Soot microphysical effects on liquid clouds, a multi-model investigation

Abstract

We use global models to explore the microphysical effects of carbonaceous aerosolson clouds. Although absorption of solar radiation by soot warms the atmosphere, sootmay cause climate cooling due to its contribution to cloud condensation nuclei (CCN)and therefore cloud brightness. Six global models conducted three soot experiments; four of the models had detailed aerosol microphysical schemes. The average cloudradiative response to biofuel soot (black and organic carbon), including both indirectand semi-direct effects, is−0.11 Wm−2, comparable in size but opposite in sign tothe respective direct effect. In a more idealized fossil fuel black carbon experiment,some models calculated a positive cloud response because soot provides a deposition sink for sulfuric and nitric acids and secondary organics, decreasing nucleation andevolution of viable CCN. Biofuel soot particles were also typically assumed to be largerand more hygroscopic than for fossil fuel soot and therefore caused more negativeforcing, as also found in previous studies. Diesel soot (black and organic carbon)experiments had relatively smaller cloud impacts with five of the models<±0.06 Wm−2 from clouds. The results are subject to the caveats that variability among models,and regional and interrannual variability for each model, are large. This comparisontogether with previously published results stresses the need to further constrain aerosolmicrophysical schemes. The non-linearities resulting from the competition of opposingeffects on the CCN population make it difficult to extrapolate from idealized experiments to likely impacts of realistic potential emission changes.