Cloud condensation nuclei at two Finnish sub-Arctic sites: long-term observations of aerosol activation, predictions based on a simple semi-empirical parameterization, and comparison to global climate model simulations

Abstract. In this study, we present long-term observations of cloud condensation nuclei (CCN) at two locations in the Finnish sub-Arctic: Pallastunturi and Värriö. Both stations belong to the ACTRIS research infrastructure. At Pallastunturi, the interactions between aerosols and clouds have been measured in situ at a fjell top, 565 m a.s.l. since 2005. From Pallastunturi data, a simple parameterisation was developed to predict the activation diameter D₅₀ and subsequent CCN- concentrations, based on air mass origin and temperature. The parameterization was applied to aerosol size distribution measurements conducted at Värriö from 1998 to 2021. The average annual CCN concentrations were 140 ± 40 # /cm³ at Pallas and 170 ± 50 # /cm³ at Värriö. The CCN- concentrations exhibited statistically significant decreasing trends at both stations: for Pallas -2.5 CCN /cm³ / year, and for Värriö -4.2 CCN /cm³ / year. The decrease was most pronounced during February and March, i.e. during the Arctic Haze period. We also compared the results with EC-Earth3 global climate model simulations. Using two measurement sites separated by ~230 km reduces the uncertainty in the spatial representativeness of observations and provides a robust basis for climate model evaluation. EC-Earth3 predicted the measured CCN- concentrations fairly well at 0.2 % supersaturation, reproducing the observed long-term trend. However, the modelled seasonal variation was slightly different, predicting higher concentrations during winter time (November to April). The study demonstrates the value of long term in-cloud aerosol measurements in validating and improving climate predictions.