among production pathways
over Southeast China (Barth & Church, 1999) and East Asia (Manktelow et al., 2007) for
1985 by calculating the percent partitioning of each production rate to the total sulfate
production rate (Table 3). There is a large discrepancy between the two prior studies.
Manktelow et al., 2007 calculates that about 37.5% production is by gas phase (OH) and
56.9% is by aqueous (H 20 2) production, whereas Barth & Church, 1999 calculate that about
18.8% of the production is by gas phase (OH) and 76.9% of the production is by aqueous
phase (H 2 0 2 ) (Table 3). This discrepancy may arise from a number of factors such as
differences in models and emission inventories used for simulations or the time frame and
spatial extent chosen for their analysis. However, the primary cause of the difference is
probably attributable to the use of distinct years on representing the level of oxidizing
agents in each simulation. Although both studies had fixed concentration of oxidizing agents
for simulation of 1985 conditions (with SO 2 emissions fixed at 1985 level), Barth & Church,
1999 held oxidant concentrations at the 1985 level, whereas Manktelow et al., 2007 held
oxidant concentrations at the 2000 level. The significant influence that oxidizing agents may
16
have on determining partitioning between sulfate production pathways is analyzed later in
this study.
The percentage of contribution made by each production pathway to the total
sulfate column contribution over China for year 1985 is calculated and averaged from
March to August. The results are in reasonable agreement with prior studies: indicating that
about 35.9% of the sulfate production is from gas phase (OH) and that 63.6% is from the
aqueous phase (H 20 2) production (Table 3, "1985_base"). The percentage of each
production pathway contributing to the sulfate column from Manktelow et al., 2007 is recalculated from Table 1 of Manktelow et al., 2007 and is shown in Table 1 of this study. For
1985, all three studies indicate that the dominant sulfate production pathway is aqueous
phase production involving H 2 0 2 chemistry (about 60-70% of total chemical production),
and followed by gas phase chemistry involving OH (about 15-40%). Note that the results
from this study indicate a relatively much smaller contribution from production through
ozone (-1%), in contrast to other two prior studies (-5 to 10%). The sulfate production
with sea-salt chemistry makes an insignificant contribution. This negligible contribution
from ozone and sea-salt production causes two major chemical pathways, gas phase
production (OH) and aqueous phase production (H 2 0 2 ), to balance each other and to total
nearly 99% (Table 1). Thus, a growth from one chemistry results in a reduction of the
partitioning in the other sulfate production pathway by a similar percentage shift.
&
Sulfate production pathway partitioning in Historical Years (1985,2001,
2010)
Three historical years (1985, 2001, & 2010) are chosen as milestones to examine
the partitioning in sulfate production pathway. From 1985 ("1985_base") to
2001("2001_base"), gas phase production (OH) pathway partitioning increases by about 12
percent and aqueous phase (H 20 2) production reduces by about 12% in Chinese
atmosphere. However, the comparison between "1985_base" and "2001_base" should be
done with caution, since "2001_base" uses emissions held at the 2010 emission level (except
Asian NOx and Asian SO 2 held at 2001 condition) whereas "1985_base" uses emissions held
at 1985 conditions. Moreover, the meteorological conditions used for simulations are not
the same (MERRA vs. GEOS-5). Thus, more precise analysis on shifts in chemistry
attributable to change in Asian emissions is conducted by comparing "2001_base" and
"2010_base". Two simulations differ only by Asian NO. and Asian SO 2 emissions. Other
factors including meteorology and emissions of other precursors are the same and are all
fixed at 2010 level.
Two regions are specific to the interest of this study: China (25-55N, 100-150E) and
the Pacific Ocean (25-55N, 1SOE-120N). These are further divided into four spatial
quadrants: lower levels over China (0-4km, 25-55N, 100-150E), upper levels over China (2-
8km, 25-55N, 100-150E), lower levels over the Pacific (0-4 km, 25-55N, 150E - 120 W), and
upper levels over the Pacific (2-8km, 25-55N, 150E-120W). These geographic divisions are
used for analyzing March to August averaged column burden of sulfate and sulfur dioxide
(Table 4) and the sulfate production rate from each production pathways (Table 5). The
percent change in sulfate production rate (Tg S/month) between "2010_base" scenario and
"2001_base" scenario is calculated (Table 2:"2001to2010 ratio") over the course of
transpacific transport and is averaged over the latitude band of 25N to 55N in order to
analyze the shifts in chemical production rate in response to simultaneous growth in Asian
NOx and Asian SO 2 emissions from 2001 to 2010 (Table 5).
In the Chinese boundary layer, spring averaged gas phase (OH) sulfate production
rate increases by more than 40% from 2001 to 2010 and the enhancement in production
rate extends to the upper atmosphere over China (about 5-15%) and over to the western
17
Pacific (Figure 3-a). The spring averaged aqueous (H 2 0 2 ) sulfate production rate increases
by relatively much smaller percentage (less than 10%) in Chinese boundary layer (Figure 3b). An enhancement in sulfate production rate in China during summer (Figure 3, bottom) is
similar to spring (Figure 3, top) in spatial pattern as well as the percentage enhancement,
but generally enhancement during summer is restricted longitudinally and extends further
vertically. This enhancement in vertical extension is probably due to the enhancement in
convective processes during summer [Thompson et al., 1994; Li et al., 2005; Luan et al.,
2012].
The average taken between March and August is used for the following analysis on
partitioning of chemical production pathways. The rapid enhancement in gas phase
production rate over most altitudes over China (Figure 3) yields about 7-8% increase in the
partitioning of gas phase (OH) and corresponding 7-8% decrease in Aqueous (H 20 2) phase
sulfate production (Table 5). The partitioning of gas phase production escalates from 47.7%
in 2001 to about 55.3% in 2010 (Table 3), and the partitioning of the H 20 2 Aqueous
production pathway decreases from 51.6% in 2001 to 44.0% in 2010. The spring and
summer averaged sulfate column burden increases from 2001 to 2010 over China, as a
result of concurrent increases in both gas phase (OH) and aqueous phase (H 20 2) chemistry:
about 0.011 Tg S (about 23%) increase in the lower atmosphere (0-4km) and 0.004 Tg S
(about 17%) enhancement in the upper atmosphere (2-8km) in units of column burden
summed for all altitudes over China (Table 4).
From these shifts, the relative significance between the two major sulfate
production pathways over China is reversed in 2010. The gas phase production (OH) plays a
more significant role (55.3%) than aqueous phase (H 20 2) production (44.0%) in 2010
(Table 5). This is in contrast to year 1985 when about 60-70% of total sulfate column over
China is produced through aqueous phase production with H 2 0 2 . In summary, the emission
change of NOx and S02 from 1985 to 2010 results in about a 20% increase in gas phase
chemistry (OH) contribution to the total column sulfate production, which leads to a
reduction of partitioning in aqueous phase (H 2 0 2 ) sulfate production by nearly 20%. Due to
notable increases in NOx emissions from 2001 to 2010, spring and summer averaged
aqueous phase sulfate production rates by ozone also experience about a 30-40%
enhancement in the boundary layer over the source region (not shown). However, as
discussed, the production by ozone is not further analyzed in this study, since the relative
contribution is much smaller than that from gas phase chemistry and aqueous chemistry by
.
H 202
)
An interesting change evident in Figure 3 is over the upper level Pacific, where the
sulfate production rate from all production pathways (ozone and sea-salt production not
shown) is reduced during both spring and summer, except for the gas phase production in
spring. The gas phase chemistry during spring does not show a significant reduction (less
than 5%) and is restricted to the upper level atmosphere over the eastern Pacific and the
US; whereas, the reduction in average sulfate production rate from aqueous (H 0
2
2
chemistry is up to 20% in the free troposphere during spring and extends from the western
Pacific to the US. The reduction in aqueous phase (H 20 2) production extends over broad
regions and altitudes including the upper level Pacific and the US for both spring and
summer, but the reduction is more pronounced during spring. Over the Pacific, this
significant reduction in aqueous production pathway (H 2 0 2 ) leads to about a 7% reduction
in partitioning of aqueous chemistry contributing to the column sulfate production
averaged during spring and summer and correspondingly increases the partitioning of gas
phase chemistry by about 7% (Table 5). The shift in partitioning of sulfate production
pathways for the upper level Pacific (2-8km) is about 2-3%, which increases the
partitioning of gas phase chemistry and reduces the aqueous chemistry (H 20 2) partitioning.
