Photochemical smogs

Equation (7.10)] by atmospheric hydrocarbons is the principal cause of photochemical smog. 

It is noticeable that the process starts with an OH radical. After one pass through the cycle, two molecules of NO are oxidized to N02. The OH radical formed in the last step [Equation (7.15)] can start the cycle again. On the other hand, 03 can also be formed from 02 reacting with hydrocarbon free radicals  

The hydrocarbon free radicals (e.g. R02) formed can react further with different species including NO, N02,02,03 and other hydrocarbons. Thus, 

The free radical R02 can react with 02 and N02 to produce peroxyacyl nitrate (PAN)  

Peroxyacyl nitrate can also be formed from a reaction involving RO3 and 

The word smog was coined to describe the combination of smoke and fog that shrouded London during the 1950s. The primaiy cause of this noxious cloud was sulfur dioxide. Today, however, photochemical smog, which is formed by the reactions of automobile exhaust in the presence of sunlight, is much more common. 

Automobile exhaust consists mainly of NO, CO, and various unbumed hydrocarbons. These gases are called prunary pollutants because they set in motion a series of photochemical reactions that produce secondary pollutants. It is the secondary pollutants—chiefly NO2 and O3— that are responsible for the buildup of smog. 

Nitric oxide is the product of the reaction between atmospheric nitrogen and oxygen at high temperatures inside an automobile engine  

Once released into the atmosphere, nitric oxide is oxidized to nitrogen dioxide  

Sunlight causes the photochemical decomposition of NO2 (at a wavelength shorter than 400 nm) into NO and O  

Electrostatic precipitators and scrubbers (see Charge them up and drop them out Electrostatic precipitators and Washing water Scrubbers, later in this chapter), combined with filters, have been effective in reducing the release of soot, ash, and sulfur dioxide into the atmosphere and have reduced the occurrence of London smog. 

Photochemical smog is produced after sunlight initiates certain chemical reactions involving unbumed hydrocarbons and oxides of nitrogen (commonly shown as NOx — which stands for a mixture of NO and NO ). The common automobile engine produces both of these compounds when it s running. 

The chemistry of photochemical smog is still not crystal clear (pun intended), but scientists do know the basics that go into creating the smog. Nitrogen from the atmosphere is oxidized to nitric oxide in internal combustion engines and then released into the atmosphere through the engines exhaust systems  

Nitrogen dioxide is a brownish gas. It s irritating to the eyes and lungs. It absorbs sunlight and then produces nitric oxide and highly reactive oxygen atoms  

Reaction 7.24 is illustrative of abstraction reactions in which an atom of H is abstracted from a hydrocarbon. The hydroxyl radical product of this reaction, HO-, is very active in carrying out abstraction reactions. Even faster addition reactions are possible with alkene hydrocarbons that have unsaturated C=C bonds. For example, hydroxyl radical adds to a molecule of propene 

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Benefits depend upon location. There is reason to beheve that the ratio of hydrocarbon emissions to NO has an influence on the degree of benefit from methanol substitution in reducing the formation of photochemical smog (69). Additionally, continued testing on methanol vehicles, particularly on vehicles which have accumulated a considerable number of miles, may show that some of the assumptions made in the Carnegie Mellon assessment are not vahd. Air quaUty benefits of methanol also depend on good catalyst performance, especially in controlling formaldehyde, over the entire useful life of the vehicle. 

Reduction of metal oxides with hydrogen is of interest in the metals refining industry (94,95) (see Metallurgy). Hydrogen is also used to reduce sulfites to sulfides in one step in the removal of SO2 pollutants (see Airpollution) (96). Hydrogen reacts directiy with SO2 under catalytic conditions to produce elemental sulfur and H2S (97—98). Under certain conditions, hydrogen reacts with nitric oxide, an atmospheric poUutant and contributor to photochemical smog, to produce N2 ... 

Three different types of chemical mechanisms have evolved as attempts to simplify organic atmospheric chemistry surrogate (58,59), lumped (60—63), and carbon bond (64—66). These mechanisms were developed primarily to study the formation of and NO2 in photochemical smog, but can be extended to compute the concentrations of other pollutants, such as those leading to acid deposition (40,42). 

G. Z. Whitten and co-workers. Modeling of Simulated Photochemical Smog with Kinetic Mechanism, Vols. I and II, Report No. EPA-600/3-79-001a, U.S. Environmental Protection Agency, Research Triangle Park, N.C., 1979. 

