Transport and direct radiative forcing of carbonaceous and sulfate aerosols in the GISS GCM

We simulate the major anthropogenic aerosols, sulfate, organic carbon and black carbon, in the Goddard Institute for Space Studies General Circulation Model (GISS GCM), and examine their transport, relative abundances, and direct radiative forcing. Both present‐day and projected future emissions are used, as provided by the IPCC SRES (A2) scenarios for 2030 and 2100. We consider the sensitivity of the black carbon distribution to the treatment of its solubility and allow solubility to depend upon exposure to gas phase production of sulfuric acid (case S), or time (case A), or a fixed rate (case C). We show that all three approaches can be tuned to give reasonable agreement with present‐day observations. However, case S has higher black carbon in the arctic winter, owing to reduced SO 2 oxidation and black carbon solubility. This improves upon the arctic deficiency in previous models, though may be somewhat excessive in this model. We also show that with a different ratio of sulfur/carbonaceous emissions, the case S mechanism can give significantly different results compared to the other mechanisms. Thus, in the 2100 simulation, with reduced sulfur and increased black carbon emissions, the black carbon burden is ∼13% higher and the direct radiative forcing is ∼40% higher in case S compared to the other cases. We consider the relative abundances of carbonaceous and sulfate aerosols in different regions. In the current simulations, carbonaceous aerosols exceed sulfate at the surface in Asia and much of Europe and throughout the column in biomass burning regions. We show that the model ratio of carbonaceous to sulfate aerosols increases with altitude over many oceanic regions, especially in summertime, as was observed during the Tropospheric Aerosol Radiative Forcing Observational Experiment campaign; however, over land and during other seasons the ratio generally decreases with altitude. The (present day) direct radiative forcings for externally mixed (case A) black carbon, organic carbon, and sulfate are calculated to be 0.35, −0.30, and −0.65 W/m 2 , respectively. In the 2100 simulation these forcings are 0.89, −0.64, and −0.54 W/m 2 , respectively. The net anthropogenic aerosol global average forcing seasonality inverts between the current and future simulations: the forcing is most negative in (Northern Hemisphere) summertime in 2000 but is least negative or even positive during (NH) summer in 2100; this inversion is more extreme in the Northern Hemisphere.