A51E-01 INVITED
Using satellite composites to examine transport of gases and aerosols in mid-latitude cyclones
Mid-latitude cyclones have been recognized as an important mechanism for venting polluted air from the continental boundary layer to the free troposphere. Warm conveyor belts, rising airstreams originating at the surface in the warm sector of mid-latitude cyclones, are particularily efficient at transporting pollutants to the middle and upper troposphere. In this study we examine the transport of gases and aerosols in mid-latitude cyclones by using multi-platform remote sensing observations of aerosol optical depth, CO, O3, and NO2 from the MODIS, MLS, OMI, and TES instruments. These atmospheric composition observations are complemented by satellite observations of QuickSCAT surface winds, MODIS clouds, AMSR-E surface rain rate and column water vapor. We first identify mid-latitude cyclones using NCEP sea level pressure, and then composite satellite observations over a 4000 km x 4000 km grid centered over each cyclone. Composites generated over clean regions (South Pacific and South Atlantic) are contrasted to composites downwind of polluted regions (western North Pacific and western North Atlantic). We find a strong signature of surface wind-induced sea-salt generation for each of these regions. The observations downwind of polluted regions exhibit enhanced CO, NO2, and aerosols in the lower troposphere behind the surface cold front, as well as enhanced CO and aerosols at high altitudes ahead of the surface warm front, especially downwind of Asia. In addition, when the warm conveyor belt originates in the clean subtropics, it has a significant dilution effect on the polluted continental outflow. We will use these observations to provide quantitative constraints on the role of mid-latitude cyclones in exporting pollution to the free troposphere.
A51E-02
Transpacific Transport of Pollution and its Impact on Atmospheric Composition over North America
We have analyzed the transport of pollution across the Pacific during springtime 2006 and examined how the long-range transport impacts atmospheric composition over North America. Our analysis combines simulations performed with both the chemistry transport model MOZART and the regional air quality model WRF-CHEM with a suite of observations. The observational data set includes aircraft measurements of trace gases and aerosols performed during the INTEX-B campaign (Intercontinental Chemical Transport Experiment), ground-based trace gas and aerosol measurements, and satellite observations of CO from the Measurements of Pollution in the Troposphere (MOPITT) instrument and of aerosol loading from the Moderate Resolution Imaging Spectroradiometer (MODIS). Model tracers are used to examine the contributions of different regions to pollution levels over the Pacific and to estimate the ozone production from NOx sources in Asia to ozone loadings over the Pacific and North America. The MOPITT and MODIS multi-year data series reaching back to 2000 are used to put 2006 into relation to the transpacific transport occurring in other years. The authors acknowledge the INTEX-B team for making their measurements available.
A51E-03
Evaluating the potential influence of inter-continental transport of sulfate aerosols on air quality
In this study, we compare the potential influence of inter-continental transport of sulfate aerosols on the air quality of continental regions. We use a global chemical transport model, Model of Ozone and Related Tracers, version 2 (MOZART-2), to quantify the source-receptor relationships of inter-continental transport of sulfate aerosols among ten regions in 2000. In order to compare the importance of foreign emissions relative to domestic emissions and estimate the effect of future changes in emissions on human exposure, we define an "influence potential" (IP). The IP quantifies the human exposure that occurs in a receptor region as a result of a unit of SO2 emissions from a source region. We find that due to the non-linear nature of sulfate production, regions with low SO2 emissions usually have large domestic IP, and vice versa. An exception is East Asia (EA), which has both high SO2 emissions and relatively large domestic IP, mostly caused by the spatial coincidence of emissions and population. We find that intercontinental IPs are usually less than domestic IPs by 1-3 orders of magnitude. SO2 emissions from the Middle East (ME) and Europe (EU) have the largest potential to influence populations in surrounding regions. By comparing the IP ratios (IPR) between foreign and domestic SO2 emissions, we find that the IPR values range from 0.00001 to 0.16 and change with season. Therefore, if reducing human exposure to sulfate aerosols is the objective, all regions should first focus on reducing domestic SO2 emissions. In addition, we find that relatively high IPR values exist among the EU, ME, the former Soviet Union (FSU) and African (AF) regions. Therefore, based on the IP and IPR values, we conclude that a regional agreement among EA countries, and an inter-regional agreement among EU, ME, FSU and north AF regions to control sulfur emissions would benefit public health in these regions.
A51E-04
Can we Detect an Influence over North America From Increasing Asian NOx Emissions?
Several studies have reported a rapid increase in Asian NOx emissions. This knowledge, combined with our observations of long-range transport and global chemical transport models, suggests that we may soon be able to identify an influence on atmospheric composition over the Eastern Pacific and North American regions. Using the GEOS-CHEM model, we find that the Asian contribution to PAN in the free troposphere over the Eastern Pacific is approximately 50%. To estimate the magnitude of change in the Eastern Pacific troposphere we might expect from the recent Asian emission changes, we have conducted two simulations, one with standard emissions inventory and one with doubled emissions of NOx and NMHC from East Asia. Based on published data, a doubling of NOx emissions would be approximately correct for the period of 1999-2007. This simulation yields small changes in NOx and O3 (20% and 7%, respectively) but larger changes in mean PAN mixing ratios (43%) in the Eastern Pacific. Thus it appears that PAN is the most sensitive indicator of Asian NOx emissions in the Pacific troposphere. However, the simulation is very sensitive to NMHC emissions, which are not known with high accuracy. We can evaluate these changes with aircraft, surface and mountain top observations of PAN, NOx and O3 over the Eastern Pacific/western North America between 1999 and 2007. Measurements of PAN, NO and O3 were made by aircraft over the Eastern Pacific in 1999 during the PHOBEA campaign (Kotchenruther et al., 2001). Aircraft observations were made in the same region in 2006 during the INTEX campaign. In addition, free tropospheric observations at the Mt. Bachelor Observatory (MBO) have been carried out since 2004. O3 in the Eastern Pacific shows strong interannual variability which makes it difficult to tease out smaller long-term changes from the aircraft data. On the other hand, long-term observations of surface O3 in western North America have shown a significant increase at numerous rural sites during this time period, but multiple factors may be responsible for these changes. For NOx, comparing aircraft observations in the Eastern Pacific between 1999 and 2006 and considering the uncertainty in the observations, we find no evidence for a significant change. For PAN, comparing aircraft observations made in 1999 and 2006, and MBO free tropospheric observations in 2006 we see evidence for a significant enhancement in PAN over this time frame. The aircraft observations yield an enhancement of 22% in the free tropospheric mixing ratios, whereas the MBO free tropospheric observations suggest a larger enhancement (~100%). The observations are consistent with an emerging trend in PAN in the Eastern Pacific region, although further observations are needed to confirm.
