Impacts of air pollutant caps on climate

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 aerosols1 that are important in air pollution and climate are often highly correlated due to shared generating processes.

Most important among the generating processes is the combustion of fossil fuels and biomass which produces carbon monoxide (CO), nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon dioxide (CO₂), black carbon(BC) aerosols, and sulphur oxides (SOx, comprised of some sulphate aerosols, but mostly SO₂ gas which subsequently forms reflective sulphate aerosols). In addition, the atmospheric lifecycles of common air pollutants such as CO, NOx and VOCs, and of the climatically important methane (CH4) and sulphate 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 Gt of reactive trace gases from the atmosphere; this amount is similar to the total mass of carbon removed annually from the atmosphere by the land and ocean combined.