Formation and Loss of O3 in the Clean Atmosphere

On the other hand, when the atmospheric NOx concentration increases, the HOx chain reaction of OH with CH4 and CO operating under the low NOx conditions described in Sect. (7.1), is coupled to the O3 formation reactions 0( P) from the photolysis of NO2 (Sect. 4.2.2) and O2, to bring O3 production. 

From Fig. 7.3, it can be seen that there is a threshold of NO concentration under which net O3 destruction and above which net O3 production occurs in the clean troposphere. Putting P(03)=L(03) in Eq. (7.53), the threshold value of NO concentration, [NO]th is calculated as. 

Under the solar irradiation in daytime, the concentration ratio of NO and NO2 can be determined by the ratio of NO production rate by the photolysis of NO2 in reaction (7.50) and the conversion rate of NO to NO2 by O3 (Sect. 5.4.1), 

Typical O3 mixing ratios are 10 ppbv and 30-50 ppbv over the clean ocean and the clean terrestrial, respectively, while those of peroxy radicals are 10 pptv, three orders of magnitude lower than O3. However, the rate constants of the reaction of NO with the peroxy radicals, e.g. HO2, CH3O2, are 8 x 10 cm molecule s which is nearly three orders of magnitude larger than that of NO and O3, 

8 X 10 cm molecule s (see Table 5.4), the contributions of these peroxy radicals and O3 are comparable to determine the equilibrium concentration ratio of NO and NO2 in the daytime. The calculated ratio from Eq. (7.56) is [N0]/[N02] 0.3. Using this value, the threshold value of NOx calculated from Eq. (7.53) is [NOx]th 80 pptv. Eorm Eig. 7.3, the threshold value of NOx can be read as 60 pptv, and thus the threshold mixing ratio of NOx to determine whether production or destmction of net O3 occurs in the clean atmosphere is in general thought to be a few 10 s to 100 pptv. 

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