18
Despite the reductions in production rates from most of the chemical pathways, the spring
and summer averaged sulfate column burden from 2001 to 2010 over the Pacific increases
by 0.012Tg S (18%) over the lower atmosphere and about 0.05 Tg S (13%) in the upper
atmosphere (Table 2), which is attributable to efficient transpacific transport of sulfate from
regions with elevated chemical production, especially from the Chinese boundary layer. For
spring, the enhancement in gas phase production in the upper free troposphere over the
Pacific is another contributing factor to increasing sulfate column burden.
Break down of the response of chemistry to shifts in Asian emissions: Four
emission scenarios ("2010_base", "201OBNOX01", "2010BSO201", and "2010_base")
Focusing on recent changes during 2000 to 2010, the concurrent emission shift of
NO. and SO 2 over Asia is further dissected into individual emission changes of NO. or S02 by
analyzing four sensitivity runs ("2010_base", "2010BNOX01", "2010B_SO201", and
"2001_base"; please refer to Table 1) to diagnose the shift in chemistry attributable to
individual emission precursor changes. The response to -22% increase in Asian SO
2
emissions from 2001 to 2010 can be represented by a comparison between "2010_base" vs.
"2010B_S0201" and "2001_base" vs. "2010BNOX01". These two pairs differ in NOx
emission levels, but the percentage change in sulfate concentration and chemistry between
two pairs are similar (not shown). Thus, only "2010_base" and "2010B_SO201" are used for
the following comparison analysis, which gives more weight to the recent NOx condition.
The feedback to -100% enhancement in Asian NOx emissions from 2001 to 2010 is
considered by comparing the "2010_base" and "2010B_NOX01".
Response to about 20% increase in Asian SO 2 emission: "1.2SO2" case
The percent ratio of sulfate and SO 2 concentration between 2010 conditions
("2010_base") and the sensitivity test with SO 2 emission scaled down to the 2001 level
("2010BSO201), hereafter referred to as "1.2SO2_ratio" in Table 2, over the transpacific
pathway is averaged over the latitude band of 25-55N for spring (Figure 4, top) and
summer (Figure 4, bottom). The sulfate concentration (Figure 4-b) increases by more than
15% in spring and 20% in summer in the lower atmosphere (0-4km) over China, indicating
a fast oxidation in the boundary layer that allows the increase in sulfate concentration to
closely correspond to changes in SO 2 emissions (-22%). The enhancement extends to the
upper atmosphere over China, the Pacific, and upper atmosphere (2-10km) over the US
(diminishing to about 5-10% in enhancement). Over the Pacific, the enhancement in sulfate
concentration is most evident in 2-8km altitudes and the enhancement is suppressed in the
boundary layer. This supports the view that an active wet deposition in marine boundary
layer causes suppression of transport in the boundary layer, which causes the main
transport of Asian aerosol to take place in the lower part of the free troposphere [Heald et
al., 2006]. Spring and summer averaged responses of the sulfate column burden over China
to about 22% increase in Asian SO 2 emission ("1.2SO2" case) are about 0.08 Tg S (about
17%) increase in the lower atmosphere (0-4km) and about 0.03 Tg S (about 14%) increase
in the upper atmosphere (Table 4). Thus, most of the enhancements in sulfate burden over
China from 2001 to 2010 can be attributed to the increase of SO 2 emissions (Table 4). The
enhancement in SO 2 concentration (Figure 4-a) from the elevation of SO 2 emissions is most
pronounced in the Chinese boundary layer, and extends to the upper free troposphere
reaching up to about 10km (diminishes to about 8-10%) in spring. During summer, the
vertical transport of elevated SO 2 concentration is more evident, and a significant
enhancement exceeding 20% is obtained, as high as 10km in altitude over China (Figure 4c), probably due to the enhanced convection during summer. The eastward transport of
sulfate and SO 2 is more efficient during spring than summer, probably because upper level
19
)
westerlies are stronger in spring, transporting pollutants eastward more actively. The SO 2
concentration enhancement over the Pacific is higher in the atmosphere (4-10km) than the
enhancement in sulfate concentration (2-10km), and is more restricted in longitudinal
extent, not showing a significant enhancement beyond the Central Pacific. The enhancement
of the chemical production rate (Figure 5) takes place mostly in a region where S02
concentration is elevated (Figure 4-a,c), with the largest enhancement by gas phase
production over the Chinese boundary layer (more than 20%), extending to 10 km in
altitude over China, diminishing to about 10% in spring and 15% in summer. Most of the
enhancement in chemistry is restricted to the atmosphere over China (Figure 5); not
showing much of elevated signal in sulfate production over the Pacific nor the US, except for
the gas phase production (OH) during spring. The spring enhancement in gas phase
production (OH) extends into the Central Pacific, but the enhancement decreases to less
than about 10% over the region.
Notable enhancements in net chemical production rate (calculated as summation
from all sulfate production pathways: Gas phase (OH), Aqueous phase (H 2 0 2 , 03 & sea-salt))
are mostly restricted over China with small signs of increases over the upper atmospheres
of the Pacific and negligible change over the US for both spring and summer (Figure 5-c & f).
A small reduction in the net production rate in the lower boundary over the US is
illustrated. From this rather spatially restricted enhancement in chemical production
(Figure 5) and SO 2 concentration (Figure 4 a&c) with small signals of enhancements beyond
the Central Pacific, eastward extension of the enhancement in sulfate concentration (Figure
4 b&d) in the free troposphere over the Pacific and the US can be interpreted as the efficient
transport of sulfate across the Pacific, allowing elevation of sulfate concentrations over the
regions with suppressed chemical production. In summary, about 22% growth in Asian SO 2
emission increases the SO 2 concentrations, enhances gas phase (OH) and aqueous phase
(H 20 2) sulfate production rates over Asia to the Central Pacific, and shifts the chemical
production pathway favorable to the gas phase production by about 2-3%. The resulting
elevation in sulfate concentration extends further eastward, reaching the free troposphere
over the US via the Pacific Ocean.
The small enhancement of gas phase production partitioning in response to the
elevation in Asian SO 2 emissions from our study is in a good agreement with two prior
studies, which conducted sensitivity studies on the shifts in sulfate production pathway
partitioning by changing the SO 2 emission over East Asia or Southeast China. Manktelow et
al., 2006 studied the influence of increasing East Asian SO 2 emission from 1985 to 2000
compared to oxidizing agent concentrations fixed at 2000 level, and also estimated a 2-3%
increase in gas phase production partitioning. The corresponding reduction in H 2 0 2
production pathway was about 1%. Barth & Church, 1999 also reported a sensitivity test of
doubling the southeast Chinese SO 2 emission from that of the 1985 level, which also
indicated about a 4% increase in partitioning in gas phase sulfate production and a
corresponding 5% reduction in partitioning of production by aqueous chemistry (H 2 0 2
over Southeast China.
Response to about 100% increase in Asian NO. emission: "2NOX" case
The rise in SO 2 emission from 2001 to 2010 does not explain the large shift in percent
partitioning among the sulfate production pathways during the period. Next, I present a
sensitivity study including doubling in NO, emission to examine the influence that oxidizing
agents have in altering the sulfate production pathways as well as the resulting impact on
transpacific transport of Asian sulfate. The calculated changes ("2NOXratio" case) of the
average concentration of sulfate and SO 2 averaged for the 25-55N latitude band are
depicted in Figure 6. Enhancements in sulfate concentration are about 1-10% over the
20
Chinese lower atmosphere (0-4km), but the enhancement is spatially restricted over the
Chinese lower atmosphere, and the upper atmosphere over China and the Pacific Ocean
does not have much of enhancement (less than 2%). This finding indicates that a doubling
of NO, emission does not have a significant influence on the Asian sulfate influence crossing
the Pacific Ocean. This is in agreement with Figure 2 e&f, which depicts almost negligible
increase in surface sulfate concentration over the Eastern Pacific (less than 1%) and the US
(almost negligible). The enhancement in the sulfate column burden averaged from March to
August (Table 2) over the Chinese boundary is about 0.03 Tg S (-6%) for the lower
atmosphere and about 0.01 Tg S (-3%) over the upper atmosphere. Over the Pacific, the
sulfate column burden (Table 2) over the lower atmosphere increases by 0.003 Tg S (-4%)
and by about 0.001 Tg S (-1%) for the upper atmosphere.
