Nitrogen dioxide, tropospheric photolysis

Molecular oxygen photodissociation is feeding reaction (1) with atomic oxygen in the stratosphere, the part of the atmosphere extending from above the troposphere to about 50 km. In the troposphere, the lowest part of the atmosphere extended up to 7-16 km, 02 photolysis is not significant. Nitrogen dioxide (N02) photolysis provides the required 03P for 03 production ... 

Tuazon et al. (1984a) investigated the atmospheric reactions of TV-nitrosodimethylamine and dimethylnitramine in an environmental chamber utilizing in situ long-path Fourier transform infared spectroscopy. They irradiated an ozone-rich atmosphere containing A-nitrosodimethyl-amine. Photolysis products identified include dimethylnitramine, nitromethane, formaldehyde, carbon monoxide, nitrogen dioxide, nitrogen pentoxide, and nitric acid. The rate constants for the reaction of fV-nitrosodimethylamine with OH radicals and ozone relative to methyl ether were 3.0 X 10 and <1 x 10 ° cmVmolecule-sec, respectively. The estimated atmospheric half-life of A-nitrosodimethylamine in the troposphere is approximately 5 min. 

Nitrogen dioxide is about 20 to 50% of the total nitrogen oxides NO, (NO, NOz, HN03, N2Os), while CIO represents about 10 to 15% of the total chlorine species CIO, (Cl, CIO, HCI) at 25 to 30 km. Hence, the rate of ozone removal by CIO, is about equal to that by NO, if the amounts of NO, are equal to those of CIO,. According to a calculation by Turco and Whitten (981), the reduction of ozone in the stratosphere in the year 2022 with a continuous use of chlorofluoromethanes at present levels would be 7%. Rowland and Molina (843) conclude that the ozone depletion level at present is about 1%, but it would increase up to 15 to 20% ifthechlorofluoromethane injection were to continue indefinitely at the present rates. Even if release of chlorofluorocarbons were stopped after a large reduction of ozone were found, it would take 100 or more years for full recovery, since diffusion of chlorofluorocarbons to the stratosphere from the troposphere is a slow process. The only loss mechanism of chlorofluorocarbons is the photolysis in the stratosphere, production of HCI, diffusion back to the troposphere, and rainout. 

Only a small percentage of the chlorine released by photolysis of CFCs is present in the active forms as Cl or CIO, however. Most of it is bound up in reservoir compounds such as hydrogen chloride and chlorine nitrate, formed respectively by hydrogen abstraction (equation 10) from methane and addition (equation 11) to nitrogen dioxide. Slow transport of these reservoir species across the tropopause, followed by dissolution in tropospheric water and subsequent rain-out, provide sink processes for stratospheric chlorine. 

Photochemical production of ozone in the troposphere occurs from the photolysis of nitrogen dioxide (N02) during the daytime, producing oxygen atoms (O) ... 

Nitrogen dioxide photolysis is a key driver of tropospheric atmospheric chemistry since it leads directly to the production and eventual consumption of ozone during the daytime as follows ... 

Nitrogen oxides play an important role in the photochemistry of the troposphere, controlling the formation of tropospheric O3, affecting the concentration of the hydroxyl radical, and contributing to acid precipitation. Nitrogen dioxide (NO2) is one of the most important reactive nitrogen species its photolysis is the primary source of O3 in the troposphere (see Section 28.6 for further details on the actual chemical processes and rates). 

The formation of oxygen atom 0( P) in the photolysis of nitrogen dioxide (NO2) is the fundamental reaction that causes direct production of O3 in the troposphere. In this section, absorption spectrum and 0( P) production quantum yields relevant to the tropospheric photochemistry are described. 

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