Attribution of the Present-Day Aerosol Direct Radiative Forcing to Anthropogenic Emission Sectors

In this study, a general circulation model coupled with a gas-phase module and an aerosol chemistry module was employed to investigate the impacts of anthropogenic emission sectors on aerosol direct radiative forcing at the top of atmosphere (TOA) in the present-day climate. The predictions were based on the emission inventories developed in support of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). Six emission sectors-agriculture, open biomass burning, domestic activities, industry, energy generation, and transport-were considered, with a special focus on nitrate aerosol that shows large uncertainties in current models. The results show that the energy sector accounts for the largest contribution (-222 mW m(-2)) to global aerosol radiative forcing, with substantial negative forcing from sulfate. Inclusion of nitrate results in the transport sector yielding a global nitrate radiative forcing of -92 mW m(-2) and an internally mixed aerosol radiative forcing of-85 mW m(-2), which is opposite to the positive radiative forcing predicted in the past, indicating that the transport emissions could not be a potential control target to counteract climate warming as expected before. The maximum change in nitrate burden is found to be associated with agricultural emissions, which accounts for about 75% of global ammonia gas (NH3) emissions. Agricultural emissions account for global nitrate radiative forcing of -186 mW m(-2) and internally mixed aerosols direct radiative forcing of -149 mW m(-2). Such agricultural radiative forcing exceeds the radiative forcing of the industrial sector and is responsible for a large portion of negative radiative forcing over the Northern Hemisphere.