Articles | Volume 15, issue 18
Atmos. Chem. Phys., 15, 10411–10433, 2015
Atmos. Chem. Phys., 15, 10411–10433, 2015

Research article 23 Sep 2015

Research article | 23 Sep 2015

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

P. S. Kim1, D. J. Jacob1,2, J. A. Fisher3, K. Travis2, K. Yu2, L. Zhu2, R. M. Yantosca2, M. P. Sulprizio2, J. L. Jimenez4,5, P. Campuzano-Jost4,5, K. D. Froyd4,6, J. Liao4,6, J. W. Hair7, M. A. Fenn8, C. F. Butler8, N. L. Wagner4,6, T. D. Gordon4,6, A. Welti4,6,9,a, P. O. Wennberg10,11, J. D. Crounse10, J. M. St. Clair10,b,c, A. P. Teng10, D. B. Millet12, J. P. Schwarz6, M. Z. Markovic4,6,d, and A. E. Perring4,6 P. S. Kim et al.
  • 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
  • 2School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • 3School of Chemistry, University of Wollongong, Wollongong, NSW, Australia
  • 4Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA
  • 5Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO, USA
  • 6Chemical Sciences Division, National Oceanic and Atmospheric Administration Earth System Research Laboratory, Boulder, CO, USA
  • 7NASA Langley Research Center, Hampton, VA, USA
  • 8Science Systems and Applications, Inc., Hampton, VA, USA
  • 9Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zurich, Switzerland
  • 10Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
  • 11Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA
  • 12Department of Soil, Water, and Climate, University of Minnesota, Minneapolis-Saint Paul, MN, USA
  • anow at: Experimental Aerosol and Cloud Microphysics, Leibniz Institute for Tropospheric Research (TROPOS), Leipzig, Germany
  • bnow at: Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • cnow at: Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD, USA
  • dnow at: Air Quality Research Division, Environment Canada, Toronto, ON, Canada

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 km2 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 PM2.5 (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 = [NH4+]/(2[SO42−] + [NO3]) 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 PM2.5 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 PM2.5. This implies that satellites can be used reliably to infer surface PM2.5 over monthly timescales if a good CTM representation of the aerosol vertical profile is available.

Final-revised paper