Dylan B. Millet

Spatial distribution of isoprene emissions from North America derived from formaldehyde column measurements by the OMI satellite sensor

Received 10 May 2007; revised 19 September 2007; accepted 25 October 2007; published 26 January 2008. [1] Space-borne formaldehyde (HCHO) column measurements from the Ozone Monitoring Instrument (OMI), with 13 � 24 km 2 nadir footprint and daily global coverage, provide new constraints on the spatial distribution of biogenic isoprene emission from North America. OMI HCHO columns for June-August 2006 are consistent with measurements from the earlier GOME satellite sensor (1996–2001) but OMI is 2–14% lower. The spatial distribution of OMI HCHO columns follows that of isoprene emission; anthropogenic hydrocarbon emissions are undetectable except in Houston. We develop updated relationships between HCHO columns and isoprene emission from a chemical transport model (GEOS-Chem), and use these to infer top-down constraints on isoprene emissions from the OMI data. We compare the OMI-derived emissions to a state-of-science bottom-up isoprene emission inventory (MEGAN) driven by two land cover databases, and use the results to optimize the MEGAN emission factors (EFs) for broadleaf trees (the main isoprene source). The OMI-derived isoprene emissions in North America (June–August 2006) with 1 � 1 resolution are spatially consistent with MEGAN (R 2 = 0.48–0.68) but are lower (by 4–25% on average). MEGAN overestimates emissions in the Ozarks and the Upper South. A better fit to OMI (R 2 = 0.73) is obtained in MEGAN by using a uniform isoprene EF from broadleaf trees rather than variable EFs. Thus MEGAN may overestimate emissions in areas where it specifies particularly high EFs. Within-canopy isoprene oxidation may also lead to significant differences between the effective isoprene emission to the atmosphere seen by OMI and the actual isoprene emission determined by MEGAN.

Submicron aerosol composition at Trinidad Head, California, during ITCT 2K2: Its relationship with gas phase volatile organic carbon and assessment of instrument performance

[1] Two Aerodyne aerosol mass spectrometers (AMSs) were deployed at Trinidad Head on the north Californian coast during the National Oceanographic and Atmospheric Administration Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) experiment, to study the physiochemical properties of submicron aerosol particles within the Pacific marine boundary layer. One AMS was modified to allow the study of sea salt-based particles, while the other used a temperature cycling system on its inlet. The reported loadings increased by a factor of 2 when the temperature approached the dew point, which is due to the inlet performance and has implications for other AMS experiments and applications. The processed data were compared with those of a particle into liquid sampler-ion chromatograph and showed that the ammonium, sulfate and organic fractions of the particles were consistently found within a single, normally acidic, accumulation mode at around 300 - 400 nm. However, when influenced by land-based sources, vehicle emissions and increased ammonium loadings were seen. The concentrations of nitrate in the accumulation mode were low, but it was also found within sea salt particles in the coarse mode and can be linked to the displacement of chloride. The organic fraction showed a high degree of chemical ageing and evidence of nitrogen-bearing organics was also observed. The particulate organic data were compared to the volatile organic carbon data derived from an in-situ gas chromatograph-mass spectrometer-flame ionization detector and relationships were found between the gas and particle phase chemicals in both the overall concentrations and the levels of oxidation.

Formaldehyde Distribution over North America: Implications for Satellite Retrievals of Formaldehyde Columns and Isoprene Emission

[1]xa0Formaldehyde (HCHO) columns measured from space provide constraints on emissions of volatile organic compounds (VOCs). Quantitative interpretation requires characterization of errors in HCHO column retrievals and relating these columns to VOC emissions. Retrieval error is mainly in the air mass factor (AMF) which relates fitted backscattered radiances to vertical columns and requires external information on HCHO, aerosols, and clouds. Here we use aircraft data collected over North America and the Atlantic to determine the local relationships between HCHO columns and VOC emissions, calculate AMFs for HCHO retrievals, assess the errors in deriving AMFs with a chemical transport model (GEOS-Chem), and draw conclusions regarding space-based mapping of VOC emissions. We show that isoprene drives observed HCHO column variability over North America; HCHO column data from space can thus be used effectively as a proxy for isoprene emission. From observed HCHO and isoprene profiles we find an HCHO molar yield from isoprene oxidation of 1.6 ± 0.5, consistent with current chemical mechanisms. Clouds are the primary error source in the AMF calculation; errors in the HCHO vertical profile and aerosols have comparatively little effect. The mean bias and 1σ uncertainty in the GEOS-Chem AMF calculation increase from <1% and 15% for clear skies to 17% and 24% for half-cloudy scenes. With fitting errors, this gives an overall 1σ error in HCHO satellite measurements of 25–31%. Retrieval errors, combined with uncertainties in the HCHO yield from isoprene oxidation, result in a 40% (1σ) error in inferring isoprene emissions from HCHO satellite measurements.

