Annual Variation of the Hydroxyl Radical in a Mid-Latitude Rural Atmosphere

B. D. Hall; P. L. Utley; J. C. Sheppard; Malcolm J. Campbell

doi:10.7273/000007808

The annual variation of the hydroxyl radical (OH) has been determined from measurements at three rural sites in eastern Washington. A distinct annual trend was observed with highest concentrations observed during summer months, and lowest concentrations detected during the winter. Average OH concentrations were 2.6 ±0.2, 3.5 ± 0.5, and 7.7 ±0.6 radicals cm· 3 during winter, spring/fall, and summer, respectively. Monthly average OH concentrations are described by a simple photochemical box model, in which actinic flux, ozone, and water vapor are the major variants. These results are consistent with our understanding of the atmospheric chemistry in a rural, relatively unpolluted region. INTODUCTION The hydroxyl radical (OH) is believed to be the primary oxidant of many atmospheric pollutants (Levy, 1971; Wuebbles et al., 1989). Although present at low concentrations, the catalytic nature of reactions involving OH makes it a very effective atmospheric cleansing agent. Furthermore, through the oxidation ofhydrocarbons, the hydroxyl radical also plays a critical role in acid formation and ozone generation. Therefore, knowledge of the spatial and temporal variations of the OH concentration are key to understanding the role of various atmospheric pollutants with regard to local and regional pollution, and climate change. Measurement of tropospheric OH has proved exceptionally difficult owing to its low concentration, short lifetime, and high reactivity. With reliable methods only recently developed (Brune, et al., 1995; Eisele and Tanner, 1991; Felton et al., 1988; Hard et al., 1992; Hofzumahaus et al., 1991; Mount, 1992) there exists relatively few measurements of OH concentration in the troposphere. A recent review of OH measurement technology by Crosley (1995) is available in the literature. The formation of OH involves the photolysis of ozone (03) and subsequent reaction of O(1D) with water. 0 3 + hv (<320 nm) ➔ 02 + O(1D) (1) O(1D) + H 2 O ➔ 2OH (2)

Annual Variation of the Hydroxyl Radical in a Mid-Latitude Rural Atmosphere

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