Investigating sources of organic carbon and tracking
transpacific transport:
Integrating aerosol observations and models
Colette L. Heald
Atmospheric Physics Seminar, University of Toronto
November 21, 2005
GROWTH IN EAST ASIA IS THE MAJOR DRIVER
FOR GLOBAL ATMOSPHERIC CHANGE
Asian CO ↑ 60%
Will influence hemispheric air quality and radiative forcing
TRANSPACIFIC TRANSPORT OF ASIAN AEROSOLS
Despite their short lifetimes, aerosols can be transported across the Pacific and can
affect North American air quality standards and visibility.
Most documented cases consist of
transport of dust:
Visibility reduction at Glen Canyon, Arizona
due to transpacific transport of Asian dust
April
16, Day
2001
Clear
BUT Model simulations suggest that anthropogenic aerosols from Asia can
ALSO be transported to the United States [Park et al., 2004]
Asian contribution
is comparable to
“natural” standard
set by EPA Haze
Rule (0.12 µgm-3)
AEROSOL CONCENTRATIONS IN THE U.S.:
contributions from natural sources and transboundary
pollution
Annual regional means from GEOS-CHEM
standard and sensitivity simulations
Ammonium
Sulfate
West
East
Ammonium
Nitrate
West
East
Elemental
Carbon
West
East
Organic
Carbon
West
East
Baseline (2001)
1.52
4.11
1.53
3.26
0.27
0.66
1.77
3.07
Background
0.43
0.38
0.27
0.37
0.08
0.06
1.30
1.22
Natural
0.11
0.11
0.03
0.03
0.06
0.04
1.25
1.17
Transboundary
0.28
0.26
0.18
0.23
0.02
0.02
0.05
0.05
Asia
0.13
0.12
-0.02
-0.02
<0.01
<0.01
0.01
<0.01
Canada & Mexico
0.15
0.14
0.20
0.25
0.02
0.02
0.04
0.04
EPA natural defaults
0.12
0.23
0.10
0.10
0.02
0.02
0.47
1.40
Regional Haze Rule objective for 2064 in parks and wilderness areas
• EPA default natural concentrations are too low by factors of 2-3 except for OC in
eastern U.S. – quantifying fire influences is critical
• Transboundary pollution influence from Asia is comparable in magnitude to that
from Canada + Mexico
[Park et al., 2003, 2004]
TRANSPACIFIC TRANSPORT OF ASIAN AEROSOLS
We have observations
of outflow from Asia
(can verify emissions)
Aerosol
Precursor
gases
SOx
NOx
NH3
VOCs
Direct
Emission
OC
BC
Dust
Need to verify that we
accurately predict
transpacific transport
 SATELLITES 
We can compare
simulated aerosols
with ground station
observations over NA
Transport
Deposition
Subsidence
EPA Haze Rule
ASIA
PACIFIC
NORTH
AMERICA
MODIS OBSERVATIONS
•
•
•
•
•
Aboard the Terra and Aqua
satellites
Measuring radiance in solar to
IR wavelengths (0.43-14 um)
Retrieve total aerosol optical
depth at 550 nm
Pixel resolution of 1km, swath
width 2330 km
 global coverage in 3 days
Different retrieval algorithms
over ocean and land
MODIS
Aerosols
Land
(difficult to characterize reflectance)
GEOS-CHEM GLOBAL CTM OXIDANT-AEROSOL
SIMULATION
http://www-as.harvard.edu/chemistry/trop/geos/index.html
Bey et al. [2001], Park et al. [2003], Park et al. [2004], Alexander et al. [2004],
Fairlie et al. [2005]
Dust
Black Carbon
Sulfate
Aerosol Optical
Depth (AOD) over
the Pacific (during
Spring) is
dominated by dust
and sulfate
Organic Carbon
Seasalt
AOD
(550 nm)
DIFFERENTIAL TRANSPORT OF AEROSOLS AND CO
OBSERVED FROM SPACE
Anthropogenic plume,
similar for CO and
aerosols (allowing for
aerosol scavenging)
March
2001
Biomass burning
plume for CO – Not
observed for aerosols
TRANSPACIFIC TRANSPORT (2001)
MODIS AOD
GEOS-Chem AOD
Sulfate AOD
Dust AOD
peak
Asian
dust
ALSO
substantial
anthropogenic
aerosol
transport
GEOS-CHEM underestimates MODIS observations by factor of ~2 in Spring
WHAT CAN AERONET OBSERVATIONS TELL US?
Is the model/MODIS bias primarily a model underestimate or a satellite
retrieval bias?
MODIS
AERONET
GEOS-CHEM
AERONET sites indicate a possible MODIS retrieval bias
(not correlated with cloud cover).
AN EXAMPLE OF TRANSPACIFIC TRANSPORT OF
ASIAN AEROSOL POLLUTION AS SEEN BY MODIS
April 25, 2001
April 26, 2001
April 27, 2001
TRANSPACIFIC TRANSPORT EVENTS AT SURFACE SITES
Midway Island (central North Pacific)
IMPROVE Sites (NW United States)
IMPROVE obs
GEOS-Chem
GEOS-Chem (Asian)
4 transpacific events tracked at surface sites
IMPACT OF ASIAN SULFATE ON U.S. AIR QUALITY
Observed
NW US:
0.72 μgm-3
Simulated
Asian
NW US:
0.18 μgm-3
Observed
during
Asian
events
NW US:
1.04 μgm-3
Asian
events
NW US:
0.60 μgm-3
Asian aerosols preferentially impact ground sites in the NW US.
