O

FIGURE 6.35 Measured OH concentrations at an altitude of 11.8 km near Hawaii and concentrations predicted using simple chemistry (adapted from Wennberg et al., 1998).

simple 03-H20-0H-C0-CH4 chemistry outlined earlier. The measured concentrations of OH are significantly larger than predicted by the model, indicating that there are additional sources of OH and other free radicals.

In general, for a compound to reach the upper troposphere in sufficient concentrations to impact the chemistry, it must not react rapidly in the lower troposphere. However, more reactive compounds can be rapidly transported (on the time scale of a few minutes) from the surface to the upper troposphere through convective events (e.g., thunderstorms) (e.g., Gidel, 1983; Chatfield and Crutzen, 1984; Chatfield and Alkezweeny, 1990; Pickering et al., 1992; Wang et al., 1995; Kley et al., 1996; Mahlman, 1997; Kley, 1997; Jaeglé et al., 1998a; Talbot et al., 1998). As a resuit, some compounds with relatively short tropospheric lifetimes can be carried into the upper troposphere and act as free radical sources.

The additional source of HOx free radicals has been proposed to be the photolysis of compounds such as CH3COCH3, HCHO, CH3OOH, and H202 carried into the upper troposphere by such convective transport (e.g., Chatfield and Crutzen, 1984; Singh et al., 1995; Arnold et al., 1997a, 1997b; McKeen et al., 1997; Prather and Jacob, 1997; Jaeglé et al., 1997, 1998a; Folkins et al., 1998; Lee et al., 1998). Thus, inclusion of acetone photolysis improves the agreement between the measured and modeled values of OH. Figure 6.36, for example, shows the measured and calculated values of OH with and without acetone photolysis in the model as well as calculated rates of production of HOx from the photolysis of 03 and acetone, respectively (Wennberg et al., 1998). At higher altitudes where acetone photolysis predominates, the agreement between the measurements and model is good. At lower altitudes, however, measured OH is still larger than the predicted values even with acetone photolysis, indicat-

0 0

Post a comment