Investigating carbonaceous aerosol and its absorption properties from fires in the Western United States (WE-CAN) and Southern Africa (ORACLES and CLARIFY)

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 downwind of Africa than in the western United States and, while the Saleh et al. (2014, doi.org/10.1038/ngeo2220) 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 +0.29 W m(-2) without whitening and +0.08 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 our standard BrC whitening scheme may be too fast over Africa.