Atmospheric volatile organic compound measurements during the Pittsburgh Air Quality Study: Results, interpretation, and quantification of primary and secondary contributions

[1]xa0Primary and secondary contributions to ambient levels of volatile organic compounds (VOCs) and aerosol organic carbon (OC) are determined using measurements at the Pittsburgh Air Quality Study (PAQS) during January–February and July–August 2002. Primary emission ratios for gas and aerosol species are defined by correlation with species of known origin, and contributions from primary and secondary/biogenic sources and from the regional background are then determined. Primary anthropogenic contributions to ambient levels of acetone, methylethylketone, and acetaldehyde were found to be 12–23% in winter and 2–10% in summer. Secondary production plus biogenic emissions accounted for 12–27% of the total mixing ratios for these compounds in winter and 26–34% in summer, with background concentrations accounting for the remainder. Using the same method, we determined that on average 16% of aerosol OC was secondary in origin during winter versus 37% during summer. Factor analysis of the VOC and aerosol data is used to define the dominant source types in the region for both seasons. Local automotive emissions were the strongest contributor to changes in atmospheric VOC concentrations; however, they did not significantly impact the aerosol species included in the factor analysis. We conclude that longer-range transport and industrial emissions were more important sources of aerosol during the study period. The VOC data are also used to characterize the photochemical state of the atmosphere in the region. The total measured OH loss rate was dominated by nonmethane hydrocarbons and CO (76% of the total) in winter and by isoprene, its oxidation products, and oxygenated VOCs (79% of the total) in summer, when production of secondary organic aerosol was highest.

Changes in the photochemical environment of the temperate North Pacific troposphere in response to increased Asian emissions

[1]xa0Measurements during the Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) field study characterized the springtime, eastern Pacific ozone distribution at two ground sites, from the National Oceanic and Atmospheric Administration WP-3D aircraft, and from a light aircraft operated by the University of Washington. D. Jaffe and colleagues compared the 2002 ozone distribution with measurements made in the region over the two previous decades and show that average ozone levels over the eastern midlatitude Pacific have systematically increased by ∼10 ppbv in the last two decades. Here we provide substantial evidence that a marked change in the photochemical environment in the springtime troposphere of the North Pacific is responsible for this increased O3. This change is evidenced in the eastern North Pacific ITCT 2K2 study region by (1) larger increases in the minimum observed ozone levels compared to more modest increases in the maximum levels, (2) increased peroxyacetyl nitrate (PAN) levels that parallel trends in NOx emissions, and (3) decreased efficiency of photochemical O3 destruction, i.e., less negative O3 photochemical tendency (or net rate of O3 photochemical production; P(O3)). This changed photochemical environment is hypothesized to be due to anthropogenic emissions from Asia, which are believed to have substantially increased over the two decades preceding the study. We propose that their influence has changed the springtime Pacific tropospheric photochemistry from predominately ozone destroying to more nearly ozone producing. However, chemical transport model calculations indicate the possible influence of a confounding factor; unusual transport of tropical air to the western North Pacific during one early field study may have played a role in this apparent change in the photochemistry.

Impact of Asian emissions on observations at Trinidad Head, California, during ITCT 2K2

[1]xa0Field measurements of a wide suite of trace gases and aerosols were carried out during April and May 2002, along with extensive chemical transport modeling, as part of the NOAA Intercontinental Transport and Chemical Transformation study. Here, we use a combination of in-situ ground-based measurements from Trinidad Head, CA, chemical transport modeling, and backward trajectory analysis to examine the impact of long-range transport from Asia on the composition of air masses arriving at the California coast at the surface. The impact of Asian emissions is explored in terms of both episodic enhancements and contribution to background concentrations. We find that variability in CO concentrations at the ground site was largely driven by North American emissions, and that individual Asian plumes did not cause any observable pollution enhancement episodes at Trinidad Head. Despite this, model simulations suggest that Asian emissions were responsible for 33% of the CO observed at Trinidad Head, providing a larger mean contribution than direct emissions from any other region of the globe. Surface ozone levels were found to depend primarily on local atmospheric mixing, with surface deposition leading to low concentrations under stagnant conditions. Model simulations suggested that on average 4 ± 1 ppb of ozone (10% of observed) at Trinidad Head was transported from Asia.

