Combining ISAM with Photochemical Indicator Transition Value Equations to Evaluate the Contribution of Pollution Sources to Ozone

Abstract

O3/NOY is a photochemical indicator that has the potential to analyze O3 sensitivity through continuous automated monitoring instruments. When O3/NOY is used in the ISAM (Integrated Source Apportionment Method) module of the CMAQ (Community Multiscale Air Quality) air quality model, two issues arise: firstly, ISAM does not incorporate all reactive nitrogen species when calculating NOY; secondly, the default transition value of the module may not be applicable to the environmental conditions worldwide. To address and explore these issues, this study used CMAQ v5.0.2 to simulate 25 anthropogenic NOx and volatile organic compounds (VOC) emission reduction scenarios, calculating the O3/NOY transition values for each target grid. Subsequently, a set of transition value equations (TVEs) describing the variation in values was obtained through regression with NOZ. The evaluation of the ability of TVEs to assess O3 sensitivity compared to the default transition value in ISAM (= 7) showed that during high O3 pollution events (daily maximum O3 concentration ≥ 100 ppb), the overall accuracy (OA) of correctly assessing the O3 sensitivity of the target grid using TVEs (OA = 70%) was significantly higher than that using the default transition value (OA = 35%). Consequently, when using ISAM to simulate major emission sources' impact on O3 pollution in Taiwan, it was found that during high O3 pollution periods at monitoring stations, the primary contribution with the default transition value was from fugitive sources (40%), then vehicle sources (28%). However, with TVEs providing better O3 sensitivity determination, vehicle sources (35%) became the main contributors, followed by fugitive sources (31%). This indicates that the developed TVEs in this study can effectively enhance the accuracy of assessing O3 sensitivity, thereby more precisely quantifying the contribution sources of O3 pollution and assisting in the formulation of more accurate ozone pollution control strategies.