Terrestrial or marine - indications towards the origin of ice-nucleating particles during melt season in the European Arctic up to 83.7° N

Ice-nucleating particles (INPs) initiate the primary ice formation in clouds at temperatures above ca. -38 degrees C and have an impact on precipitation formation, cloud optical properties, and cloud persistence. Despite their roles in both weather and climate, INPs are not well characterized, especially in remote regions such as the Arctic. We present results from a ship-based campaign to the European Arctic during May to July 2017. We deployed a filter sampler and a continuous-flow diffusion chamber for offline and online INP analyses, respectively. We also investigated the ice nucleation properties of samples from different environmental compartments, i.e., the sea surface microlayer (SML), the bulk seawater (BSW), and fog water. Concentrations of INPs (N-INP) in the air vary between 2 to 3 orders of magnitudes at any particular temperature and are, except for the temperatures above -10 degrees C and below -32 degrees C, lower than in midlatitudes. In these temperature ranges, INP concentrations are the same or even higher than in the midlatitudes. By heating of the filter samples to 95 degrees C for 1 h, we found a significant reduction in ice nucleation activity, i.e., indications that the INPs active at warmer temperatures are biogenic. At colder temperatures the INP population was likely dominated by mineral dust. The SML was found to be enriched in INPs compared to the BSW in almost all samples. The enrichment factor (EF) varied mostly between 1 and 10, but EFs as high as 94.97 were also observed. Filtration of the seawater samples with 0.2 mu m syringe filters led to a significant reduction in ice activity, indicating the INPs are larger and/or are associated with particles larger than 0.2 mu m. A closure study showed that aerosolization of SML and/or seawater alone cannot explain the observed airborne N-INP unless significant enrichment of INP by a factor of 10(5) takes place during the transfer from the ocean surface to the atmosphere. In the fog water samples with -3.47 degrees C, we observed the highest freezing onset of any sample. A closure study connecting N-INP in fog water and the ambient N-INP derived from the filter samples shows good agreement of the concentrations in both compartments, which indicates that INPs in the air are likely all activated into fog droplets during fog events. In a case study, we considered a situation during which the ship was located in the marginal sea ice zone and N-INP levels in air and the SML were highest in the temperature range above -10 degrees C. Chlorophyll a measurements by satellite remote sensing point towards the waters in the investigated region being biologically active. Similar slopes in the temperature spectra suggested a connection between the INP populations in the SML and the air. Air mass history had no influence on the observed airborne INP population. Therefore, we conclude that during the case study collected airborne INPs originated from a local biogenic probably marine source.

Automated summary

The study revealed that the concentrations of ice-nucleating particles (INPs) in the air in the European Arctic may be the same or even higher in certain temperature ranges compared to midlatitudes, while lower in other temperature ranges. Additionally, it was found that the sea surface microlayer (SML) is often enriched in INPs compared to bulk seawater.