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Analysis of Air Transport and Oxidation Chemistry in the NorthAtlantic Region from Interpretations of Non-Methane Hydrocarbon
(NMHC) Measurements at Pico Mountain, Azores
D. Helmig1, D. Tanner1, R.C. Owen2 and R. E. Honrath2 and D. Parrish3
1
Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO
80309, USA
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Department of Civil and Environmental Engineering, Michigan Technological University,
Houghton, Michigan, USA
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Chemical Sciences Division, National Oceanic and Atmospheric Administration, Boulder, CO
80303, USA
Manuscript in preparation for Journal of Geographical Research
June 27, 2006
Abstract
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One year of continuous measurements of non-methane hydrocarbons at the mountaintop PICO-
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NARE observatory on Pico Island, Azores were used to investigate seasonal oxidation chemistry
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and transport patterns in the central North Atlantic Region. NMHC at this site exhibited seasonal
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and short-term variations and cycles that reflect the distance of the island from continental
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sources of NHMC emissions and oxidation of NMHC by the seasonally highly variable OH
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radical. Substantially enhanced NMHC levels during the summer of 2004 were attributed to the
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impact of long-range transport of biomass burning plumes resulting from Northern Canada and
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Alaskan wildfires.
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NMHC concentrations and their relative ratios were valuable in identifying transport situations
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where anthropogenically influenced air from the mid and western U.S. was transported to Pico in
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5-8 days.
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processing (‘photochemical clock’) was shown to yield results that were in qualitative agreement
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with trajectory and FLEXPART analysis interpretations.
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measured at PICO-NARE throughout all seasons. Enhanced ozone levels were observed in air
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that had relatively ‘fresh’ photochemical signatures (e.g. ln [propane]/[ethane] > -2.5). Ozone at
During summer, air samples were the most photochemically processed.
Interpretations of NMHC ratios for use as a relative scale for photochemical
Ozone in excess of 35 ppbv was
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lower levels (< 40 ppbv) was always correlated with more processed air patterns (‘older’ air with
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ln [propane]/[ethane] < -2.5.
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1. Introduction
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Non-methane hydrocarbons (NMHC) in the atmosphere show considerable variations on spatial
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and temporal scales, their concentrations being determined by the strength of emission sources
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and atmospheric removal processes. Atmospheric oxidation is mostly due to reaction with the
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OH radical, with reaction rate constants increasing significantly with the molecule size.
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Consequently, lighter hydrocarbons (having the slowest reaction rates) exhibit much longer
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lifetimes and their atmospheric concentrations decline at slow enough rates for NMHC
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concentrations to remain high enough after several days of transport to impact air chemistry at
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remote downwind locations. Since many individual NMHC have common emission sources and
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since their emission ratios vary comparatively little, changes in absolute concentrations and
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NMHC ratios can be used as tools to decipher atmospheric transport and oxidation chemistry.
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Several researchers have investigated this utility and have presented a framework for the
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interpretation of atmospheric light (C2-C6) hydrocarbon observations.
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Selected NMHC can be used as tracer for specific emission sources or events. For instance,
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isoprene is a selective tracer for biogenic emissions (Fehsenfeld et al., 1992), acetylene has been
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found to be significantly enhanced in biomass burning plumes (DeGouw et al., 2004) and light,
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saturated and unsaturated NMHC (e.g. ethane, propane) have been used to identify influences
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from urban energy use and petrochemical industries (Blake and Rowland, 1995; Jobson et al.,
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2004). Diurnal concentration changes of light, unsaturated NMHC (ethene, propene) allowed
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identifying occurrences of upslope and downslope flow conditions at Mauna Loa Observatory
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(Greenberg et al., 1996).
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Certain NMHC (e.g. butanes) have similar atmospheric removal rates and hence their,
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atmospheric ratios show little variations during atmospheric transport and processing. Their
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analytical data can therefore be used as a quality control tool in NMHC measurements (Parrish et
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al., 1998).
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The variability of NMHC concentrations can provide information on the impact or distance of a
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measurement site from pollution sources. A ‘remoteness’ scale has been proposed, that is
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derived from a plot of the natural logarithm (ln) of the standard deviation of ambient NMHC
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concentrations at a given site versus their estimated lifetime (Jobson et al., 1999). Relative
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changes of the ratio of branched versus straight n-alkanes have been used to infer the importance
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of halogen and nitrate radical versus OH radical chemistry as the reaction rates of these two
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different oxidation routes are significantly different enough to cause shifts in the atmospheric
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concentration ratios of these isomeric compounds (Penkett et al., 1993; Finlayson-Pitts, 1993).
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NMHC ratios and concurrent measurements of ozone were also applied to investigate potential
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changes in the oxidation chemistry of the atmosphere. In particular, the relative increase of
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ozone at observed NMHC ratios was used as an argument for an increased ozone production
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(respectively reduced ozone loss rates) in long-range transport across the Pacific Ocean (Parrish
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et al., 2004). NMHC measurements from remote, marine environments have also been applied
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for estimating mean OH radical fields during transport of air in the marine boundary layer and
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lower free troposphere (Ehhalt et al., 1998; Williams et al., 2000).
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The possibilities for using NMHC data for interpretations of atmospheric oxidation processes are
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particularly promising in situations where observations can be obtained in air that has traveled
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for extended periods of time without being influenced by recent emissions or surface processes.
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Hence, remote islands that are high enough to probe free tropospheric air are ideal locations for
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this research. These aforementioned considerations motivated the monitoring of NMHC at the
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mountaintop PICO-NARE site on Pico Island, Azores. These measurements commenced in the
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summer of 2004 and have been continuous since during most times when the station was on
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power. In this paper we present data from the first year of these new observations and examples
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of interpretations that demonstrate the utility of the NMHC data for interpretations of oxidation
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and transport processes in the North Atlantic region.
