Description
Located in California on the US/Mexico border, Imperial County is designated as a PM2.5 nonattainment area for National Ambient Air Quality Standards. We performed a historical PM2.5 trend analysis to determine if PM2.5 concentrations have significantly decreased from 2000-2018 and to investigate the effects of wind direction and U.S-side agricultural burning on PM2.5 levels. Archived 24-hour averaged PM2.5 concentrations, meteorological wind data, and agricultural burn records were obtained from government agencies for the three cities of: Brawley, El Centro, and Calexico. This study identified that all three cities had an overall declining trend of mean and median PM2.5 concentrations throughout the 19 years (0.23-0.63 μg-m-3/year, R2 = 0.38 -0.87). The cooler months had more days of exceedances in the PM2.5 daily standard of 35 μg/m3 (91% of exceedances), although PM2.5 levels had median increases in summer months. For 2010-2018, days with a majority of Calm or South winds, Calexico experienced significantly (p<0.01) higher mean 24-hour average PM2.5 concentrations compared to other wind direction days. Examination of days with agricultural burn activity also revealed a steady decline of city-combined burn events (15/year) from 2010-2018. In all three cities, no significant differences in PM2.5 concentrations were observed when comparing burn days (0) to a range of days before and after the burn, although there was an observable increase in PM2.5 concentrations one day after the burns. For the years 2015-2018, El Centro’s and Calexico’s non-permitted burn days were found to have significantly higher (p<0.001) PM2.5 concentrations (median 13.0 μg/m3 & 15.2 μg/m3, respectively), than both the permitted burn days where a burn occurred (median 7.5 μg/m3 & 11.2 μg/m3, respectively), and such days where a burn did not occur (median 7.5 μg/m3 & 11.0 μg/m3, respectively). Wind findings indicate that the increased air PM2.5 pollution in Imperial County is associated with days with poor dispersion, and winds from the direction of Mexico, implying that cross-border transport of PM2.5 from Mexicali may negatively affect the city of Calexico. KEY WORDS: Imperial Valley, Fine Particulate Matter, Calexico, Cross-border Pollution, Crop Burning, U.S.-Mexico Border
