Examining the implications of photochemical indicators on the O3-NOx-VOC sensitivity and control strategies: A case study in the Yangtze River Delta (YRD), China.

Ozone (O₃) has become a significant air pollutant in China in recent years. O₃ abatement is challenging due to the nonlinear response of O₃ to precursors nitrogen oxides (NOx) and volatile organic compounds (VOCs). Photochemical indicators are widely used to estimate the O₃-NOx-VOC sensitivity, and this has important policy implications. However, the effectiveness of the indicators has seldom been evaluated. This study examines the applications of four indicators that include the ratio of the production rates of H2O2 and HNO₃ (PH₂O₂/PHNO₃), HCHO/NO₂, HCHO/NO_y, 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 O₃ isopleths, it was found that HCHO/NO₂ and HCHO/NO_y showed the most consistency, whereas PH₂O₂/PHNO₃ (NO_y) tended to underestimate (overestimate) the positive response of O₃ to NO_x. Additionally, PH₂O₂/PHNO₃ was less likely to attribute the O3 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. [ABSTRACT FROM AUTHOR]