Tropospheric Aerosol Impacts on Trace-Gas Budgets through Photolysis

Aerosols affect the global budgets of O3, OH and CH4 in part through their alteration of photolysis rates and in part through their direct chemistry interactions with gases (a.k.a. “heterogeneous chemistry”). The first effect is evaluated here with a global tropospheric chemistry transport model using recently developed global climatologies of tropospheric aerosols: a satellite-derived aerosol climatology over the oceans (AVHRR) and a model-generated climatology for land plus oceans (CCSR). Globally averaged, the impact of aerosols on photolysis alone is to increase tropospheric O3 by 0.63 DU and increase tropospheric CH4 by 130 ppb (via tropospheric OH decreases of 8%). These greenhouse gas increases lead to an aerosol indirect effect (counting both natural and anthropogenic aerosols) of +0.08 W/m. Although the CH4 increases are of course global, the changes in tropospheric OH and O3 are mainly regional with largest impacts in northwest Africa for January and in India and southern Africa for July. The influence of aerosols is greater in July than in January, greater in the Northern Hemisphere than in Southern Hemisphere, as expected for pollution sources in the Northern Hemisphere. The predominant impact is due to the aerosols over land, aerosols over ocean contribute less than a third to globally integrated changes.