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In remote marine regions where there are not significant sources of large, biogenic VOCs, OH is removed by reaction with CO and CH4:

OH + CH4 CH302 + HzO. (104) Both H02 and OH can react with 03:

In the absence of NO, radical-radical reactions of H02 and of R02 occur. The self-reaction of H02 is both pressure and water concentration dependent:

m,h2o

As indicated by the involvement of water vapor and an inert third body, this reaction has several channels (see DeMore et al., 1997, for a review). There is both a bimolecular channel, which is pressure independent, and a termolecular channel, which is pressure dependent. In addition, the rate constant increases in the presence of gaseous water, suggesting that the reaction proceeds through a mechanism such as

HO2 + H2O <-> (HO2 • H2O), HO2 + (HO2 • H2O) -»-» H2O2 + o2 + H2O,

The binding energy of the H02 • HzO complex has been calculated to be 6.9 kcal mol~' (Aloisio and Francisco, 1998). The recommended overall rate constant (in units of cm3 molecule-1 s_l) for reaction (107) is given by (DeMore et al., 1997)

kwl = [2.3 x 10"'V»"/7 + 1.7 x 10-33[M]e1()(K)/7']

At 1 atm pressure, 298 K, and 50% relative humidity, k[{]1 = 5.5 x 10~12 cm3 molecule-1 s_1 (see Problem 13). The pressure and water vapor dependences are quite significant. For example, Stockwell (1995) points out that the relative error can be as much as 75% near the earth's surface and 30% at 10 km, leading to underestimates of the rate of formation of H202 and overestimates of the rates of formation of organic peroxides (formed from H02 + R02; see the following) and of 03.

In addition to the HOz self-reaction, there are also H02-R02 and R02-R02 reactions:

Although reaction (108) is generally accepted to represent the major, if not sole, reaction path, it has been suggested by Ayers et al. (1997), based on measurements of HCHO in clean marine air, that a portion may proceed by an alternate path to produce HCHO + H20 + 02. As discussed earlier, path (109a) is the major path in the CH302 self-reaction at room temperature, with a branching ratio of ~0.7 and the remainder occurring via (f09b); however, km.d/kmh is temperature dependent, with the relative importance of reaction (109b) decreasing at lower temperatures. For example, the recommended temperature dependence for these channels gives a branching ratio for (109b) of ~ 0.13 at 245 K compared to 0.3 at 298 K (Atkinson et al., 1997a).

The result of this chemistry is the photochemical destruction of 03 and the formation of peroxides.

On the other hand, when sufficient concentrations of NO are present, H02 and R02 both react with NO:

As discussed earlier, the NOz then photolyzes to 0(3P), which adds to 02 to form 03. Under these conditions, 03 will be formed. The concentration of NO at which this crossover from ozone destruction to ozone formation occurs is central to the chemistry of both remote and polluted regions.

At night, there can be significant concentrations of N03 radicals present, along with HOz and ROz. HOz reacts with N03,

with a recommended rate constant of 3.5 x 10"12 cm3 molecule~1 s"1 at 298 K (DeMore et al., 1997). Given that this rate constant is similar to that for the H02 self-reaction under typical tropospheric conditions near the surface, this reaction can be a significant contributor to the removal of HOz at night. It accomplishes the same thing as NO, i.e., converts H02 to OH and generates N02.

In addition to gas-phase chemistry, aqueous-phase chemistry discussed in Chapter 8.C.3 taking place in clouds can also be important in remote regions. For example, modeling studies by Lelieveld and Crutzen (1990) suggest that clouds may decrease the net production of 03 by uptake of H02, dissociation to H+ + and reaction of 03 with 02 in cloud droplets.

A test of our understanding of the chemistry of remote regions thus requires measurements of not only

OH Formation

HO2 Formation

CH3O2 Formation

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