Photochemical Urban Atmospheric Pollution

Photochemical Urban Atmospheric Pollution.—Many of the reactions discussed in the headings above are of importance in an understanding of photochemically induced atmospheric pollution in urban situations, and are also important in stratospheric aeronomy. The remote sensing of airborne pollutants has been covered in Section 11, but we present here briefly the subject of some papers of general interest to atmospheric pollution. 

Many papers have been concerned with the mechanism of photochemical air pollution and its modelling,399 and reports on photochemical smog from 390 D. Siebert and G. Flynn, J. Chem. Phys., 1975, 62, 1212. 

Incorvia and Zink14 have used ligand field theory to interpret the photosolvation quantum yields for d9 complexes of Civ and Dlh symmetry. The theory 

18 Prevrashch. Kompleks. Soedin Deistviem Sveta, Radiats. Temp. , ed. G. A. Lazerko, Izd. Beloruss. Gos. Univ., 1973. 

Models for photoredox reactions of transition-metal ammine complexes have been critically examined.1 The role of the solvent is stressed in particular it will participate in the processes involved in the relaxation of the Franck-Condon excited state to the primary radical pair products. 

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Benzene in the atmosphere exists predominantly in the vapor phase (Eisenreich et al. 1981). The most significant degradation process for benzene is its reaction with atmospheric hydroxyl radicals. The rate constant for the vapor phase reaction of benzene with photochemically produced hydroxyl radicals has been determined to be 1.3 10"12 cm3/molecule-second, which corresponds to a residence time of 8 days at an atmospheric hydroxyl radical concentration of 1.1 x 106 molecules/cm3 (Gaffney and Levine 1979 Lyman 1982). With a hydroxyl radical concentration of 1 x 108 molecules/cm3, corresponding to a polluted atmosphere, the estimated residence time is shortened to 2.1 hours (Lyman 1982). Residence times of 472 years for rural atmospheres and 152 years for urban atmospheres were calculated for the reaction of benzene with ozone (03) using a rate constant for 03 of 7 /1 O 23 cm3/molecule-second (Pate et al. 1976) and atmospheric concentrations for 03 of 9.6/1011 molecules/cm3 (rural) and 3/ 1012 molecules/cm3 (urban) (Lyman 1982). 

This section is devoted to the results of some theoretical studies on solar radiation, absorption rates, and primary photochemical processes in smog, undertaken by Leighton (18). In work sponsored by the Air Pollution Foundation, Leighton made a critical analysis of the chemical effects that sunlight and sky radiation may have on smog formation in urban atmospheres. The radiant energy available for photochemical... 

We saw in Section 3.6.1 that the acid-laden smoke particles in the London atmosphere caused great harm to human health in the past. Pollutants in the atmosphere still cause concern because of their effect on human health, although today we need to consider a wider range of potentially harmful trace substances. The photochemical smog encountered ever more widely in modern cities gives urban atmospheres that are unlike the smoky air of cities in the past. Petrol as a fuel, unlike coal, produces little smoke. 

Benzene (see Fig. 2.4) is another pollutant component of automotive fuels. It occurs naturally in crude oil and is a useful component because it can prevent pre-ignition in unleaded petrol (the production process is usually adjusted so that the benzene concentration is about 5%). There is evidence that in some locations, where there has been a switch to fuels with high concentrations of aromatic hydrocarbons, there has been a sharp increase in photochemical smog. This is due to the high reactivity of these hydrocarbons in the urban atmosphere. This problem should draw our attention to the way in which the solution of one obvious environmental problem (lead from petrol) may introduce a second rather more subtle problem (i.e. increased photochemical smog from reactive aromatic compounds). 

Urban air pollution remains an issue of much public concern. While it is true that in many cities the traditional problems of smoke and S02 from stationary sources are a thing of the past, new problems have emerged. In particular, the automobile and heavy use of volatile fuels have made photochemical smog a widespread occurrence. This has meant that there has been a parallel rise in legislation to lower the emission of these organic compounds to the atmosphere. 