Observations at IMPROVE sites are elevated from mean when simulated Asian
influence is high
[Heald et al., 2005b]
PROJECTED SOx EMISSIONS IN ASIA
One projection suggests that
emissions of SOx will more than
double in China between
1995-2020
[Streets & Waldhoff, 2000]
courtesy: David Streets
Increasing SOx emissions from Asia could degrade North American air quality
and present a further barrier to attainment of domestic air quality regulations
in the United States (eg. EPA Haze Rule)
ORGANIC CARBON AEROSOL
*Numbers from IPCC [2001]
Reactive
Organic
Gases
Secondary Organic
Aerosol (SOA): 8-40 TgC/yr
Nucleation or Condensation
OC
Oxidation
by OH, O3, NO3
Monoterpenes
FF: 45-80 TgC/yr
BB: 10-30 TgC/yr
Aromatics
Direct
Emission
Fossil Fuel
BIOGENIC SOURCES
Biomass
Burning
ANTHROPOGENIC SOURCES
FIRST SUGGESTIONS OF HIGH ORGANIC CARBON
AEROSOL CONCENTRATIONS IN THE FREE TROPOSPHERE
High organic loading
in the FT
High organic loading
in the UT
Single particles over NA
[Murphy et al., Science, 1998]
TARFOX (E US)
[Novakov et al., JGR, 1998]
ACE-ASIA: OC AEROSOL MEASUREMENTS IN THE
FREE TROPOSPHERE
(ACE-Asia aircraft campaign conducted off of Japan during April/May 2001)
Seinfeld group
Huebert group
Russell group
What is the source
of this FT organic
carbon aerosol?
+
Mean Observations
Mean Simulation
Observations
High Levels of OC were observed in the FT during ACE-Asia by 2 independent
measurement techniques. We cannot simulate this OC with direct emissions
DO WE UNDERSTAND OTHER AEROSOLS?
Secondary
production
Scavenging
Scavenging
Mean Observations
Mean Simulation (GEOS-Chem)
GEOS-Chem simulates both the magnitude and shape of sulfate and EC
concentrations throughout the troposphere  what is different about OC?
ANY INDICATION THAT DIRECT EMISSIONS ARE
UNDERESTIMATED?
Biomass Burning:
• Satellite firecounts show no active fires in Siberia
• OC aerosol from agricultural burning in SE Asia emitted earlier in the season, at
lower latitudes and is not injected into the FT
Pollution:
• Although the highest aerosol observations are associated with elevated CO, there
is a free tropospheric background of 1-3 μg sm-3 that is not correlated with CO or
sulfate.
IS SECONDARY ORGANIC AEROSOL (SOA) THE
EXPLANATION?
Secondary
Organic Aerosol
Condensation of low
vapour pressure ROGs
on pre-existing aerosol
Reactive
Organic Gases
Oxidation by
OH, O3, NO3
SOA is a good candidate because
can condense more easily with
colder temperature (i.e. higher altitude)
GEOS-CHEM April Biogenic SOA
[Chung and Seinfeld, 2002]
mechanism
Biogenic VOCs
(eg. monoterpenes)
FT observations ~ 4mg/m3
Biogenic SOA far too small!
IMPLICATIONS FOR TRANSPACIFIC TRANSPORT
Observed
Simulated
Asian air masses
Sulfate: 0.24 µgm-3
OC: 0.53 µgm-3
ASIA
High concentrations of OC
aerosols measured in the FT
over Asia (not captured by models)
[Heald et al., 2005a]
PACIFIC
NORTH
AMERICA
Twice as much OC
aerosol as sulfate
observed at Crater Lake
[Jaffe et al., 2005]
CARBON CYCLE AND POTENTIAL RADIATIVE IMPLICATIONS
4 ug/sm3 (ACE-Asia)
AOD @ 50% RH: 0.057
TOA Radiative Forcing = -1.2 W/m2
OC AEROSOL
1 µg/sm3 from 2-7 km globally = 105 TgC/yr
VOC EMISSIONS
500-1000 TgC/yr
[IPCC, 2001]
DISSOLVED
ORGANIC CARBON
IN RAINWATER
430 TgC/yr
[Willey et al., 2000]
[Heald et al., 2005a]
ICARTT: COORDINATED ATMOSPHERIC CHEMISTRY
CAMPAIGN OVER EASTERN NORTH AMERICA AND NORTH
ATLANTIC IN SUMMER 2004
Multi-agency,
International Collaboration
MOPITT Observations of CO Transport
(July 17-19) [Turquety et al., in prep]
2004 fire season in North America:
• worst fire season on record
in Alaska
Emissions derived from MODIS
hot spots [Turquety et al., in prep]
OC: 1.4 TgC
OC emissions from biomass burning were 4 times climatological average!
