A Oxidation Of No To Noz And The Leighton Relationship

In the early 1950s, the major "ingredients" in photochemical air pollution had been identified by Haagen-Smit and co-workers as VOC and NOx, and the photolysis of N02 had been identified by Blacet as the source of the high ozone levels (see Chapter 1.B.3). Initially, the atmospheric conversion of emitted NO to N02 was thought to be due to its reaction with 02:

30 40 VOC/NOx

FIGURE 7.2 Ratio of final concentrations of PAN to HNO, (PAN/HN03) versus initial VOC/NOt in a series of smog chamber experiments. The HN03 includes both that in the gas phase and that estimated to be adsorbed on chamber walls (from Spicer, 1983).

30 40 VOC/NOx

FIGURE 7.2 Ratio of final concentrations of PAN to HNO, (PAN/HN03) versus initial VOC/NOt in a series of smog chamber experiments. The HN03 includes both that in the gas phase and that estimated to be adsorbed on chamber walls (from Spicer, 1983).

Indeed, this reaction can be easily demonstrated by mixing relatively high (i.e., ~Torr) concentrations of NO with air. The colorless NO is rapidly converted to the brown-orange N02, and one can feel the reaction vessel warm as the reaction exothermicity is released.

Despite the fact that reaction (1) is often cited, erroneously, as responsible for the NO to NOz conversion in the atmosphere, elementary reaction kinetics can be used to demonstrate that this cannot be the case. Even in a highly polluted atmosphere, the conversion of NO to N02 occurs over a period of several hours. Reaction (1) is kinetically second order in NO in both the gas and liquid phases (e.g., DeMore et al., 1997; Lewis and Deen, 1994). Following the conventions discussed in Chapter 5.A.1, the rate law for reaction (f) can be written as follows:

As a result of the reaction being second order in NO, the rate of oxidation of NO to N02 decreases by a factor of fOO as the NO concentration falls by a factor of 10.

The recommended value of the third-order rate constant at room temperature is kt = 2.0 X 10~3X cm6 molecule"2 s~ 1 (Atkinson et al., 1997a). At 1 Torr NO, for example, the initial rate of oxidation in 1 atm air is about 40% per minute, whereas at 1 mTorr (1.3 ppm at 1 atm, 298 K), it is only 0.04% per minute (see also Problem 1). Thus, at a concentration of NO of even 0.1 ppm, found as a peak concentration in some polluted areas, the rate is too slow to be consistent with observed conversion to N02 on a time scale of hours.

However, the second-order nature of reaction (I) does provide a qualitative diagnostic for emissions of NO from certain power plants, smelters, etc. Occasionally, an orange-brown plume characteristic of NOz can be seen starting a short distance above the stack exit. In such a case, the concentration of the NO exiting the stack is sufficiently high that it is being rapidly oxidized to NOz by 02 via reaction (1) (see Problem 2). At lower concentrations, oxidation of the NO by 03 at the edges of the plume can also be important, giving oxidation rates as high as 20% per minute (e.g., see Cheng et al., 1986).

In most cases, however, such a plume is not visible because the NO concentrations are sufficiently small that reaction (f) is very slow. Once this was recognized in the f950s, the puzzle was to identify the reactions responsible for the NO to N02 conversion. As discussed in Chapters l.B and 6, it is now known that hydroperoxy and alkylperoxy free radicals are the oxidizing agents:

Sources of H02 and R02 are discussed in Chapter 6.

In a hypothetical atmosphere containing only NO, N02, and air, that is, no organics, the reactions controlling the concentrations of NO and NOz are (4), (5), and (6):

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