Rokjin J. Park

A large organic aerosol source in the free troposphere missing from current models

average 4 m gs m 3 in the 2–6.5 km column with little vertical gradient. These values are 10–100 times higher than computed with a global chemical transport model (CTM) including a standard 2-product simulation of secondary organic aerosol (SOA) formation based on empirical fits to smog chamber data. The same CTM reproduces the observed vertical profiles of sulfate and elemental carbon aerosols, which indicate sharp decreases from the boundary layer to the FT due to wet scavenging. Our results suggest a large, sustained source of SOA in the FT from oxidation of long-lived volatile organic compounds. We find that this SOA is the dominant component of aerosol mass in the FT, with implications for intercontinental pollution transport and radiative forcing of climate. Citation: Heald, C. L., D. J. Jacob, R. J. Park, L. M. Russell, B. J. Huebert, J. H. Seinfeld, H. Liao, and R. J. Weber (2005), A large organic aerosol source in the free troposphere missing from current models, Geophys. Res. Lett., 32, L18809, doi:10.1029/2005GL023831.

Sources of carbonaceous aerosols over the United States and implications for natural visibility

emissions, captures most of the variance in the observations (R 2 = 0.84 for EC, 0.67 for OC) with a low bias of 15% for EC and 26% for OC. Multiple linear regression to fit the IMPROVE data yields best estimates of 1998 U.S. sources of 0.60 Tg year 1 EC and 0.52 Tg year 1 OC from fossil fuel; 0.07 Tg year 1 EC and 0.89 Tg year 1 OC from biofuel; 0.08 Tg year 1 EC and 0.60 Tg year 1 OC from wildfires; and 1.10 Tg year 1 OC from vegetation. We find that fires in Mexico and Canada contributed 40–70% of annual mean natural EC in the United States for 1998 and 20–30% of annual mean natural OC. Transpacific transport from Asian pollution sources amounted to less than 10% of the natural EC and less than 2% of the natural OC; in contrast to ozone, we find that intercontinental transport of anthropogenic carbonaceous aerosols does not enhance significantly the natural background. IMPROVE observations and model simulations for the summer of 1995 show that Canadian fire emissions can produce large events of elevated EC and OC in the southeastern United States. Our best estimates of mean natural concentrations of EC and OC in the United States, using a model simulation with climatological monthly mean fire emissions, are 2–3 times higher than the default values recommended by the U.S. Environmental Protection Agency for visibility calculations, except for OC in the eastern United States (16% lower). INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 4801 Oceanography: Biological and Chemical: Aerosols (0305); KEYWORDS: carbonaceous aerosols, natural visibility, natural aerosol concentrations, trans-Pacific transport of aerosol, transboundary transport of aerosol, biomass burning aerosol

Sulfate Formation in Sea-Salt Aerosols: Constraints from Oxygen Isotopes

imparts a large D 17 O signature to the resulting sulfate (8.8%) relative to oxidation by H2O2 (0.9% )o r by OH or O 2 (0%). Ship data from two Indian Ocean Experiment (INDOEX) cruises in the Indian Ocean indicate D 17 O values usually 70%) and increases MBL sulfate concentrations by typically >10% (up to 30%). Globally, this mechanism contributes 9% of atmospheric sulfate production and 1% of the sulfate burden. The impact on H2SO4 (g) formation and implications for the potential formation of new particles in the MBL warrants inclusion in models examining the radiative effects of sulfate aerosols.

Estimating ground‐level PM2.5 using aerosol optical depth determined from satellite remote sensing

[1] We assess the relationship of ground-level fine particulate matter (PM 2.5 ) concentrations for 2000-2001 measured as part of the Canadian National Air Pollution Surveillance (NAPS) network and the U.S. Air Quality System (AQS), versus remote-sensed PM 2.5 determined from aerosol optical depths (AOD) measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Multiangle Imaging Spectroradiometer (MISR) satellite instruments. A global chemical transport model (GEOS-CHEM) is used to simulate the factors affecting the relation between AOD and PM 2.5 . AERONET AOD is used to evaluate the method (r = 0.71, N = 48, slope = 0.69). We find significant spatial variation of the annual mean ground-based measurements with PM 2.5 determined from MODIS (r = 0.69, N = 199, slope = 0.82) and MISR (r = 0.58, N = 199, slope = 0.57). Excluding California significantly increases the respective slopes and correlations. The relative vertical profile of aerosol extinction is the most important factor affecting the spatial relationship between satellite and surface measurements of PM 2.5 ; neglecting this parameter would reduce the spatial correlation to 0.36. In contrast, temporal variation in AOD is the most influential parameter affecting the temporal relationship between satellite and surface measurements of PM 2.5 ; neglecting daily variation in this parameter would decrease the correlation in eastern North America from 0.5-0.8 to less than 0.2. Other simulated aerosol properties, such as effective radius and extinction efficiency have a minor role temporally, but do influence the spatial correlation. Global mapping of PM 2.5 from both MODIS and MISR reveals annual mean concentrations of 40-50 ug/m 3 over northern India and China.

