Radiative forcing of organic aerosol in the atmosphere and on snow: Effects of SOA and brown carbon

Abstract - Organic aerosols (OA) play an important role in climate change. However, very few calculations of global OA radiative forcing include secondary organic aerosol (SOA) or the light-absorbing part of OA (brown carbon). Here, we use a global model to assess the radiative forcing associated with the change in primary organic aerosol (POA) and SOA between present day and pre-industrial conditions, both in the atmosphere and land snow/sea-ice. Anthropogenic emissions are shown to substantially influence the SOA formation rate, causing it to increase by 29 Tg/yr (93%) since pre-industrial times. We examine the effects of varying the refractive indices, size distributions for POA and SOA, and brown carbon fraction in SOA. The increase of SOA exerts a direct forcing ranging from -0.12 to -0.31 Wm-2 and a first indirect forcing in warm phase clouds ranging from -0.22 to -0.29 Wm-2, with the range due to different assumed SOA size distributions and refractive indices. The increase of POA since pre-industrial times causes a direct forcing varying from -0.06 to -0.11 Wm-2, when strongly and weakly absorbing refractive indices for brown carbon are used. The change in total OA exerts a direct forcing ranging from -0.14 to -0.40 Wm-2. Atmospheric absorption from brown carbon ranges from +0.22 to +0.57 Wm-2, which corresponds to 27% ~ 70% of the black carbon (BC) absorption predicted in the model. The radiative forcing of OA deposited in land snow and sea ice ranges from +0.0011 to +0.0031 Wm-2, or as large as 24% of the forcing caused by BC in snow and ice simulated by our model.

Lin, G., J. E. Penner, M. G. Flanner, S. Sillman, L. Xu, & C. Zhou (2014) Radiative forcing of organic aerosol in the atmosphere and on snow: Effects of SOA and brown carbon, Journal of Geophysical Research: Atmospheres (Early Edition).