A Comprehensive Analysis of Ice Nucleating Particles in a Subarctic Urban Environment During the ALPACA Campaign

Ice nucleating particles (INPs) influence cloud properties, precipitation, and radiative forcing, yet knowledge regarding their sources and characteristics remains incomplete, especially in polluted Arctic and subarctic environments. This study investigates the size and composition of INPs during the Alaskan Layered Pollution And Chemical Analysis (ALPACA) campaign, conducted in Fairbanks from January 17 to February 25, 2022. Fairbanks is a unique location situated in a valley with extreme cold, frequent temperature inversions, and persistent pollution during winter, conditions that favor ice fog formation and make local cloud microphysics highly sensitive to INPs. We report INP data from online measurements using a continuous flow diffusion chamber and offline filter and size-resolved impactor samples. Interpretation of the observations was supported by contextual aerosol size and composition data. Our findings show that while the aerosol population was dominated by submicron particles by number and mass, INP activity was concentrated in the supermicron (1.21-12 µm) range. INPs were primarily composed of heat-labile organic material across all size ranges and freezing temperatures, with over 90% of INPs at -15°C and -20°C identified as proteinaceous or biological in origin. Local sources, such as lichen and road gravel, were likely contributors to the observed INP population. Compared to other urban and Arctic sites, Fairbanks exhibited higher INP concentrations at lower temperatures and lower concentrations at warmer temperatures. These results provide new insight into the nature of INPs in subarctic, polluted-urban regions and emphasize the importance of particle size and local emissions in determining INP activity and abundance.