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

Abstract

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 (NOx-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 NOx-limited and VOC-limited conditions, respectively. Satellite data on TROPOspheric Monitoring Instrument (TROPOMI) HCHO/NO2 ratios represent relative NOx-sensitivity or VOC-sensitivity (i.e., higher and lower ratios indicating increasing NOx-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 NOx-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 NOx- 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 NOx emission controls can worsen O3 air quality until the O3 chemical regime reaches NOx-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 NOx-limited conditions during the warm seasons and vice versa during the cool seasons, depending on the seasonality of NOx 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.