A51E-05
Measurements of atmospheric mercury at Storm Peak Laboratory in the central Rocky Mountains: Evidence for local/ regional emissions and influence of long-range transport from Asia
The goal of this study was to test if Asian long-range transport (ALRT) of pollutants can be observed significantly inland in North America. Our point of observation is the Storm Peak Laboratory (SPL), a high-elevation mountaintop research facility (3200m asl) in the Rocky Mountains located 1500 km inland from the Pacific where near-coastal stations, such as Mt. Bachelor, have previously measured mercury enhancements due to ALRT. Measurements of gaseous elemental mercury (GEM) from October 2006 to May 2007 averaged 1.83 ± 0.15 ng m-3 with minimum and maximum concentrations of 1.28 ng m-3 and 2.61 ng m-3, respectively. GEM showed pronounced diurnal patterns with peak concentrations during daytime and lowest concentrations during late night/early morning hours. In fall and winter, the GEM patterns almost identically follow diurnal fluctuations of water vapor, aerosol concentration, and ozone. The daily fluctuations are due to daytime surface heating which causes the local boundary layer (BL) to move up to SPL during the day and contract downslope during the night, allowing for measurement of free tropospheric air. Thus, the daily GEM enhancements for the fall and winter seasons are likely due to local and regional sources of mercury to the BL from anthropogenic and natural emissions. In spring, diurnal patterns of GEM and carbon monoxide (CO) continue to exhibit diurnal patterns but concentration increases occurred time-shifted with respect to water vapor, aerosol, and ozone concentrations. This suggests that sources other than upward mixing of BL air contributed to the observed mercury during that time period. One very pronounced GEM enhancement occurred between March 31 and April 6, 2007. CO levels increased with GEM during the event. Calculated 10-day back-trajectories (Hysplit) indicate that air masses measured at SPL had a likely source area of Eastern Asia. Our data indicate that mercury loads from ALRT are not limited to the coastal areas but can be carried far inland and affect areas such as the Rocky Mountains. http://www.stormpeak.dri.edu
A51E-06
Transboundary Influences on Ozone Pollution in the United States: Present Conditions and Future Projections
The GEOS-Chem global chemical transport model (v7-02-01) with 1° x1° horizontal resolution over North America is used to investigate the influence of transboundary pollution from Canada and Mexico on surface ozone in the United States. We conduct a series of 4 simulations for present day conditions: (1) a standard simulation with 2001 anthropogenic emissions worldwide, (2) with zero U.S. anthropogenic emissions, (3) with zero anthropogenic emission in North America (Canada, U.S. and Mexico), and (4) with 2006 East Asian emissions and no emission in North America. For future projections, we repeat (1)-(3) with a 2020 worldwide emissions inventory, in addition to a run with U.S. Power plant emissions switched off. The standard simulation is evaluated with observations for U.S. regions most sensitive to transboundary pollution (northeast for Canada, southwest for Mexico). Monthly mean ozone enhancements from transboundary pollution are in the 5-10 ppb range for these regions in June-August, but can exceed 30 ppbv under conditions when ozone is above the U.S. air quality standard (84 ppbv). Unlike intercontinental pollution influence which mainly contributes to the ozone background and is depleted during regional smog episodes, transboundary pollution influence from Canada and Mexico is highly variable and can contribute significantly to these episodes.
A51E-07
Global Budgets of Atmospheric Glyoxal and Methylglyoxal, and Implications for Formation of Secondary Organic Aerosols
We present the first global budgets of atmospheric glyoxal and methylglyoxal with the goal of quantifying their potential for global secondary organic aerosol (SOA) formation via irreversible uptake by aqueous aerosols and clouds. Our explicit simulation of glyoxal and methylglyoxal is based on the best current knowledge of source and sink processes. Global sources of glyoxal and methylglyoxal are 45 Tg y-1 and 140 Tg y-1, respectively. Isoprene is the largest global source of both dicarbonyls, producing 45% of glyoxal and 85% methylglyoxal. Acetylene and acetone act as background dicarbonyl sources due to their long lifetime. Atmospheric lifetimes of glyoxal and methylglyoxal are 2.9 hours and 1.6 hours, respectively, mainly due to removal by photolysis. Our simulated dicarbonyl concentrations at northern mid-latitudes during the growing season are in the range 10-100 ppt, consistent with in situ measurements. On a global scale, the highest simulated dicarbonyl concentrations are over biomass burning regions, in agreement with satellite observations. The global source of SOA from the irreversible uptake of dicarbonyls is estimated to be 11 Tg C y-1, including 2.6 Tg C y-1 from glyoxal and 8 Tg C y-1 from methylglyoxal. The dicarbonyl pathway could make a large contribution to SOA in the free troposphere over source regions, particularly under low-NOx conditions.