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

Abstract

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 (NO_x) 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.

Introduction

The emission of a variety of pollutant gases (e.g., nitrogen oxides, NO_x, and volatile organic compounds, VOCs) into the troposphere may present a health risk either directly, or as a result of their oxidation. This can lead to a variety of secondary oxidised products, many of which are potentially more harmful than their precursors. Because much of the chemistry is driven by the presence of sunlight, the oxidised products are commonly referred to as secondary photochemical pollutants, and include photochemical oxidants such as ozone (O₃). The production of elevated levels of O₃ at ground level is of particular concern, since it is known to have adverse effects on human health, vegetation (e.g., crops) and materials (PORG, 1997). Established air quality standards for O₃ are frequently exceeded, and the formulation of control policies is therefore a major objective of environmental policy (UNECE, 1992, UNECE, 1993, UNECE, 1994). Nevertheless, other pollutants that are formed on local or regional scales in the planetary boundary layer may also have direct health impacts (e.g. peroxy acetyl nitrate, PAN), and/or play wider roles in global atmospheric chemistry.

Photochemical air pollution, first identified in Los Angeles in the 1940s, is now a widespread phenomenon in many of the world's population centres (e.g., see NRC, 1991; PORG, 1997). Consequently, considerable attention has been given to identifying and quantifying chemical processes leading to the generation of O₃ and other secondary photochemical pollutants in the planetary boundary layer. This has involved the laboratory study of many hundreds of chemical reactions, and a significant body of evaluated chemical kinetics and photochemical data has accumulated for elementary atmospheric reactions (e.g., Atkinson et al., 1997a, Atkinson et al., 1997b; DeMore et al., 1997). Computer models have provided a useful means of assembling these data, and of describing the likely behaviour and interconversion of various atmospheric pollutants, and such models play a central role in policy development and implementation. This work has been driven, of course, by the need to interpret the results of field studies of atmospheric chemical processes. In recent years, an enormous variety of observational data has become available for molecular and free radical species involved in atmospheric chemical processes in both polluted and clean environments.

The aim of this review is to provide a comparatively detailed description of our current understanding of the chemical mechanisms leading to the generation of secondary photochemical pollutants in the troposphere, 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 and volatile organic compounds, and their role in the formation of O₃. The chemistry producing a variety of other oxidants and secondary pollutants, which are often formed in conjunction with O₃, is also described. Some discussion of nighttime chemistry and the formation and role of secondary organic aerosols (SOA) in tropospheric chemistry is also given, since these are closely linked to the gas-phase photochemical processes. Where possible, the relative importance of the various processes is discussed and illustrated by observational data, with emphasis generally placed on conditions appropriate to the UK and northwest continental Europe. Although some reference to heterogeneous reactions and aqueous uptake is made, multiphase chemical processes are not considered in detail, and the reader is referred to other texts (e.g., Jonson and Isaksen, 1993; Ravishankara, 1997) for further information on this important area of tropospheric chemistry.