Chapter 3: Atmospheric Chemical Processes of Importance in Cities

Chemical processes affect the current and future composition of the atmosphere, controlling the abundance of a range of pollutants harmful to health, vegetation and materials. Atmospheric composition above a city is a function of the incoming air mass, emissions associated with the urban environment and chemical processing which affects the levels of primary pollutants and creates secondary species. In the city environment, local emissions and their short-timescale chemical processing tend to dominate atmospheric composition. Key atmospheric components include hydrocarbons and oxides of nitrogen emitted from vehicles and a range of domestic and industrial activities, and particulate matter produced mechanically and chemically within the atmosphere. In this article the key emissions and the dynamical environment within which the chemistry occurs are briefly introduced, followed by a description of the predominant chemical processes occurring in cities. In the gas phase the focus is upon the evolution of nitrogen oxides over the city scale, the role of atmospheric free radicals, nitrogen oxides and hydrocarbons in the production of ozone, and the different emission control regimes which are encountered. In the condensed phase, the origins, composition and chemistry of atmospheric particles found in urban environments are described, together with their interactions with the gas-phase component of the aerosol mixture. The chapter concludes by considering the limitations introduced by the chemical complexity of the urban environment, and some current uncertainties in our understanding of the chemistry occurring above our cities.