Strategies to reduce PM2.5 and O3 together during late summer and early fall in San Joaquin Valley, California

PM2.5 and O-3 controls are traditionally considered separately because PM2.5 is usually high in winter while O-3 is generally high in summer. In this study, we explore the opportunity of controlling the two pollutants simultaneously through a better understanding of their intra-seasonal correlation and chemical-coupling behaviors under different meteorological conditions during the late summer and early fall (August-September) episodes in the San Joaquin Valley (SJV), California. A correlation analysis is first used to identify the temporal correlations between O-3 and PM2.5 and their underlying physical and chemical drivers. Sensitivity analysis is then applied to determine the chemical coupling between PM2.5 and O-3 and subsequent multipollutant control opportunities under two contrasting meteorological conditions using the Community Multi-Scale Air Quality (CMAQ) model. We find that O-3 and PM2.5 are positively correlated on the daily timescale because both are sensitive to atmospheric stagnation. However, O-3 and PM2.5 are negatively correlated on the hourly timescale determined by the negative correlation between hourly NO3- and O-3, which is mainly due to the opposite effects of T and RH on the diurnal variations of NO3- and O-3. Reducing NOx on average lead to O-3 increase, but it can facilitate reducing O-3 at higher O-3 (>75 ppb) locations under the more stagnant conditions. NOx emission control could become beneficial for both O-3 and PM2.5 when the NOx emissions in 2005 are further reduced by 15% under the more stagnant meteorological conditions and by 30% under the more ventilated meteorological conditions.