Contributions of local and long-range sources to the annual cycle of Arctic ice nucleating particles

Understanding ice nucleating particle (INP) concentrations, activation temperatures, and sources in the Arctic is necessary for constraining their contribution to Arctic amplification, due to their influence on the optical properties and lifetime of clouds. Despite this, Central Arctic INP observation studies are limited, and the relative contributions of local and long-range sources of INPs are not yet well understood. In this work, we present a dataset of year-round INP concentrations at 2-day resolution from aerosol filter samples taken during the 2019-20 MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) campaign, analysed using an immersion-mode droplet technique. We investigate the influence of long-range transportation events and local wind-blown sources on INP freezing spectra through comparisons with INPs measured from snow samples, major ions detected using ion chromatography, and aerosol concentrations in the diameter size range of 0.5–20 μm. We further constrain the impact of potential aerosol sources on total INP concentrations using a dilution series and background comparison method.

Our results show the presence of a distinct seasonal cycle, in agreement with previously reported observations. Warm-temperature INPs peak during summer, with INP concentrations at -15°C increasing to an average of 0.53 L^-1  from 0.01 L^-1 during the rest of the year. We also observe a period of low INP activity during November and December, ending with the onset of Arctic haze, and characterised by many samples being indistinguishable from background levels.

We observe a correlation between aerosol concentration and median INP activation temperature (R² = 0.412). Our results show shifts to warmer activation temperatures on the order of 1 – 4 °C during aerosol peaks  of more than 10 cm^-3 above background associated with both blowing snow, and long-range transportation events. We use this to highlight the influence of total INP concentration on freezing spectra observed from droplet freezing experiments. To further constrain the impact of blowing snow events and long-range transportation on INP populations, we present a collection of case studies that have been analysed using a series of targeted dilutions and comparisons with background INP spectra.