Impacts of global open-fire aerosols on direct radiative, cloud and surface-albedo effects simulated with CAM5

Aerosols from open-land fires could significantly perturb the global radiation balance and induce climate change. In this study, Community Atmosphere Model version 5 (CAM5) with prescribed daily fire aerosol emissions is used to investigate the spatial and seasonal characteristics of radiative effects (REs, relative to the case of no fires) of open-fire aerosols including black carbon (BC) and particulate organic matter (POM) from 2003 to 2011. The global annual mean RE from aerosol–radiation interactions (REari) of all fire aerosols is 0.16± 0.01 W m−2 (1σ uncertainty), mainly due to the absorption of fire BC (0.25± 0.01 W m−2), while fire POM induces a small effect (−0.05 and 0.04± 0.01 W m−2 based on two different methods). Strong positive REari is found in the Arctic and in the oceanic regions west of southern Africa and South America as a result of amplified absorption of fire BC above low-level clouds, in general agreement with satellite observations. The global annual mean RE due to aerosol–cloud interactions (REaci) of all fire aerosols is −0.70± 0.05 W m−2, resulting mainly from the fire POM effect (−0.59± 0.03 W m−2). REari (0.43± 0.03 W m−2) and REaci (−1.38± 0.23 W m−2) in the Arctic are stronger than in the tropics (0.17± 0.02 and −0.82± 0.09 W m−2 for REari and REaci), although the fire aerosol burden is higher in the tropics. The large cloud liquid water path over land areas and low solar zenith angle of the Arctic favor the strong fire aerosol REaci (up to−15 W m−2) during the Arctic summer. Significant surface cooling, precipitation reduction and increasing amounts of low-level cloud are also found in the Arctic summer as a result of the fire aerosol REaci based on the atmosphere-only simulations. The global annual mean RE due to surface-albedo changes (REsac) over land areas (0.03± 0.10 W m−2) is small and statistically insignificant and is mainly due to the fire BC-in-snow effect (0.02 W m−2) with the maximum albedo effect occurring in spring (0.12 W m−2) when snow starts to melt.