Abstract
Abstract. The hygroscopic growth of the atmospheric aerosol is a critical parameter for quantifying the anthropogenic radiative forcing. Until now, there has been a lack of long term measurements due to limitations in instrumental techniques. In this work, for the first time the seasonal variation of the hygroscopic properties of a continental background aerosol has been described, based on more than two years of continuous measurements. In addition to this, the diurnal variation of the hygroscopic growth has been investigated, as well as the seasonal variation in CCN concentration. These physical properties of the aerosol have been measured with a Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA), a Differential Mobility Particle Sizer (DMPS), and a Cloud Condensation Nuclei Counter (CCNC). The results show that smaller particles are generally less hygroscopic than larger ones, and that there is a clear difference in the hygroscopic properties between the Aitken and the accumulation mode. A seasonal cycle was found for all particle sizes. In general, the average hygroscopic growth is lower during wintertime, due to an increase in the relative abundance of less hygroscopic or barely hygroscopic particles. Monthly averages showed that the hygroscopic growth factors of the two dominating hygroscopic modes (one barely hygroscopic and one more hygroscopic) were relatively stable. The hygroscopic growth additionally showed a diurnal cycle, with higher growth factors during day time. CCN predictions based on H-TDMA data underpredicted the activated CCN number concentration with 7 % for a 1 % water supersaturation ratio. The underprediction increases with decreasing s, most likely due to a combination of measurement and modeling uncertainties. It was found that although the aerosol is often externally mixed, recalculating to an internal mixture with respect to hygroscopicity did not change the CCN concentration as a function of supersaturation significantly.
Highlights
The hygroscopic properties of atmospheric aerosols are of utmost importance for the climate
As aerosol particles move around the atmosphere, they continuously interact with the surrounding water vapour, changing in size with the relative humidity (RH)
The number of activated particles in a cloud is highly dependent on the hygroscopic properties of the particles (McFiggans et al, 2006) as well as the size distribution of the aerosol (Dusek et al, 2006) which determines how many activated cloud droplets will form for a given supersaturation (s) ratio
Summary
The hygroscopic properties of atmospheric aerosols are of utmost importance for the climate. The number of activated particles in a cloud is highly dependent on the hygroscopic properties of the particles (McFiggans et al, 2006) as well as the size distribution of the aerosol (Dusek et al, 2006) which determines how many activated cloud droplets will form for a given supersaturation (s) ratio. This in turn affects both the reflective properties and precipitation pattern of the cloud (Rosenfeld, 1999, 2000), effects which are Published by Copernicus Publications on behalf of the European Geosciences Union
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