Deducing atmospheric conditions that contribute to elevated pollution events in the Salton Sea Air Basin

Abstract

The Salton Sea Air Basin (SSAB) has struggled to comply with National Ambient Air Quality Standards (NAAQS) for three federally regulated air pollutants: ozone, PM_2.5, and PM₁₀. Seasonal and diurnal patterns are presented along with their long-term decadal trends to better understand the relationship between the meteorological setting and air pollution levels as well as their deviations. This analysis revealed that ozone exceedances are no longer dominated by regulated NO_x emissions but rather are primarily controlled by regional agricultural soil emissions. These emissions are also likely influenced by stratospheric ozone transport to the deep convective boundary layers unique to the area. Observed peak ozone levels are still ∼2–5 ppb lower on weekends indicative of a NO_x-limited regime during the warm season. This implies that local ozone production is directly dependent on NO_x levels, which emphasizes the need to address unregulated sources of NO_x pollution such as those from heavily fertilized, arid agricultural soils. Additionally, strong correlations of PM_2.5 with NO_x, PM₁₀, and CO, indicate that influences from combustion sources as well as agricultural soils, secondary formation, and mechanical processes are all important sources of PM_2.5 production in Calexico, the main city of PM_2.5 nonattainment on the southern border with Mexico. These PM_2.5 exceedances occur in winter months, when low ventilation accumulates localized PM_2.5 precursor emissions. Source apportionment of PM_2.5 was assessed using non-negative matrix factorization of data from the Chemical Speciation Network (CSN) site in Calexico. The CSN data analysis for Calexico identifies biomass burning as the dominant source of high PM_2.5 concentrations, followed by agricultural soil emissions and wind-blown dust. In the immediate vicinity of the Salton Sea, less frequent PM_2.5 exceedances seem to be associated with occasional windstorms. Further, analysis of PM₁₀ exceedance days indicates that high winds, primarily westerly, are a critical factor, and the low PM_2.5/PM₁₀ ratios suggest minimal contribution from photochemical or combustion sources. Furthermore, the correlation of PM₁₀ with wind speed across various sites underscores the importance of dust resuspension and soil erosion. This comprehensive assessment highlights the complexity of air quality problems in the SSAB. Additionally, it emphasizes the need for targeted and localized air quality management strategies for this region which, despite its low population, suffers some of the worst air pollution impacts in the state of California.