Development and application of observable response indicators for design of an effective ozone and fine particle pollution control strategy in China

Abstract. Designing effective control policies requires efficient quantification of the nonlinear response of air pollution to emissions. However, neither the current observable indicators nor the current indicators based on response-surface modeling (RSM) can fulfill this requirement. Therefore, this study developed new observable RSM-based indicators and applied them to ambient fine particulate matter (PM 2.5 ) and ozone (O 3 ) pollution control in China. The performance of these observable indicators in predicting O 3 and PM 2.5 chemistry was compared with that of the current RSM-based indicators. H 2 O 2  × HCHO/NO 3 and total ammonia ratio, which exhibited the best performance among indicators, were proposed as new observable O 3 - and PM 2.5 -chemistry indicators, respectively. Strong correlations between RSM-based and traditional observable indicators suggested that a combination of ambient concentrations of certain chemical species can serve as an indicator to approximately quantify the response of O 3 and PM 2.5 to changes in precursor emissions. The observable RSM-based indicator for O 3 (observable peak ratio) effectively captured the strong NO x -saturated regime in January and the NO x -limited regime in July, as well as the strong NO x -saturated regime in northern and eastern China and their key regions, including the Yangtze River Delta and Pearl River Delta. The observable RSM-based indicator for PM 2.5 (observable flex ratio) also captured strong NH 3 -poor condition in January and NH 3 -rich condition in April and July, as well as NH 3 -rich in northern and eastern China and the Sichuan Basin. Moreover, analysis of these newly developed observable response indicators suggested that the simultaneous control of NH 3 and NO x emissions produces greater benefits in provinces with higher PM 2.5 exposure by up to 12 µg m −3 PM 2.5 per 10 % NH 3 reduction compared with NO x control only. Control of volatile organic compound (VOC) emissions by as much as 40 % of NO x controls is necessary to obtain the co-benefits of reducing both O 3 and PM 2.5 exposure at the national level when controlling NO x emissions. However, the VOC-to-NO x ratio required to maintain benefits varies significantly from 0 to 1.2 in different provinces, suggesting that a more localized control strategy should be designed for each province.