Severe wintertime air pollution, particularly in high-latitude cities, is understudied and gaining significant attention due to seasonal deterioration of air quality and adverse human health effects. Specifically, Fairbanks, Alaska, experiences some of the highest levels of PM<sub>2.5</sub> (particulate matter &lt; 2.5 µm) in the United States. Previous measurements show that organic carbon and sulfate dominate PM<sub>2.5</sub> during pollution events, but uncertainties surround the contributing sources and the mixing state of the sulfate aerosol. Therefore, the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign was conducted in Fairbanks, AK, from Jan. 17 – Feb. 26, 2022. Here, we focus on measurements made using an aerosol time-of-flight mass spectrometer (ATOFMS), which measures the real-time size and chemical composition of individual aerosol particles, providing valuable information on the distribution of chemical species throughout the aerosol population. Individual particle mass spectra give a “chemical fingerprint” of the primary particle source and atmospheric chemical reactions leading to secondary aerosol formation. Simultaneous size-resolved aerosol number concentration measurements were also conducted. Together these methods provide quantitative identification of individual aerosol sources and chemical mixing state. In particular, we distinguish primary particulate sulfate from domestic heating (oil, wood), local power generation (coal and oil), and vehicle emissions (gasoline and diesel). In addition, we examine the internal mixing state of secondary organo-sulfur species, including hydroxymethanesulfonate, within individual particles from various sources. These results improve understanding of wintertime aerosol chemical composition, sources, and processes under cold and dark conditions.

Mixing state of primary and secondary sulfate within individual aerosols during wintertime in polluted Fairbanks, Alaska

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