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2. Methods
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2.1. PICO-NARE Station
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The PICO-NARE observatory is located in the summit caldera of the inactive Pico Mountain
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volcano (38.47N, 28.40W), the highest mountain on Pico Island, and in the Azores, Portugal. At
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2225 m asl, lower, free tropospheric air is sampled at the station during most times. More
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information on the geo- and topographical features are provided by Kleissl et al. (2006). A
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detailed analysis of boundary layer height, and of mechanical uplifted and buoyant flow
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conditions showed that air from lower elevations was potentially lifted up to the station height up
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to 50% of the days during some months, but to a lesser extent (~ 25% of the days) during the
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summer. However, chemical measurements of nitrogen oxides and carbon monoxide at the
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observatory showed very little (resp. negligible) influence from island emission sources even
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during upslope conditions, which infers that even during uplifting events, mostly lower
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tropospheric air is transported to the station (Kleissl et al., 2006). Data and interpretations from
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other research, including studies of oxidized nitrogen species, ozone, carbon monoxide and of
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aerosol properties at PICO-NARE have been presented previously (Honrath et al., 2004; Fialho
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et al., 2006) and in other contributions to this special issue (Val Martin et al., 2006).
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2.2 NMHC Measurements
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The remoteness of the PICO-NARE site and the limitations for power and for supply of cryogen
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and consumable gases determined the design of an analytical system that was tailored towards
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this unique situation. All consumable gases and blank air were prepared at the site with low-
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power gas generators. The instrument was designed to follow automated startup and shuddown
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procedures and could be remotely controlled from our Boulder, CO offices. Ozone was removed
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by flowing the sample air through an ozone scrubber prepared from sodium-thiosulfate-
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impregnated glass wool. After sample drying and NMHC focusing on a mulit-stage solid
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adsorbent trap, NMHC were analyzed by thermal desorption with gas chromatography (GC)
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separation and flame ionization detection (FID). The instrument was calibrated by regular
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injections of a multi-component, rural air standard that was quantified prior to shipment against a
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gravimetric hydrocarbon standard scale in the NOAA Aeronomy Laboratory. A second, remote,
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ambient air standard (collected at Niwot Ridge, Colorado) was injected every 3-4 days for
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quality control.
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Sample volumes of 600 ml (10 min collection time) and 3000 ml (50 min collection time) were
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alternated for quantification of ethane and NMHC > C2, respectively. Typically, a total of 12
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ambient air samples, one standard and one blank sample were analyzed daily. Data were
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transferred daily for instant quality control and analysis. The primary calibration standard was
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returned to Boulder in spring 2006 and the control analysis on the independently calibrated
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NOAA GC system showed that C2-C6 NMHC mixing ratios were within +/- xx % (DAVID, WE
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PROBABLY WILL HAVE TO PUT OUR BEST GUESS NUMBERS HERE AS WE STILL
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DON’T HAVE THIS ANALYSIS BACK FROM PAUL? ANY UPDATE ON THAT? WE
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DEFININTELY NEED TO HAVE THIS DONE BY THE TIME THE REVIEWS COME
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BACK) of the values determined two years earlier, prior to the shipment to Pico. NMHC were
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quantified using compound-specific FID response factors, as determined from the primary
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standard injections. Quantified NMHC included ethane, propane, n-butane, i-butane, i-pentane,
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n-pentane, n-hexane and isoprene (the ethane record doesn’t begin until in fall 2004 when some
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modifications in the focusing procedure allowed its quantitative analysis).
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described experiments analytical precision and accuracy were estimated to be better than 10%
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for mixing ratios > 100 ppt and approximately a factor of 2 higher for levels between the
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detection limit (which typically were ~ 30, 11, and 1-2
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respectively) and 100 pptv. More instrumental details have been provided elsewhere (Tanner et
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al., 2006).
From the above
pptv for C2, C3, and C4-C6,
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2.3 Trajectory Analysis
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Backward trajectories were calculated with the Hybrid Single-Particle Lagrangian Integrated
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Trajectories (HYSPLIT) model (Draxler and Rolph, 2003). HYSPLIT uses meteorological data
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from the National Weather Service’s National Center for Environmental Prediction (NCEP) final
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analysis (FNL). Data are available at 6-hour resolution with 13 pressure altitude levels. A set of
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six trajectories were calculated, one terminating at the station, four terminating at grid points
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adjacent to the station and separated from it by 1o, and one terminating directly below the station
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at 2000 m asl.. Trajectories were run 10 days backward in time.
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2.4 FLEXPART Simulations
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Besides the trajectory analysis, the FLEXPART particle dispersion model (versions 5.2 and 6.2,
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(Stohl et al., 1998; Stohl and Thomson, 1999; Stohl et al., 2005)) was used to evaluate derived
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NMHC transport times with synoptic transport modeling results. FLEXPART version 6.2 was
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driven with data from the European Centre for Medium Range Weather Forecasts (ECMWF)
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(ECMWF, 2005) with a 1 degree horizontal resolution, 60 vertical levels and a temporal
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resolution of 3 hours, using meteorological analyses at 0000, 0600, 1200, and 1800 UTC, and
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ECMWF 3-hr forecasts at intermediate times (3, 9, 15, 21 UTC). FLEXPART version 5.2 was
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driven with data from wind fields from the NOAA NCEP FNL. The FNL data were downloaded
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from the National Center for Atmospheric Research data archive, available every 6-hours with a
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horizontal grid spacing of 1
1 , and 21 vertical levels between 1000 and 100 hPa
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Version 5.2 of the model was run in its forward mode to simulate CO enhancements at the PICO-
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NARE station resulting from the transport of North American and Asian emissions. These
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emissions were divided into one day age classes, which was also useful for determining the time
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since emission. CO emissions were released into the lowest 300 m of the atmosphere over North
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America and Asia. Emissions were based on the EDGAR 3.2 Fast Track 2000 dataset [Olivier et
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al., 2001] for anthropogenic sources only with a 1 degree resolution.
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Version 6.2 of the model was run in its backward mode to create “retroplumes”, similar to
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backward trajectories. Retroplumes are simulated from the release of thousands of particles at
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the receptor that are advected backwards in time. Retroplumes are superior to trajectories in that
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they allow for an assessment of the deformation of an air mass as it travels and for determining
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source regions for observed enhancements (Seibert and Frank, 2004).
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initiated every three hours with 20,000 particles released over a three hour time interval into a 1
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degree x 1 degree grid box centered on the PICO-NARE station, over an altitude range of 1750
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m asl to 2750 m asl. Particles were followed backward in time for 20 days.
