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

We investigated the relationships between ambient O-3 concentrations and the concentrations of its precursors, NO, and NMHC, in Osaka, Japan. The levels of O'(x) (where [O'(x)] = [O'(x)] + [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 O-3 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 O'(x) concentrations remained constant after the mid 1990s. The trend for O'(x) concentrations could not be explained in terms of an increase in local O-3 production, and the trend was found to reflect background 03 concentrations in Japan. There was a clear relationship between the NO2/O'(x) ratio and NOx, concentration: the ratio decreased with decreasing NOx concentration. As a consequence, O-3 increased with decreasing NO, concentration. The reduction of NOx emissions was deemed to be an important factor for the recent trend of increasing O-3 concentrations in Osaka City. (c) 2007 Elsevier B.V. All rights reserved.