How Many New Sulfate Aerosols Can Be Produced From Anthropogenic SO2?

To better understand human-induced sulfur perturbations on the climate system, including intentional injections of sulfur designed to geo-engineer climate (Crutzen 2006), it is important to quantify the number of new sulfate aerosols that can be produced from anthropogenic SO2. This quantity has been elusive because the conditions best suited for aerosol nucleation occur in the upper troposphere (UT), where it is cold and the air is relatively free of existing aerosols. It is not certain how much of the anthropogenic SO2 emitted at the surface reaches the UT and undergoes nucleation. To address this question, tagged tracers are implemented in a 3D global atmospheric transport model containing sulfur chemistry and different aerosol nucleation mechanisms. Five tagged sulfur tracers are considered (anthropogenic, oceanic, volcanic, aircraft and stratospheric). Using this model, we have estimated the tropospheric contributions of anthropogenic SO2 to the total SO2 and H2SO4(g) burdens and the new particle formation rates. Among our findings, anthropogenic SO2 is shown to have a pronounced impact on the sulfur burdens and new particle formation rates in the northern UT. In the southern UT, however, oceanic-based SO2 is the dominant source for producing new sulfate aerosols. Furthermore, we also find that oceanic-based SO2 accounts for major fractions of the sulfur burdens and new particle formation rates across the tropical UT, and, in contrast to other reports, is still an important source at some locations in the northern UT.