Biogenic versus anthropogenic sources of CO in the United States

Aircraft observations of carbon monoxide (CO) from the ICARTT campaign over the eastern United States in summer 2004 (July 1–August 15), interpreted with a global 3-D model of tropospheric chemistry (GEOS-Chem), show that the national anthropogenic emission inventory from the U.S. Environmental Protection Agency (93 Tg CO y−1) is too high by 60% in summer. Our best estimate of the CO anthropogenic source for the ICARTT period is 6.4 Tg CO, including 4.6 Tg from direct emission and 1.8 Tg CO from oxidation of anthropogenic volatile organic compounds (VOCs). The biogenic CO source for the same period from the oxidation of isoprene and other biogenic VOCs is 8.3 Tg CO, and is independently constrained by ICARTT observations of formaldehyde (HCHO). Anthropogenic emissions of CO in the U.S. have decreased to the point that they are now lower than the biogenic source in summer.

Temperature dependence of volatile organic compound evaporative emissions from motor vehicles

[1]xa0A chemical mass balance approach is used to determine the relative contributions of evaporative versus tailpipe sources to motor vehicle volatile organic compound (VOC) emissions. Contributions were determined by reconciling time-resolved ambient VOC concentrations measured downwind of Sacramento, California, in summer 2001 with source speciation profiles. A composite liquid fuel speciation profile was determined from gasoline samples collected at Sacramento area service stations. Vapor-liquid equilibrium relationships were used to determine the corresponding headspace vapor composition. VOC concentrations measured in a highway tunnel were used to define the composition of running vehicle emissions. The chemical mass balance analysis indicated that headspace vapor contributions ranged from 7 to 29% of total vehicle-related VOC depending on time of day and day of week, with a mean daytime contribution of 17.0 ± 0.9% (mean ± 95% CI). A positive association between the headspace vapor contribution and ambient air temperature was found for afternoon hours. We estimate a 6.5 ± 2.5% increase in vapor pressure-driven evaporative emissions and at least a 1.3 ± 0.4% increase in daily total (exhaust plus evaporative) VOC emissions from motor vehicles per degree Celsius increase in maximum temperature.

Volatile organic compound measurements at Trinidad Head, California, during ITCT 2K2: Analysis of sources, atmospheric composition, and aerosol residence times

[1] We report hourly in-situ observations of C(1)-C(8) speciated volatile organic compounds (VOCs) obtained at Trinidad Head CA in April and May 2002 as part of the NOAA Intercontinental Transport and Chemical Transformation study. Factor analysis of the VOC data set was used to define the dominant processes driving atmospheric chemical composition at the site, and to characterize the sources for measured species. Strong decreases in background concentration were observed for several of the VOCs during the experiment due to seasonal changes in OH concentration. CO was the most important contributor to the total measured OH reactivity at the site at all times. Oxygenated VOCs were the primary component of both the total VOC burden and of the VOC OH reactivity, and their relative importance was enhanced under conditions when local source contributions were minimal. VOC variability exhibited a strong dependence on residence time (s(lnX) = 1.55(tau)(-0.44), r(2) = 0.98; where s(lnX) is the standard deviation of the natural logarithm of the mixing ratio), and this relationship was used, in conjunction with measurements of (222)Rn, to estimate the average OH concentration during the study period (6.1 x 10(5) molec/cm(3)). We also employed the variability-lifetime relationship defined by the VOC data set to estimate submicron aerosol residence times as a function of chemical composition. Two independent measures of aerosol chemical composition yielded consistent residence time estimates. Lifetimes calculated in this manner were between 3 - 7 days for aerosol nitrate, organics, sulfate, and ammonium. The lifetime estimate for methane sulfonic acid ( similar to 12 days) was slightly outside of this range. The lifetime of the total aerosol number density was estimated at 9.8 days.

Chemical speciation of organic aerosol during the International Consortium for Atmospheric Research on Transport and Transformation 2004: Results from in situ measurements

[1]xa0We report the first ever hourly in situ measurements of speciated organic aerosol composition. Measurements were made during July and August 2004 at Chebogue Point, Nova Scotia, using a novel thermal desorption aerosol GC/MS-FID (TAG) instrument as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). Hourly time resolution measurements of organic marker compounds were used to define several different source types contributing to the aerosols observed, including two aged anthropogenic sources from the United States, oxidized biogenic aerosols from Maine and Canada, local biogenic contributions to secondary organic aerosol (SOA), local anthropogenic contributions to hydrocarbon-like organic aerosol (HOA), and a potential marine or dairy source. These TAG-defined sources were used to separate aerosol mass spectrometer (AMS) measurements of total organics, SO42−, NO3−, and NH4+, along with total aerosol black carbon (BC) into several distinct particle types. Average organic aerosol mass ranged from 33% of the total aerosol mass during anthropogenic U.S. outflow events to 81% of total aerosol mass during biogenic oxidation events arriving from Maine and Canada during 26 July to 15 August 2004.