The analysis of size-segregated cloud condensation nuclei counter (CCNC) data and its implications for cloud droplet activation
Ambient aerosol, CCN (cloud condensation nuclei) and hygroscopic\nproperties were measured with a size-segregated CCNC (cloud condensation\nnuclei counter) in a boreal environment of southern Finland at the SMEAR\n(Station for Measuring Ecosystem-Atmosphere Relations) II station. The\ninstrumental setup operated at five levels of supersaturation S covering\na range from 0.1-1% and measured particles with a size range of 20-300\nnm; a total of 29 nonconsecutive months of data are presented. The\nmedian critical diameter D-c ranged from 150 nm at S of 0.1% to 46 nm\nat S of 1.0 %. The median aerosol hygroscopicity parameter kappa ranged\nfrom 0.41 at S of 0.1% to 0.14 at S of 1.0 %, indicating that ambient\naerosol in Hyytiala is less hygroscopic than the global continental or\nEuropean continental averages. It is, however, more hygroscopic than the\nambient aerosol in an Amazon rainforest, a European high Alpine site or\na forested mountainous site. A fairly low hygroscopicity in Hyytiala is\nlikely a result of a large organic fraction present in the aerosol mass\ncomparative to other locations within Europe. A considerable difference\nin particle hygroscopicity was found between particles smaller and\nlarger than similar to 100 nm in diameter, possibly pointing out to the\neffect of cloud processing increasing kappa of particles > 100 nm in\ndiameter. The hygroscopicity of the smaller, similar to 50 nm particles\ndid not change seasonally, whereas particles with a diameter of similar\nto 150 nm showed a decreased hygroscopicity in the summer, likely\nresulting from the increased VOC emissions of the surrounding boreal\nforest and secondary organic aerosol (SOA) formation. For the most part,\nno diurnal patterns of aerosol hygroscopic properties were found.\nExceptions to this were the weak diurnal patterns of small, similar to\n50 nm particles in the spring and summer, when a peak in hygroscopicity\naround noon was observed. No difference in CCN activation and\nhygroscopic properties was found on days with or without atmospheric new\nparticle formation. During all seasons, except summer, a CCN-inactive\nfraction was found to be present, rendering the aerosol of 75-300 nm in\ndiameter as internally mixed in the summer and not internally mixed for\nthe rest of the year.