Long-Term in-Situ Observations of Aerosol–Cloud Interactions in Northern Finland

The interactions between aerosol particles and clouds cause the largest uncertainty in our estimate of present day radiative forcing compared to pre-industrial time. The most important question is cloud formation, specifically which particles activate into cloud droplets and which do not.

Finnish Meteorological Institute's Pallas Atmosphere-Ecosystem Supersite station is located in Finnish Lapland in the border between Arctic and sub-Arctic regions. It is part of the national park, with limited access. Sammaltunturi station (67^°58´N, 24^°07´E, 560 m a.s.l.) is the main measurement site. Being in the border of the Arctic and sub-Arctic, the station is influenced by air masses from both pristine Arctic areas and from more polluted lower latitudes. The aerosol concentrations are highly variable from ~10 to 10 000 cm^-3 with clearly different physical and chemical properties. These makes it ideal place to study the effect of transported pollution aerosol from lower latitudes to cloud properties. It is also considered as a great place for ground-based observations of low level and orographic clouds. During autumn, Sammaltunturi measurement site is usually about 50% of time inside a cloud.

Our measurement of visibility (indicating when station is inside a cloud) and aerosol total number concentration started at 1995, particle number size distribution data without cloud droplets in 2000 and with cloud droplets in 2005. All these measurement series are ongoing and continuous except for short gaps. For long term continuous measurements of aerosol – cloud interactions we have used particle number size distribution measurement from two separate inlets, one allowing the cloud droplets to enter the measurements and one blocking them. Once water is evaporated from both particle populations, they are comparable and the difference between them refers to those particles that have been activated into cloud droplets. Conducting this type of measurements at a mountain or hill top site (being frequently within cloud) allows long time series of ambient cloud droplet activation data. This enables investigation of seasonal and inter-annual variation and trends in climate-relevant time scales. These measurements are complemented with several campaign based measurements with extended experimental set up and different measurement platforms (UAV, soundings and both in situ and satellite remote sensing).

The site was found to be inside cloud (in-cloud time, t_cloud) for 23 % of time during the entire time span starting from 1996. Annual mean t_cloud varied from year to year, ranging from 15 % in 2003 to 29 % in 2012.

Aerosol particle number concentration, N_p, (for particles with diameter between 50 nm and 500 nm) showed a similar inter-annual pattern at the site when compared to t_cloud. No significant correlation between these parameters at annual level were found. The seasonal patterns of t_cloud and N_p were, however, very different. Where t_cloud peaks strongly in autumn, N_p peaks in summer. This also means that even though the low clouds at the site are most frequent in fall, the more rare low summer clouds have highest number of potential cloud concensation nuclei. The high particle number concentration in summer clouds leads to increase of D50 activation diameter (diameter at which 50% of particles with that diameter activate). Also the optically thickest clouds (in-cloud periods with lowest visibility) were observed in summer.

Compared to other sites at lower latitudes the activation spectrum changes significantly throughout the year. Even at fairly high supersaturation (~ 1 %) less than half of the total aerosol population are activated into cloud droplets. This finding highlights the fact that at pristine environment even little changes in aerosol population have relatively larger impact to cloud properties than at lower latitudes, more aerosol rich, environments.

ACKNOWLEDGEMENTS

This work was supported by KONE foundation, Nordforsk Contract number 26060, CRAICC Amendment on CRAICC-PEEX Collaboration, Academy of Finland project: Greenhouse gas, aerosol and albedo variations in the changing Arctic (project num­ber 269095), Academy of Finland Center of Excellence program (project number 272041), BACCHUS (EU 7th Framework program), Natural Environment Research Council (NERC), grant number NE-L011514-1 and ACTRIS-2, the European Research Infrastructure for the observation of Aerosol, Clouds, and Trace gases. This project has received funding from the European Union’s Horizon 2020 research and inno­vation programme under grant agreement No 654109.