A notable influence that changes in Asian NO, emissions do have is a large reduction
of local and tropospheric S02 concentrations as depicted in Figure 6-a&c. The SO 2
concentration over China in the lower atmosphere is reduced by more than 20%, and the
influence propagates to broad regions over the Pacific with a secondary maximum
reduction of about 15% over the upper level Pacific in spring. The SO 2 concentration
reduction extends to the US, but the influence diminishes to about 5-10% with more
pronounced reduction in the upper atmosphere (up to more than 10%). The eastward
extension of the SO 2 concentration reduction and sulfate concentration increase are more
significant during spring than during summer. A strong vertical band of reduction in SO
2
concentration in the upper atmosphere (6-10km) over China during summer overlaps with
the narrow vertical band of elevated concentration of sulfate, indicating that the large
reduction in sulfate concentration is due to enhanced sulfate production from the elevated
NOX concentration, resulting in a reduction in available SO 2 concentration. As is depicted in
the figure, the reduction in SO 2 concentration is not strongly pronounced in the boundary
layer of the source region, but is more pronounced in the lower free troposphere (1-5km)
for both spring and summer; and even in the upper free troposphere (5-9km) over the
Pacific during spring. This reduction of SO 2 concentration in the upper atmosphere is also
depicted in the column SO 2 mass budget analysis (Table 2). The column of SO 2 mass
reduction as a result of enhancement in NOx emission from 2001 to 2010 is about 0.002 Tg S
(-10%) in the Chinese lower atmosphere, and about 0.0005 Tg S (-15%) in the Chinese
upper atmosphere. Over the Pacific, the reduction is about 0.002 Tg S(-9%) in the lower
atmosphere and about 0.0005 Tg S (-12%) in the upper atmosphere.
In contrast to the simple congruent increase in sulfate production rate from all
sulfate production pathways in the "1.2 S02" case, the feedback of sulfate production
pathways to a doubling of Asian NOx emission from 2001 to 2010 is rather more
complicated (Figure 7). Despite a small rise (1-2%) in the H 20 2 column budget over all four
quadrants along the transpacific transport pathway (Table 4), which supports the idea of a
general enhancement in oxidizing power from the increased NOx emission, the aqueous
sulfate production rate by H 20 2 chemistry decreases over all four quadrants: from China by
more than 20% (15%) in spring (summer) to the US through all altitudes in the troposphere
(Figure 7 b&e). Gas phase sulfate production is increased near the Asian boundary layer by
10-25%, extending up to about 4km-Skm for spring and 2-3km for summer. The
enhancement stretches to the Pacific in the lower atmosphere in spring (by about 5% for
altitudes lower than 2km) and for summer (up to about 3km in altitude). The upper free
troposphere over China, Pacific Ocean and the US all experience up to a 10% reduction in
gas phase production. The reduction is pronounced in the upper free troposphere over
China (3-10km) as well as over the Central and Eastern Pacific (6-10km). The reduction
extends all the way to the upper free troposphere over the US. The spring and summer
averaged percent shift in sulfate column production pathway partitioning is by about 6 over
21
)
China and is by 5% over the Pacific % toward the gas phase (Table 5). The upper pacific
shows a smaller shift in percent partitioning (Table 5) of about 3% shift toward the gas
phase. Corresponding percentage reduction in aqueous phase production (H 2 0 2
partitioning exists for all three regions. From these results, we can conclude that the shift in
sulfate production pathways over China and the Pacific Ocean from 2001 to 2010 is
dominantly driven by the doubling of the Asian NOx emission during the time, with nonnegligible contributions from increase in Asian SO 2 emission. The net chemistry (Figure 7
c&f) reflects the balancing between the two chemistries, i.e., a reduced enhancement
(dominant positive influence from gas phase and smaller negative influence from aqueous
phase (H 2 0 2 ) chemistry) in the Chinese boundary layer (-10%) that extends to about 23km in altitude, and the general reduction in production through the upper atmosphere
with a strong reduction just above the Chinese boundary at about 3-7km for both spring
and summer. The notable reduction in sulfate production over the Central/Eastern Pacific
at 6-10km is also evident in the net chemistry.
&
The relationship between sulfate production pathways, column budget
lifetime
The change in partitioning between gas phase production and aqueous phase
production may have an influence on the efficiency of the Asian sulfate transport because
the column burden and the lifetime of sulfate may increase due to the shifting of the
production pathway toward gas phase production by OH [Barth & Church, 1999; Unger et
al., 2006]. Generally, the lifetime of sulfate produced from the aqueous production pathway
is shorter than that produced from the gas phase due to the higher risk of removal through
wet scavenging [Barth & Church, 1999; Unger et al., 2006] and the subsequent cloud
precipitation during the ascent into the free troposphere. This relationship is applicable not
only at the boundary layer over the source region, but throughout the transpacific transport
pathway.
Hence, increase in Asian NO, emissions can lead to an enhancement in total sulfate
column burden as well as an increase in the lifetime of sulfate over the source region by
increasing the percent of sulfate produced by gas phase production. Increases in NO,
emission enhance SO 2 oxidization to form sulfate by strengthening oxidizing power in the
boundary layer of the source region (Figure 7 c&f). The consequence of these changes is
aggravation of regional air pollution over China and reduction of available sulfur dioxide
concentration for the long-range transport (Figure 6 a&c).
Transport efficiency to the Pacific Ocean
In order to understand the negligible enhancement in Asian sulfate enhancements in
the model surface layer over the US as a response to almost doubling of Asian NO. emission,
we analyzed chemistry and wet deposition, which are essential processes involved during
lifting from the Asian boundary layer to upper atmospheres over the Pacific Ocean. Over the
source region, the sulfate burden and lifetime are expected to increase, which is supported
by both the anticipated strengthening in oxidizing power from the increase in total available
oxidant levels, and by the shift toward gas phase production, which reduces exposures to
wet removal processes. Over the Pacific, shifts in column burden and lifetime of sulfate as
responses to the Asian NOx emission growth are complicated, because wet deposition is
commonly involved during the ascent. The escalated sulfate production s in the boundary
layer of the source region from growth in Asian NOx emissions may enhance transport of
Asian sulfate over the Pacific Ocean, depending on the wet scavenging efficiency of sulfate
during the lift to the free troposphere. If wet scavenging is efficient, enhancement of SO
2
oxidation in the Asian boundary layer can lead to reductions in total available column
22
amount and shorten the lifetime of sulfate over the Pacific, because the active conversion
from SO 2 to sulfate is associated with transformation of semi-soluble gas species to
hygroscopic species. Thus, active sulfate formation through SO 2 oxidation in conjunction
with wet scavenging may result in a general reduction in sulfate concentration as well as
sulfate precursor (SO 2) concentrations in the free troposphere over the Pacific. However, if
wet scavenging of sulfate is not efficient during the ascent, then the increased oxidizing
power could enhance the total sulfate concentration in the Pacific by efficiently transporting
the air masses with enhanced sulfate content from the boundary layer to the upper
atmosphere over the Pacific Ocean without losses, while continuing the sulfate formation
during the course of transport.
The net sulfate production rate over the Pacific decreases when Asian NOx emission
is doubled (Figure 7 c&f), so the small percentage increase (less than 2%) in sulfate
concentration over the Pacific (Figure 6 b&d) cannot be attributed to the enhancement in
chemical production in the region. Rather, the elevated sulfate burden over the Pacific can
be a result of transport of the excess sulfate produced from the escalated chemical
production over the Asian boundary region. Our model results indicate that the
enhancement in chemical production over the lower atmosphere in the Asian boundary is
largely counter balanced by increases in the two major sink processes, dry and wet
depositions, for aerosols. Wet deposition is a continuous sink for sulfate, so that a large
amount of sulfate is actively removed during wet processes during the lift to the upper
altitudes and toward the Pacific. In response to the shift in NOx emissions, enhancements in
sulfate burden and the corresponding increases in wet deposition rates are similar in terms
of the spatial pattern as well as the rough magnitude (Figure 8).
