Relationship of NOX and NMHC to photochemical O3 production in a coastal and metropolitan areas of Japan

Atmospheric trace gases such as oxidants, nitrogen oxides (NOX) and non-methane hydrocarbons (NMHC) measured by the local Japanese government authorities were analyzed to elucidate the relationship between O3 and its precursors (NOX and NMHC) in metropolitan Nagoya and its coastal outskirts in Japan. Hourly data on oxidants, NOX and NMHC concentrations from 1991 to 2001 were used in order to clarify the seasonal variations, diurnal variations and photochemical potentials. Seasonal variation of O3 shows a general trend at mid-latitude in the northern hemisphere such as a maximum from late spring to summer and minimum in winter. However, coastal O3 concentrations are higher than urban ones even though the NOX and NMHC concentrations in the coastal areas are much lower than those in the urban areas. The data on NOX and NMHC concentrations are also used to describe O3 isopleth diagrams to elucidate the photochemical contributions to O3 production in these two areas. From the diagrams, O3 was found to be produced most effectively when the NOX/NMHC ratio is around 0.1 in both the urban and coastal areas. This value roughly corresponds to the numerical simulation results. Photochemical O3 production in the urban areas tends to be suppressed by the excess NOX concentrations. Photochemical O3 production in the coastal areas, on the other hand, is likely to be more active because the NOX/NMHC ratio has the most effective role in producing O3. This NOX/NMHC ratio shows a seasonal variation, which is close to the optimum ratio of O3 production from late spring to summer.