Investigating carbonaceous aerosol and its absorption properties from fires in the western US (WE-CAN) and southern Africa (ORACLES)

Abstract:

Biomass burning (BB) produces large but uncertain quantities of aerosol (both black and organic carbon, BC and OC, respectively), which have significant air quality consequences and impacts on the Earth’s radiative balance. The two biomass burning aerosol (BBA) species (BC and OC) interact with radiation differently, with BC generally absorbing (heating) and OC generally thought of as scattering (cooling). However, both experimental and modeling work has shown that some OC, termed brown carbon (BrC), also absorbs radiation; although, its actual properties are not well constrained. We explore three aircraft campaigns from large BB source regions with different dominant fuel and fire types (WE-CAN in the western US and ORACLES and CLARIFY off southern Africa) and compare observations with simulations from the global chemical transport model, GEOS-Chem to better understand the impact of carbonaceous aerosol from fires. The model generally captures the vertical profile and magnitude of BBA in the regions, with a few notable exceptions. We find that the model (using GFED4s emissions) captures OA and CO concentrations, but appears to underestimate BB BC emissions during ORACLES. We show that we need to include a factor of 1.5 absorption enhancement for BB BC in our model to match the observed mass absorption coefficient (MAC) at longer wavelengths (660 nm) in both regions. Our comparisons demonstrate that the absorption efficiency of BrC differs in the two regions and that the parameterization of Saleh et al. (2014) based on the BC:OA ratio improves model-observation agreement. We also investigate whether photochemical whitening is needed to match observed absorption. In addition, we use the OMI UV aerosol index product to qualitatively explore the spatial extent and whitening of BrC absorption efficiency. Finally, we discuss the implications of this differing brownness for the global direct radiative effect of carbonaceous aerosol from fires.