Nitrogen dioxide-hydrocarbon interaction

In photochemical smog episodes, secondary air pollutants such as ozone, nitrogen dioxide, aldehydes, and peroxyacetyl nitrate are formed as a result of the chemical interaction of the primary air pollutants, principally nitric oxide and hydrocarbon vapors, with sunlight and air (Fig. 2.6) [49]. This interpretation of the processes involved has been verified by smog chamber experiments (Fig. 2.7), and has since been confirmed by field measurements as the sensitivity of ambient air instrumentation has improved [50, 51]. In photochemical smog episodes, it is the secondary pollutants that cause severe eye irritation and upper respiratory effects felt by people and at the same time causes serious damage to plants. 

Sulfur dioxides, hydrocarbons, nitrogen oxides, and particulate matter such as sand, dust, dirt and soot are known to be common air pollutants. These atmospheric pollutants may cause degradation in combination with solar radiation and other weather factors. Sulfur dioxide is a pollutant that arises from industrial sources and soot is acidified by the interaction with moisture and other particles in the presence of sulfur dioxide and oxygen. 

The lifetime of these species in the atmosphere is relatively short and if they were distributed evenly their harmful effects would be minimal. Unfortunately these man-made effluents are usually concentrated in localized areas and their dispersion is limited by both meteorological and topographical factors. Furthermore, synergistic effects mean that the pollutants interact with each other in the presence of sunlight, carbon monoxide, nitrogen oxide(s), and unburned hydrocarbons lead to photochemical smog, while when sulfur dioxide concentrations become appreciable, sulfur oxide-based smog is formed. 

It is nearly 40 years since the first catalytic devices were commercially produced and fitted into cars, after the recognition that car exhaust primary pollutants, that is, unburned hydrocarbons (HCs), nitrogen oxides (NO ), and carbon monoxide (CO), interact with sun light resulting in the formation of secondary pollutants (e.g., ozone, oxygenated hydrocarbons and hydrocarbon radicals, peroxyacetyl nitrate PAN, and nitric dioxide), which are responsible for the photochemical smog in capital cities [1]. The phenomenon had become of such a concern in almost all the big cities, that forced environmental legislation firstly introduced in 1970 by the US Clean Air Act (US-CAA), and practically applied in 1975 [2]. 

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