In situ vertical profiles of aerosol extinction, mass, and composition over the SEUS during the SENEX and SEAC4RS studies

Nicholas L Wagner1, Charles A Brock2, Doug A Day3, Glenn S Diskin4, Timothy Dean Gordon5, Martin Graus6, John S Holloway7, L Gregory Huey8, Jose L Jimenez9, Daniel Lack7, Jin Liao5, Xiaoxi Liu8, Milos Z Markovic10, Ann Middlebrook2, Anne Elizabeth Perring11, Mathews Richardson12, Dr. Joshua Peter Schwarz, PhD13, Carsten Warneke5, Andre Welti14, Armin Wisthaler15, Luke D Ziemba4, Daniel M Murphy16 and Pedro Campuzano-Jost17, (1)NOAA ESRL, Boulder, CO, United States, (2)NOAA Chemical Sciences Laboratory, Boulder, United States, (3)Cooperative Institute for Research in Environmental Sciences (CIRES), Boulder, United States, (4)NASA Langley Research Center, Hampton, United States, (5)NOAA Boulder, Boulder, CO, United States, (6)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (7)CIRES, Boulder, CO, United States, (8)Georgia Institute of Technology, Atlanta, GA, United States, (9)Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado Boulder, Boulder, United States, (10)Environment Canada Toronto, Toronto, ON, Canada, (11)CIRES, Boulder, United States, (12)NOAA/CIRES, Boulder, United States, (13)NOAA, Chemical Sciences Laboratory, Boulder, United States, (14)ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, (15)University of Oslo, Department of Chemistry, Oslo, Norway, (16)NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, United States, (17)University of Colorado Boulder, Department of Chemistry and Cooperative Institute for Research in Environmental Sciences, Boulder, United States
Abstract:
Shallow cumulus convection enhances vertical transport of trace gases and aerosol and creates a cloudy transition layer on top of the sub-cloud mixed layer. Two recent studies have proposed that an elevated layer of enhanced organic aerosol over the southeastern United States (SEUS) could explain the discrepancy in the summertime enhancement of aerosol optical depth (AOD) and summertime enhancement of surface measurements of aerosol mass. We investigate the vertical profile of aerosol over the SEUS during the summertime using in situ aircraft-based measurements of aerosol from the SENEX and SEAC4RS studies. During shallow cumulus convection over the SEUS, we found that aerosol and trace gas concentration in the transition layer are diluted by cleaner air from the free troposphere, and the absolute aerosol loading decreases with altitude in the transition layer. However, after normalizing the vertical profiles to the CO boundary layer enhancement to correct for the dilution, the aerosol mass, volume, and extinction relative to the boundary layer CO enhancement is ~20% greater in the transition layer than in the mixed layer. The enhancement of aerosol loading suggests production of aerosol mass in the transition layer, although biomass burning could also be the source of the enhancement. The median composition of the aerosol in the mixed layer is ~70% organics and ~18% sulfate, while it is 65% organics and 23% sulfate in the transition layer. The composition of the aerosol enhancement in the transition layer is roughly equal parts sulfate and organics by mass. The enhancement of aerosol extinction in the transition layer is not sufficient to explain the summertime enhancement of AOD over SEUS.
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