Photolysis in the Stratosphere

In this section, absorption spectra and cross sections, and photolytic processes of atmospheric molecules that are not photolyzed by the solar actinic flux in the troposphere and photolyzed only in the stratosphere are described. Photolyses of many inorganic halogen molecules, which are important in the stratosphere, are 

Wave-length 10 o (cm Wave-length 10 o (cm Wave-length 10 o (cm Wave-length 10 o (cm  

Source NASA/JPL Panel Evaluation No. 17 (Sander et al. 2011) (Data for 336-348 nm are smoothed) 

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Environmental Fate. The fate of bromomethane in the environment is dominated by rapid evaporation into air, where it is quite stable (EPA 1986b). The rates of volatilization from soil and water have been studied and are known with reasonable precision (although such rates are typically site-specific) (Jury et al. 1984 Lyman et al. 1982). The rates of breakdown by hydrolysis, reaction with hydroxyl radical, and direct photolysis in the stratosphere have also been estimated (Castro and Belser 1981 Davis et al. 1976 Robbins 1976). Further studies to improve the accuracy of available rate constants for these processes would be helpful, but do not appear to be essential in understanding the basic behavior of bromomethane in the environment. 

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. 

Minschwaner, K., and D.E. Siskind, A new calculation of nitric oxide photolysis in the stratosphere, mesosphere, and lower thermosphere. J Geophys Res 98, 20,401, 1993. 

CC12F2 (FI 2) 305 285b 5.2 -150 Anthropogenic Photolysis in the stratosphere Rising at a rate of 26 pptv/yr... 

C2C13F3 (FI 13) 23 2lb 0.6 -100 Anthropogenic Photolysis in the stratosphere Growth rate estimated as about 15% annually... 

PROBABLE FATE photolysis could be important, photooxidation half-life in water 54.1-541 days, direct photolysis in the stratosphere may occur, but is insignificant in the troposphere, reaction with photochemically produced hydroxyl radicals yields a half-life of 1.45 yrs oxidation atmospheric photooxidation by hydroxyl radicals to COBT2 is relatively rapid hydrolysis too slow to be important, first-order hydrolytic half-life 687 yrs volatilization volatilization has been demonstrated, could be an important transport process, volatilization from moist soil surfaces expected to occur sorption no information is available biological processes slight potential for bioaccumulation/metabolization is known to occur in some organisms other reactionsAnteractions possibly produced by halogen reaction... 

At steady state the emissions of a species into the atmosphere is just balanced by its rate of removal. The global mean abundance (expressed, say, in Tg) divided by the emission rate (Tg yr ) is equal to the global mean lifetime (yr). The inverse of the overall global mean lifetime is the sum of the inverse lifetimes of each of the removal processes. For a substance that is removed from the atmosphere by reaction with OH radicals, photolysis in the stratosphere, uptake by oceans, destruction at the land surface, and precipitation (washout). 

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