Relationship of NO X and NMHC to photochemical O 3 production in a coastal and metropolitan areas of Japan

Abstract

Atmospheric trace gases such as oxidants, nitrogen oxides (NO X ) and non-methane hydrocarbons (NMHC) measured by the local Japanese government authorities were analyzed to elucidate the relationship between O 3 and its precursors (NO X and NMHC) in metropolitan Nagoya and its coastal outskirts in Japan. Hourly data on oxidants, NO X and NMHC concentrations from 1991 to 2001 were used in order to clarify the seasonal variations, diurnal variations and photochemical potentials. Seasonal variation of O 3 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 O 3 concentrations are higher than urban ones even though the NO X and NMHC concentrations in the coastal areas are much lower than those in the urban areas. The data on NO X and NMHC concentrations are also used to describe O 3 isopleth diagrams to elucidate the photochemical contributions to O 3 production in these two areas. From the diagrams, O 3 was found to be produced most effectively when the NO X /NMHC ratio is around 0.1 in both the urban and coastal areas. This value roughly corresponds to the numerical simulation results. Photochemical O 3 production in the urban areas tends to be suppressed by the excess NO X concentrations. Photochemical O 3 production in the coastal areas, on the other hand, is likely to be more active because the NO X /NMHC ratio has the most effective role in producing O 3. This NO X /NMHC ratio shows a seasonal variation, which is close to the optimum ratio of O 3 production from late spring to summer.