The sources and diurnal variations of submicron aerosols in a coastal-rural environment near Houston, US

Abstract. Aerosol properties were characterized at a rural site southwest of Houston from May to September 2022 during the intensive operation periods (IOP) of the Tracking Aerosol Convection Interactions ExpeRiment (TRACER). Backward trajectory analysis reveals three major air mass types, including marine air mass from the Gulf, urban air mass influenced by urban emissions, and regional air mass. Marine aerosols typically show a bimodal size distribution and have the lowest particle number and mass concentrations of PM₁ (particulate matter with an aerodynamic diameter of less than 1 μm), while the aerosols from air masses strongly influenced by urban emissions exhibit the highest concentrations. Organic aerosol (OA) accounts for more than 50 % of PM₁ for urban and regional air masses, whereas sulfate is comparable to OA in marine air masses. Positive Matrix Factorization (PMF) analysis of aerosol mass spectra identifies 6 OA factors, including hydrocarbon-like OA (HOA), OA from the oxidation of monoterpenes (91FAC), OA from the reactive uptake of isoprene epoxydiols by acidic sulfate particles (isoprene-SOA), and three oxygenated OA factors with high O:C ratios (OOA1, 2, and 3). We find OOA1, a factor with a high f55 signal and f55/f57 ratio, is related to shipping emissions, instead of cooking emissions suggested in previous studies. OOA3 has the highest O:C ratio and exhibits elevated mass concentration in the afternoon. Similar diurnal variation of highly oxidized OA factors was commonly observed in the Houston area during previous studies and attributed to the SOA formation by photochemistry and mixing from aloft. Here, using air mass backward trajectories and 1-D box model, we show the diurnal trend of OOA3 mass concentration is instead driven by changes in air mass arriving at the rural site. The air mass changes are likely caused by the shift between land breezes and sea/bay breezes. Within the same air mass type (e.g., either urban or marine air mass), OOA3 mass concentration is largely independent of wind direction and shows essentially no diurnal variation, suggesting OOA3 is related to aged OA with minimal influence by local emissions. This study helps identify the major sources of OA in the Houston region and highlights the impacts of both atmospheric chemistry and meteorology on aerosol properties in the coastal-rural environment.