The analysis of size-segregated cloud condensation nuclei counter (CCNC) data and its implications for cloud droplet activation

摘要

Ambient aerosol, CCN (cloud condensation nuclei) and hygroscopic properties were measured with asize-segregated CCNC (cloud condensation nuclei counter) in a boreal environment of southern Finland at the SMEAR(Station for Measuring Ecosystem-Atmosphere Relations) II station. The instrumental setup operated at five levels of supersaturation covering a range from 0.1–1% and measured particles with a sizerange of 20–300 nm; a total of 29 non-consecutive months of data arepresented. The median critical diameter ranged from 150 nm at of 0.1% to 46 nm at of 1.0%. The median aerosolhygroscopicity parameter κ ranged from 0.41 at of 0.1% to0.14 at of 1.0%, indicating that ambient aerosol in Hyytiäläis less hygroscopic than the global continental or European continentalaverages. It is, however, more hygroscopic than the ambient aerosol in anAmazon rainforest, a European high Alpine site or a forested mountainoussite. A fairly low hygroscopicity in Hyytiälä is likely a result of alarge organic fraction present in the aerosol mass comparative to otherlocations within Europe. A considerable difference in particle hygroscopicitywas found between particles smaller and larger than ~100 nm indiameter, possibly pointing out to the effect of cloud processing increasingκ of particles > 100 nm in diameter. The hygroscopicity of thesmaller, ~50 nm particles did not change seasonally, whereasparticles with a diameter of ~150 nm showed a decreased hygroscopicityin the summer, likely resulting from the increased VOC emissions of thesurrounding boreal forest and secondary organic aerosol (SOA) formation. Forthe most part, no diurnal patterns of aerosol hygroscopic properties werefound. Exceptions to this were the weak diurnal patterns of small,~50 nm particles in the spring and summer, when a peak inhygroscopicity around noon was observed. No difference in CCN activation andhygroscopic properties was found on days with or without atmospheric newparticle formation. During all seasons, except summer, a CCN-inactivefraction was found to be present, rendering the aerosol of 75–300 nm indiameter as internally mixed in the summer and not internally mixed for therest of the year.