Influence of air mass origin on microphysical properties of low-level cloudsin a subarctic environment

In this work, an analysis was performed to investigate how different long-range transport air masses can affect the microphysical properties of low-level clouds in a clean subarctic environment. The cloud measurements included in situ and remote sensing ground-based techniques and were conducted during eight Pallas Cloud Experiments (PaCEs) held in the autumn between 2004 and 2019. Each PaCE was carried out at the Pallas Atmosphere-Ecosystem Supersite, located in the Finnish subarctic region. Two cloud spectrometer ground setups were installed on the roof of the station to measure cloud microphysical properties: the cloud, aerosol and precipitation spectrometer (CAPS) and the forward-scattering spectrometer probe (FSSP). Air mass histories were analyzed using the Lagrangian FLEXible PARTicle dispersion model (FLEXPART) in order to investigate the differences between five distinct source regions ( "Arctic ", "Eastern ", "Southern ", "Western " and "Local "). We observed clear differences in the cloud microphysical properties for the air mass source regions. Arctic air masses were characterized by low liquid water content (LWC), low cloud droplet number concentration (Nc) and comparatively large median volume and effective droplet diameter. The Western region (marine North Atlantic) differed from the Arctic by both higher Nc and LWC. The Eastern region (continental Eurasia) only had a little higher LWC than the Arctic but substantially higher Nc and a smaller droplet diameter. The Southern region (continental Europe) had high Nc and LWC and a very similar droplet diameter to the Eastern region. Finally, the relationship between Nc and droplet size (i.e., the Twomey effect) was characterized for the different source regions, indicating that all region clouds were sensitive to increases in Nc.