Introduction

The troposphere is chemically complex. Many of the important species in the troposphere are short-lived, with lifetimes less than or of order of a month. Hence, the composition of the troposphere is regionally diverse, leading to regionally diverse chemical processes which control that composition. With the advent of three-dimensional models and regionally-specific estimates of emissions, however, it has become clear that human activity has perturbed the composition of even short-lived species over vast regions of the globe. The list of short-lived species of concern includes the reactive nitrogen oxides, reactive sulfur, ozone, non-methane hydrocarbons and carbon monoxide. Observations have established that increases in the long-lived species such as carbon dioxide, methane, nitrous oxide, and the chlorofluorocarbons are taking place with important resulting impacts on stratospheric chemistry. Further, there is some indication that carbonyl sulfide (a major precursor to stratospheric aerosols) may have an important anthropogenic source. Growth of pollutants such as these are of concern because they act as greenhouse gases or aerosol precursors (CO, CH, O, NO, chlorofluorocarbons, carbonyl sufide, reactive sulfur), as agents for depletion of stratospheric ozone (NO, chlorofluorocarbons), are harmful to vegetation (O, acids) or act as nutrients (nitrate, sulfate, trace metals). The chemical interactions are important to understand, because the build up of pollutants depends not only on the rates of their release into the troposphere but on their rate of removal in the troposphere. Removal rates depend on processes which determine the rate of oxidation of the component (which for most species proceeds mainly by reaction with the hydroxyl radical) or through precipitation scavenging or dissolution in the ocean (which requires that the pollutant or its oxidation product(s) be soluble in water).

This review is necessarily restricted due to space limitations. Here we review research on those species which are either produced or consumed within the troposphere, excluding components such as CO, NO, and the chlorofluorocarbons for which the troposphere can be viewed as an inert tracer (see Toohey, this issue, for a review of stratospheric chemistry), and excluding the sulfur budget and aerosols excepting their effect on photochemistry (see Kreidenweis, this issue, for a review of aerosol research). In addition, we emphasize regional and global issues, rather than urban photochemistry, as this topic area has seen an explosion of research in the last four years. Finally, we do not discuss laboratory studies. These are periodically reviewed by the NASA panel for data evaluation (DeMore et al., 1992) and by the IUPAC subcommittee on gas kinetic data evaluation for atmospheric chemistry (Atkinson et al., 1992).