Examining the implications of photochemical indicators for O-3-NOx-VOC sensitivity and control strategies: a case study in theYangtze River Delta (YRD), China

Ozone (O-3) has become a significant air pollutant in China in recent years. O-3 abatement is challenging due to the nonlinear response of O-3 to precursors nitrogen oxides (NOx) and volatile organic compounds (VOCs). Photochemical indicators are widely used to estimate the O-3-NOx-VOC sensitivity, and this has important policy implications. However, the effectiveness of the indicators has seldom been evaluated. This study examined the applications of four indicators that include the ratio of the production rates of H2O2 and HNO3 (PH2O2/PHNO3), HCHO/NO2, HCHO/NOy, and reactive nitrogen (NOy) in the Yangtze River Delta (YRD) with localized thresholds. The overall accuracy was high (> 92%) for all indicators and not significantly reduced with different simulation periods or in different locations of the region. By comparing with the O-3 isopleths, it was found that HCHO/NO(2 )and HCHO/NOy showed the most consistency, whereas PH2O2/PHNO3 (NOy) tended to underestimate (overestimate) the positive response of O-3 to NOx. Additionally, PH2O2/PHNO3 was less likely to attribute the O-3 formation to mixed sensitivity than the other indicators, and this demonstrated a preference for a single-pollutant control strategy. This study also revealed that the details in the methodology used to derive the threshold values impacted the results, and this may produce uncertainties in the application of photochemical indicators.