Aerosol absorption in global models from AeroCom Phase III

Aerosol induced absorption of shortwave radiation can modify the climate through local atmospheric heating, which affects lapse rates, precipitation, and cloud formation. Presently, the total amount of aerosol absorption is poorly constrained, and the main absorbing aerosol species (black carbon (BC), organic aerosols (OA) and mineral dust) are diversely quantified in global climate models. As part of the third phase of the AeroCom model intercomparison initiative (AeroCom Phase III) we here document the distribution and magnitude of aerosol absorption in current global aerosols models and quantify the sources of intermodel spread, highlighting the difficulties of attributing absorption to different species. 15 models have provided total present-day absorption at 550 nm (using year 2010 emissions), 11 of which have provided absorption per absorbing species. Of the summed component AAOD, 60 % (range 36-84%) is estimated to be due to BC, 31 % (12-49%) is due to dust and 11% (0-24%) is due to OA, however the components are not independent in terms of their absorbing efficiency, and in models with internal mixtures of absorbing aerosols, a major challenge is the lack of a common and simple method to attribute absorption to the different absorbing species. We discuss challenges of attributing absorption to different species, we compare burden, refractive indices, and density, and we contrast models with internal mixing to models with external mixing. The difference in spectral dependency between the models is striking.