Retroplumes were
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3. Results and Discussion
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3.1 NMHC Mixing Ratios
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Plots with the individual sample data (representing a total of 1958 analyzed air samples) for
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ethane, propane, and n-butane from Aug. 2004–Sept. 2005 were presented by Tanner et al.
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(2006). Here we combined these data to monthly whisker plots that show the minimum, 5, 25,
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50, 75, and 95 percentile, and the maximum values of measured mixing ratios during each month
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of available measurements (Fig. 1). Sinusoidal best fit curves were calculated from the (diurnal
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resolution) data and are included in these graphs to illustrate a smoothed seasonal NMHC cycle.
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These data show the typical Northern Hemisphere seasonal cycle of NMHC with lower mixing
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ratios in the summer and maximum values in late winter. This behavior to a large extent is
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driven by the annual concentration changes of the OH radical, which is closely linked to the
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latitudinal solar radiation cycle. High variability in NMHC mixing ratios was observed at any
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given time of year. It is noteworthy that all of these features show relations and dependencies
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toward the individual NMHC reactivity with OH and the resulting NMHC lifetime. The longest-
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lived NMHC, ethane, shows the relatively smallest amplitude between the mean winter and
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summer mixing ratio and the smallest relative variability on short (e.g. weeks) time scales. All
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of these features increase with increasing molecule size (respectively shorter OH lifetime). The
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seasonal maximum and minimum of ethane occurs the latest of all compounds (March 3 and
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September 3, respectively), as due to its slower OH reaction, ambient levels respond with a
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longer delay to the seasonal OH cycle. Heavier NMHC were found to maximize earlier, up to
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around January 20 for the most reactive compounds, and also had their seasonal minimum
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earlier, around July 18. These features in the Pico NMHC data are in agreement with data from a
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number of other sites, which, along with their seasonal OH dependencies, have been presented
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and discussed in detail in the literature (e.g. Jobson et al., 1994; Goldstein et al., 1995; Gautrois
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et al., 2003).
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Comparison of the NMHC >C3 for summer 2004 with data from the corresponding period
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during 2005 shows a higher variability as well overall higher mixing ratios during 2004. As
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discussed in detail in other contributions to the ICARTT issue, the summer of 2004 was
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characterized by an unusually high occurrence of boreal wild fires in Northern Canada and
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Alaska, outflow of which was frequently observed at Pico (Val Martin et al., 2006). Substantial
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enhancements in NMHC, at times increasing to twice their seasonal background levels, were
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observed in these boreal fire plumes. Overall, during times when the station was impacted by
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boreal fire plumes (as defined in Val Martin et al., 2006), e.g. propane 25/50/75 percentile
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mixing ratios were 65/81/143 pptv, whereas outside of fire events, they were 25/51/87 pptv
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during the 2004 summer.
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A number of data sets have been presented in the literature that allow comparisons with the
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PICO-NARE measurements. We included two related data series in Figure 1, notably from two
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years of measurements at the continental, remote boreal site in Fraserdale, Ontario (50oN, 82oW)
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(Jobson et al., 1994) and from the Mauna Loa Observatory Photochemical Experiment-2 (19oN,
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xxoW) (Greenberg et al., 1996), which, as shown below, bracket the Pico measurements and
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allow a further interpretation of the particular conditions encountered at Pico.
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Probably the most related study are the MLOPEX-2 measurements. Similar to PICO-NARE, the
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Mauna Loa site is a remote mountaintop island location, where, during downslope conditions,
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free tropospheric air is sampled that has traveled over the ocean for several days.
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more prominent diurnal upslope-downslope cycle and data presented by Greenberg et al. were
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broken up into the occurrences of these two flow regimes.
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25/50/75 percentiles for the time periods spanned by the width of the boxes. The shown MLO
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data are from downslope (e.g. free tropospheric air) conditions. Upslope data for MLO typically
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were higher, with relative enhancements increasing with decreasing molecule liftetime.
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MLOPEX-2 data are consistently lower for all NMHC and during all seasons. The differences in
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Pico and MLO NMHC mixing ratios increases with decreasing lifetime, e.g. while ethane mixing
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ratios compare to within ~20%, n-butane values at MLO are more than 5 times lower than at
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Pico.
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experiences overall higher NMHC values than Pico. Again, differences in these two data series
MLO has a
Included data in Figure 1 are the
In contrast to MLO, Fraserdale, a remote, low elevation continental forest site,
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become more pronounced with molecular weight, although this time the PICO-NARE data are
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the ones becoming increasingly lower.
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Higher NMHC levels at Fraserdale than at Pico, and higher NMHC levels at PICO-NARE than
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at MLO are likely due to several reasons. 1. Aircraft profiles have shown that NMHC mixing
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ratios generally decline with height within the free troposphere (e.g. Blake et al., 1997), with
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larger concentration changes being observed for shorter-lived compounds. Pico is higher than
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Fraserdale, and MLO is at about 1200 m higher altitude than the PICO-NARE station,
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consequently NMHC mixing ratios would be expected to be highest at Fraserdale, followed by
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PICO-NARE and MLO. Secondly NMHC mixing ratios in the lower troposphere decrease
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towards lower latitude (Rudolph, 1995). Again, this dependency would infer higher NMHC
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levels at Fraserdale (50oN), followed by Pico (38oN) and MLO (19oN). Of further importance is
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the distance to the adjacent continents, which is about two times as much for MLO, and which
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will cause transport and photochemical processing times from continental sources to be longer,
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resulting in more depleted NMHC ratios at MLO compared to PICO-NARE.
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comparisons of the PICO-NARE data with other data sets from higher (than Pico) northern
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latitudes in Canada, the Atlantic Region and Europe (as summarized by Gautrois et al. (2003))
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shows that Pico NMHC levels are unanimously lower, both during the winter and in the summer
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compared to these locations.
