Ice nucleating particle (INP) measurements were performed in the boreal environment of Southern Finland at the Station for Measuring Ecosystem-Atmosphere Relations SMEAR II in the winter-spring of 2018. Measurements with the Portable Ice Nucleation Chamber (PINC) were conducted at 242 K and 105% relative humidity with respect to water. The median INP number concentration [INP] during a six-week measurement period was found to be 13 L-1. [INP] varied by three orders of magnitude and showed a general increase from mid-February until early April. No local or regional sources of INPs in the boreal environment of Southern Finland could be identified. Rather, it is believed that the INPs at SMEAR II are a result of mixing during long-range transport. Despite high variability, the measured [INP] values fall within the range expected for INP number concentrations measured elsewhere at similar thermodynamic conditions. [INP] did not correlate with any of the examined relevant parameters during the entire field campaign, indicating that no one single parameter can be used to predict the INP number concentration at the measurement location during the examined time period. The absence of correlation across the entire field campaign also suggests that a variety of particles are acting as INPs at different times, although it was indirectly determined that, on average, ambient INPs are most likely in the size range of 0.1–0.5 μm in diameter. On shorter time scales, several particle species correlated well with [INP] implying their potential role as INPs. Depending on the meteorological conditions, signatures of black carbon BC, supermicron biological particles and sub-0.1 μm particles have been found in the INP signal. However, an increase in the concentration of any of these particle species may not necessarily lead to the increase in [INP], reasons for which remain unknown.
A laboratory study of the ice nucleation efficiency of several types of mineral and soil dust was also finished during the project duration. The study showed that it is not possible to predict the ice nucleation activity of surface-collected soil dust based on the presence and amount of certain minerals or any particular class of compounds, such as soluble or proteinaceous/organic. Instead, at temperatures of 238–243 K the ice nucleation activity of the untreated, surface-collected soil dust in the condensation freezing mode can be roughly approximated by one of the existing surrogates for atmospheric mineral dust, such as illite NX.
The project resulted in one peer-reviewed, Open Access publication, the one included in the Publications list. The publication related to the field campaign is still in its preparatory phase. It will, however, be also published with an Open Access publisher within the next couple of months.
The work carried out during the project duration has been presented in several conferences and workshops. Main results were presented during the International Conference on Nucleation and Atmospheric Aerosols 2017, International Aerosol Conference 2018 and European Aerosol Conference 2019. The project was also presented during several visits to the University of Helsinki and Gothenburg University, as well as internally within the ETH group.
