Comment on acp-2021-1090

Abstract

Black carbon (BC) is one of the dominant absorbing aerosol species in the atmosphere. It normally has complex fractal-like structures due to the aggregation process during combustion. A wide range of aerosol-radiation interactions (ARI) of BC has been reported throughout experimental and modeling studies. One reason for the large discrepancies among multiple studies is the application of the over-simplified spherical morphology for BC in ARI estimates. Here, we employ a regional chemical transport model coupled with a radiative transfer code which utilizes the non-spherical BC optical simulations to re-evaluate the effects of particles' morphologies on BC ARI. Anthropogenic activities and wildfires are two major sources of BC emissions. Therefore, we choose four typical polluted cities in East China which are dominated by urban emissions, and three locations in the northwest US that are dominated by fire emissions in this study. Our modeling results show that spherical BC models overestimate the aerosol optical depth (AOD) at 550 nm wavelength up to 0.03 and 0.15 at typical polluted cities in East China and fire regions in the northwest US, respectively, than fractal BC models with a fractal dimension (Df) of 1.8. Besides, spherical BC model underestimates BC aerosol absorption optical depth (AAOD) at 450 nm up to 0.016 and 0.04 at typical polluted cities in East China and fire sites in the US, respectively, than the fractal BC model. BC morphologies have relatively small impacts on the single scattering albedo (SSA) and extinction Ångström exponent (EAE), while these morphological effects on the absorption Ångström exponent (AAE) are rather significant. The spherical BC models underestimate AAE by approximately 0.17 at 450 and 850 nm wavelength pair than the fractal counterparts. Besides, BC morphologies have non-negligible impacts on the BC ARI. The calculated mean ARI using the spherical BC model is approximately 3.47–4.45 Wm−2 at typical polluted cities in China, while the values increase to approximately 3.83–4.92 Wm−2 when using the fractal aggregate model, and the relative variations are approximately 10.4 %–15.3 %. The mean BC ARI increases from approximately 4.91–6.61 Wm−2 to 5.52–7.05 Wm−2 and the relative variations for BC ARI are approximately 6.2 %–6.9 % when we modify the BC from spheres to fractal aggregates with a Df of 1.8 in the northwest US. Therefore, the effects of BC morphologies on the regional radiative effects should be carefully evaluated in different regions.