Further
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The cumulative distribution of NMHC during fall 2004 (September 22 to December 20), winter
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2004-2005 (December 21 to March 19), spring 2005 (March 20 to June 20) and summer 2005
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(June 21 to September 21) is shown in Figure 2. Data series that do not extend in the lower
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percentage range resulted from respective fractions of these data being reported below the
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instrument detection limit (for instance, i- and n-pentane were below the detection limit in about
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4% of the measurements during fall 2004, whereas during summer 2005, ~85% and 60% of
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chromatograms had i- and n-pentane peaks that were too small to quantify). The regression line
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slopes through these individual data series indicate the variability of the atmospheric
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concentrations of a given compound. Steeper slopes are observed for longer-lived NMHCs (e.g.
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ethane) as these compounds have a higher atmospheric background concentration which reduces
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the relative variability caused by emission influences. It is noteworthy that regression line slopes
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are lower for the summer, which likely can be attributed to the shorter seasonal atmospheric
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lifetime and resulting lower background concentrations, which will cause higher relative
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variabilities.
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Results for isoprene measured at the station were presented by Kleissl et al. (2006). Isoprene,
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was typically not detected (< 1 pptv) in winter and nighttime samples. During spring, isoprene
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was occasionally observed in samples collected during morning to evening hours. Occurrences
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and mixing ratios of isoprene increased towards late summer.
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was detected on 60% of all days in the afternoon with maximum mixing ratios reaching up to 27
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pptv. The isoprene data clearly show seasonal and diurnal dependencies that are determined by
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both the expected seasonal changes in isoprene emission rates from vegetation growing at lower
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elevation on Pico Island and by occurrences of buoyant and mechanical uplift flow that
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transports air from lower parts of Pico to the observatory (Kleissl et al., 2006).
During August 2005, isoprene
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3.2 NMHC Variability
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The variability of NMHC during each of the four differentiated seasons is directly related to the
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slopes of regression lines through the data in Figure 2. On a cumulative distribution plot, the
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slope of the regression line is the standard deviation of the data, assuming the data (including
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points falling below the detection limit) are log-normally distributed. The seasonal lifetimes of
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NMHC were determined from their OH reaction constants and seasonal OH radical
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concentration. [OH] was estimated at a 1-day resolution according to (Goldstein et al., 1995):
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[OH] = A [1-B cos(2*pi * t/365)],
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where A=1.6*106 and B=0.80.
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(Spivakovsky et al., 2000) for 800 hPa, 36.0oN, 27.5oW. Daily OH concentrations were then
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averaged to seasonal OH values within the defined time periods. Reaction rate constants were
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adjusted to the temperatures measured at the PICO-NARE station during the respective season.
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Please note that this local lifetime represents an estimate for the conditions at the receptor site;
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the actually encountered lifetime during transport of a NMHC to Pico may have deviated from
This calculation utilizes monthly average OH values
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this estimate dependant on the geographical and atmospheric conditions during the transport
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path. Also note that this analysis in not very sensitive towards the applied [OH], but more so
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towards the relative reactivity differences between individual compounds. Consequently, errors
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in the estimated, total [OH] will only have little effect on the results for the regression
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coefficients (Jobson et al., 1999).
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and for each of the four seasons. The results shown in Figure 3 show well correlated linear
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relationships for all four seasons. Regression line slopes, according to lnx = A -b. for each
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seasonal data set are included in the figure. Seasonal differences in the b-values were not
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statistically significantly different at P > 95% for seasons where data of all C2-C5 NMHC were
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included. The best fit linear regression analysis through the data for all seasons yielded lnx =
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1.60 -0.44.
lnx – lifetime estimates were obtained for each compound
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The exponent b in this equation has been noted to describe the importance of sink terms in the
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regional variability budget whereas the coefficient A can be related to the degree of
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photochemical aging; A-values have been used to derive estimates of transit times for different
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sample sets (Jobson et al., 1990).
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Interpretation of observed values for b from different sites has shown that b approaches 0 near
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urban areas, where the variability is strongly influenced by differences in the strength of local
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emission sources, whereas b-values close to 1 are found in stratospheric data sets, where the
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variability is low and dominated by chemical loss alone. The mean Pico value of 0.44 +/- 0.03.
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compares well with data from three aircraft data sets collected over other diverse remote areas,
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including the Arctic Boundary Layer (ABLE3A), the equatorial Atlantic (TRACE-A) and the
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western Pacific (PEM-West B) experiment, which resulted in b-values of 0.46–0.53 (ref). This
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comparison illustrates a rather high similarity between the continuous, seasonal Pico data and the
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results from the comparatively short aircraft campaigns that have been previously presented in
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the literature.
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3.3 Ratios of NMHC
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Correlation plots of all C2-C6 NMHC as well as of these NMHC with CO are shown in Figure 4
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and results for the linear regression analyses are given in Table 1. The common feature in these
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data is that regression line slopes of (NMHCA/NMHCB with carbon number NMHCA < NMHCB
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increase monotonically with increasing carbon number of NMHCB. This behavior is expected as
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the atmospheric mixing ratios (and lifetimes) of NMHC generally decrease with increasing
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carbon number. For an individual pair of NMHC, the regression line slopes become larger
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towards the summer, as the longer-chain NMHC are removed from the atmosphere faster than
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the more stable, shorter-chain NMHC. Regression coefficients generally decrease towards the
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summer, as shorter liftetimes, lower concentrations and higher relative variability cause the
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correlation between individual compounds to become weaker. In general, compounds with
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similar lifetimes generally show better correlations than compound pairs with much different
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lifetimes. However, the correlation between CO and ethane (which have very similar lifetimes)
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is notably weaker, likely because of their differences in primary and secondary sources.
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Although fossil fuel combustion is a common source of both gases, CO is also a degradation
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product of hydrocarbons (mainly methane) in the atmosphere.
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The OH reaction rate constants of the isomeric pairs iso-butane and n-butane, and of iso-pentane
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are very similar and consequently, the atmospheric ratios of these two compound pairs is
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expected to change very little during transport and photochemical oxidation. Their correlations
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in the data from Pico, differentiated by the four seasons, as well as their ratio against the absolute
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levels of n-butane and iso-pentane, respectively, are shown in Figure 5. The tight correlation
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between these two compounds is very obvious. Deviations and larger scatter at lower (e.g < 50
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pptv) mixing ratios to some extent can be attributed to the loss of precision when mixing ratios
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approach the detection limit.
