Sources, seasonality, and trends of southeast US aerosol: an integrated analysis of surface, aircraft, and satellite observations with the GEOS-Chem chemical transport model

Creators: Kim, P. S.; Wennberg, P. O.; Crounse, J. D.; St. Clair, J. M.; Teng, A. P.

Abstract

We use an ensemble of surface (EPA CSN, IMPROVE, SEARCH, AERONET), aircraft (SEAC4RS), and satellite (MODIS, MISR) observations over the southeast US during the summer–fall of 2013 to better understand aerosol sources in the region and the relationship between surface particulate matter (PM) and aerosol optical depth (AOD). The GEOS-Chem global chemical transport model (CTM) with 25 × 25 km^2 resolution over North America is used as a common platform to interpret measurements of different aerosol variables made at different times and locations. Sulfate and organic aerosol (OA) are the main contributors to surface PM_(2.5) (mass concentration of PM finer than 2.5 μm aerodynamic diameter) and AOD over the southeast US. OA is simulated successfully with a simple parameterization, assuming irreversible uptake of low-volatility products of hydrocarbon oxidation. Biogenic isoprene and monoterpenes account for 60 % of OA, anthropogenic sources for 30 %, and open fires for 10 %. 60 % of total aerosol mass is in the mixed layer below 1.5 km, 25 % in the cloud convective layer at 1.5–3 km, and 15 % in the free troposphere above 3 km. This vertical profile is well captured by GEOS-Chem, arguing against a high-altitude source of OA. The extent of sulfate neutralization (f = [NH_4^+]/(2[SO_4^(2−)] + [NO_3^−]) is only 0.5–0.7 mol mol^(−1) in the observations, despite an excess of ammonia present, which could reflect suppression of ammonia uptake by OA. This would explain the long-term decline of ammonium aerosol in the southeast US, paralleling that of sulfate. The vertical profile of aerosol extinction over the southeast US follows closely that of aerosol mass. GEOS-Chem reproduces observed total column aerosol mass over the southeast US within 6 %, column aerosol extinction within 16 %, and space-based AOD within 8–28 % (consistently biased low). The large AOD decline observed from summer to winter is driven by sharp declines in both sulfate and OA from August to October. These declines are due to shutdowns in both biogenic emissions and UV-driven photochemistry. Surface PM_(2.5) shows far less summer-to-winter decrease than AOD and we attribute this in part to the offsetting effect of weaker boundary layer ventilation. The SEAC4RS aircraft data demonstrate that AODs measured from space are consistent with surface PM_(2.5). This implies that satellites can be used reliably to infer surface PM_(2.5) over monthly timescales if a good CTM representation of the aerosol vertical profile is available.

Additional Information

© Author(s) 2015. This work is distributed under the Creative Commons Attribution 3.0 License. Received: 05 May 2015. Published in Atmos. Chem. Phys. Discuss.: 01 Jul 2015. Revised: 03 Sep 2015. Accepted: 05 Sep 2015. Published: 23 Sep 2015. We are grateful to the entire NASA SEAC^4RS team for their help in the field. We thank Aaron van Donkelaar, Eloise Marais, Loretta Mickley, Randall Martin, Chuck Brock, Ann Dillner, Ralph Kahn, Armin Sorooshian, Tran Nguyen, and Jenny Hand for helpful discussions and Sajeev Philip for assistance with downloading meteorological fields. We also thank Jack Dibb, Bruce Anderson and the LARGE team, Phil Russell, Jens Redemann and the 4STAR team, and Greg Huey for the data shown in the Supplement. This work was funded by the NASA Tropospheric Chemistry Program and by a Department of Energy Office of Science Graduate Fellowship to PSK made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under contract no. DE-AC05-06OR23100. P. Campuzano-Jost and J. L. Jimenez were supported by NASA NNX12AC03G and NSF GS-1243354/1360834. K. D. Froyd and J. Liao are supported by NASA grant NNH12AT29I from the Upper Atmosphere Research Program, Radiation Sciences Program, and Tropospheric Chemistry Program, and by NOAA base funding. D. B. Millet acknowledges support from NSF (Grant #1148951). P. O. Wennberg, J. D. Crounse, J. M. St. Clair, and A. P. Teng acknowledge support from NASA (NNX12AC06G and NNX14AP46G). We thank the US EPA for providing the 2010 North American emission inventory. The inventory is intended for research purposes and was developed for Phase 2 of the Air Quality Model Evaluation International Initiative (AQMEII) using information from the 2008-based modeling platform as a starting point. A technical document describing the 2008-based 2007v5 modeling platform can be found at http://epa.gov/ttn/chief/emch/2007v5/2007v5_2020base_EmisMod_TSD_13dec2012.pdf. A report on the 2008 NEI can be found at www.epa.gov/ttn/chief/net/2008report.pdf. GEOS-Chem is managed by the Harvard University Atmospheric Chemistry Modeling Group with support from the NASA Atmospheric Composition Modeling and Analysis Program. The GEOS-FP data used in this study were provided by the Global Modeling and Assimilation Office (GMAO) at NASA Goddard Space Flight Center. Edited by: K. Tsigaridis

Attached Files

Published - acp-15-10411-2015.pdf

Files

acp-15-10411-2015.pdf

Files (4.4 MB)

Name	Size	Download all
acp-15-10411-2015.pdf md5:a386958946cfba6c31d662aaaced9077	4.4 MB	Preview Download

Additional details

	All versions	This version
Views	0	0
Downloads	0	0
Data volume	0 Bytes	0 Bytes