Ozone is a specific atmospheric pollutant characteristic of urban and some industrial areas of the troposphere. Its concentration is very variable. Because of both the photochemical character of the origin of ozone and its high reactivity with organic atmospheric pollutants as well as some organic materials on the Earth s surface, its night concentration drops to zero. Only ground state molecular oxygen attacks rubber in this period of the day, but mechanically initiated processes and reactive ozonation intermediates and products remain involved. 

The above mentioned urban air pollution in Asian cities drives the tropospheric chemical reactions. This tropospheric chemistry is dominated by the oxidation of trace atmospheric components, as aresult ofwhich organic compounds such as methane and other hydrocarbons are converted into carbon dioxide and water. The consequences of these chemical transformations are known as photochemical smog (photosmog) and the associated problem of ground level ozone. Here we should consider also the effects of particulate matter, one of the major pollutants of urban air in Asia. 

Because PAN is in thermal equilibrium with NO2 and the peroxyacetyl radical, it can act as a means of transporting these more reactive species over long distances. The NO2 released by thermal decomposition of PAN is photolyzed rapidly in the troposphere to form O3 by Reaction 19.1 and Reaction 19.2. Ozone is a criteria air pollutant and is a major health concern. Thus, the PANs play important roles as a chemical means of transporting key species such as NO2 and formaldehyde to remote locations. As such, PANs are globally important atmospheric molecules, as well as urban air pollutants. Since the original observation of PANs in Los Angeles photochemical smog, PANs have been measured in every corner of the world. 

Once thought to be of importance only in polluted urban atmospheres, PANs are now recognized to be ubiquitous, having been detected in urban, rural, and global environments (Roberts, 1990). By virtue of their photochemical inertness, relative insolubility in water, and low OH rate constant, PANs can have an appreciable atmospheric lifetime. The principal loss mechanism is thermal decomposition by reaction 5.91 or 5.95 back to the peroxy-acyl radical and NO2. The thermal decomposition is highly temperature dependent at temperatures of the upper troposphere PANs are quite stable and can be transported long distances. 

In this paragraph, dependence of O3 production on NO and VOC concentrations under the conditions of polluted urban atmosphere is summarized referring to the photochemical smog chamber experiments (see Column 2 p. 317). The almost sole reaction of direct O3 formation in the troposphere is the reaction of O2 with the ground state oxygen atom 0( P) from the photolysis of NO2,... 

Oxidation of SO2 to H2SO4 by free radicals present in the polluted urban atmosphere (particularly photochemical)... 

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

Certainly, photochemical air pollution is not merely a local problem. Indeed, spread of anthropogenic smog plumes away from urban centers results in regional scale oxidant problems, such as found in the NE United States and many southern States. Ozone production has also been connected with biomass burning in the tropics (79,80,81). Transport of large-scale tropospheric ozone plumes over large distances has been documented from satellite measurements of total atmospheric ozone (82,83,84), originally taken to study stratospheric ozone depletion. 

Nitrogen Dioxide (NO2) Is a major pollutant originating from natural and man-made sources. It has been estimated that a total of about 150 million tons of NOx are emitted to the atmosphere each year, of which about 50% results from man-made sources (21). In urban areas, man-made emissions dominate, producing elevated ambient levels. Worldwide, fossil-fuel combustion accounts for about 75% of man-made NOx emissions, which Is divided equally between stationary sources, such as power plants, and mobile sources. These high temperature combustion processes emit the primary pollutant nitric oxide (NO), which Is subsequently transformed to the secondary pollutant NO2 through photochemical oxidation. 

A reaction of ozone provides an example of concentration effects. Ozone in the atmosphere near the Earth s surface is a serious pollutant that damages soft tissues such as the lungs. In major urban areas, smog alerts are issued whenever there are elevated concentrations of ozone in the lower atmosphere. Nitmgen oxide, another component of photochemical smog, is a colorless gas produced in a side reaction in automobile engines. One reaction that links these species is the reaction of NO and O3 to produce O2 and NO2 ... 

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