Urban photochemistry in central Tokyo: 2. Rates and regimes of oxidant (O₃ + NO₂) production

Net photochemical production rates of oxidant (O x = O 3 + NO 2 ), F‐D (O x ), were determined in Tokyo during the winter and summer of 2004 using observed and calculated HO 2 radical concentrations. In both cases, calculated RO 2 (organic peroxy) radical concentrations were used. The rates calculated using the two HO 2 data sets are similar. In summer, morning F‐D (O x ) values on smog days (those with midday O 3 concentrations exceeding 100 ppbv) were higher than those on smog‐free days (with typical midday O 3 concentrations of 30 ppbv); however, the amount of ozone produced in a single day, as estimated by integrating F ‐ D (O x ) over the daytime, was not significantly different for the two periods. This analysis suggests that the occurrence of smog events in the city center cannot readily be explained by day‐to‐day variations in the strength of in situ photochemistry. On smog days, the coupling of photochemistry and meteorology appears to be important, as air masses in which oxidants accumulated over successive days arrive at the city center at approximately midday, transported by land‐sea breeze circulation. The average maximum daytime F‐D (O x ) values in summer, 11 and 13 ppbv h −1 using observed and calculated HO 2 levels, respectively, were only 1.5 and 2.2 times higher than those in winter (8 and 6 ppbv h −1 ). In winter, an underestimation of HO 2 levels at high NO concentrations resulted in an underestimation of F‐D (O x ) when calculated using modeled HO 2 . While the model predicted a volatile organic compounds (VOC)‐limited regime for O x production in winter, F‐D (O x ) based on observed HO 2 did not show features of the VOC‐limited regime and only steadily increased with increasing NO mixing ratio, even when it exceeded 20 ppbv. In summer, the dependence of F‐D (O x ) on nonmethane hydrocarbons (NMHCs) and NO x concentrations was similar in the two cases, in which observed and calculated HO 2 levels were used. A VOC‐limited regime, predicted on smog‐free days, changed to a NO x ‐limited regime on smog days. The F‐D (O x ) values determined for Tokyo are also compared with values for other cities.