Abstract View
Rethinking the Lifetime of Aged Biomass Burning Aerosol over the Southeast Atlantic Ocean
AMIE DOBRACKI, Steven Howell, Pablo Saide, Steffen Freitag, Allison Aiken, James Podolske, Arthur J. Sedlacek, Kenneth Thornhill, Jonathan Taylor, HuiHui Wu, Hugh Coe, Paquita Zuidema, University of Miami
Abstract Number: 318
Working Group: Aerosol Chemistry
Abstract
Southern Africa produces approximately one-third of the world’s fire-emitted carbon, a relative contribution that is expected to increase with time. A westward outflow off the continent allows the aerosol to remain over the southeast Atlantic ocean (SEA) for one to two weeks, as large-scale subsidence prevents aerosol removal by deep convection and maintains a coherent vertical structure that is resistant to mixing with cleaner environmental air. These meteorological conditions along with chemical uniformity of the sources and the range of plume ages provided a unique natural laboratory for three field campaigns (NASA-ORACLES, DOE-LASIC, and UK Met Office-CLARIFY) (2016-2018) to study biomass burning aerosol aged beyond two days. In situ aerosol mass concentrations (organics, nitrates, sulfates, ammonium) derived from the ORACLES Aerosol Mass Spectrometer, combined with aerosol age (time since emission) estimates from the in-field aerosol forecasting model indicate biomass-burning aerosol ages typically exceed 4 days. Estimates of the organic nitrate, or the portion of the biomass-burning aerosol that is most susceptible to loss through photodegradation, indicate a decrease from 80% to <50% in the free troposphere over 4 days. During this time the relative percentage of black carbon increases from 5% to 13% as it is transported over the SEA. One important consequence is that the single-scattering-albedo decreases from 0.875 to 0.84 with age as organic aerosol, primarily nitrate and secondarily ammonium, is lost and black carbon is conserved, although changes in particle size, which may also be influential, have not yet been accounted for. We further assume that thermodynamic processes only affect the partitioning of the inorganic nitrate. Global aerosol models do not currently model the photodegradation of biomass-burning aerosol; it is hoped that these empirical results can provide a valuable model metric. The loss of organic aerosol after 2 days is predicted by chamber studies, however, in situ analysis of long range transport biomass burning aerosol, especially in the southern hemisphere, remains sparse.