High-Latitude Urban Winter Aerosol Sources: Combustion, Dust, and Road Salt and the Role of Primary and Secondary Sulfur

Wintertime urban air pollution poses severe health risks. Yet, few studies have investigated the sources and aging of urban particulate matter at high latitudes, where low temperatures impact emissions and particle chemistry. Atmospheric particles (0.18–3.2 μm) were collected from January to February 2022 during the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign in Fairbanks, Alaska. The morphology and elemental composition of 33,422 individual particles were measured by computer-controlled scanning electron microscopy with energy-dispersive X-ray spectroscopy. The smallest measured particles (0.18–0.32 μm) were primarily fractal soot from residential wood burning and fossil fuel combustion, and carbon, oxygen, and sulfur (C/O/S)-rich particles, most of which were aqueous/liquid (12 ± 2% semisolid, by number). The largest particles (1.8–3.2 μm) were mechanically-generated road dust and road salt, as well as superaggregate soot from inefficient combustion. Sulfur-containing fractal soot particles provide evidence of significant primary sulfur aerosol emissions. C/O/S, road salt, and dust particles showed evidence of secondary sulfur formation. The C/O/S particles reflect the transformation of hygroscopic combustion particles to larger particles containing mainly secondary organic and sulfate aerosol. These results demonstrate the heterogeneity of the high-latitude urban aerosol population and the participation of multiple sources in primary and secondary sulfur chemistry.