Ozone production in transpacific Asian pollution plumes and implications for ozone air quality in California

[1] We examine the ozone production efficiency in transpacific Asian pollution plumes, and the implications for ozone air quality in California, by using aircraft and surface observations in April–May 2002 from the Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) campaign off the California coast and the Pacific Exploration of Asian Continental Emission–B (PEACE-B) campaign over the northwest Pacific. The observations are interpreted with a global three-dimensional chemical transport model (GEOS-CHEM). The model reproduces the mean features observed for CO, reactive nitrogen oxides (NOy), and ozone but underestimates the strong (20 ppbv) stratospheric contribution to ozone in the middle troposphere. The ITCT 2K2 aircraft sampled two major transpacific Asian pollution plumes, one on 5 May at 5–8 km altitude with CO up to 275 ppbv but no elevated ozone and one on 17 May at 2.5–4 km altitude with CO up to 225 ppbv and ozone up to 90 ppbv. We show that the elevated ozone in the latter plume is consistent with production from peroxyacetylnitrate (PAN) decomposition during subsidence of the plume over the northeast Pacific. This production is particularly efficient because of the strong radiation and low humidity of the subsiding environment. We argue that such PAN decomposition represents a major and possibly dominant component of the ozone enhancement in transpacific Asian pollution plumes. Strong dilution of Asian pollution plumes takes place during entrainment in the U.S. boundary layer, greatly reducing their impact at U.S. surface sites. California mountain sites are more sensitive to Asian pollution because of their exposure to the free troposphere. Model results indicate a mean Asian pollution enhancement of 7 ppbv ozone at Sequoia National Park in May 2002 on those days when the 8-hour average ozone concentration exceeded 80 ppbv. INDEX TERMS: 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); KEYWORDS: ozone, Asian pollution, ITCT 2K2, PEACE-B, transpacific transport

Export efficiency of black carbon aerosol in continental outflow: Global implications

[1] We use aircraft observations of Asian outflow from the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) mission over the NW Pacific in March-April 2001 to estimate the export efficiency of black carbon (BC) aerosol during lifting to the free troposphere, as limited by scavenging from the wet processes (warm conveyor belts and convection) associated with this lifting. Our estimate is based on the enhancement ratio of BC relative to CO in Asian outflow observed at different altitudes and is normalized to the enhancement ratio observed in boundary layer outflow (0-1 km). We similarly estimate export efficiencies of sulfur oxides (SO x = SO 2 (g) + fine SO 2- 4 ) and total inorganic nitrate (HNO T 3 = HNO 3 (g) + fine NO - 3 ) for comparison to BC. Normalized export efficiencies for BC are 0.63-0.74 at 2-4 km altitude and 0.27-0.38 at 4-6 km. Values at 2-4 km altitude are higher than for SO x (0.48-0.66) and HNO T 3 (0.29-0.62), implying that BC is scavenged in wet updrafts but not as efficiently as sulfate or nitrate. Simulation of the TRACE-P period with a global three-dimensional model (GEOS-CHEM) indicates that a model timescale of 1 ± 1 days for conversion of fresh hydrophobic to hydrophilic BC provides a successful fit to the export efficiencies observed in TRACE-P. The resulting mean atmospheric lifetime of BC is 5.8 ± 1.8 days, the global burden is 0.11 ± 0.03 Tg C, and the decrease in Arctic snow albedo due to BC deposition is 3.1 ± 2.5%.

Mapping annual mean ground‐level PM2.5 concentrations using Multiangle Imaging Spectroradiometer aerosol optical thickness over the contiguous United States

[1] We present a simple approach to estimating ground-level fine particulate matter (PM2.5, particles smaller than 2.5 mm in diameter) concentrations by applying local scaling factors from a global atmospheric chemistry model (GEOS-CHEM with GOCART dust and sea salt data) to aerosol optical thickness (AOT) retrieved by the Multiangle Imaging Spectroradiometer (MISR). The resulting MISR PM2.5 concentrations are compared with measurements from the U.S. Environmental Protection Agency’s (EPA) PM2.5 compliance network for the year 2001. Regression analyses show that the annual mean MISR PM2.5 concentration is strongly correlated with EPA PM2.5 concentration (correlation coefficient r = 0.81), with an estimated slope of 1.00 and an insignificant intercept, when three potential outliers from Southern California are excluded. The MISR PM2.5 concentrations have a root mean square error (RMSE) of 2.20 mg/m 3 , which corresponds to a relative error (RMSE over mean EPA PM2.5 concentration) of approximately 20%. Using simulated aerosol vertical profiles generated by the global models helps to reduce the uncertainty in estimated PM2.5 concentrations due to the changing correlation between lower and upper tropospheric aerosols and therefore to improve the capability of MISR AOT in estimating surface-level PM2.5 concentrations. The estimated seasonal mean PM2.5 concentrations exhibited substantial uncertainty, particularly in the west. With improved MISR cloud screening algorithms and the dust simulation of global models, as well as a higher model spatial resolution, we expect that this approach will be able to make reliable estimation of seasonal average surface-level PM2.5 concentration at higher temporal and spatial resolution. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0394 Atmospheric Composition and Structure: Instruments and techniques; KEYWORDS: MISR AOT, GEOS-CHEM, PM2.5