A budget analysis over a restricted volume (low altitude China defined as 0-4km,
100-150E, 25-55N) indicates that as a response to changes in Asian NOx emissions,
enhancements in deposition rates largely balances the growths in sulfate production rates
(Table 6). The enhancement in sulfate production rate within a given volume is calculated
by subtracting the sulfate production rate in "2010B_NOX01" from that of "2010_base". The
enhancement in production rate is hereafter referred to as "chem". Increases in the loss rate
from dry and wet deposition are also calculated following the same method, and are
referred to as "sink". The summation of "chem" and "sink" within a volume represents how
much of the enhanced sulfate production over the lower level Chinese boundary layer
escapes without being lost through deposition processes. For spring, about 75% of the
increase in sulfate chemical production rate is counter balanced by enhancements of the
wet deposition and dry deposition rates ("loss ratio": Table 4). Thus, only 25% of sulfate
produced from the enhanced chemistry can survive the loss mechanisms and escape to the
upper atmosphere and to the Pacific from the lower atmosphere (0-4km) over the Chinese
boundary layer. For summer, about 88% of elevated chemical production is counter
balanced by the strengthened wet and dry depositions, allowing only 13% to escape. In
summary, for "2NOX" case, a moderate increase in sulfate production rate (0.033 Tg
S/month for spring and 0.038 Tg S/month for summer) is largely counter balanced by
enhancements in deposition rates, resulting in only a small enhancement in sulfate
production rate (0.0085 Tg S/month for spring, 0.0046 Tg S/month for summer) to
proliferate to the upper atmosphere over China and to the Pacific Ocean.
The budget analysis indicates that for the "1.2SO2" case, about 80% of the increase
in sulfate chemical production rate is counter-balanced by increases of wet deposition and
dry deposition rates for spring. Thus, about 20% of the increased sulfate production is not
counter balanced the enhancements in loss mechanisms. For summer, about 90% of
increased chemical production is counter balanced by wet and dry deposition, allowing only
10% to escape without being balanced by loss mechanisms. This indicates that the loss rate
23
for the "1.2SO2" case is higher than that of the "2NOX" case. However, over the Pacific, the
enhancement in sulfate concentration from -22% increase in Asian S02 emissions is more
significant (Figure 4 b&d vs. Figure 6 b&d) than that from doubling of Asian NO. emission.
The difference can be attributed to a significant increase in sulfate production rate (about
0.13 Tg S/month for spring and 0.10 Tg S/month for summer) in the "1.2SO2" case (about 3
to 4 times greater than that of "2NOX" case), which allows the small percentage that escapes
the volume (about 20% in spring and about 10 % in summer) to be significant in absolute
amount (about 0.024 Tg S/month for spring and 0.01 Tg S/month for summer),
proliferating to the free troposphere over the Pacific to increase the average sulfate
concentration significantly (Figure 4). Moreover, reasons for stronger enhancements in
sulfate concentration over the Pacific in the "1.2SO2" case than that of "2NOX"case can be
that the "1.2SO2" case has a positive enhancement in net chemical production rate over the
Pacific (Figure 5 c&f); in contrast to "2NOX" case, which has a reduction in net chemical
production rate except for the lower atmosphere over Asia (mostly in overlap with the
volume defined above: Figure 7 c&o.
Column budget, wet and dry deposition loss, and lifetime over China and
upper Pacific
Lifetime is defined as sulfate column burden divided by sulfate loss rate (wet and
dry depositions). Barth & Church, 1999 found that the sulfate burden over Southeast China
more than doubles when Southeast Chinese SO 2 emissions were doubled, while emissions of
other pollutants (including the precursors for oxidizing agents) are fixed at the 1985 level.
This non-linear response - percent of enhancement in sulfate burden is greater than the
percent increase of Chinese SO 2 emission-- was explained with the shifts in production
pathways. Barth & Church, 1999 stated that due to limited H 20 2 concentration over China,
doubling increase in Southeast Chinese SO 2 emissions increases the percent of sulfate
produced by gas phase production with OH. Thus, the study found that the lifetime of
sulfate increases from 3.5 days to 4.1 days over the region when the Southeast Chinese SO 2
emissions are doubled. In contrast, Manktelow et al., 2007 used oxidant levels held at the
2000 level, and obtained a relatively slower increase in sulfate burden (-28%) as a
response to the increase in East Asian SO 2 emissions (-32%). Although lifetime was not
calculated in Manktelow et al., 2007, shortening of the lifetime of sulfate over East Asia as a
response to the growth in Asian SO 2 emission is expected because the percent of increase in
the column burden of sulfate (-28%) is smaller than the enhancement rate of deposition
(-40%) as is indicated in Table 1 of Manktelow et al., 2007.
Our results indicate that for spring as well as summer, both sulfate column burden
and wet deposition rate increase when NO, and/or SO 2 emission shifts from 2001 level.
However, the lifetime may increase or decrease, depending on the relative rate of change
between the column burden and the wet deposition rate. Our results for the "1.2 S02" case
(Table 7; 2010_base vs. 2010B_SO201, and 2010BNOX01 vs. 2001.base) are in closer
agreement with Manktelow et al., 2007, indicating a reduction in the lifetime of sulfate over
China for both spring and summer; however, the change in lifetime is almost negligible and
the shift accounts for about 1% of the lifetime at 2010 emission level. Sulfate burdens over
China increases by about 15% as a response to about 20% growth in Asian SO 2 emissions,
which is balanced by slightly more rapid growth in deposition rate (about 16%), resulting in
about a percent reduction in the lifetime of sulfate. The change in lifetime as a response to
the increase in NOx emission is more interesting in that the response differs between the
seasons. For spring, the lifetime increases by 1-2% as a result of a balance between about a
5% increase in column burden and a 2-3% increase in wet deposition. In summer, the
sulfate lifetime is shortened by about 1-2% as a result of about a 5% enhancement in sulfate
24
column burden and slightly greater enhancement in wet deposition. From these results, it is
possible to conclude that for all the scenarios and seasons, the shift in sulfate lifetime over
China due to changes in Asian emissions from 2001 to 2010 is almost negligible.
Shift in lifetime in the upper level Pacific Ocean (Table 6) is greater than that over
China, with maximum reduction of more than 5% in lifetime during summer as a response
to shift in Asian NOx and Asian SO 2 emissions from 2001 (4.33 days) to 2010 (4.07 days)
(Table 6). The dry deposition is not included in the calculation as a loss mechanism because
it is only applicable in the model surface layer. The reduction in lifetime as a response to an
increase in NOx emission is almost negligible during spring (much less than 0.1%), but is
about 3-4% during summer. Lifetime is reduced over the upper level Pacific by 3-4% in
both spring and summer as a response to increase in SO 2 emission.
Interestingly, the results indicate that the lifetime over the upper level Pacific
decreases from 2001 to 2010, despite the shift in sulfate production pathway toward the
gas phase production. In fact the expected increase in lifetime that can be associated with
the increase in gas phase production is over China during spring when only NOx emission is
increased; all the other possible combinations of shifts in Asian NOx and/or Asian SO 2
emission and seasons indicate reductions in lifetime (although mostly less than 1%) from
2001 to 2010 both over China as well as over the Pacific (mostly about 3-4%).
Summary and conclusion
In this study, Global CTM (GEOS-Chem) with coupled oxidant-aerosol chemistry was
used to broaden the knowledge on the influence of recent (2001 to 2010) growth in Asian
NOX and SO 2 emission on transpacific transport of Asian particulate matter, especially
sulfate, by quantifying and analyzing the details of chemistry and its sequential influence on
the transpacific transport. This work is motivated by prior studies, Park et al., 2004 and
Heald et al., 2006, which used GEOS-Chem to conclude that Asian sulfate may make notable
enhancements at the US surface layer, which may hinder the efforts to mitigate visibility
impairment problems and to achieve an ultimate goal of obtaining "natural visibility
conditions" by 2064 set by the Regional Haze Rule. Two other prior studies Barth & Church,
1999 and Manktelow et al., 2007, provided the motivation for analyzing the shifts in
chemistry and the sequential influence on lifetime and sulfate column burden along the
transpacific pathway.
Our model results indicate that Asian pollution has a notable positive influence on
sulfate concentrations but negative contribution on nitrate concentrations at the model
surface layer over the US. The thermodynamic relationship between sulfate and nitrate
explains the negative contributions Asian pollutions have on nitrate, because enhanced
sulfate formation from Asian sulfur dioxide emission reduces ammonium concentrations
available to neutralize nitrate. From 2001 to 2010, Asian sulfate enhancements in the US
model surface layer increase by 0.01-0.03 g M- 3 , which corresponds to about 4% of the
sulfate concentration at the 2010 emission level. This increase in Asian sulfate enhancement
is mostly attributable to about a 22% increase in Asian SO 2 emission. Almost doubling in
Asian NOx emission during the period has a negligible influence on Asian sulfate
enhancement over the US, but Asian NOx and SO 2 emissions have a small constructive
relationship in augmenting the Asian sulfate enhancement over the US model surface layer.
