FIGURE 6.29 Estimated rates of formation and loss of OH, H02, and CH302 in what is primarily free tropospheric air (adapted from Cantrell et al., 1996).

stable species such as 03, CO, CH4, and NO but also free radicals such as H02, R02, and OH. Such studies have been carried out in a number of locations (see Chapter If.A.4), including remote regions. For example, Cantrell et al. (1996) measured total peroxy radical concentrations (H02 + R02) at the Mauna Loa Observatory and compared them to model calculations. On one day in which mostly free tropospheric air reached the sampling site, the measurements and model calculations were in relatively good agreement. However, on other days, the measured values were significantly smaller than predicted. This was hypothesized as being due to the possible loss of peroxy radicals on aerosol particles and/or reaction of OH with unidentified organics.

Figure 6.29 shows the estimated rates of removal and formation of OH, H02, and CH302 for the day having primarily free tropospheric air at the sampling site. The photolysis of 03 to O('D) is estimated to be a major source of OH, but not the only source, with its contribution varying from ~20 to 55% of the total rate of OH production. The major source of H02 is the reaction of OH with CO, which is also the major sink for OH. Interestingly, the production of CH302 is predicted to occur to a small, but significant, extent from the reaction of OH with CH3OOH formed in the H02 + CH302 reaction.

Figure 6.30 shows the predicted rates of 03 formation and loss for the day corresponding to Fig. 6.29. The net is a loss of 03; i.e., the air mass is in the low-NOx regime.

Penkett, Ayers, Galbally, and co-workers have measured [HOz + R02] radicals using the radical amplifier

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