Balancing the Chemistry of Air Pollution and Climate:
Co-Benefits and Penalties

Earth’s atmosphere, with its abundant oxygen and intense solar ultraviolet radiation, is strongly photo-oxidizing. This chemistry is responsible for the removal of many gases emitted by natural and human sources (including CO, H2, CH4 and other VOCs, NOx, SO2, HFCs and HCFCs), while generating byproducts (such as O3 and particulate matter) that affect both local air quality and global climate. Given the non-linear and coupled nature of this system, it is important to realize that future regulations aimed at improving air quality could also influence climate-relevant properties of the atmosphere, and vice versa, in ways that may or may not be intended or even beneficial. The net effects are still not fully understood – a fact that will be illustrated with some discrepancies between state-of-the-art models and recent field observations, particularly with respect to formation and loss of radicals (OH and HO2), oxidants (O3, peroxides, and acids), and aerosol particles. Uncertainties from transformation and removal processes, especially by multiphase chemistry within aerosols and clouds, are still generally larger than those from emissions, and will need to be reduced to improve prediction of how atmospheric composition and climate will respond to future changes in human activities.

Sasha Madronich

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