[Differences of Three Methods in Determining Ozone Sensitivity in Nanjing]

In recent years， ozone （O₃） has become an increasingly important air pollutant in China. Identifying the sensitivity of O₃ to the precursors volatile organic compounds （VOCs） and nitrogen oxides （NO） can help make effective abatement strategies. This study compared three methods for determining O₃-VOCs-NO sensitivity： simulated photochemical indicator values and sensitivity coefficients derived from a three-dimensional air quality model and an observation-based model （OBM）， with a case study involving an O₃ pollution event that occurred in Nanjing in late July 2017. The results showed that O₃ sensitivity based on the photochemical indicator and sensitivity coefficients demonstrated similar spatial variations （over 50% of the grid cells of Nanjing exhibiting identical O₃ sensitivity）. However， sensitivity coefficients identified a larger number of areas within a transitional O₃ sensitivity regime， as opposed to the VOCs- or NO-limited regime identified by the photochemical indicator. The determination of the latter was affected by the adopted threshold values. The OBM relied on the quality of the observational data. For example， positive biases in observed NO₂ could lead to an underestimation of O₃ sensitivity to NO with the OBM. During the high pollution period， the three methods exhibited significant disparities. The photochemical indicator tended to suggest the VOCs-limited condition， whereas the OBM and sensitivity coefficients indicated the NO-limited or transitional regimes.