Sensitivity modeling of ozone and its precursors over the Chengdu metropolitan area

Abstract

The comprehensive air quality model with extensions (CAMx)–high-order decoupled direct method (HDDM) was used to simulate ozone (O3)-NOx-VOCs sensitivity from June 1 to September 30 in 2019 over the Chengdu metropolitan area. In terms of the diurnal variations of O3 sensitivities to NOx and VOCs emissions, most cities in the Chengdu metropolitan area, except Ya'an, Suining and Ziyang, show a shift in O3 sensitivity from NOx-limited to VOC-limited from daytime to nighttime, when 8:00 and 19:00 were the cut-off points for the switch in O3-sensitive regimes. The average daily maximum 8-h O3 sensitivity to regional NOx and VOCs emissions show that the Chengdu metropolitan area is mainly NOx-limited. The O3 sensitivity to local emissions in urban areas of each city is VOC-limited or in a transition regime, while O3 sensitivities to overall regional emissions are prone to NOx-limited. Therefore, when proposing O3 control strategies, it is necessary to consider the different measures for the local and regional emissions. First-order sensitivity coefficients combined with the path integral method (PIM) are used to calculate the contributions of precursors' emissions to O3 concentrations in different sectors, revealing that mobile source accounted for 31.1–36.9%, followed by industrial source, with contribution of 22.2–30.5% to O3 concentrations. Finally, five emission reduction scenarios were designed with different NOx/VOC reduction ratios, and O3 concentration changes with different NOx/VOC reduction ratios were calculated based on O3 sensitivity coefficients. The results show that the influence of local emission reductions on the changes in O3 concentrations varies significantly among cities. The decrease in O3 concentrations reach maximum when the NOx/VOC reduction ratio is 3:1 in most of the cities, except Chengdu and Mianyang, with the highest O3 decrement occurs at the NOx/VOC reduction ratio of 1:3.