Photochemical formation of smog

In metropolitan cities, with a lot of industries of different kinds and dense automobile traffic, the atmosphere is polluted by the smoke coming out from the chimneys of the factories and exhaust of the petroleum fuel driven automobiles. Smog is the combination of smoke and fog, which undergoes a lot of photochemical changes. The photochemical smog so formed, have high concentration of ozone and nitrogen dioxide. 

This reaction makes depletion of ozone which is regenerated by the reaction with 0-atoms. 

Hydrocarbons and other organic compounds in the atmosphere are assumed to oxidize through a series of steps of photochemical reactions. They do not react directly with sun-light, but react with photochemically formed species. The hydrocarbons identified in atmosphere are methane, ethane, ethene, ethylene, propane, n-butane, isopentane, etc. Hydrocarbons are removed from the atmosphere by several chemical and photochemical reactions. For instance, oxidation, to convert them into CO2, acids and aldehydes are formed. 

The automobile pollutants in the atmosphere are exposed to intense sunlight, which yields photochemical oxidants. This phenomenon give rise to photochemical smog. Three specific eye irritants have been already identified in the photochemical smog— Formaldehyde, acrolein and peroxyactyl nitrate (PAN). The possible reaction sequence is as follows  

There are certain serious bad-effects of smog on human-life, which are already recognized and mentioned below  

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VOCs - A VOC is any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metal carbides or carbonates and ammonium carbonate, which participate in atmospheric photochemical reactions1. VOCs are precursors to ground-level ozone production and various photochemical pollutants and are major components in the formation of smog through photochemical reactions2,3. There are many sources of VOCs, as will be discussed later. 

Phase diagram. A diagram showing the conditions at which a substance exists as a solid, liquid, or vapor. (11.9) Photochemical smog. Formation of smog by the reactions of automobile exhaust in the presence of sunlight. (17.6) Photoelectric effect. A phenomenon in which electrons are ejected from the surface of certain metals exposed to light of at least a certain minimum frequency. (7.2)... 

Chemistry of air pollution, (e.g., photochemical origin of smog acid rain discovery of the relevance of biogenic emissions aerosol chemistry, formation, and microphysics)... 

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. 

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). 

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. 

Nitrogen dioxide in the atmosphere undergoes the same reaction and contributes to the formation of acid rain. It also initiates a complex sequence of smog-forming photochemical reactions. 

Urban smog. Urban smog is commonly found in modem cities especially where air is trapped in a basin. It is observable as a brownish colored air. The formation of urban smog is through complex photochemical reactions that can be characterized by ... 

Research dating back to the mid 1950 s has shown that volatile orgamc compounds (VOC s) photochemically react m the atmosphere and contribute to the formation of ground level ozone, a precursor to smog [1]. Medical studies have shown that human exposure to ozone can result in eye and smus tract irritation, and can lead to respiratory related illnesses [2]. Due to the unique and severe smog problems that affected many cities in the state of California, studies of the causes of ah pollution were initiated m the 1950 s [3]. Based on its findings, California formed the Motor Vehicle Pollution Control Board m 1960 to regulate pollution from automobiles. 

Nitric oxide is the primary nitrogen oxide emitted from most combustion sources. The role of nitrogen dioxide in photochemical smog has already been discussed. Stringent emission regulations have made it necessary to examine all possible sources of NO. The presence of N20 under certain circumstances could, as mentioned, lead to the formation of NO. In the following subsections the reaction mechanisms of the three nitrogen oxides of concern are examined. 

The following reaction is involved in the formation of photochemical smog. 

Despite uncertainties concerning the causative agents and their effects, we must proceed with the regulation of emissions that lead to the formation of photochemical smog. At the same time, research should continue on identifying the individual harmful agents in photochemical smog and... 

Smog chamber studies have documented similar aerosol growth mechanisms. For example, in the photochemical oxidation of dimethyl sulfide, the formation and growth of particles in an initially particle-free system was observed. However, if seed particles with 34-nm mean size were present, an oscillation in the... 

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