Ice nucleating particle measurement at Helsinki

Ice nucleating particles (INPs) are particles in the atmosphere that are able to initiate the freezing of water droplets, a process known as ice nucleation. INPs are important to study because they play a crucial role in many atmospheric processes, including the formation of clouds, precipitation, and the radiative properties of clouds. For example, INPs can influence the concentration, size, and shape of ice crystals in clouds, which can in turn affect the reflectivity and lifetime of the clouds. This can have significant impacts on Earth's radiative balance and climate. The most relevant process of ice crystal formation in mixed-phase clouds (MPC) is by immersion freezing (Ansmann et al., 2009). Immersion freezing takes place when an INP is immersed in a water droplet and freezing is triggered on the particle's surface. In this research, we present a series of INP concentration measurements obtained using a novel assay under development at the Finnish Meteorological Institute in Helsinki. The focus of our research is to evaluate the consistency and replicability of these measurements.  To determine the contribution of different aerosols to the INP spectrum at different temperatures, we collect atmospheric particles onto membrane filters and analyze the concentration of INPs in a laboratory freezing experiment. In this experiment, we produce an aqueous solution with collected atmospheric particles and monitor identical aliquots of the aqueous solution while they are cooled until freezing. Finally, using the frozen fraction of aliquots at a given temperature, the volume of each aliquot, and the sample's air volume, the cumulative number of INPs in the given sample at each temperature is calculated following (Vali, 2019). Three automatic samplers were run in parallel to collect particles onto the membrane filters using different sampling schemes. For example, collecting daily samples with three different sampling flows. With these measurements, we will analyze the influence of the sampled air volume, air flow, and sampling duration on the INP concentration measurements.

 

References

Ansmann, A. et al. (2009) Evolution of the ice phase in tropical altocumulus: SAMUM lidar observations over Cape Verde Atmosphere, 9(9), 357

Vali, G., (2019). Revisiting the differential freezing nucleus spectra derived from drop-freezing experiments: Methods of calculation, applications, and confidence limits. Atmos. Meas. Tech., 12(2), 1219-1231. https://doi.org/10.5194/amt-12-1219-2019