Troposphere reduced

For completeness the stratospheric inventory is also plotted in Fig. 19. The value of stratospheric sulfur burden is based on data for 1971-1973 as reported by Karol (1977). The term representing the sedimentation of sulfate particles from the stratosphere to the troposphere is estimated by assuming a 1 year stratospheric residence time (Junge, 1963). Moreover, the arrow going from the tropospheric-reduced S reservoir to the S04—S one in the stratosphere represents the possibility, proposed by Crutzen (1976), that COS may be a source of stratospheric S02. 

When NMHC are significant in concentration, differences in their oxidation mechanisms such as how the NMHC chemistry was parameterized, details of R02-/R02 recombination (95), and heterogenous chemistry also contribute to differences in computed [HO ]. Recently, the sensitivity of [HO ] to non-methane hydrocarbon oxidation was studied in the context of the remote marine boundary-layer (156). It was concluded that differences in radical-radical recombination mechanisms (R02 /R02 ) can cause significant differences in computed [HO ] in regions of low NO and NMHC levels. The effect of cloud chemistry in the troposphere has also recently been studied (151,180). The rapid aqueous-phase breakdown of formaldehyde in the presence of clouds reduces the source of HOj due to RIO. In addition, the dissolution in clouds of a NO reservoir (N2O5) at night reduces the formation of HO and CH2O due to R6-RIO and R13. Predictions for HO and HO2 concentrations with cloud chemistry considered compared to predictions without cloud chemistry are 10-40% lower for HO and 10-45% lower for HO2. 

SinghHB. 1977. Atmospheric halocarbons Evidence in favor of reduced average hydroxyl radical concentration in the troposphere. Geophys Res Lett 4 101-104. 

The concentration of ozone near the Earth s surface is very low, typically in the range of 15-45 pphv (parts per billion by volume). In contrast, ozone is more abundant in the Earth s stratosphere, where it is formed by the action of ultraviolet radiation on molecules of dioxygen. A distinction is sometimes made between stratospheric ozone ("good ozone") and tropospheric (low-level or surface ozone "bad ozone"). This distinction arises from the fact that stratospheric ozone reduces the amount of ultraviolet radiation that reaches the Earth, reducing the rate of skin cancer and other medical problems... 

Table 8.17 summarizes the rate constants and estimated tropospheric lifetimes of some of these sulfur compounds with respect to reaction with OH or NO-,. The assumed concentrations of these oxidants chosen for the calculations are those characteristic of more remote regions, which are major sources of reduced sulfur compounds such as dimethyl sulfide (DMS). It is seen that OH is expected to be the most important sink for these compounds and that NO, may also be important, for example, for DMS oxidation (see also Chapter 6.J). 

As a result, we focus here on what is known about the tropospheric chemistry of DMS. As we shall see, the chemistry of even this relatively simple compound is complex, and much remains to be learned about its reaction mechanisms. For larger reduced sulfur com-... 

Toumi et al. (1994) also suggested there is a feedback between reduced stratospheric ozone and particles in that the increased UV due to ozone depletion may increase sulfate particle formation by increasing the concentrations of tropospheric OH. 

One of the approaches to the development of alternate CFCs is to use compounds with one or more abstractable hydrogen atoms so that their tropospheric lifetimes are reduced and less reaches the stratosphere. 

Understanding the chemical and physical processes discussed throughout this book is key to the development of cost-effective and health-protective air pollution control strategies. Application of atmospheric chemistry to reducing stratospheric ozone depletion was discussed in Chapter 13. Here we focus on its key role in strategies for controlling tropospheric pollutants, including ozone, acids, particles, and hazardous air pollutants. 

Numerous studies have been conducted to determine the effect of chemicals, in particular solvents, into the atmosphere. Information is learned from studies, but conclusions are often incomplete, due to the lack of a fully rigorous and universally apphcable model. The observed reductions in stratospheric ozone since the late 1970s are likely to have affected the penetration of UV radiation into the troposphere. Ozone is reduced in most areas when the UV radiation increases, but the percentage reductions in ozone are significantly smaller than the percentage UV increases (Fuglestvedt et al., 1994). 

Thus, rather surprisingly, the key to reducing tropospheric ozone pollution is to minimize the release of hydrocarbon vapors from such sources as unburnt fuel in automobile exhausts, vaporization of fuel at service station pumps (modern pumps recover fuel vapors from automobile fuel tanks while refilling them), kitchen exhausts from fast-food restaurants and, as in Mexico City, leakages of liquefied petroleum gas (mainly butane) used for domestic heating and cooking.26... 

Photochemical air pollution in the troposphere results from a complex interplay between sunlight and primary air pollutants emitted in ambient air that leads to the formation of ozone and other oxidizing and cye-irritaling agents. On the other hand, pollutants injected into the stratosphere by such human activities as supersonic transports (SST s) and release ofchlorofiuoro-methancs in air by their use as aerosol propellants and refrigerants may eventually reduce the protective layer of ozone from harsh solar ultraviolet radiation. Although the full impact of injected air pollutants in the stratosphere is not apparent at present, various model calculations show conclusively that the continuous future release of chlorofluoromethanes and NO (NO and N02) would result in substantial reduction of ozone in the stratosphere. 

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