Long Term Trends in Atmospheric OH

Assessing human influence on the oxidizing power of the atmosphere is intricate. On the one hand, anthropogenic emissions of CO and hydrocarbon emissions act to deplete OH; on the other hand, anthropogenic emissions of N0X and the thinning of the stratospheric 03 layer act to boost OH. Human-induced changes in Earth's climate (temperature, cloudiness, circulation) add to the complication. Large regional differences may be expected in the response of OH to human activity, depending on the relative importance and coupling of the above factors.

A number of global tropospheric chemistry model studies, reviewed by Thompson (1992), have examined the changes in OH concentrations since preindustrial times as driven by trends in emissions of CO, hydrocarbons, and N0X. These studies report 10 to 30% decrease in the global mean OH concentration from preindustrial times to today, a relatively small effect considering that emissions of CO, CH4, and N0X increased severalfold over that period (Table 2). The global three-dimensional model study of Wang and Jacob (1998) indicate a 9% decrease in the global mean

TABLE 2 Comparison of Present and Preindustrial Atmospheres"


CH4 Nonmethane CO NOv 03 Source* [OHf

(Tg CH4/yr) Hydrocarbons (Tg CO/yr) (Tg N/yr) (Tg 03/yr) (molecules/ (Tg C/yr) cm3)

Preindustrial 160 610rf 50 9 2300 1.15 x 106

Present 460 710 1040 42 4500 1.04 x 106

"Global data from the three-dimensional model study of Wang and Jacob (1998).

''Tropospheric 03 source including transport from the stratosphere (400 Tg 03 /yr in both preindustrial and present cases) and chemical production within the troposphere.

'Global mean tropospheric concentration weighted by atmospheric mass.

^Biogenic isoprene and acetone.

OH concentration since preindustrial times and suggests that the OH trend should follow roughly the trend of the SN0/S^J2 ratio, where SN0 is the global source of NO and Sc is the global source of CO and hydrocarbons; the parallel changes in SN0 and Sc over the past century would thus have had nearly cancelling effects on OH concentrations. This study points out that estimates of past and future trends in OH are highly sensitive to assumed trends in tropical biomass burning because NO, emitted in the tropics is particularly efficient for generating 03 and OH.

Observational constraints on long-term OH trends are largely limited to the CH3CCI3 record since 1978. An analysis of this record by Krol et al. (1998) indicates a 0.5%/yr increase in global mean OH concentrations over the period 1978 to 1993. This result is consistent with radiative transfer model calculations by Madronich and Granier (1992), which indicate a 0.4%/yr increase in OH concentrations over the 1979 to 1989 decade as a result of stratospheric 03 depletion.

Estimates of OH trends since preindustrial and glacial times have been made using polar ice core records of CH20 and H202. Interpretation of these records is complicated by postdepositional exchange with the atmosphere and reactions within the ice (Neftel et al., 1995). Also, since CH20 and H202 have atmospheric lifetimes of about a day, they can only diagnose trends in polar OH, which may be different from global tropospheric trends. Analysis of the CH20/CH4 ratio in a Greenland ice core (Staffelbach et al., 1991) suggests that OH concentrations were 30% higher in the preindustrial atmosphere than today, and 2 to 4 times lower in the last glacial maximum (LGM) than today. Such depletion of OH in the LGM is not consistent with results from tropospheric chemistry models, which indicate higher OH concentrations in glacial than interglacial periods due to lower emissions of CH4 (Thompson, 1992). Staffelbach et al. (1991) suggested that a thicker stratospheric 03 layer could be responsible for low OH levels during glacial periods.

Data for H202 in Greenland ice going back to A.D. 1300 show constant concentrations until about 1970, and a doubling of concentrations since then (Sigg and Neftel, 1991; Anklin and Bales, 1997). Although the rise in H202 would imply a rise in HO„ the CH3CC13 record shows no large trends in global mean OH concentrations during that same period.

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