Introduction

Urban air pollution has a significant impact on the chemistry of the atmosphere and thus potentially on regional and global climate. Already, air pollution is a major issue in an increasing number of megacities around the world, and new policies to address urban air pollution are likely to be enacted in many developing countries irrespective of the participation of these countries in any explicit future climate policies. The emissions of gases and microscopic particles (aerosols) that are important in air pollution and climate are often highly correlated because of shared generating processes. Most important among these processes is combustion of fossil fuels and biomass which produces carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx), volatile organic compounds (VOCs), black carbon (BC) aerosols, and sulfur oxides (SOx, consisting of some sulfate aerosols, but mostly SO2 gas which subsequently forms white sulfate aerosols). In addition, the atmospheric lifecycles of common air pollutants such as CO, NOx, and VOCs, and of the climatically important methane (CH4) and sulfate aerosols, both involve the fast photochemistry of the hydroxyl free radical (OH). Hydroxyl radicals are the dominant “cleansing” chemical in the atmosphere, annually removing about 3.7 gigatonnes (1 GT = 1015 g) of reactive trace gases from the atmosphere; while the environmental impact of each gas is different, this amount is similar to the total mass of carbon removed annually from the atmosphere by the land and ocean combined (Ehhalt, 1999; Prinn, 2003).