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FIGURE 1.3 Diurnal variation of NO, N02, and total oxidant in Pasadena, California, on July 25, 1973 (adapted from Finlayson-Pitts and Pitts, 1977).

centers, the profiles are shifted and 03 may peak in the afternoon, or even after dark, depending on emissions and airshed transport phenomena. Thus, although 03 is no longer formed after sunset, a dirty, urban air mass containing 03 and other secondary pollutants formed during the day can be transported many kilometers downwind to an otherwise relatively clean rural site.

In the early 1950's, soon after the new phenomenon of photochemical air pollution had been reported, the fundamental chemistry responsible for many of these general features began to be established. Thus, as first suggested by F. E. Blacet in 1952, photodissociation of N02 in air was shown to form 03 (Blacet, 1952):

Reaction (2) still remains the sole significant source of anthropogenically produced ozone.

The nitric oxide formed in reaction (7) was also shown to react relatively rapidly with 03, re-forming N02:

Because of reaction (8), significant concentrations of 03 and NO cannot co-exist, and the delay in the oxidant (03) peak until NO has fallen to low concentrations, shown in Fig. 1.3, is explained.

Three major questions on the overall atmospheric chemistry of photochemical "smog," not readily answered in the early studies, are:

• What is the role played by organics?

• What reactions are responsible for the rapid loss of organics?

It was first suggested in the 1950's that NO was thermally oxidized by 02:

Indeed, in the laboratory, at Torr concentrations, the clear, colorless NO is oxidized in air virtually instantaneously to dark, red-brown N02.

However, the rate of this reaction is second order in NO; that is, the speed of oxidation increases as the square of the NO concentration. Thus when one lowers the NO from high (Torr) concentrations to ambient part per trillion (ppt) or part per billion (ppb) levels (ppt = parts in 1012; ppb = parts in 109), the speed of the oxidation drops to the point where the rate is very small. For example, at 100 Torr NO 1.3 x 105 ppm), about 85% of the NO is oxidized in ~ 15 s. However, at 100 ppb NO, approximately 226 days would be required to achieve the same net oxidation! As a result, the so-called thermal (i.e., nonphotochemical) oxidation of NO by reaction (9) is generally too slow to be of importance in the atmosphere.

One exception to this generalization is the case where high concentrations of NO (e.g., several thousand ppm) may be emitted from sources such as uncontrolled power plants. In the initial seconds as the plume enters the atmosphere before it has had a chance to become completely diluted with the surrounding air, the NO may be sufficiently concentrated that the oxidation (9) by 02 is significant. For example, at 2000 ppm NO, 90% of the reactant would be oxidized to NOz within 30 min if this high concentration were to be maintained for that long. The plume integrity is generally not maintained for this period of time; however, under some meteorological conditions, the plume can be sufficiently stable that a significant fraction of the NO can undergo thermal oxidation by 02, and N02 can be directly formed many kilometers from the stack.

In summary, it soon became evident that in ambient photochemical smog, the thermal oxidation of NO could not explain the relatively rapid conversion of NO to N02.

With respect to the role of the organics, it was suggested about 1969-1970 that the hydroxyl radical drives the daytime chemistry of both polluted and clean atmospheres (Heicklen et al., 1969; Weinstock, 1969; Stedman et al., 1970; Levy, 1971). Thus, OH initiates chain reactions by attack on VOC or CO. These chains are then propagated through reactions such as those in Fig. 1.4. In this cycle, the organic is oxidized to a ketone, two molecules of NO are converted to NOz, and OH is regenerated. Of course, the ketone can then photodissociate into free radicals or itself be attacked by OH, and a similar cycle occurs, leading to further NO oxidation.

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