Caqs

Ozone Formaldehyde Nitric PAN

acid

Formic Total associated acid gaseous pollutants

FIGURE 2.22 Maximum concentrations of some trace pollutants and the percentage of the peak ozone they form when summed. Also shown is the California Air Quality Standard for 03 and the various alert levels (from Tuazon et al, 1981).

Of the gaseous air pollutants, only N02 absorbs visible light to a significant extent and thus contributes to visibility reduction. It is an orange-brown gas that absorbs radiation strongly at A < 430 nm; hence it acts as a filter for blue light. The brownish color of many polluted urban areas and the accompanying spectacular sunsets are at least partly due to the presence of N02. For example, NOz has been found to contribute to the "brown cloud" over Albuquerque, New Mexico (Franzblau et al., 1993). However, the contribution of N02 to the total extinction is generally small, for example, < 10%.

Scattering of light by gaseous air pollutants is negligible compared to the Rayleigh scattering by 02 and N2 because of the very low concentrations of pollutants compared to those of the major components of air.

Thus, particulate matter suspended in air is generally responsible for the majority of light scattering and absorption and hence for visibility reduction associated with air pollution (see Chapter 9). Small particles with diameters of ~ 0.1-1 ¡xm contribute the greatest amount to light scattering and hence visibility reduction is closely related to the fine-particle concentration. The size range that has the potential for maximum health effects also contributes the greatest amount to aesthetic effects. It is believed that scattering by particles is responsible for something in the range of 50-95% of the total extinction due to air pollutants, depending on the conditions.

Elemental carbon is believed to be the major significant light-absorbing species in particles. In remote regions, absorption may only account for ~ 5-10% of the total extinction, whereas in urban areas, its contribution is greater, up to ~ 50%.

In terms of chemical composition, visibility reduction due to particulate matter is generally related most closely to scattering by sulfate and nitrate and absorption and scattering by elemental carbon. Relative humidity (RH) is also an important factor, with a significant reduction in visibility occurring as the RH increases from 50 to 90%.

Effects on materials vary widely, depending on the composition of the material and on the particular air pollutant. For example, rubber products and polymers are especially susceptible to 03, whereas metals such as iron are particularly sensitive to acids. Indeed, Haa-gen-Smit and co-workers (1959) used the cracking of rubber by 03 to measure its concentration in air in the "early" days of photochemical air pollution. A detailed description of the effects of air pollutants on various types of materials is found in the article by Graedel and McGill (1986).

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