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For the butanes, no statistically significant difference was found in the regression line slope
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between the four seasonal data series. The regression line slope for all data was calculated to be
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0.51 +/- 0.01 (R2 = x.xx). Similar values (range 0.37–0.55) have been reported in data from a
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multitude of other sites in both continental and marine environments (e.g. Bottenheim and
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Shepherd, 1995; Bottenheim et al., 1997; Greenberg et al., 1996; Parrish et al., 1998). The same
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analysis yielded a slope of slope of 0.69 +/- 0.08 (R2 = 0.93) for the n-pentane/i-pentane data.
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The graphs on the right side of Figure 5 investigate possible changes in the oxidation chemistry
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of these compound pairs by season as well as by their absolute concentrations. Other than for the
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increase in scatter at lower mixing ratios, the butanes do not show any systematic seasonal
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changes. The pentane plot looks somewhat different, as higher n-pentane/iso-pentane ratios are
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observed for the spring and summer data. This point is also visible in the cumulative distribution
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plots (Fig. 2) where, only for the summer data, the i-pentane data distribution falls above the n-
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pentane values. This behavior points to either different source region emission ratios or to
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different oxidation chemistry during the summer months. Changes in n-pentane ratios have
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previously been investigated by several other researchers.
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pentane ratios during summer months and in low-concentration (well aged) samples were also
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evident in the Fraserdale data (Jobson et al., 1994) as well as during ICARTT and during
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NEAQS from the Ron Brown (Parrish xxxx). Notably, the n/i-pentane ratios in the Pico data
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overall seem to be higher than in the data from (respectively closer to) the continental regions.
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This behavior is contrary to expected OH kinetics, as a relative decrease of n-pentane/i-pentane
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would be expected during summer, due to the slightly higher n-pentane OH reaction rate
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constant (3.94 x 10-12 cm3 molecule-1s-1 at 298 K) (Atkinson, 1994) compared to 3.9 x 10-12 cm3
389
molecule-1s-1 at 298 K ???? for iso-pentane (2-methylbutane) (Atkinson, 1989), which should
390
result to lower n/i-pentane ratios in summer. These dependencies may point towards seasonally
391
changing competition between alternative destruction pathways, such as by the NO3 radical or by
392
chlorine chemistry (Penkett et al., 1993).
Similarly enhanced n-pentane/i-
393
394
As a note of caution, it should be pointed out that our current interpretations of the summer
395
pentane data are somewhat limited. Even though the cumulative distribution plot shows n-
396
pentane data about twice as high as i-pentane, the pentane ratio plot shows the summertime value
397
to be around 1, with quite a substantial degree of scatter. For i-pentane, only 14% of the summer
398
data, and for n-pentane, only 41% of the summer data were above the detection limit. Only 11%
399
of the samples had both i- and n-pentane. So, any conclusions about these data are from the
400
highest 11% of concentrations observed during the summer. Also, it may be possible that at such
401
low concentrations (< 5 ppt) increasing sampling or measurements artifacts have to be taken into
402
account. The pentane observations and their preliminary interpretations presented here are
13
403
nonetheless a motivation for future, more thorough and accurate studies of pentane chemistry at
404
Pico.
405
406
The distribution of NMHC data in a double natural logarithm plot of [n-buante]/[ethane] versus
407
[propane]/[ethane] (Figure 6) can be used to investigate the degree of photochemical processing
408
that occurred in air reaching Pico. Data in these plots is distributed between two theoretical lines
409
that are determined by the assumptions that air with a common ratio of these compounds at a
410
source would have only been altered by OH photochemistry (kinetic line) or by dilution with air
411
that has zero concentrations of both compounds of consideration (dilution line). The seasonal
412
differences in NMHC oxidation are clearly visible in these data. During winter, most data have
413
larger ratios and are less variable, indicative of less photochemical processing, and/or more
414
homogenously distributed source regions and air transport. In contrast, spring and summer data
415
are more scattered (weaker R2 values); the lower [NMHC]/[ethane] ratios are indicative of the
416
higher degree of air processing that occurred during transport. The regression through all data
417
yields a slope of 1.60 (±0.04), which is within the range of slopes reported for this analysis from
418
several other experiments (Parrish et al., 2004). DAVID P., ANYTHING ELSE YOU WOULD
419
LIKE TO ADD?
420
421
3.4 NMHC Processing and Ozone
422
423
In general understanding the temporal variability of tropospheric ozone at any particular location
424
is complex because several processes can have significant impacts, and these impacts vary
425
strongly on different time scales. In situ photochemical production and destruction proceed at
426
rates that vary with the ambient levels of ozone precursors and variables such as sunlight and
427
water vapor levels. Surface deposition and destruction by reaction with local emissions of NO or
428
reactive NMHC can drastically reduce near-surface ozone concentrations at rates that vary with
429
the characteristics of the planetary boundary layer and the flux of local emissions. Transport of
430
ozone to the site from the stratosphere or from upwind regions of strong photochemical
431
production can greatly increase ozone concentrations.
432
433
The PICO-NARE site is ideally situated to isolate the effects of the regional photochemical
14
434
production and destruction in the central North Atlantic from the effects of the other processes.
435
Kleissl et al. (2006) show that air sampled at the site is characteristic of the lower free
436
troposphere with essentially no opportunity for significant effects from surface deposition,
437
destruction by reaction with local emissions, or photochemical production from locally emitted
438
precursors. The varying influence of the transport of stratospheric ozone often dominates the
439
variability of ozone in free tropospheric data sets. Since ozone from the stratosphere has a steep,
440
negative correlation with CO (see e.g., (Danielsen et al., 1987)) the influence of stratospheric
441
ozone transport can be evaluated from the correlation of ozone with CO. Honrath et al. (2004)
442
discuss the ozone-CO correlation at PICO-NARE; their Figure 7 shows only a few scattered
443
points with such correlation (relatively high ozone at low CO). Consistent with other PICO-
444
NARE analyses (Honrath et. al., 2004; Lapina et al., 2006; Owen et al., 2006; and Val Martin et
445
al., 2006) the ozone variability is expected to reflect the influence of the regional photochemical
446
production and destruction in the central North Atlantic.
