The effect of local sources on particle size and chemical composition and their role in aerosol–cloud interactions at Puijo measurement station

Interactions between aerosols and liquid water clouds were studied during autumns 2010–2011 at a semiurban measurement station on Puijo tower in Kuopio, Finland. Cloud interstitial and total aerosol size distributions, particle chemical composition and hygroscopicity and cloud droplet size distribution were measured, with a focus on comparing clean air masses with those affected by local sources. On average, the polluted air contained more particles than the clean air masses, and generally the concentrations decreased during cloud events. Cloud processing was found to take place, especially in the clean air masses, and to a lesser extent in the polluted air. Some, mostly minor, differences in the average particle chemical composition between the air masses were observed. The average size and number concentration of activating particles were quite similar for both air masses, producing average droplet populations with only minor distinctions. As a case study, a long cloud event was analyzed in detail, with a special focus on the emissions from local sources, including a paper mill and a heating plant. This revealed larger variations in particle and cloud properties than the analysis of the whole data set. Clear differences in the total (between 214 and 2200 cm−3) and accumulation mode particle concentrations (between 62 and 169 cm−3) were observed. Particle chemical composition, especially the concentrations of organics (between 0.42 and 1.28 μg m−3) and sulfate (between 0.16 and 4.43 μg m−3), varied considerably. This affected the hygroscopic growth factor: for example, for 100 nm particles the range was from 1.21 to 1.45 at 90 % relative humidity. Particularly, large particles, high hygroscopicities and elevated amounts of inorganics were linked with the pollutant plumes. Moreover, the particle hygroscopicity distributions in the polluted air were clearly bimodal, indicating externally mixed aerosol. The variable conditions also had an impact on cloud droplet formation, with the droplet concentration varying between 138 and 240 cm−3 and mean diameter between 9.2 and 12.4 μm.