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Figure 6.3 shows the ozone production efficiency for CO oxidation at 298 K at the Earth's surface as a function of [NO*] = [NO] + [N02] for NO* mixing ratios from 1 ppt to lOOppb. At the ground-level conditions of Figure 6.3, we assume that pho, = 1 ppts-1, that [N0]/[N02] = 0.1, and a CO mixing ratio of 200 ppb. The OPE is largest at the lowest concentration of NO*; at these low levels, NO* termination by OH + N02 is suppressed and each NO* participates in more 03 production cycles. At 100 ppb NO*, OPE approaches zero, as the concentration of N02 is so large that the OH + N02 reaction occurs preferentially relative to propagation of the cycle.

FIGURE 6.3 Ozone production efficiency (OPE) for atmospheric CO oxidation as a function of the NO, (NO + NOs) level. Conditions are 298 K at the Earth's surface, Pno, = 1 ppts"1, [NO]/ [N02] = 0.1, and CO mixing ratio of 200 ppb.

Dependence of P0i <>n NOr Abundance in CO Oxidation One of the key aspects of tropospheric chemistry is the dependence of ozone production on the NO* abundance. We have derived relationships for Pq3 for the CO system in the limits of low and high NO*. Here we examine how Pq, depends on the NOA abundance over the complete range of NO* levels. To do this, we will fix the rate of HO* production, Pho,> and vary the NO concentration at a fixed N02/N0 ratio. Under conditions of high H02 radical abundance relative to NO,, the primary chain-terminating reaction is the HO* + HO* reaction, H02 + H02. This condition is referred to as NOx-limited. At sufficiently high NO* levels, chain termination results from the HO* + NO, reaction, OH 4- N02. This condition is called NOx-saturated. By varying the NO* concentration, we can explore the point at which the system crosses over from NO*-limited to NO*-saturated conditions. The crossover point occurs at the NO concentration where 0jPOl /6[NO] = 0. The actual value of the NO concentration at this crossover point depends on the values of Pho, and the N02/N0 ratio.

The calculation described above is presented by Thornton et al. (2002), in modeling ozone concentrations downwind of Nashville, Tennessee. For numerical values, assume 298 K at the Earth's surface, [N02]/[N0] = 7, and Pho, = 0.1,0.6, or 1.2 ppt s 1. The N02/N0 ratio of 7 approximates that at midday in a regional continental atmosphere. In order to avoid having to represent the detailed chemistry of the many hydrocarbon species present in such an atmosphere, Thornton et al. (2002) assumed that the ozone-forming characteristics of the airmass could be simulated as if all the hydrocarbons were replaced by CO at a mixing ratio of 4500 ppb. This is feasible because the atmospheric oxidation of CO exhibits most of the major chemical features of that of more complex organic molecules.

Figure 6.4 shows [H02], Po,, and HHL and NHL as a function of [NO] from 10 to 2500 ppt. The behavior exhibited in Figure 6.4 is very basic to tropospheric

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FIGURE 6.4 Characteristics of the atmospheric oxidation of CO as a function of NO.v level. Conditions are 298 K at the Earth's surface, [N03]/[N0) = 7; three values olPHOl (0.l.0.6,and 1.2 ppl s 1) arc considered. A similar calculation has been presented by Thornton et al. (2002). (a) HO2 concentration; (b) ozone production rate, P0i\ (c) HHL and NHL [see (6.1 Hand (6.12)].

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FIGURE 6.4 Characteristics of the atmospheric oxidation of CO as a function of NO.v level. Conditions are 298 K at the Earth's surface, [N03]/[N0) = 7; three values olPHOl (0.l.0.6,and 1.2 ppl s 1) arc considered. A similar calculation has been presented by Thornton et al. (2002). (a) HO2 concentration; (b) ozone production rate, P0i\ (c) HHL and NHL [see (6.1 Hand (6.12)].

chemistry, and its ramifications will appear again and again. [H02] decreases as [NO] increases; this behavior is a result of the fact that as the overall NOt level increases, the OH 4- NOt termination reaction becomes increasingly important, more effectively removing H0A from the system. The 03 production rate j°Oi achieves a maximum at a particular value of [N01; the larger the value of Pho, > the larger the value of both Pq^ and [N01 at the maximum. Panel (c) shows the H0t and NOt- loss rates, HHL and NHL. HHL is at its maximum at low [NO], where [HO;] is at its maximum. Likewise, NHL is largest at high [NO], where [NOJ is at its maximum. As PuoA increases, [H02] is larger at any given NO level. Pq, attains a maximum reflecting the competition between decreasing H02 as NO increases and the NO increase itself. At higher Pho., [H02] will remain high for larger values of [NO], thus shitting the maximum in Po2- The maximum in Po, occurs at a larger value ofNO than that at which HHL = NHL. HHL depends on |H02]2 versus P0i ~ [H02j [NO], At any value of [NO], HHL-1-NHL = PHOt. As [NO] increases, HHL decreases with the square of [H02], while the decrease of [H02] is somewhat compensated for by the increase of [NO] in PoAs a result, the HHL/NHL crossover occurs at a smaller value of [NOJ than that at which Po, reaches its maximum.

6.3.4 Theoretical Maximum Yield of Ozone from CO Oxidation

The theoretical maximum yield of 03 per CO + OH reaction would occur if NO* concentrations were sufficiently high that every H02 radical reacts with NO rather than with itself and termination of the chain by the OH -f N02 reaction were neglected. The resulting mechanism would be

While it is informative to see that one O3 molecule could theoretically result from each CO + OH reaction, this condition can never be achieved. If NOt levels are sufficiently high to keep H02 from reacting with itself, they are also sufficiently high so that some N02 must react with OH to form HNO3, thereby terminating the chain reaction.

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