Introduction

The atmosphere is an oxidizing medium. Many environmentally important trace gases are removed from the atmosphere by oxidation, including methane and other organic compounds, carbon monoxide, nitrogen oxides, and sulfur gases (Table 1). Understanding the processes and rates by which species are oxidized in the atmosphere, i.e., the oxidizing power of the atmosphere, is crucial to our knowledge of atmospheric composition. Changes in the oxidizing power of the atmosphere would have a wide range of implications for air pollution, aerosol formation, greenhouse radiative forcing, and stratospheric ozone depletion (Thompson, 1992).

The most abundant oxidants in Earth's atmosphere are 02 and 03. They have large bond energies and are hence relatively unreactive. With a few exceptions, oxidation of nonradical atmospheric species by 02 or 03 is negligibly slow. Photochemical modeling of stratospheric chemistry in the 1950s first implicated the strong radical oxidants O and OH, generated from photolysis of 03 and H20, in the oxidation of CO and CH4 (Bates and Witherspoon, 1952). The importance of photo-chemically generated radicals in the chain oxidation of hydrocarbons leading to urban 03 smog was also recognized in the 1950s (Leighton, 1961). Smog models of that time hypothesized that O atoms produced in urban air from the photolysis of N02 and 03 would provide the main pathway for hydrocarbon oxidation (Altshuller and Bufalini, 1965, 1971). This mechanism was thought unimportant outside of urban areas because of low 03 and N02 concentrations, and transport to the stratosphere was viewed as necessary for oxidation of CO, CH4, and other gases present in the global troposphere (Cadle and Allen, 1970). Long atmospheric lifetimes for these gases were implied because of the 10-year residence time of air in the troposphere.

Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471-21489-2 © 2003 John Wiley & Sons, Inc.

TABLE 1 Atmospheric Lifetimes of Selected Species

Species

Lifetime"

Reference

CH3CCI3

4.8 yr (5.7yr)

WMO (1999)

CH4

8.4yr (8.9yr)

WMO (1999)

CHF2C1

11.8 yr (12.3 yr)

WMO (1999)

CHjBr

0.7 yr (1.7yr)

WMO (1999)

Isoprene*

~ 1 h 1 h)

Jacob et al. (1989)

CO

2 mo (2 mo)

Logan et al. (1981)

NO, (NO + N02)

~ 1 d (~1 d)c

Dentener and Crutzen (1993)

S02

~ 1 d (2 wks)d

Chin et al. (1996)

(CH3)2S

~ 1 d 1 d)

Chin et al. (1996)

"The atmospheric lifetime of a species is defined as the average time that a molecule of the species remains in the atmosphere before it is removed by one of its sinks. It can be calculated as the atmospheric mass of the species divided by the species loss rate. The first number given for each entry in the column is the mean atmospheric lifetime, and the second number in parentheses is the mean atmospheric lifetime against oxidation by OH.

aCH = C(CH3)—CH = CH2, a major hydrocarbon emitted by vegetation.

cLoss of NOj- in summer and in the tropics is mostly by reaction of N02 with OH;

loss in winter at extratropical latitudes is mostly by a nonphotochemical pathway involving formation of N205 and hydrolysis to HN03. The sum of these two processes results in a lifetime of NO^ of the order of a day.

dThe principal S02 sinks are deposition and in-cloud oxidation by H202(ag).

"The atmospheric lifetime of a species is defined as the average time that a molecule of the species remains in the atmosphere before it is removed by one of its sinks. It can be calculated as the atmospheric mass of the species divided by the species loss rate. The first number given for each entry in the column is the mean atmospheric lifetime, and the second number in parentheses is the mean atmospheric lifetime against oxidation by OH.

aCH = C(CH3)—CH = CH2, a major hydrocarbon emitted by vegetation.

cLoss of NOj- in summer and in the tropics is mostly by reaction of N02 with OH;

loss in winter at extratropical latitudes is mostly by a nonphotochemical pathway involving formation of N205 and hydrolysis to HN03. The sum of these two processes results in a lifetime of NO^ of the order of a day.

dThe principal S02 sinks are deposition and in-cloud oxidation by H202(ag).

This view of a chemically inert troposphere was first challenged by Weinstock (1969) who found from 14CO measurements that the atmospheric lifetime of CO is only ~0.1 years, requiring a dominant sink in the troposphere. Levy (1971) then presented photochemical model calculations for the unpolluted troposphere showing that high concentrations of OH could be generated from photolysis of 03 in the presence of water vapor and account for the missing sink of CO in the Weinstock (1969) analysis. Further work in the early 1970s confirmed the importance of tropo-spheric oxidation by OH as the main sink of CO and CH4 (McConnell et al., 1971; Weinstock and Niki, 1972; Levy et al., 1973) and further showed that OH, not O, is the main oxidant of hydrocarbons in urban air (Heicklen, 1971; Kerr et al., 1972; Demeijian et al., 1974). Considerable evidence over the past three decades supports the view that tropospheric OH is the main oxidant for nonradical species in the atmosphere.

Indirect estimates of global mean OH concentrations have been made since the 1970s using a number of proxies, the most useful of which has been CH3CC13, a long-lived gas emitted by industry and removed from the atmosphere by oxidation by OH (Lovelock, 1977; Singh, 1977). The most recent analyses of CH3CC13 data, based on observations at a worldwide network of sites (Prinn et al., 1995), imply a global mean OH concentration in the troposphere of (1.1 ±0.1) x 106 molecules/cm3 (Krol et al., 1998; Spivakovsky et al., 2000). Techniques for direct measurement of tropospheric OH were first developed in the 1970s but suffered from interferences or poor sensitivity. Only in the 1990s have reliable techniques been developed and successfully intercompared (special issue of Journal of the Atmospheric Sciences, October 1995; Crosley, 1997). Direct measurements provide the means to test our understanding of the local processes controlling OH concentrations (e.g., McKeen et al., 1997; Jaegle et al., 1997, 2000; Frost et al., 1999). By simulating these processes in global models, one can assess the sensitivity of the oxidizing power of the atmosphere to different anthropogenic perturbations (Wang and Jacob, 1998).

This chapter reviews current understanding of the factors controlling abundances and long-term trends of OH. It also briefly reviews (Section 3) other atmospheric oxidants that are important in certain environments or for certain nonradical molecules. It docs not cover the oxidation of short-lived radical species, which often involves reaction with 02 or 03 (Atkinson, 1990). It does not cover either oxidation in the stratosphere, whose importance as a sink for species emitted at the surface is limited by the long time for transfer of air from the troposphere to the stratosphere.

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