The chemistry of smog formation: A review of current knowledge

This paper describes smog chemistry and the methods used to develop our knowledge of its complex chemistry. These methods employ computer modeling, fundamental chemistry, smog chamber experiments, sophisticated analytical instrumentation, and process analysis techniques. The photochemistry leading to smog formation involves a kinetically controlled and coupled competitive process. The essential pathway for formation of nitrogen oxides starts with emissions composed primarily of NO, which are converted to NO₂, mostly via reactions with peroxy radicals; NO₂ is converted to photochemically inert nitric acid primarily by reaction with OH. Organics in smog chemistry are eventually oxidized to CO₂ and water; before this, they typically react with OH to form peroxy radicals. The peroxy (RO₂·) radicals couple the organic and nitrogen chemistry by converting NO to NO₂; the RO₂· radicals are converted to RO radicals, which typically lead to oxygenated intermediate organics that continue through OH·---RO₂·---RO· cycles. These OH·---RO₂·---RO· cycles produce CO, CO₂, and radical products. The radical products, which usually derive from photolysis of oxygenated intermediate organic products, are central to the overall process of smog formation. This is because the balance of these radicals affects the rapidity and severity of smog development. The radical balance is, in turn, controlled by the sources and sinks that depend on the HC/NO_x ratio, the types of organics, and the light flux. With only a rudimentary understanding of smog chemistry as a process, many of the effects observed from precursor controls can be explained and the basic shape of Empirical Kinetics Modeling Approach (EKMA) isopleth curves can be accounted for. The next step beyond this basic level of understanding involves a host of subprocesses composed of a complex series of chemical reactions. Current research in smog chemistry centers on the assessment and elucidation of these complex subprocesses. Atmospheric models currently in use rely on condensed chemical mechanisms. All such modern mechanisms treat the same basic processes, but differ both in their method of condensation and in their manner of addressing the complex subprocesses of smog chemistry.