A comprehensive analysis of ice nucleating particles in a subarctic urban environment during the ALPACA campaign

Abstract

Ice nucleating particles (INPs) influence cloud properties, precipitation, and radiative forcing, yet their sources and characteristics remain poorly understood, especially in polluted Arctic and subarctic environments. This study investigates the size and composition of immersion-mode INPs during the Alaskan Layered Pollution And Chemical Analysis (ALPACA) field campaign, conducted in Fairbanks, Alaska, from January 17 to February 25, 2022. Fairbanks is a unique location with extremely cold conditions, frequent temperature inversions, and persistent wintertime pollution. We report on INP data from online measurements from a continuous flow diffusion chamber (CFDC) 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 smaller particles by both number and mass concentrations, INP activity was concentrated in the larger (1.21-12 µm) size ranges. 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 colder temperatures and lower concentrations at warmer ones. These results provide new insight into subarctic INP sources during poor air quality and emphasize the importance of particle size and local emissions in determining INP activity and abundance.