447
448
The evolution of NMHC ratios through photochemical processing provides a means to
449
investigate the photochemical evolution of ozone (Parrish et al., 1992; 2004). Figure 7 shows the
450
dependence of ozone concentrations on the natural logarithm of [propane]/[ethane] as the
451
indicator of the photochemical processing in each season.
452
concentrations are relatively constant with no dependence on the NMHC ratios. In spring and
453
summer ozone has higher variability, both toward higher and lower concentrations.
454
relationships in Figure 7 indicate that higher ozone levels were consistently observed in air that
455
had relatively ‘fresh’ photochemical signatures (e.g. ln [propane]/[ethane] > -2.5), and that lower
456
ozone correlated with more processed air (i.e. ln [propane]/[ethane] < -2.5). These relationships
457
suggest that in spring and summer the highest ozone concentrations are observed when air
458
masses most recently transported from continental source regions impact the site, and lower
459
concentrations are observed in air masses that have been processed for longer times in the marine
460
troposphere. Evidently the photochemical environment of aged air masses in the central North
461
Atlantic is characterized by net photochemical destruction of ozone in spring and even more
462
strongly in summer.
During fall and winter ozone
The
463
15
464
Table 2 compares the springtime and summertime slope of the ozone - ln [propane]/[ethane]
465
relationship found at PICO-NARE with those reported from the north temperate Pacific marine
466
boundary layer (data for fall and winter were excluded because of the weak correlation result or
467
the regression analyses (Fig. 7)). Based on these results Parrish et al. (2004) argue that the recent
468
Pacific studies (ITCT-2K2, PHOBEA, TRACE-P) find evidence for only weak net ozone
469
destruction (small positive slopes) in the more remote Pacific marine boundary layer. This weak
470
photochemical destruction is in sharp contrast with the much stronger photochemical destruction
471
indicated by a study at Point Arena from nearly two decades earlier. The exception to this
472
picture is the strong photochemical production (large negative slope) in the PEM West-B study,
473
which focused on the region of strong outflow of ozone precursor emissions from Asia to the
474
western North Pacific. Comparison of the Pacific results to those from PICO-NARE suggest that
475
spring- and summertime photochemistry in highly aged air masses more effectively destroys
476
ozone in the central North Atlantic than in the North Pacific.
477
478
3.5 Transport Event Case Study
479
480
In Figure 8a six weeks of data for four NMHC during spring 2005 are shown (note the
481
logarithmic concentration scale). NMHC concentrations are highly variable, close to 10-fold
482
increases were observed several times during this observation window. The high correlation,
483
with concurrent minima and maxima of these four individual NMHC is noteworthy.
484
amplitudes of relative mixing ratio increases are highest for the shorter-lived compounds.
485
Underneath these variable data, the springtime decline in the NMHC mixing ratios can be
486
discerned. The ln [propane]/[ethane] and ln [butane]/[ethane] analysis for the same data (Figure
487
8b) can be used to investigate the short-term changes in the inferred photochemical age of air
488
reaching PICO-NARE. Again, a high variability is found, with high ln [NMHCi]/[ethane] ratios
489
(indicating ‘fresh’, e.g. little processed air) coinciding with periods of enhanced absolute NMHC
490
mixing ratios and low ln [NMHCi]/[ethane] ratios (indicating ‘old’, e.g. well processed air)
491
coinciding with periods of low absolute NMHC mixing ratios.
The
492
493
Three periods when air switched from a ‘fresh’ signature to an ‘old’ signature and back to a
494
‘fresh’ character were subjected to a closer investigation. These three, ~ 1-2 day intervals are
16
4
495
indicated by the colored circles in Figure 8b. The corresponding data points are marked by the
496
same colors and compared with all data during this April-May period in the ln [butane]/[ethane]
497
versus ln [propane]/[ethane] plot in Figure 9. Here, we derived the line constraining the mixing
498
boundary by assuming that air, influenced with recent emissions, was not mixed with zero-
499
concentration background air (as done for Figure 5) but instead, more realistically and as
500
suggested by McKeen and Liu (1993) was mixed with background (B) air that had inferred
501
seasonal ethane, propane and butane mixing ratios of 900, 20, and 2 pptv, respectively. These
502
modifications result in the shaped form of the mixing boundary that constrains these data. 24-
503
hour [OH] was estimated at 2 x 106 molecules cm-3 (according to Spivakovsky et al 2000 et al.,
504
for 800 hPa, April, 36.0oN, 27.5oW). Molar emission ratios were set to 0.63 for propane:ethane
505
and 0.35 for n-butane:ethane, which are the averages of results from Goldstein, et al (1995), and
506
Swanson, et al. (2003). The concentration change of these NMHC and their ratios can then be
507
calculated according to
508
model:
509
d N
Background:
510
OH only
ethane=900
Mixing
and(0.1
OHh-1) and k = 0.18 x 10-12, 0.89 x 10-12 and 2.05 x 10-12
511
with K being the mixing
constant
propane=60
16:00
4/19
1:00respectively (Atkinson and Arey [2003] with
512
molecules cm-3s-1 for4/17
ethane,
propanetoand
n-butane,
butane=4
4/19
17:00
to and
4/20
15:30 of hydrocarbon pairs5d
513
T=273
of this
equation
consideration
will yield
K=0.01
h-1 K). Integration
3/26 to 5/14
OH=2E6
 A0  LA 
 A  LA 
ln 
  ln Mixing Only
B  LB 

 B0  LB 
514
t
Starting:
 kB  k A  OH 
ethane:10
propane:5
515
where,
butane:4
10d
516
K  AB
kOH
517
,
LA 


K

k
OH
ethane:2.04e-13
A 

propane:9.77e-13
518
butane:2.29e-12
519
0d
15d
and A and B are a NMHC pair.
520
-3.5
-3
-2.5
-2
-1.5
ln(propane/ethane)
-1
17
-0.5
521
This equation, using the above parameters, allows assigning a hypothetical average
522
photochemical aging, respectively transport time from the occurrence of fresh emissions to
523
arrival at Pico, which is marked as the boxed numbers (in days) on the regression line of the data
524
depicted in Figure 9. Under these assumptions, the data of the three episodes marked in Figure
525
8b are defined with ages of 9-11, 15-16, and 8-10 days respectively.