From 2001 to 2010, the net sulfate production rate increases in the Asian boundary
layer but is reduced in the upper level Pacific and the US for both spring and summer. The
reduction in chemical production over the upper level Pacific is mostly attributable to the
increase in NOx emission, which was found to reduce the sulfur dioxide concentration in the
upper Pacific and the US. Asian NOx emission increase leads to more active sulfate
25
production in the boundary layer, which reduces Asian SO 2 concentration available for
transport from Asian boundary layer to the upper Pacific. Efficient wet scavenging
processes are involved during the lift, which remove sulfate aerosols. In contrast, sensitivity
test with about a 22% increase of Asian SO 2 emissions show enhancement in sulfate and
sulfur dioxide concentrations, and the net chemical production over China as well as Pacific.
Gas phase production by OH and aqueous production by H 20 2 are the two major
production pathways along the course of transpacific transport. Over China, from 1985 to
2010, the most significant production pathway is changed from aqueous production
pathway by H 2 0 2 in 1985 (60-70%) to gas phase production with OH by 2010 (53%). From
2001 to 2010, column burden sulfate production by gas phase processes increases by -6%
with corresponding reductions in aqueous phase production by H 2 0 2 (-8%) over China.
Over the Pacific Ocean, the shift corresponds to about 7% with increase in gas phase
production and reduction in aqueous phase (H 20 2) production. Upper level Pacific has a
moderate shift (2-3%) toward the gas phase production. The shift in production pathway is
primarily driven by the growth in Asian NOx emission, with minor contributions from
increase in Asian SO 2 emission.
A budget analysis over a restricted volume of Asian lower boundary explains the
reasons for the differences in Asian sulfate enhancement over the Pacific and the US surface
layer in response to the shifts in Asian SO 2 or Asian NOx emission. Sensitivity test with Asian
NOx emission indicates that the enhancement in sulfate production rate is three to four
times smaller than that with Asian SO 2 emission. This modest enhancement in sulfate
production from doubling in Asian NOx emission is largely balanced by elevation in
deposition rates (75 % is balanced in spring and 88% is balanced in summer) so that
significant amounts of enhanced sulfate cannot escape the lower Chinese boundary layer to
increase the sulfate concentration over the Pacific and the US. In contrast, despite even
more active deposition rates (80 % during spring and 90% during summer), a significant
amount of sulfate escapes from the Asian boundary layer as a response to a 20% increase in
Asian SO 2 emissions because enhancement in sulfate production rate is much greater (3 to 4
times) in absolute amount than when Asian NOx emissions are increased. Moreover, net
chemistry over the upper level Pacific is enhanced when Asian SO 2 emission is elevated,
which allows more sulfate production along the path; in contrast to the reduction in net
chemical production rate outside of the Chinese boundary layer when Asian NOx emissions
are increased.
The shift in chemical production pathway toward the gas phase is supposed to
increase the column burden and lifetime of sulfate. Our results indicate that the sulfate
column burden shows a notable increase as a response to the changes in Asian emissions.
However, for all scenarios and seasons, the shifts in sulfate lifetime over China due to
changes in Asian SO 2 and/or Asian NOx emission are found to be almost negligible
(generally 1-2%). The shift in lifetime over the upper level Pacific is more pronounced, with
maximum reductions of more than 5% in lifetime during summer as a response to a
simultaneous increase in Asian NO. and SO 2 emissions. Generally, lifetime over the upper
level Pacific Ocean is reduced by 3-4% as a response to the imposed growth in Asian NO,
and SO 2 emission; except during spring with Asian NO. emission change, which indicated
almost negligible reduction in lifetime. Thus in summary, the expected increase in lifetime
that can be associated with shift toward the gas phase production is found only when NO,
emission is increased during spring over China; all the other possible combinations of shift
in Asian NOx and/or Asian SO 2 emissions and seasons indicate reduction in lifetime from
2001 to 2010 both over China as well as over the Pacific.
26
(a) SPR 2001Asian S04(2-) [0.01 ug/m3]
(b) SPR 2001 Asian/2001 S04(2-) [%]
0
80 N
80N
70N
70*N
60*N ------
60 N ---
50*N
50*N
40N
40*N
30*N
30*N
20*N
20*N
1 0*N
1 0*N
0
00
0
120 E
0.
-5.
6.
8.
1800
10.
12.
15.
30.
70.
00
0
120*W
60 W
1 ZU-::
500. 2000.3000. -5.
0.
5.
80*N
0
70 N
70*N
0
60 N
60*N
50*N
50*N
40N
40*N
30*N
30*N
0
20 N
20*N
1 0*N
1 0*N
00
120*E
17
-20.
1800
120*W
0
120 E
0
1800
120 W
60*W
n--
-10.
70*N
-
00
0
60 W
-5.
-3.
-2. -1.
0.
1. 200. 1500.2000.2500.-250-100.-60. -30. -10. -5. 0. 5. 20. 50. 90. 95. 97. 99. 100.
(e) SPR 2001Asian NH4+ [0.01 ug/m3]
(f) SPR 2001Asian/2001 NH4+ [%]
0
80*N
.---.
60*W
10. 15. 20. 25. 30. 40. 60. 90. 95. 98. 99.
(d) SPR 2001Asian/2001 N03- [%J
(c) SPR 2001 Asian N03- [0.01 ug/m3]
80*N
120*W
1800
80 N
--------- -..-. - -
---
-
-
---
70*N
60*N
60*N
-q
..
-..
---
---------
50N
SO*N
40*N
40*N
30*N
30*N--r
20*N
20*N
- --
- -- ---
-*A--
.
..
10*N
0
00
-5.
0.
0
120 E
1.
3.
4.
1800
6.
120OW
0
60 W
0
0
120 E
1800
rm-MMMMOMM
20. 100. 500.1000.1200.1300.1500. -5. 0. 5. 10. 15. 20. 25. 40.
60.
70.
120*W
60*W
80.- 9 9
~ 99.~70.
Figure 1. Asian inorganic particulate matter (sulfate, nitrate and ammonium) enhancement
in the model surface layer: (a,c,e) concentration of Asian enhancement (sulfate, nitrate,
ammonium, respectively) [Vg M-3 ] in model surface layer at 2001 emission level; (b,d,f) the
percentage of Asian enhancement (sulfate, nitrate, ammonium, respectively) to aerosol
concentration (sulfate, nitrate, ammonium, respectively) at 2001 emission level.
*Note that contour intervals are different for every figure for the best representation for
each figure. Please refer to Table 1 and 2 for abbreviations used.
(a)
SPR
2001 Asian S04(2-) [ug/m3]
(c) SPR 2001 to201 0 S04(2-) [ug/m3]
(b) SPR 2001 Asian/2001 S04(2-) [%]
(d)
60ON
60*N
60*N
60*N
50*N
50*N
50*N
50*N
40*N
40*N
40*N
40*N
30*N
30*N .30N
20*N
20ON
20*N
11*
1*N
1*N
0
1800
150OW
1200 W
90*W
0
60*W
1800
150*W
1200 W
900 W
60*W
20*N
M
1 O"N
00
1800
150*W
120*W
90*W
600 W
-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
-1. 0. 1 3. 6. 9. 12.15.20.25.30.35.40.45.
-0.010.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
(e) SPR 2NOX S04(2-) [ug/m3]
(f) SPR 2NOX/2010 S04(2-) [%]
(g) SPR 1.2SO2 S04(2-) [ug/m3]
1800
0.0
60*N
60*N
50*N
50*N
50*N
50"N
40*N
40*N
30*N
30*N
30*N
30*N
20*N
20*N
20*N
20*N
0
10 N
00
1800
0
150*W
1200 W
90*W
60W
1800
10 N
150*W
120*W
900 W
600 W
-0.010 -0.007 -0.003 0.000 0.003 0.007 0.010 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
150*W
1.0
2.0
120*W
90*W
3.0
4.0
(h) SPR 1.2SO2/2010
60*N
40N40*N
.....
00
60*N
10*N
2001 t0201 /201 0 S04(2-) [%]
30*N
0
0
SPR
60OW
5.0
10*N
00
1800
150*W
120*W
90*W
60*W
1800
-0.010.00 0.01 0.02 0.03 0.04 0.05 0.06 0.070.08
0.0
150*W
1.0
2.0
120*W
90*W
3.0
40
60W
5.0
Figure 2. Differences on sulfate enhancements between various emission scenarios in US surface layer during spring: (a) the
"2001Asian" sulfate concentration [pg m-]; (b) the percentage of "2001Asian" to sulfate concentration at 2001 emission level; (c,e,g)
shifts in Asian enhancement from various emission scenarios; (d,fh) changes in percentage of Asian enhancement over sulfate
concentration at 2010 emission level by shifts in emission scenarios (b,d,fh). *Please note the change of scales for (e). "2001to2010"
case (c,d); "2NOX" case (e,f); "1.2SO2" case (gh).
