Ozone and other secondary photochemical pollutants: chemical processes governing their formation in the planetary boundary layer

The chemical processing of pollutants emitted into the atmosphere leads to a variety of oxidised products, which are commonly referred to as secondary pollutants. Such pollutants are often formed on local or regional scales in the planetary boundary layer, and may have direct health impacts and/or play wider roles in global atmospheric chemistry. In the present review, a comparatively detailed description of our current understanding of the chemical mechanisms leading to the generation of secondary pollutants in the troposphere is provided, with particular emphasis on chemical processes occurring in the planetary boundary layer. Much of the review is devoted to a discussion of the gas-phase photochemical transformations of nitrogen oxides (NOx) and volatile organic compounds (VOCs), and their role in the formation of ozone (O3). The chemistry producing a variety of other oxidants and secondary pollutants (e.g., organic oxygenates; oxidised organic and inorganic nitrogen compounds), which are often formed in conjunction with O3, is also described. Some discussion of nighttime chemistry and the formation of secondary organic aerosols (SOA) in tropospheric chemistry is also given, since these are closely linked to the gas-phase photochemical processes. In many cases, the discussion of the relative importance of the various processes is illustrated by observational data, with emphasis generally placed on conditions appropriate to the UK and northwest continental Europe.