CHAPTER Oxidant Formation in the Troposphere JOHN W. BIRKS 10 ABSTRACT. In the troposphere, ozone concentrations have approximatelv doubled over the past century, almost certainly as a result oj increasing emissions oj carbon monoxide rCOI. hydrocarbons, and oxides oj nitrogen. Ozone and other oxidants derived irom ozone are damaging to biological tissues and thus have detrimental eiiects on natural ecosYstems. crops, and human health. Ozone is a secondary pollutant produced as a byproduct oj the oxidation oj CO and hydrocarbons In the presence oj oxides oj nitrogen. The level oj nitrIC oxide (NOI may be thought 01 as a "chemical switch" that changes the pathways bv which CO and hydrocarbons are oxidized jrom ones that naturally destro,;-ozone to ones that iorm ozone. Ozone iormation may be greatlv enhanced in rural areas as a result oi the re- actions oj biogenic hydrocarbons such as isoprene and the monoterpenes with OXides oj ni- trogen which are transported irom Clites. ThiS chapter describes the chemistry oj ozone jar- malton precursors and discusses the role oi oxidant jormation in acid precipitation. INTRODUCTION Ozone. 0 •. is one of the most imponanr trace gases in the atmosphere. In the strato- sphere it serves as a protective shield against biologically hannful ultraviolet radiation JI wavelengths between 20(l and 320 nm. Ultraviolet light in the L"\'-B region. 2RO-320 nm. i, damaging to life because of its strong ab- sorption by protein,. nucleic and other biological molecules. Hence the great concern for ozone depletion in the stratosphere resulting from anthropogenic emissions of halocarbons and other ozont: deplt:tmg chemicals. By conrrast. in the troposphere the principal concern is the production of enhanced levels of ozone and other oxidanrs derived from ozone. The identification of ozone for- mation in photochemical smog was first made by Haagen-Smit and co-workers and de- scribed in a classic series of papers in the 1950s (Haagen-Smit et al.. ) 95). ) 952. ) 953. 11)54. ) 955. ) 959). Oxidanr production in the troposphere is enhanced by anthropogenic emissions of the pollutanrs carbon monoxide. hydrocarbons. and the oxides of nitro- 233