526
527
Previous attempts of deriving photochemical transport times, or ‘photochemical clocks’ from
528
analysis of NMHC ratios have raised the question to what extent quantitative information from
529
this analysis can be valid. The fact that naturally the transport path, reaction history and dilution
530
of air parcels arriving at a receptor site may vary substantially, poses severe limitations on this
531
utility (ref). In the following paragraphs two synoptic transport and air parcel mixing analysis
532
approaches are applied for an evaluation of the above interpretations derived from the NMHC
533
ratio analysis.
534
535
Back trajectories for the three episodes marked periods in Figure 8b are shown in Figure 10. Air
536
sampled at Pico on 4/18 had previously been traveled across the Northern Pacific and then been
537
rapidly transported over California, the Midwestern U.S. and Northern Canada.
538
plots show that 4-6 days prior to arrival at Pico, these trajectories passed over the mid-western
539
and Western U.S.. at relatively low, e.g. 2-4 km above the surface. In contrast, air trajectories
540
arriving during 4/20 had resided within the oceanic, mid-Atlantic region for a minimum of 10
541
days. Two days later, air arriving on 4/22 had passed over Eastern Canada and the Central
542
United States 2-10 days prior. The transport altitude during this third period was lower than
543
during 4/18, indicating possibly stronger influence and more recent injections for surface
544
emissions.
The altitude
545
546
The comparison of the NMHC interpretations with these trajectory analysis imply that periods
547
identified with ‘photochemically fresh’ air coincided with air transport over populated, U.S.
548
continental regions, where, most likely, an injection of recent anthropogenic emissions had
549
occurred. In contrast, the period that was identified as ‘photochemically old’ was attributed to
550
conditions where air had resided over the Atlantic ocean for an extended (> 10 days) period of
551
time.
The photochemical age classification derived from our calculations presented above
18
552
qualitatively agree well with the trajectory interpretations; calculated mean photochemical
553
transport times are generally longer than inferred trajectory transport times from the most
554
recently encountered influence of expected surface influenced during the transport path.
555
556
These conclusions are further supported by the FLEXPART simulations.
Retroplumes for the
557
three episodes are shown in Figure 11a, c, d are in general agreement with the trajectory analysis.
558
FLEXPART provides additional information on the spatial distribution of emission sources that
559
contributed to CO enhancements in the retroplume. The source contribution map (Figure 11b)
560
indicates that for the first event the bulk of the emissions originated over central Colorado about
561
six days prior to arrival. Significant contributions were also from the western US with age
562
ranges from 6-8 days. The contribution of individual CO enhancements (at 1-day transport tme
563
resolution) to the overall, anthropogenic CO enhancement as well as the derived average age of
564
CO from FLEXPART is shown in Figure 8c, in comparison with the NMHC time series. The
565
average age during the first event from this analysis was 7-8 days, most of the CO injections had
566
occurred 8-10 days prior to arrival. Interestingly, the transport pathway appears to be similar to
567
typical warm conveyor belt transport, a mechanism that has been observed to transport emissions
568
from the North American continent directly to the PICO-NARE station (Owen et al, 2006).
569
Indeed, there was a cold front located over this region of the U.S. on April 13-15, indicating
570
frontal transport was partially responsible for this episode. The transport pathway took the
571
emissions from the source region to high altitudes above regions with little emissions which
572
resulted in little mixing with polluted air masses of different ages and composition, giving the
573
emissions observed at the station a fairly small range of age.
574
575
In contrast, FLEXPART results show that air sampled during 4/19-20 originated over the
576
southeastern Pacific, traveled briefly over Mexico and the Gulf of Mexico, before spending 10-
577
12 days within the mid-Atlantic region, circulating around the Azores High at relatively low
578
altitudes, before arriving at the PICO-NARE station (Figure 11c). The CO time series (Figure
579
8c) shows no CO present at the station less than 10 days old, with most falling into the 15-20 day
580
old bin. While there is a strong signal from southwestern Mexico, these aged emissions come
581
from all across the eastern U.S. and the Caribbean (Figure 11d), indicating significant mixing of
582
several polluted air masses as well as dilution with unpolluted marine air. The average age of the
19
583
CO in the resulting mixture, which only has a small modeled CO enhancement, ranges from 16-
584
19 days old.
585
586
Finally, air sampled during 4/21-23 had passed over large portions of North America before
587
arriving at the PICO-NARE station (Figure 11e). The path for the retroplume is consistent with
588
another typical transport pathway to the station, export from the U.S. and subsequent transport at
589
relatively low levels in the westerly wind (Owen, et al, 2006). The height of the plume during
590
transit over the U.S. was relatively low, generally less than 3 km, and remained low during
591
transport over the Atlantic to the station. The contribution map indicates that sources across the
592
eastern U.S. were responsible for the enhancements observed at this time (Figure 11f), with a
593
wide range of CO ages present. The resulting average age of CO for this episode ranges from 6-
594
9 days, though the individual ages range widely, from 4-15 days old (Figure 8c). Both the
595
retroplumes and distribution of CO ages indicate the mixing of many air masses of relatively
596
fresh emissions (particularly compared to the air sampled during 4/19-20).
597
598
In general, FLEXPART simulates not only the timing of CO enhancements well, but also
599
appears to simulate the relative ages of emissions. The average age appears to be the best value
600
to compare with the “photochemical age” derived from the HC ratios. Conclusively, both the
601
back-trajectory analysis and FLEXPART calculations yield reasonable confirmation of the
602
aforementioned interpretations from observed concentrations of NMHC and their photochemical
603
processing and mixing during transport.
604
605
Can we improve/expand last paragraph? Which comparison gives better agreement? Why?
606
What are limitations of photochemical clock? How can the photochemical clock possibly be
607
improved?
608
609
4. Summary and Conclusions
610
611
Air sampled at PICO-NARE shows high variability in NMHC and their ratios during all times of
612
the year. This observation is indicative of the variable atmospheric transport conditions that
613
bring in air with variable flow and with much different origin and photochemical history.
20
614
Overall, concentrations of NMHC at PICO-NARE are higher than at MLO, which reflects the
615
higher influence of the adjacent continents to air composition in the central Atlantic region in
616
comparison to the Northern Mid-Pacific.
