The aim of this study is to elucidate the relationship between the formation of photochemical oxidants (Ox) and the emission rate of the primary pollutants in the Osaka bay area. In this second report, an investigation on the relationship between formation of photochemical oxidants and emission levels of primary pollutants such as nitrogen oxides (NOx) and reactive hydrocarbons (RH) over a typical summer time (daily precipitation=0.0 mm/day, daily maximum temperature ≥35°C, daily maximum radiation ≥3.0MJ/m²/h, daily average wind speed ≤4.0 m/s, weak general wind) in Harima (36 km×28 km), Hanshin (32 km×32 km), and Osaka (28 km×36 km) areas was made by using a three-dimensional air quality model OASIS. Emission ratio of RH to NOx, RH/NOx, in Harima, Hanshin, and Osaka areas was calculated to be 6.1, 3.5, and 4.3, respectively. On the basis of mass-balance analysis of odd-hydrogen radicals generated during the photochemical reaction process, Harima area was classified“the low-NOx regime”defined by Sillman et al. while Hanshin and Osaka areas were classified“the high-NOx regime”. In Harima area, the low-NOx regime, reduction in the primary pollutant emission led to simple decrease in the maximum Ox concentration and OH and HO2 radicals concentrations, so that the reduction in NOx emission was more effective against than the environmental protection against Ox pollution. In Hanshin and Osaka areas, the high-NOx regimes, unlike the Harima area, possibilities in which a reduction in NOx emission led to an increase in OH and HO₂ radicals concentrations, and conversely to an increase in Ox concentration. These important results suggest that policy for a well-balanced reduction in both NOx and RH emissions is required for decrease in Ox concentration in typical summer period in Hanshin and Osaka areas. Additionally, it was also found in both the low- and high-NOx regimes that the peak time of the maximum Ox concentration would appear sooner with reduction in NOx emission, or later with reduction in RH emission.

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