Synthesis of the ACTRIS Network Cloud Condensation Nuclei Measurements
Abstract:We present results of cloud condensation nuclei (CCN) measurements between 2011 and 2014 from several ACTRIS stations (http://www.actris.net/), two urban datasets, and Barrow, Alaska (2007/08). Aerosol number concentration, size distribution and chemical composition are also analyzed to derive further key variables such as the hygroscopicity parameter kappa and the activation diameters at several supersaturations (SS). The sites cover a large area allowing for temporal and spatial characterization of CCN variability in different atmospheric regimes such as marine, continental, boreal, Arctic and Mediterranean environments, boundary layer and free tropospheric conditions. Additionally, autocorrelation analysis is performed to investigate the persistence of variables over different timescales and to explore meaningful averaging periods for global modelling of CCN.
The aerosol populations and their activation behavior show significant differences at the stations. While peak concentrations of CCN are observed in summer at the high altitude sites, in the Arctic the highest concentrations occur during the Haze period in spring. The rural-marine and rural-continental sites exhibit similar CCN concentration characteristics with a relatively flat annual cycle. At some stations, e.g. in the boreal environment, the annual cycle is more pronounced for higher SS. Geometric mean diameters of aerosol populations as well as the activation ratios on the basis of particles > 50 nm vary strongly among sites and throughout the seasons.
In terms of CCN persistence, there are three different regimes: At some sites the autocorrelation drops within a week and shows little seasonal pattern, while at others it remains relatively high for 7 or more days exhibiting also seasonal patterns, and in the third group it has a high correlation for two days and then drops rapidly. Several but not all sites show diurnal cycles.