UNDERESTIMATE OF OC AEROSOL DURING ICARTT
Observations
GEOS-Chem Simulation
NOAA ITCT-2K4 flight tracks
WSOMCx2
SOA
(R. Weber’s PILS instrument aboard)
OMC WSOMC
Same underestimate in OC aerosol
observed over North America.
Note: biomass burning plumes were removed
INCLUDING ISOPRENE AS A SOURCE OF SOA
Recent study: yield of SOA from isoprene is 0.9-3.0%[Kroll et al., 2005].
Isoprene oxidation products have been observed in the particulate phase
[Matsunaga et al., 2005]
GEIA Emissions July/August 2004
3% yield
= 0.4 Tg SOA
10% yield
= 0.8 Tg SOA
Applying smog chamber estimates [Kroll et al., 2005] to isoprene emissions
inventories suggests a 50% increase in the SOA source over NA.
IS SCAVENGING OF OC AEROSOLS OVERESTIMATED IN
MODELS?
Hydrophillic aerosols are wet scavenged assuming 100% solubility.
Recent analysis of cloud events at Puy de Dome suggest scavenging efficiency of
OC is much lower [Sellegri et al., 2003]
Observations
GEOS-Chem Simulation
GEOS-Chem Simulation
(with scavenging e=0.14)
A large decrease in scavenging efficiency increases OMC concentrations
throughout the troposphere, however discrepancies remain in the BL
ORGANIC CARBON IN THE FT: AN ONGOING QUESTION
Chemistry leading to SOA production is not well understood!
Global Models currently do not include:
1. SOA formation in clouds [Lim et al., 2005]
VOC
Oxidation by OH  water-soluble
Evaporation
products dissolve into cloud droplets
2. SOA from isoprene [Claeys et al., 2004; Kroll et al., 2005; Matsunaga et
al., 2005]
3. Uptake of carbonyls on aerosols [Liggio et al., 2005]
4. Multi-stage oxidation of long-lived oxidation intermediates
Further Investigation Required:
1. Chamber study yields overestimate yields at low NOx
2. Chamber study yields performed at room temperature  chemistry at
cold T?
3. Do very reactive BVOCs inferred over forest canopies [Goldstein et al.,
2004] produce appreciable SOA?
4. Do models over-estimate scavenging?
BEFORE: ORGANIC CARBON AEROSOL
*Numbers from IPCC [2001]
Reactive
Organic
Gases
Secondary Organic
Aerosol (SOA): 8-40 TgC/yr
Nucleation or Condensation
OC
Oxidation
by OH, O3, NO3
Monoterpenes
FF: 45-80 TgC/yr
BB: 10-30 TgC/yr
Aromatics
Direct
Emission
Fossil Fuel
BIOGENIC SOURCES
Biomass
Burning
ANTHROPOGENIC SOURCES
ORGANIC CARBON AEROSOL
Cloud
Processing
SOA: ?? TgC/yr
ROG
Nucleation or Condensation
OC
Heterogeneous Reactions
Oxidation
by OH, O3, NO3
Isoprene Monoterpenes
FF: 45-80 TgC/yr
BB: 10-30 TgC/yr
Aromatics
Direct
Emission
Fossil Fuel
BIOGENIC SOURCES
Biomass
Burning
ANTHROPOGENIC SOURCES
ACKNOWLEDGEMENTS
CO-AUTHORS
• Daniel Jacob (Harvard), Rokjin Park (Harvard), Becky Alexander (formerly
Harvard, now U of Washington), Duncan Fairlie (Harvard/Langley), Bob
Yantosca (Harvard), Allen Chu (GSFC), Lynn Russell (Scripps), Barry
Huebert (U of Hawaii), John Seinfeld (Caltech), Hong Liao (Caltech), Rodney
Weber (Georgia Tech)
SCIENCE TEAMS/CAMPAIGNS
• The ACE-Asia Science Team
• The TRACE-P Science Team
• The MODIS Science Team
• The MOPITT Science Team
FUNDING:
• NASA Earth System Science Fellowship and ACMAP
• Natural Science and Engineering Research Council of Canada
SUMMARY
TRANSPACIFIC TRANSPORT OF ANTHROPOGENIC AEROSOLS
• Annual Asian contributions to sulfate in the W US exceed the “natural” visibility
objectives of the EPA Regional Haze Rule [Park et al., 2004]
• During the spring (2001) ¼ of the sulfate observed at ground site in the NW
US comes from Asia (0.18 μgm-3 ± 50%)
• MODIS and ground observations corroborate the transpacific transport of
aerosols simulated by GEOS-Chem, despite quantitative differences in
observed and simulated AOD
ORGANIC CARBON AEROSOL
• Organic carbon aerosol observed in the FT off of Asia (~4 μgm-3) during spring
2001 is a factor of 10-100 higher than currently simulated by a global model
• Concentrations of OC are also underestimated over NA during summer 2004
• These discrepancies highlight an incomplete understanding of formation and
loss processes of OC and SOA in the troposphere
• The high loading of OC aerosol in the FT is likely to have important
implications for intercontinental transport and radiative forcing