Nitrogen Oxides. From the combustion of fuels containing only C, H, and O, the usual ak pollutants or emissions of interest are carbon monoxide, unbumed hydrocarbons, and oxides of nitrogen (NO ). The interaction of the last two in the atmosphere produces photochemical smog. NO, the sum of NO and NO2, is formed almost entkely as NO in the products of flames typically 5 or 10% of it is subsequently converted to NO2 at low temperatures. Occasionally, conditions in a combustion system may lead to a much larger fraction of NO2 and the undeskable visibiUty thereof, ie, a very large exhaust plume. 

Materials The damage that air pollutants can do to some materials is well known ozone in photochemical smog cracks rubber, weakens fabrics, and fades dyes hydrogen sulfide tarnishes silver smoke dirties laundry acid aerosols ruin nylon hose. Among the most important effects are discoloration, corrosion, the soiling of goods, and impairment of visibility. 

Fig. 8-3. Relationship between Los Angeles Basin s urban sources of photochemical smog and the San Bernardino Mountains, where ozone damage has occurred to the ponderosa pines. The solid lines are the average daily 1-hr maximum dose of ozone (ppm), )uly-September 1975-1977. Source Adapted from Davidson, A., Ozone trends in the south coast air basin of California, in "Ozone/Oxidants Interaction with the Total Environment.". A ir Pollution Control Association, Pittsburgh, 1979, pp. 433-450.

Table 12-4 are sufficient for an understanding of smog chemistry. The major undesirable components of photochemical smog are NOj, O3, SOj, H2SO4, PAN, and aldehydes. Air quality standards have been established in several countries for SO2, NO2, and O3 H2SO4 contributes to acidic deposition and reduction in visibility and PAN and aldehydes can cause eye irritation and plant damage if their concentrations are sufficiently high. 

The important hydrocarbon classes are alkanes, alkenes, aromatics, and oxygenates. The first three classes are generally released to the atmosphere, whereas the fourth class, the oxygenates, is generally formed in the atmosphere. Propene will be used to illustrate the types of reactions that take place with alkenes. Propene reactions are initiated by a chemical reaction of OH or O3 with the carbon-carbon double bond. The chemical steps that follow result in the formation of free radicals of several different types which can undergo reaction with O2, NO, SO2, and NO2 to promote the formation of photochemical smog products. 

Eschenroeder, A. Q., and Martinez, J. R., "Mathematical Modeling of Photochemical Smog," No. IMR- 1210. General Research Corp., Santa Barbara, CA, 1969. 

OZONE A reactive form of oxygen the molecule of which contains 3 atoms of oxygen. In the ozone layer it protects the earth by filtering out ultra-violet rays. At ground level, as a constituent of photochemical smog, it is an imtant and can cause breathing difficulties. 

Photochemical smog Air conditioning/ventilation units Reaction between chemicals and ozone... 

Combustion processes are the most important source of air pollutants. Normal products of complete combustion of fossil fuel, e.g. coal, oil or natural gas, are carbon dioxide, water vapour and nitrogen. However, traces of sulphur and incomplete combustion result in emissions of carbon monoxide, sulphur oxides, oxides of nitrogen, unburned hydrocarbons and particulates. These are primary pollutants . Some may take part in reactions in the atmosphere producing secondary pollutants , e.g. photochemical smogs and acid mists. Escaping gas, or vapour, may... 

Latency refers to the period of time that elapses between the first contact of a harmful agent and a host, and the development of identifiable symptoms or disease. Latency may be as short as a few hours, the time required for photochemical smog to induce watery eyes. Or it may stretch to 20 - 30 years for a chronic condition such as asbestosis or malignant neoplasm of the lung. The association between a gi en e.xposure and a disease is all that more difficult because of the passage of time. 

PBM (Photochemical Box Model) is a simple stationary single-cell model with a variable height lid designed to provide volume-integrated hour averages of ozone and otlier photochemical smog pollutants for an urban area for a single day of simulation. 

The physiological effects of N2O (laughing gas, anaesthetic) and NO2 (acrid, corrosive fumes) have been known from the earliest days, and the environmental problems of NOj from automobile exhaust fumes and as a component in photochemical smog are well known in all industrial countries. 

VOCs react in the presence of sunlight to produce photochemical smog, a mixture of organic chemicals that can irritate the eyes and other mucous membranes. VOCs also constitute a major precursor chemical leading to ozone production. VOC levels across the United States fell, on average, by 20.4 percent between 1989 and 1998. 

Photochemical smogs arise worldwide because of the action of sunlight on emissions from gasoline-powered vehicles. Decreased visibility, increased morbidity, and crop damage as a result of photochemical smogs led to introduction of the catalytic converter on automobiles in the United States. This has had only a small impact on the occurrence of photochemical smogs in the United States. 

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