Developing a novel decision support tool for improving O3 air quality through strategic precursor gas emission controls

In many parts of the world, ambient O₃ levels have increased despite regulatory efforts to improve O₃ air quality. The unexpectedly increasing O₃ levels are largely attributed to a lack of understanding about O₃ chemical regimes. The O₃ mitigation strategies related to precursor gas emission controls have not adequately considered the O₃ chemical regimes (NO_x-limited or VOC-limited conditions). This study develops a novel approach to identifying the spatiotemporal variations of O₃ chemical regimes collectively using ground and satellite data to support the decision-making of precursor gas emission controls. Using ground NO₂ and O₃ concentrations measured in both the Seoul Metropolitan Area (SMA) of the Republic of Korea and the LA County of California, U.S. for May 2018-April 2021, a mixed effects model is employed to generate daily relationships between NO₂ and O₃ concentrations. Positive and negative relationships between NO₂ and O₃ concentrations provide strong evidence of NO_x-limited and VOC-limited conditions, respectively. Satellite data on TROPOspheric Monitoring Instrument (TROPOMI) HCHO/NO₂ ratios represent relative NO_x-sensitivity or VOC-sensitivity (i.e., higher and lower ratios indicating increasing NO_x-sensitivity and VOC-sensitivity, respectively). The modeling and satellite approaches are complementary to each other because (1) the model does not account for VOC concentrations due to a lack of VOC measurements and (2) TROPOMI HCHO/NO₂ alone does not provide the threshold level of separating NO_x-limited from VOC-limited conditions. The monthly slopes of NO₂ concentrations against O₃ concentrations are highly correlated with monthly TROPOMI HCHO/NO₂ both in the SMA (0.75) and LA County (0.87). Threshold levels distinguishing NO_x- from VOC-limited conditions are defined by the TROPOMI HCHO/NO₂ values when the NO₂ slopes are equal to 0. In the SMA and LA County, the threshold levels are 3.0 (95% CI= 2.6-3.4) and 1.4 (95% CI= 1.3-1.6), respectively. During the study period, the O₃ chemical regime is mostly VOC-limited in the SMA (35 out of 36 months), meaning that NO_x emission controls can worsen O₃ air quality until the O₃ chemical regime reaches NO_x-limited conditions. In the SMA, VOC emission controls can help reduce the O₃ levels. On the other hand, in LA County, the O₃ chemical regimes transition from VOC-limited to NO_x-limited conditions during the warm seasons and vice versa during the cool seasons, depending on the seasonality of NO_x and VOC emissions. The O₃ mitigation strategies in LA County can be the most effective with season-specific emission controls. This study offers a novel method for determining the most effective strategy of precursor gas emission controls and informing decision-making to enhance O₃ air quality.