Observations of Reactive Gaseous Mercury in the Free Troposphere at the Mount Bachelor Observatory

August 2005. The mean mercury concentrations (at standard conditions) were 1.54 ng/m 3 (GEM), 5.2 pg/m 3 (PHg), and 43 pg/m 3 (RGM). RGM enhancements, up to 600 pg/m 3 , occurred at night and were linked to a diurnal pattern of upslope and downslope flows that mixed in boundary layer air during the day and free tropospheric air at night. During the night, RGM was inversely correlated (P < 0.0001) with CO (r = � 0.36), GEM (r = � 0.73), and H2 O( r =� 0.44), was positively correlated with ozone (r = 0.38), and could not be linked to recent anthropogenic emissions from local sources or long-range transport. Principal component analysis and a composite of change in RGM versus change in GEM during RGM enhancements indicate that a nearly quantitative shift in speciation is associated with increases in ozone and decreases in water vapor and CO. This argues that high concentrations of RGM are present in the free troposphere because of in situ oxidation of GEM to RGM. A global chemical transport model reproduces the RGM mean and diurnal pattern but underestimates the magnitude of the largest observed enhancements. Since the only modeled, in situ RGM production mechanisms are oxidation of GEM by ozone and OH, this implies that there are faster reaction rates or additional RGM production mechanisms in the free troposphere.

Transition metal-catalyzed oxidation of atmospheric sulfur: Global implications for the sulfur budget

[1] We use observations of the oxygen-17 excess (Δ 17 O) of sulfate in the Arctic to quantify the sulfate source from aqueous SO 2 (S(IV)) oxidation by O 2 catalyzed by transition metals. Due to the lack of photochemically produced OH and H 2 O 2 in high latitudes during winter, combined with high anthropogenic SO 2 emissions in the Northern Hemisphere, oxidation by O 3 is predicted to dominate sulfate formation during winter in this region. However, Δ 17 O measurements of sulfate aerosol collected in Alert, Canada, are not consistent with O 3 as the dominant oxidant and indicate that a S(IV) oxidant with near-zero Δ 17 O values (O 2 ) is important during winter. We use a global chemical transport model to interpret quantitatively the Alert observations and assess the global importance of sulfate production by Fe(III)- and Mn(II)-catalyzed oxidation of S(IV) by O 2 . We scale anthropogenic and natural atmospheric metal concentrations to primary anthropogenic sulfate and dust concentrations, respectively. The solubility and oxidation state of these metals is determined by cloud liquid water content, source, and sunlight. By including metal-catalyzed S(IV) oxidation, the model is consistent with the Δ 17 O magnitudes in the Alert data during winter. Globally, we find that this mechanism contributes 9-17% to sulfate production. The inclusion of metal-catalyzed oxidation does not resolve model discrepancies with surface SO 2 and sulfate observations in Europe. Oxygen isotope measurements of sulfate aerosols collected near anthropogenic and dust sources of metals would help to verify the importance of this sulfur oxidation pathway.

Impacts of enhanced biomass burning in the boreal forests in 1998 on tropospheric chemistry and the sensitivity of model results to the injection height of emissions

derived inventories for the fire emissions that differ by a factor of two. We find that it is essential to use both surface and column observations of CO to constrain the magnitude of the fire emissions and their injection altitude. Our results show that the larger of the two inventories appears more reliable and that about half of the emissions were injected above the boundary layer. The boreal fire emissions cause a much larger enhancement in ozone when about half the emissions are released above the boundary layer than when they are releasedexclusivelyintheboundarylayer,asaconsequenceoftheroleofPANasasourceof NOx as air descends in regions far from the fires. Citation: Leung, F.-Y. T., J. A. Logan, R. Park, E. Hyer, E. Kasischke, D. Streets, and L. Yurganov (2007), Impacts of enhanced biomass burning in the boreal forests in 1998 on tropospheric chemistry and the sensitivity of model results to the injection height of emissions, J. Geophys. Res., 112, D10313, doi:10.1029/2006JD008132.