Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
2.1.2.3. Sources and Sinks of OH and HO 2
HO x is produced in the UT and LS via a number of processes. One important primary
source of OH is the reaction of water vapor with O(1D) produced in the photolysis of ozone:
O 3 + sunlight O(1D) + O 2 (6a)
O(1D) + H 2 O OH + OH (7)
Additional sources of OH and HO 2 in the troposphere include the photolysis of acetone
(Singh et al., 1995), peroxides (Chatfield and Crutzen, 1984), and formaldehyde. In the
stratosphere, the photolysis of HOBr and HNO 3 produced in reactions occurring on aerosols
may also be important HO x sources (Hanson et al., 1996). The existence of important
primary HO x sources in addition to reactions 6a and 7 has been demonstrated by recent
measurements of OH and HO 2 in the UT and LS (McKeen et al., 1997; Jaeglé et al., 1997,
1998; Brune et al., 1998; Wennberg et al., 1998). These primary sources of OH can be
further amplified during the oxidation of CH 4 and other hydrocarbons.
Figure 2-1: Net ozone production (24-hr average) as a function of
NOx in the upper troposphere (adapted from Jaeglé et al., 1998).
OH is chemically coupled to HO 2 by reactions such as 1-3 that convert these species on time scales of seconds to minutes. Removal of HO x (leading to the formation
of H 2 O) occurs on a longer time scale (5-30 min) and is dominated by processes such as:
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OH + HO 2 H 2 O + O 2 (8)
OH + NO 2 + M HNO 3 + M (9)
OH + HNO 3 H 2 O + NO 3 (10)
Net: OH + OH + NO 2 H 2 O + NO 3