Sources of pollution and interrelationships between aerosol and precipitation chemistry at a central California site

This study examines co-located aerosol and precipitation chemistry data between 2010 and 2016 at Pinnacles National Monument ~65 km east of the coastline in central California. Positive matrix factorization analysis of the aerosol composition data revealed seven distinct pollutant sources: aged sea salt (25.7% of PM_2.5), biomass burning (24.2% of PM_2.5), fresh sea salt (15.0% of PM_2.5), secondary sulfate (11.7% of PM_2.5), dust (10.0% of PM_2.5), vehicle emissions (8.2% of PM_2.5), and secondary nitrate (5.2% of PM_2.5). The influence of meteorology and transport on monthly patterns of PM_2.5 composition is discussed. Only secondary sulfate exhibited a statistically significant change (a reduction) over time among the PM_2.5 source factors. In contrast, PM_coarse exhibited a significant increase most likely due to dust influence. Monthly profiles of precipitation chemistry are summarized showing that the most abundant species in each month was either SO₄ ²⁻, NO₃ ⁻, or Cl⁻. Intercomparisons between the precipitation and aerosol data revealed several features: (i) precipitation pH was inversely related to factors associated with more acidic aerosol constituents such as secondary sulfate and aged sea salt, in addition to being reduced by uptake of HNO₃ in the liquid phase; (ii) two aerosol source factors (dust and aged sea salt) and PM_coarse exhibited a positive association with Ca²⁺ in precipitation, suggestive of directly emitted aerosol types with larger sizes promoting precipitation; and (iii) sulfate levels in both the aerosol and precipitation samples analyzed were significantly correlated with dust and aged sea salt PMF factors, pointing to the partitioning of secondary sulfate to dust and sea salt particles. The results of this work have implications for the region's air quality and hydrological cycle, in addition to demonstrating that the use of co-located aerosol and precipitation chemistry data can provide insights relevant to aerosol-precipitation interactions.