617
618
Short-chain NMHC remain elevated in air plumes that have been influenced by either
619
anthropogenic injections or biomass burning after time scales in excess of 1 week during their
620
transport to the PICO-NARE station. Isoprene data convincingly describe summertime (mostly
621
buoyant) upslope flow occurrences. Isoprene was found as the best of all chemical tracers to
622
identify upslope flow.
623
624
A good correlation was determined between seasonally differentiated NMHC variability and the
625
NMHC OH lifetimes. Regression analysis of the lnx=A-b relationship for these data yields a b-
626
value of -0.44, which confirms the remote, marine island character of the Pico site and the lack
627
of major local influences on NMHC levels.
628
629
Summertime ozone/(ln [propane]/[ethane]) correlations show higher variability, indicating more
630
variability in photochemical conditions than during wintertime. Net ozone destruction typically
631
only occurs, after photochemical processing has reduced ln [propane]/[ethane] to values < -2.5; a
632
conditions only observed during the summer.
633
634
The reasonable agreement that was found between indirectly derived photochemical ages of
635
NMHC in air plumes sampled at Pico and back-trajectory and FLEXPART analysis suggests that
636
assumptions that went into the model calculations were reasonable and that interpretations of
637
NMHC ratios provide a meaningfull tool for deciphering photochemical age and transport of air
638
sampled at Pico.
639
640
Acknowledgments
641
642
We thank P. Goldan, NOAA Aeronomy Laboratory, Boulder, CO for the reference analysis of
643
the primary NMHC standard prior and after its use at Pico, M. Dziobak and M. Val Martin,
644
Michigan Technological University, for GC instrument maintenance tasks at the Pico, D.
21
645
Henriques, Institute of Meteorology, Ponta Delgada, Portugal for retrieving the ECMWF data
646
used in this work, Andreas Stohl, Norsk Institutt for Luftforskning (NILU), Kjeller, Norway for
647
providing and assisting in running the FLEXPART mode, the Data Support Section of NCAR’s
648
Scientific Computing Division for making the NCEP FNL analyses available for download and
649
T. Jobson, Washington State University, for the Fraserdale data. This research was funded by a
650
grant from the NOAA Office of Global Programs (award # NA03OAR4310072).
651
652
653
654
655
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24
Figure 1
Whisker plots of monthly data for ethane, propane, i-butane, n-butane, i-pentane and pentane. The 5, 25, 50, 75, 95 percentiles are
indicated by the horizontal lines of each box, the vertical lines extend to the minimum and maximum observed values. The width of
the box indicates the time period over which data was acquired for a given month. The vertical dotted lines show the windows that
were applied in the seasonal (winter, spring, summer, fall) analysis. Data from a remote boreal forest site in Canada (Jobson et al.,
1994) and 25, 50, and 75 percentile data from Mauna Loa Observatory (Greenberg et al., 1996) were added for comparison.
25
Figure 2
Cumulative distributions of NMHC at the PICO-NARE station during the four measurement seasons.
26
Figure 3
The standard deviation of the natural logarithm of the NMHC mixing ratio during the four seasons at its seasonal OH lifetime.
27
the seasonal data distribution.
28
Figure 5
Mixing ratio of i-butane versus n-butane (left) and ratio of i-butane/n-butane versus n-butane
(right) in the upper graphs and of n-pentane versus i-pentane (left) and ratio of n-pentane/ipentane versus i-pentane (right) in the lower graphs The error bars in the right graph show the
standard deviation of the data within 10-percentile bins of the data distribution. Dotted lines
illustrate the estimated uncertainties in the measurement. REMOVE dotted uncertainty
ranges?
Figure 6
Relationship between the natural logarithms of [n-buante]/[ethane] versus [propane]/[ethane] for
the fall, summer, spring and summer data as defined in Fig. 1.
29
Figure 7
Ozone in relation to the natural logarithm of [propane]/[ethane] (indicating degree of
photochemical processing) during the fall, winter, spring and summer. The lines indicate the
linear least-squares fits to the log-transformed data, and their slopes with 95% confidence limits
and correlation coefficients are annotated.
30
A
B
4/17 16:00 to
4/19 1:00
4/21 5:00 to
4/23 15:30
4/19 17:00 to
4/20 15:30
31
C
Figure 8
Mixing ratios of four NMHC (A) and the natural logarithms of [propane]/[ethane] and
[butane]/[ethane] for 21 days in spring 2005 (B). Panel (C) shows for the same time window the
Flexpart calculations for the contribution of emissions of CO over North America on the CO
enhancement in the retroplume arriving at Pico. The derived average CO enhancement transport
time is shown on the secondary y-axis. .
Still need missing May data. Try to redo figure c in Excel so that format matches panels a and
b? please change y-axis title to CO (ppbv).
32
Figure 9
Distribution of the marked data points in Fig. 8 (graph B) in the ln-ln photochemical age/dilution
analysis plot in comparison to all spring 2005 data (top). A photochemical age scale was
calculated using variables described in the text. Resulting ages (in days of transport) are
indicated in the diamonds on the regression line fit.
33
Figure 10
Geographical and altitude back trajectory analysis of the events marked in the Fig. 8b. The solid
lines illustrate back trajectories (every 6 hours) arriving at six grid points surrounding the station
for 4/18, 00.00 UTC. The dotted lines show the corresponding analysis for 4/20 00.00 UTC and
the dashed back trajectories are for arrival at 4/22 00:00 UTC. Dot marks on the trajectory lines
indicate 2-day transport distances.
Right now this is a composite of the same figure cut and pasted twice, zoomed to different sizes.
Can we make this one nice figure at a somewhat improved resolution?
34
Figure 11
Results for retroplumes initiated at 00-03 on July 18 (A&B) July 20 (C&D) and July 22 (E&F).
The left column shows the total column (0-10km) residence times. The right column shows the
foot print layer (0-300m) response to emissions sources (residence time folded with emission
strength). Colors are logarimithically scaled (100-1%) according to the maximum value for each
plot type (14000 seconds for total residence time, 4700 grams of CO for foot print layer
response), as shown by the scale on the bottom.
35
Table 1
36