Impact of NOx reduction on long-term ozone trends in an urban atmosphere

We investigated the relationships between ambient O3 concentrations and the concentrations of its precursors, NOx and NMHC, in Osaka, Japan. The levels of Ox ′ (where [Ox ′]=[O3]+[NO2]−0.1×[NOx] where the last term accounts for primary emissions of NO2) were uniform within the city even in the photochemically active season. We suggested that NO oxidation by peroxy radicals was a minor contributor, and that oxidation of locally emitted NO by background O3 in the city was the primary control on NO2 concentrations. Ozone concentrations increased linearly from 1985 to 2002 at a rate of 0.6 ppbv/yr, even though Ox ′ concentrations remained constant after the mid 1990s. The trend for Ox ′ concentrations could not be explained in terms of an increase in local O3 production, and the trend was found to reflect background O3 concentrations in Japan. There was a clear relationship between the NO2/Ox ′ ratio and NOx concentration: the ratio decreased with decreasing NOx concentration. As a consequence, O3 increased with decreasing NOx concentration. The reduction of NOx emissions was deemed to be an important factor for the recent trend of increasing O3 concentrations in Osaka City.