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

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 preindustrial conditions in both the atmosphere and the 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 preindustrial 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 -0.31W m(-2) and a first indirect forcing in warm-phase clouds ranging from -0.22 to -0.29W m(-2), with the range due to different assumed SOA size distributions and refractive indices. The increase of POA since preindustrial times causes a direct forcing varying from -0.06 to -0.11W m(-2), when strongly and weakly absorbing refractive indices for brown carbon are used. The change in the total OA exerts a direct forcing ranging from -0.14 to -0.40W m(-2). The atmospheric absorption from brown carbon ranges from +0.22 to +0.57W m(-2), which corresponds to 27%similar to 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 +0.0031W m(-2) or as large as 24% of the forcing caused by BC in snow and ice simulated by the model.