*Please refer to Table 1 and 2 for abbreviations
.
O#@t11VAj -
ffiIMM~ #MW" -
!,
,
----
-
--
-
--
-
-
............
i
6.0
S04(2-) [%]
6.0
28
(a) SPR 2001to2010_ratio Gas (OH) [%]
-
10-
(b) SPR 2001to2010_ratio AQ (H202) [%]
-
8
E
-
6
0
60.
100
70.
80.
200
Longitude
90.
300
0
100. 110. 120. 130. 140.
60.
(c) SUM 2001to2010_ratio Gas (OH) [%]
100
70.
80.
200
Longitude
90.
300
100. 110. 120. 130. 140.
(d) SUM 2001to2010_ratio AQ (H202) [%]
10
8
a)
V
a)
*0
0
60.
70.
80.
90.
100. 110. 120. 130. 140.
60.
100
70.
80.
200
Longitude
90.
300
100. 110. 120. 130. 140.
.
Figure 3. Sulfate production rates: "2001to2010"case. Response of sulfate production rates
[kg S/ month] to shifts in Asian emissions from 2001 to 2010 calculated as percent ratios
between "2010_base" scenario and "2001_base" scenario averaged for 25-55 N latitude
band: (a) spring averaged percent ratio of (a) gas phase sulfate production by OH, (b)
aqueous phase sulfate production by H 2 0 2 , and summer averaged percent ratio of (c) gas
phase sulfate production by OH, (d) aqueous phase sulfate production by H 2 0 2
*Please refer to Table 1 and 2 for abbreviations used. The color bars are saturated after
140% for (a) and (c).
29
(a) SPR 1.2SO2_ratio S02 [%]
10 - - - -.
(b) SPR 1.2SO2ratio S04 [%]
. . .10
8-
8-
EE
_,6 -
-
6
a)D
4
2
o
2
0
100
200
Longitude
0 - -300
75. 80. 85. 90. 95. 100.-105.110.115.120.125.
(c) SUM 1.2SO2_ratio S02
0
-- - -
-
4-4
100
200
Longitude
300
75. 80. 85. 90. 95. 100.105.110.115.120.125.
[%J
(d) SUM 1.2SO2_ratio S04 [%]
10.
10
8-
8
EE
4-
4
2
2
100
- - - - - - - -0200
300
Longitude
75. 80. 85. 90. 95. 100.105.110.115.120.125.
- - -
-
0 .. .
0
0
100
200
Longitude
300
75. 80. 85. 90. 95. 100.105.110.115.120.125.
Figure 4. Sulfur dioxide and sulfate concentrations: "1.2S02" case. Response of sulfur
dioxide and sulfate concentrations to about 20% increase in Asian SO emission calculated
2
as percent ratios between "2010_base" scenario and "2010B_SO201" scenario averaged for
25-55 N latitude band: spring averaged percent ratios on (a) sulfur dioxide concentrations,
and (b) sulfate concentrations; and summer averaged percent ratios on (c) sulfur dioxide
concentrations, and (d) sulfate concentrations.
*Please refer to Table 1 and 2 for abbreviations used. The color bars are saturated after
125% for (a) and (c).
30
(a) SPR 1.2SO2_raWo GAS (OH) [%J
10
(b) SPR 1.2S92_ratIo AQ (H202) [%]
10
(c)
SPR 1.2SO2 ratio Net chemistry 1%]
10:1
86--
6-
4-
4
2-
2
0
200
100
Longitude
60.
70.
80.
90.
300
100. 110. 120. 130. 140.
(d) SUM 1.2SO2_ratio Gas (OH) [%j
2
0
60.
100
70,
80.
200
Longitude
90.
300
100.110. 120. 130. 140.
0
60.
(a) SUM 1.2SO2_ratio AQ (H202) [%]
100
200
Longitude
300
70. 80. 90. 100. 110. 120. 130. 140.
(f)SUM 1.2SO2_ratio Net chemistry [%]
10
E
te
-C
8-
8
6-
6-
4-
4-
2-
2,
0
60.
100
70.
80.
200
Longitude
90.
300
100. 110. 120. 130. 140.
0
60.
100
70.
80.
200
Longitude
90.
300
100. 110. 120. 130. 140.
Figure 5. Sulfate production rates: "1.2S02" case. Response of sulfate production rates [kg
S/ month] to about 20% increase in Asian SO 2 emissions calculated as ratios between
"2010_base" scenario and "2010B_0S201" scenario averaged for 25-55 N latitude band:
spring averaged ratios of (a) gas phase sulfate production by OH, (b) aqueous phase sulfate
production by H 2 0 2 , (c) net chemical production (sum of gas phase (OH), aqueous phase
(H 2 0 2 , 03, sea-salt), and summer averaged ratio of (d) gas phase sulfate production by OH,
(e) aqueous phase sulfate production by H 2 0 2 , (f) net chemical production (sum of gas
phase (OH) and aqueous phases (H 2 0 2 , 03, sea-salt)).
*Please refer to Table 1 and 2 for abbreviations used.
31
(b) SPR 2NOX ratio
10 -- -- - - - - - - - -
10
8
E
-.-.-.--
8-
6
E
6
4
4-
2
2
0
0
S04 [%]
-
(a) SPR 2NOXratio S02 [%]
100
200
Longitude
0
300
100
200
Longitude
300
80. 84. 88. 92. 96. 100.104.108.112.116.120.
95. 96. 97. 98. 99. 100.101.102.103.104.105.
(c) SUM 2NOXratio S02 [%]
(d) SUM 2NOXratio S04 [%]
80)
TIC
0
4-
2
0
100
200
Longitude
300
80. 84. 88. 92. 96. 100.104.108.112.116.120.
0
100
200
Longitude
300
95. 96. 97. 98. 99. 100.101.102.103.104.105.
Figure 6. Sulfur dioxide and sulfate concentrations: "2NOX" case. Responses of sulfur
dioxide and sulfate concentrations to about 100% increase in Asian NOx emission calculated
as ratio between "2010_base" scenario and "2010B_NOX01" scenario averaged for 25-55 N
latitude band: spring averaged percent ratio of (a) sulfur dioxide concentrations, and (b)
sulfate concentrations, and summer averaged percent ratio of (c) sulfur dioxide
concentrations, and (d) sulfate concentrations.
*Please refer to Table 1 and 2 for abbreviations used. The color bars are saturated at 80%
for (a) and (b) and at 105% for (c) and (d).
32
(b) SPR 2NOX-ratio AO (H202) [%]
u
80.
200
Longitude
100
85.
90.
95.
300
0
100
200
300
(c)
0
SPR 2NOX-ratio Net chemistry [%]
100
200
Longitude
Longitude
100. 105. 110. 115. 120.
70.
(d) SUM 2NOX-ratio Gas (OH) 1%]
78.
85.
92.
100. 108. 115. 122. 130.
(e) SUM 2NOX-ratio AQ (H202) [%]
80.
85.
90.
95
300
100. 105. 110. 115. 120.
(f) SUM 2NOX-ratio Net chemistry [%]
T
ZE
(D
0
80.
100
85.
90.
200
Longitude
300
95. 100. 105. 110. 115. 120.
0
70.
100
78.
85.
200
Longitude
92.
300
100. 108. 115. 122. 130.
Figure 7. Sulfate production rates: "1.2SO2" case. Responses of sulfate production rates [kg
S/ month] to about 100% increase in Asian NO, emission calculated as ratio between
"2010_base" scenario and "2010B_NOX01" scenario averaged for 25-55 N latitude band:
spring averaged ratios of (a) gas phase sulfate production by OH, (b) aqueous phase sulfate
production by H 2 0 2 , (c) net chemical production (sum of gas phase (OH), aqueous phase
(H 2 0 2 , 03, sea-salt), and summer averaged ratio of (d) gas phase sulfate production by OH,
(e) aqueous phase sulfate production by H 2 0 2 , (f) net chemical production (sum of gas
phase (OH), aqueous phase (H 2 0 2 , 03, sea-salt)).
