A new approach for estimation of the effect of NOx emission reduction on roadside NO2 concentration in Tokyo

The mean NO2/NOx ratios of vehicle emission, the mean duration time of NO reaction with O3 and background concentrations at the five most polluted roadside stations in Tokyo are estimated statistically using monitoring data at both roadside sites and urban background sites. These statistics are then used to categorize roadside NO2 into three components; NO2 emission from vehicles (primary NO2), oxidation of NO by O3 (secondary NO2) and background NO2 (BG NO2). Secondary NO2 and BG NO2 make a greater contribution to the high NO2 concentrations in spring and summer. On the other hand, BG NO2 contributes the dominant portion to high NO2 concentrations in winter. The effects of vehicle NOx emission reduction on the concentration of primary, secondary and BG NO2 are evaluated using an NO2 emission reduction scenario, an oxidation model proposed in this study and an air quality simulation, respectively. It is predicted that the shift to vehicles meeting the stringent emission regulation beginning in 2009 will lower the roadside NO2 concentration due mainly to the large decrease in secondary NO2. On the other hand, the reduction of NOx in vehicle emissions results in a smaller decrease in roadside NO2 in winter.

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► Roadside NO2 concentrations are apportioned into three sources.
► Secondary NO2 and background NO2 make a greater contribution in spring and summer.
► Background NO2 contributes the dominant portion in winter.
► The NO oxidation model is constructed to predict secondary NO2 concentration.
► Vehicle NOx emission reduction makes secondary NO2 concentration lower largely.