Atmospheric Chemistry of Tropospheric Ozone Formation: Scientific and Regulatory Implications

Abstract

Tropospheric ozone continues to be of significant concern worldwide due to its effect on human health, forest ecosystems and agricultural crops. Its levels have also increased globally over the last century. Photolysis of ozone (O₃) is the major source of the hydroxyl (OH) radical in the remote troposphere, so that an increase in O3 will produce more OH, resulting in decreased lifetimes of many trace species, such as methane and the hydrochlorofluorocarbons (HGFG), which are of stratospheric importance. A brief overview of the chemistry of volatile organic compounds (VOC) and oxides of nitrogen (NO_X) which lead to ozone formation is included. The question of whether controlling VOC or NO_X or both is most effective in urban/suburban and downwind/rural regions is discussed. The impact of uncertainties in emissions inventories is briefly discussed. Finally, the relative contributions of individual VOC to ozone formation and how this information forms the technical basis for California’s new regulations for VOC and concurrent NO_x emissions from passenger cars and light-duty trucks is described. We believe that the weight of past experience and current evidence supports strict concurrent controls on both NO_X and VOC as essential to effective overall control of tropospheric ozone.