*Please refer to Table 1 and 2 for abbreviations used. The color bars are saturated at 120%
for (a) and (d).
33
(a) SPR [2010_babe-2010- NOXOI 25-55N avg. S04 [ug/n3]
(b) SPR [2010_.baae-201B0NOX01 25-55N avg. WETDEP [ug/m3
10
6-
R
I
42-
o
100
200
300
1
IM
200
LongkKdG
0.00 0.01 0.02 0.03 0.05 0.10 0.15 0.20 0.50 1.00
0.00 001 0.02 0.03 0.
.oT10 0.15
3W
0.20 0.50
1.Wo
Figure 8. Balancing between enhancements in Asian sulfate concentrations and
wet
deposition rates in spring as a response to about 100% increase in Asian NOx emission:
(a)
increases in sulfate concentrations [Ig M- 3 ], (b) increases in sulfate
wet deposition rates [rIg
m-3].
*Please refer to Table 1 and 2 for abbreviations used. *Color bars are saturated after
1 [ptg
m-3].
34
Table 1. Summary of emission scenarios used for simulations.
Scenarios
2001_base
Asian SO 2 and
NO, emissions
Both at 2001
level
2010_base
Both at 2010
level
2010B_zero
Both zero
emission
2010B_SO201
Asian SO 2: at
2001 level
Asian NO,: at
2010 level
201OB_NOX01
Asian NOx: at
2001 level
Asian SO2: at
2010 level
1985_base
Both at 1985
level
Goal
Simulate nitrate, ammonium
and sulfate concentrations and
sulfate production rates with
Asian NO, and SO 2 emissions at
2001 level
Simulate nitrate, ammonium
and sulfate concentrations and
sulfate production rates with
Asian NO. and SO 2 emissions at
2010 level
Simulate nitrate, ammonium
and sulfate concentrations and
sulfate production rates
without Asian emissions
Simulate nitrate, ammonium
and sulfate concentrations and
sulfate production rates with
Asian SO 2 emission at 2001
level and Asian NO. emissions
at 2010 level
Simulate nitrate, ammonium
and sulfate concentrations and
sulfate production rates with
Asian NOx emission at 2001
level and Asian SO 2 emissions at
Meteorology
GEOS-5;
2010
condition
GEOS-5;
2010
condition
GEOS-5;
2010
condition
GEOS-5;
2010
condition
GEOS-5;
2010
condition
2010 level
Simulate nitrate, ammonium
MERRA;
1985
and sulfate concentrations and
condition
sulfate production rates with
Asian SO 2 and NOx emissions at
1985 level
*Emissions of all other pollutants are held at 2010 emission level. Only Asian NOx and
Asian SO 2 are scaled for each scenario (see methods for details).
35
Table 2. Summary of "cases" considered.
Goal
Calculate concentrations and
production rates with Asian
emissions at 2001 level
Asian aerosol
concentrations
2001Asian/2
001
Percent of Asian enhancements to
the pollution concentrations in
2001
Percent ratio of
concentrations
2001to2010
Calculate change in concentrations
or production rates as responses to
the growth in Asian NOx and S02
emissions from 2001 to 2010.
2001to2010/
2010
Calculate change in concentrations
or production rates as responses to
the growth in Asian NOx and SO2
emissions from 2001 to 2010.
Calculate the percentage of change
in Asian aeorosol concentration (as
a response to Asian NOx and S02
emissions changes from 2001 to
2010) to sulfate concentrations at
2010 condition
Calculate the changes in
concentrations or production rates
as responses to the growth in Asian
S02 emissions from 2001 to 2010.
Shift in Asian
aerosol
concentrations
or production
rates
Percent ratios
1.2SO2/2010
2NOX
2NOX/2010
Calculate the changes in
concentrations or production rates
as responses to the growth in Asian
S02 emissions from 2001 to 2010.
Calculate the percentage of change
in Asian sulfate concentrations (as
a response to shift in S02 emissions
from 2001 to 2010) to sulfate
concentrations at 2010 condition
Calculate the changes in
concentrations or production rates
as responses to the growth in Asian
NOx emissions from 2001 to 2010.
Calculate the changes in
concentrations or production rates
as responses to the growth in Asian
NOx emissions from 2001 to 2010.
Shift in Asian
aerosol
concentrations
or production
rates
Percent ratios
2001Asian/2001
=(2001_base 201OB-zero)
/2001_base*100
2001to2010
= 2010_base
2001_base
Figure 1
Figure 2
2001to2010_ratio
=2010_base/2001_bas
e*100
2001to2010/2010
=(2010_base2001_base)
/2010_base*100
Figure3
1.2SO2
=2010_base 2010B_SO201
Figure 2
Table 6
1.2SO2_ratio
=2010_base/2010BS
Figure 4
Figure 5
Calculate the percentage of change
in Asian sulfate concentrations (as
a response to shift in NOx emissions
from 2001 to 2010) to sulfate
1 concentrations at 2010 condition
Figure 2
Figure2
0201*100
Percent ratios
1.2SO2/2010
=(2010_base
2010BSO201)
/2010-base*100
Figure 2
Shift in Asian
aerosol
concentrations
or
production/los
s rates
Percent ratios
2NOX
=2010_base 2010BNOX01
Figure 2
Figure 8
Table 6
2NOX-ratio
=2010_base/2010BN
Figure 6
Figure 7
-
1.2SO2
Percent ratios
Figures
& Tables
Figure 1
Figure 2
OX01*100
Percent ratios
2NOX/2010
=(2010_base
2010BNOX01)/2010
base*100
-
2001Asian
Abbreviation and
Calculation
2001Asian
= 2001_base 201OBzero
-
Cases
Figure 2
36
&
Table 3. Chemical production Pathways: comparison with prior studies.
The percent of sulfate produced through gas phase ("OH %") production pathway and
aqueous phase production pathway by H202 ("H202 %") is calculated from this study
(2001_base, 1985_base), and two prior studies: Manktetelow et al., 2007 and Barth
Church, 1999.
2001 base
Manketelow 00 E.Asia
1985 base
Maneeo
5EAi
Barth &Chad gs
Temporal range CHEM
Oh
OH %
[Tg S/ yr] [Tg S/ yr]
avg. March-Aug
_avg. June-Dec
avg. March-Aug
avg. Jurne-Dec
wk2*
4rldrni
1
H202
H202 %
Tg S/ yr]
7.52
3.59
47.7
39.9
3.88
5.38
1.93
35.9
37.51
3.42
2
1
9.0
+H202
%
PSO4 China Column
51.6
55.9
63.6
56.91
7,
99.3
95.8
99.5
9.
90.3
China: 100-150 E, 25-55 N.
Manktelow 00 E.Asia: Result from [Manktelow et al., 2007] with East Asian emissions at
2000 level.
Manktelow 85 E.Asia: Result from [Manktelow et al., 2007] with East Asian emissions at
1985 level.
*Please note that [Manktelow et al., 2007] had oxidant levels fixed at 2000 level.
Barth & Church 85: Results from [Barth & Church, 1999] with Southeast Chinese emissions
at 1985 level.
Barth & Church 85_SO2*2: Results from [Barth & Church, 1999] with Southeast Chinese SO
2
emissions doubled from 1985 level; all the other emissions (including oxidants) are fixed at
1985 level.
*Please refer to Table 1 and 2 for abbreviations used.
37
Table 4. Column burdens of sulfate and sulfur dioxide concentrations.
Left: the column burdens over the four quadrants -- lower level China (LWCHColumn:0-
4km, 25-55N, 100-150E), upper level China (UPCH Column:2-8km, 25-55N, 100-150E),
lower level Pacific (LWPCColumn:0-4 km, 25-55N, 150E - 120 W), and upper level Pacific
(UPPCColumn:2-8km, 25-55N, 150E-120W )-- are calculated.
Right: the ratios of the column burdens calculated (left) against that of 2010_base to
calculate the shifts in the column burdens as a response to changes in Asian SO and Asian
2
NOx emissions.
*All the results are averaged from March to August.
*Please refer to Table 1 for abbreviations used in this table.
