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

Wednesday, 17 December 2014
Nicholas L Wagner1, Charles A Brock1, Douglas A Day2, Glenn S Diskin3, Timothy Dean Gordon4, Martin Graus2, John S Holloway5, Greg Huey6, Jose L Jimenez7, Daniel Lack5, Jin Liao4, Xiaoxi Liu6, Milos Z Markovic8, Ann M Middlebrook4, Anne Elizabeth Perring9, Mathews Richardson1, Joshua Peter Schwarz1, Carsten Warneke4, Andre Welti10, Armin Wisthaler11, Luke D Ziemba12, Daniel M Murphy1 and Pedro Campuzano Jost13, (1)NOAA ESRL, Boulder, CO, United States, (2)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (3)NASA Langley Research Ctr, Hampton, VA, United States, (4)NOAA Boulder, Boulder, CO, United States, (5)CIRES, Boulder, CO, United States, (6)Georgia Institute of Technology, Atlanta, GA, United States, (7)University of Colorado at Boulder, Boulder, CO, United States, (8)Environment Canada Toronto, Toronto, ON, Canada, (9)NOAA Earth System Research Lab, Boulder, CO, United States, (10)ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, (11)University of Oslo, Department of Chemistry, Oslo, Norway, (12)NASA Langley Research Center, Hampton, VA, United States, (13)University of Colorado Boulder, Boulder, CO, United States
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.