Seasonal variation of aerosol composition in Orange County, Southern California
2021
Mackey, Katherine R.M. | Stragier, Stephanie | Robledo, Laura | Cat, Linh Anh | Xu, Xiaomei | Capps, Scott | Treseder, Kathleen K. | Czimczik, Claudia I. | Faiola, Celia
Southern California is an urban metropolis bordered by mountains to the north and east and the Pacific Ocean to the west. Its unique geography and climatological features lead to annual patterns in atmospheric aerosol content from natural and anthropogenic sources. Here we sought to determine the nitrogen (N, including NO₃⁻, NH₄⁺, and total N) content of Southern California particulate matter (PM₂.₅) collected in Orange County, CA, and to identify relevant sources and processes that affect PM₂.₅ abundance and chemistry in different seasons. PM₂.₅ carbon (C) and N isotopic compositions (δ¹³C and δ¹⁵N, respectively) suggested that the C content of the PM₂.₅ derived partly from the marine inversion layer that occurs in the months of May through July. The high N content and δ¹⁵N values of PM₂.₅ collected in summer suggest a contribution from anthropogenic emissions, which are likely trapped in the basin by the inversion layer. In contrast, winter PM₂.₅ had a lower N content per volume of air, and δ¹³C values indicated a greater terrestrial signal, consistent with easterly winds. This trend was particularly evident in samples collected during high velocity Santa Ana wind events, which bring terrestrial material from the Great Basin and mountainous regions to the east of Southern California. Based on these observations, we suggest that atmospheric deposition of N is more likely to affect terrestrial ecosystems in the summer, when N content is high and aerosols are trapped by the inversion layer, whereas coastal marine ecosystems are more likely to be affected in the winter when westward winds are more common.
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