Abstract. Ice-nucleating particles (INPs) can initiate ice formation in clouds at temperatures above −38 ∘C through heterogeneous ice nucleation. As a result, INPs affect cloud microphysical and radiative properties, cloud lifetime, and precipitation behavior and thereby ultimately the Earth's climate. Yet, little is known regarding the sources, abundance and properties of INPs, especially in remote regions such as the Arctic. In this study, 2-year-long INP measurements (from July 2018 to September 2020) at Villum Research Station in northern Greenland are presented. A low-volume filter sampler was deployed to collect filter samples for offline INP analysis. An annual cycle of INP concentration (NINP) was observed, and the fraction of heat-labile INPs was found to be higher in months with low to no snow cover and lower in months when the surface was well covered in snow (> 0.8 m). Samples were categorized into three different types based only on the slope of their INP spectra, namely into summer, winter and mix type. For each of the types a temperature-dependent INP parameterization was derived, clearly different depending on the time of the year. Winter and summer types occurred only during their respective seasons and were seen 60 % of the time. The mixed type occurred in the remaining 40 % of the time throughout the year. April, May and November were found to be transition months. A case study comparing April 2019 and April 2020 was performed. The month of April was selected because a significant difference in NINP was observed during these two periods, with clearly higher NINP in April 2020. In parallel to the observed differences in NINP, also a higher cloud-ice fraction was observed in satellite data for April 2020, compared to April 2019. NINP in the case study period revealed no clear dependency on either meteorological parameters or different surface types which were passed by the collected air masses. Overall, the results suggest that the coastal regions of Greenland were the main sources of INPs in April 2019 and 2020, most likely including both local terrestrial and marine sources.

Abstract. Ice nucleating particles (INPs) can initiate ice formation in clouds at temperatures above −38 °C through heterogeneous ice nucleation. As a result, INPs affect cloud microphysical and radiative properties, cloud life time and precipitation behavior and thereby ultimately the Earth’s climate. Yet, little is known regarding the sources, abundance and properties of INPs especially in remote regions such as the Arctic. In this study, two-year-long INP measurements (from July 2018 to September 2020) at Villum 5 Research Station (VRS) in Northern Greenland are presented. A low-volume filter sampler was deployed to collect filter samples for off-line INP analysis. An annual cycle of INP concentration (NINP) was observed and the fraction of biogenic INPs was found to be higher in snow-free months and lower in months when the surface was snow-covered. Samples were categorized into three different types based only on the slope of their INP spectra, namely into summer, winter and mix type. For each of the types a temperature dependent INP parameterization was derived, clearly different depending on the time 10 of the year. Winter and summer type occurred only during their respective seasons and were seen 60 % of the time. The mixed type occurred in the remaining 40 % of the time throughout the year. April, May and November were found to be transition months. A case study comparing April 2019 and April 2020 was performed. The month of April was selected because a significant difference in NINP was observed during these two periods, with clearly higher NINP in April 2020. NINP in the case study period revealed no clear dependency on either meteorological parameters or different surface types which were passed 15 by the collected air masses. Overall, the results suggest that the coastal regions of Greenland were main sources of INPs in April 2019 and 2020, most likely including both local terrestrial and marine sources. In parallel to the observed differences in NINP, also a higher cloud ice fraction was observed in satellite data for April 2020, compared to April 2019.

<p>Ice nucleating particles (INPs) are a rare but important type of atmospheric aerosol particles, contributed mostly by mineral dust particles and by biogenic macromolecules expressed by a range of different microorganisms. INPs’ ability to nucleate ice in cloud droplets already far above their homogeneous freezing temperature (at ~ -38°C) influences for instance cloud radiative processes and precipitation formation.</p> <p>Despite a recent surge of research on INPs, neither are all important sources of atmospheric INP sufficiently known, nor their atmospheric abundance which varies with location and season.</p> <p>Here we examine the heat sensitivity of some types of INP of biogenic origin by exposing them to a range of different heating temperatures (60°C, 85°C, and 90°C) for one hour. The heating is expected to destroy proteinaceous ice active macromolecules. The ice activity of different samples was examined before and after heating.</p> <p>Examined samples included birch pollen (<em>Betula pendula</em>), fungi (<em>Mortierella alpina, Fusarium acuminatum</em>), the bacteria <em>Pseudomonas syringae</em> (from a commercially available SNOMAX sample) and aspen leaves (from <em>Populus tremuloides</em>) which had been sampled and freeze-dried decades ago. We compare their heat sensitivity to that of INPs from airborne aerosol samples collected on filters in summer months at Villum Research Station (VRS) in North Greenland, which were exposed to the same heating procedure.</p> <p>For samples from <em>F. acuminatum</em> and <em>P. syringae</em>, a continuing decrease in ice activity (expressed as INP per sample mass) was observed for each of the heating steps. The decrease was larger than one order of magnitude for each heating step across the examined temperature range (roughly -5°C to -25°C). For the <em>B. pendula</em> sample, highly ice active macromolecules inducing ice nucleation at > -10°C were already destroyed by heating to 60°C, while the signal below -15°C was changed much less by any of the heating steps. The <em>M. alpina</em> sample showed no change in ice activity after heating to 60°C, but a strong decrease across the examined temperature range after heating further to 85°C, and some additional decrease (roughly one order of magnitude) after heating to 90°C. The aspen leave samples showed no noticeable reaction to heating at freezing temperatures below -15°C, but behaved similar to the <em>M. alpina</em> sample at -10°C.</p> <p>Interestingly, for freezing temperatures > -10°C, INP concentrations of VRS summer samples also showed no or only a small decrease in ice activity upon heating to 60°C, similar as the <em>M. alpina</em> and aspen leave samples. Also similar to these two, VRS samples showed a very pronounced decrease upon heating to 85°C and some further decrease upon heating to 90°C. This is interesting in the light that recent research suggested that <em>M. alpina</em>, together with the bacteria species <em>Pantoea ananatis</em>, are likely sources of the INPs present in a aspen leave sample of the same batch as the one examined here. Combining these findings, we speculate that <em>M. alpina</em> may be of considerable importance as terrestrially sourced atmospheric INPs for regions even including the summer Arctic.</p>