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Investigating Carbonaceous Aerosol and its Absorption Properties from Fires in the Western US (WE-CAN) and Southern Africa (ORACLES and CLARIFY)
THERESE CARTER, Colette L. Heald, Christopher Cappa, Jesse Kroll, Teresa Campos, Hugh Coe, Michael Cotterell, Nicholas Davies, Delphine K. Farmer, Cathryn Fox, Lauren A. Garofalo, Lu Hu, Justin Langridge, Ezra Levin, Shane Murphy, Rudra Pokhrel, Yingjie Shen, Kate Szpek, Jonathan Taylor, HuiHui Wu, MIT
Abstract Number: 14
Working Group: Wildfire Aerosols
Abstract
Biomass burning (BB) produces large quantities of carbonaceous aerosol (black carbon and organic aerosol, BC and OA, respectively), which significantly degrade air quality and impact climate. Black carbon absorbs radiation, warming the atmosphere, while OA typically scatters radiation, leading to cooling. However, some OA, termed brown carbon (BrC), also absorbs visible and near UV radiation; although, its properties are not well constrained. We explore three aircraft campaigns from large BB regions with different dominant fuel and fire types (WE-CAN in the western US and ORACLES and CLARIFY off southern Africa) and compare them with simulations from the global chemical transport model, GEOS-Chem using GFED4s. The model generally captures the observed vertical profiles of carbonaceous BB aerosol concentrations; however, we find that BB BC emissions are underestimated in southern Africa. Our comparisons suggest that BC and/or BrC absorption is substantially higher off Africa than in the western US and that, while the Saleh et al. (2014) and FIREX parameterizations based on the BC:OA ratio improve model-observation agreement in some regions, they do not sufficiently differentiate absorption characteristics at short wavelengths. We find that photochemical whitening substantially decreases the burden and direct radiative effect of BrC (annual mean of +0.292 W m‑2 without whitening and +0.083 W m-2 with). Our comparisons suggest that whitening is required to explain WE-CAN observations; however, the importance of whitening for African fires cannot be confirmed. Qualitative comparisons with the OMI UV aerosol index suggest that our standard BrC whitening scheme may be too fast over Africa.