Modeling study on the spatial variation of the sensitivity of photochemical ozone concentrations and population exposure to VOC emission reductions in Japan

Despite ozone precursor emission reduction efforts in most polluted areas worldwide, photochemical ozone remains among the most concerning air pollutants in some regions, possibly because of spatial heterogeneity of ozone sensitivity to individual precursors. However, regional differences in ozone sensitivity to volatile organic compounds (VOCs), a major precursor, have not been quantitatively shown. Therefore, this study estimated the spatial variations in the efficacy of VOC emission reductions to mitigate photochemical ozone concentrations and population exposure (as related to ozone-related health problems) over Japan, and to inform policy makers pursuing effective ozone reduction strategies in polluted regions. Using air quality simulations, we found that the abovementioned efficacy differed substantially, up to 10-fold, among 20 × 20-km areas in which VOCs had been reduced. Using the above-calculated spatial distribution of the efficacy and data on emission source locations of several industries, upstream industries (e.g., petroleum and coal products) showed greater reduction efficacies than downstream industries (e.g., plastic products). By comparing the calculated ozone sensitivity to VOCs with the ozone–NOx–VOC sensitivity derived from space-based observations of the HCHO/NO 2 column ratio from the Ozone Monitoring Instrument, the general spatial trends in ozone sensitivity to VOC emission reductions were verified qualitatively. Thus, regional countermeasures should be considered rather than the existing uniform nationwide measures in Japan. Moreover, to prioritize VOC emission reductions by industry, considering the reactivity or ozone formation potential of each emitted VOC exclusively, as is currently done, is insufficient, the emission locations of each industry should also be considered.