TgS
2010_base
2010BNOX01
2010_SO201
2001_base
TgS
0.059
0.056
0.051
0.048
Tg S
2010_base
2010BNOXO1
20108_SO201
2001_base
Tg S
0.024
0.023
0.021
0.020
TgS
2010_base
20108_NOX01
2010BSO201
2001_base
TgS
2010-base
1.000
1.000
099
2010B S0202
1.165
1.199
0.995
0.287
0.283
0.287
0.284
2010_base
20108_NOX01
2010BSO201
2001_base
0.670
0.659
0.672
0.662
2010_base
2010BSO201
20&-_6w~e
0.703
0.697
0.704
0.698
2010_base
20WO-ENOX01
201BSO201
2001Obase
2010_base/* 2010_base/* 2010_base/*
1.000
1.000
1.000
1.026
0.861
1.012
1.138
1.155
0.997
1.165
1.004
1.009
Tg
0.027
0.029
0.023
0.025
TgS
0.044
0.043
0.040
0.039
0.306
0.300
0.307
0.302
Tg
0.0028
0.0033
0.0024
0.0028
0.083
0.080
0.073
0.071
TgS
2010_base
20108_NOX01
20108_SO201
2001_base
Tg
0.023
0.025
0.019
0.021
1.000
1.000
1.000
L.042
M.913
1.132
1.169
I.V6
1.074
1.0,16
0.996
1.012
1.000
1.000
1.000
1.017
0.882
1.008
1.114
1.110
0.998
1.130
0.990
1.006
Tg
0.0041
0.0046
0.0037
0.0041
38
Table 5. Sulfate production rates column burdens of the two major chemical production
pathways.
Column burdens of sulfate produced by gas phase production pathway with OH and
aqueous phase production pathway with H202 [Tg S/month]. The percentage of sulfate
produced from gas phase pathway ("OH %") and aqueous phase production pathway by
H202 ("H202 %") is calculated.
China: 100-150 E, 25-55 N
Pacific Ocean (PC): 150E-120W, 25-55N
Upper level Pacific Ocean (UPPC): 150E-12OW, 25-55N, 2-8km
*All results are averaged from March to August.
*Please refer to Table 1 for abbreviations used in this table.
Oh
[Tg S/
[Tg S/
QH% H202
[Tg S/
month] month]
2010 base
2010B NOX01
2010B S0201
2001 base
1985 base
AVG
AVG
AVG
AVG
AVG
SPR&SUM
SPR&SUM
SPR&SUM
SPR&SUM
SPR&SUM
PS04 PC Column
0.77
0.74
0.65
0.63
0.45
CHEM
[Tg S/
month]
0.43
0.36
0.35
0.30
0.16
Oh
[Tg S/
55.28
49.35
53.53
47.72
35.90
AVG
AVG
AVG
AVG
AVG
SPR&SUM
SPR&SUM
SPR&SUM
SPR&SUM
SPR&SUM
PS04 UPPC
[2-8km] Column
0.95
0.92
0.83
0.81
0.6
CHEM
[Tg S/
0.4
0.39
0.38
0.33
0.21
Oh
[Tg S/
AVG SPR&SUM
AVG SPR&SUM
AVG SPR&SUM
AVG SPR&SUM
AVG SPR&SUM
0.035
0.038
0.031
0.034
0.040
0.017
0.018
0.015
0.016
0.024
43.99
49.96
45.81
51.63
63.63
99.28
99.31
99.35
99.35
99.53
H202 % OH+H202
month]
48.03
42.94
45.67
40.80
3059
0.49
0.52
0.45
0.47
0.4
OH % H202
[Tg S/
month] month]
2010 base
2010B NOX01
2010B S0201
2001 base
1985 base
0.34
0.37
0.30
0.32
0.29
OH % H202
[Tg S/
month] month]
2010 base
2010B NOX01
2010B S0201
2001 base
1985 base
H202% OH+H202%
%
CHEM
51.21
56.29
53.59
58.45
68.61
99.2
99.23
99.26
99.26
99.58
H202 % OH+H202
%
PS04 China
Column
month]
49.15
46.14
49.37
46.52
59.4
0.018
0.021
0.016
0.018
0.016
50.69
53.73
50.4
53.36
40.4
9
99.85
99.86
9986
99.87
99.93
39
Table 6. Budget Analysis: a balance between enhancements in chemical production rates
and wet and dry deposition rates.
Changes in chemical production rates and the sink rates (dry and wet depositions) over
defined volume [0-4km, 100-150 E, 25-55N] to doubling in Asian NOx emissions [top;
2010_base vs. 2010BNOX01] and to increasing Asian SO 2 emissions by about 22% [bottom;
2010 base vs. 2010B_SO201]. The budgets are summed over the defined volume [0-4km,
100-150 E, 25-55N], representing the lower altitudes of China. In units of [Tg S/month].
*Please refer to Table 1 for abbreviations used in this table.
2NOX: 2010_base vs. 2010B_NOX01
SPR
0.034
-0.025
0.0085
0.75
SUM
0.038
-0.034
0.0046
0.88
1.2SO2: 2010_base vs. 201OB_SO201
SPR
0.13
-0.10
0.024
0.81
SUM
0.10
-0.09
0.011
0.90
chem= Difference in chemical production
sink= Difference in wet and dry deposition (negative indicates enhancement in sink)
chem+sink= net balance between enhancements in chemical production rates and sink
rates. (*positive means enhancement in chemical production rate is greater than that of the
sink.)
Loss ratio = 1- (chem+sink)/chem = the ratio of enhancement in chemistry that is counterbalanced by the enhancement in sink processes.
40
Table 7. Column budgets, wet depositions, dry depositions, sinks and lifetimes over China.
Seasonal averages of sulfate column burdens, wet deposition rates, dry deposition
rates,
sink rates (defined as a sum of dry and wet deposition rates), and lifetimes of sulfate
over
China (100-150 East, 25-55N) for various emission scenarios.
*Please refer to Table 1 for abbreviations used in this table.
China Whole Column
Column [Ti SI
2010 base
SPR
2010B NOX01 2010B S0201 2001 base
0.0782
.0718
. N686
SUM
.0819
.0593
0.0564
0.0510
0.0487
Wet dep [Tg S/month]
SPR
.6655
0.6464
.5730
M.550
0.5968
0.4822
.0994
0.5655
0.0931
0.5071
Dry dep [Tg S/month]
SUM
SPR
0.0860
.0810
SUM
.0740
0.0697
0.0641
Sink [Tg S/monthl
0.0606
SPR
1.7649
0.7395
0.6590
0.6708
3.2830
2.7091
0.6352
Ufetime [days]
SUM
SPR
SUM
3.2416
0.5712
3.34M_
2.7379
.6390
0.5428
2.7240
3.3009
2.7519
Table 8. Column budgets, wet depositions, dry depositions, sinks and lifetimes in the upper
level Pacific Ocean.
Seasonal averages of sulfate column burdens, wet deposition rates, dry deposition rates,
sink rates (wet deposition rates), and lifetimes of sulfate over upper level Pacific (150E to
120W, 25-55N, 2-8km) for various emission scenarios.
*Please refer to Table 1 for abbreviations used in this table.
2-8km Pacific
Column [Tg S]
SPR
SUM
Wet dep [Tg S/month] SPR
SUM
Lifetime [days]
SPR
SUM
2010_base
0.0573
0.0310
0.3126
0.2331
5.6215
2010B NOX01 2010BS0201
0.0564
0.0517
0.0305
0.0276
0.3076
0.2717
0.2234
0.2003
5.6233
5.8340
4.0798
4.1819
4.2252
2001 base
0.0509
0.0272
0.2676
0.1926
5.8381
4.3259
41
Supplemental Figure 1. Ratio of nitrate concentrations during spring for "1.2SO2_ratio" case
(left) and for "2NOXratio" case (right). *Please refer to Table 1 and Table 2 for
abbreviations used in this figure. The color bars are saturated at 30% (left) and at 170%
(right).
SPR 1.2SO2jsio NOS-(%]
SR 2
I
- --
-
10
XraO W3-1%1
4
2
0
100
__~ngftuds
0[
0
0
100
100
300
30. 44. 56. 72. 66. 100 114.125. 142Z 156.170
Supplemental Figure 2. The spring nitrate concentration ratio ("2001Asian/2001") focused
on Arctic region. *Please refer to Table 1 and Table 2 for abbreviations used in this figure.
The color bars are saturated at 0% and at 90%.
SPR 2001AsIan/2001 NO3- 1%J
S0*N
70*N
W O*N ..
-...
..
50%N
..
.. "
40*N
A .........
30*N
......
-...-..-..
..-
20ON
10*N
-
-
........-
--
0 V
120*E
0. 0.
5.
180*
120*W
WOW
10. 15. 20. 25. 30. 35. 40. 50. 60. 70. 80. 90.
42
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