No2 o2

The Leighton relationship, named after Philip Leighton, who wrote the first definitive monograph on air pollution in 1961, is given by

no 31 k4

According to this relationship, also referred to frequently as the "photostationary state," the ratio of concentrations of 03, NO, and NOz should be a constant given by the ratio of rate constants for photolysis of N02 and for the reaction of NO with 03. Since [usually referred to as /cp(NOz)] changes with the solar zenith angle, this ratio of concentrations is also expected to change during the day.

In general, the Leighton relationship is expected to hold when reactions (4) and (6) are the major loss processes for N02 and 03 [reaction (5) is essentially always the loss process for 0(3P)]. Under such circumstances, the Leighton relationship can be used in computer models of tropospheric chemistry to minimize computation time. Thus, instead of carrying out numerical integration procedures separately to obtain [03], [NO], and [NOz], if two of the three concentrations are known, one can obtain the third by using Eq. (A).

Figure 7.3 illustrates a test of Eq. (A) carried out by measuring NO, N02, and 03 simultaneously (Ritter et al., 1979). Rearranging Eq. (A), one obtains

Thus, if 03, NO, and N02 are measured simultaneously, with the values of k6 and k4 being well known, the left-hand side of Eq. (B) can be calculated and compared to the value of unity, which is expected if the Leighton relationship holds. Alternatively, the logarithm of the left-hand side should be zero. Figure 7.3 shows the logarithm of the left side of Eq. (B) as a function of time over a 2-h period midday in a rural

FIGURE 7.3 Test of photostationary state in rural Michigan on June 4, 1977, 11:30 to 13:30 hours (adapted from Ritter et al., 1979).

FIGURE 7.3 Test of photostationary state in rural Michigan on June 4, 1977, 11:30 to 13:30 hours (adapted from Ritter et al., 1979).

area in Michigan (U.S.) (Ritter et al., 1979). The values do indeed lie close to zero, although deviations from this are clearly seen. This is not unexpected, given the assumptions made in deriving Eqs. (A) and (B).

Deviations occur when the conversion of NO to N02 occurs by reactions other than that with 03, i.e., by reactions (2) and (3) with HOz and R02 (e.g., Parrish et al., 1986; Carroll and Thompson, 1995); Eq. (A) then becomes

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