One-year climatology of the aerosol hygroscopicity at the High Alpine Research Station Jungfraujoch (3580 m asl.)

Hygroscopic aerosol particles are able to absorb water at high relative humidity (RH). The hygroscopic growth factor (GF) is defined as the ratio of the particle diameter at humidified and dry conditions. Hygroscopic growth influences both the aerosol direct and indirect effects on climate, therefore being highly relevant for climate models: Particles that have grown to larger diameters due to water uptake scatter the incident sunlight to a greater extent and are more likely to act as cloud condensation nuclei (IPCC, 2007). The diameter-dependent water uptake of aerosol particles is typically measured by a HTDMA (Hygroscopicity Tandem Differential Mobility Analyzer). The instrument selects particles of a specific mobility diameter in the first DMA (Differential Mobility Analyzer) after having dried and neutralized the particles. The selected size class is then humidified, typically to 90% RH. The size distribution after the humidification is finally obtained by the second DMA and a CPC (Condensation Particle Counter). The HTDMA used in this study is based on the setup presented by Weingartner et al. (2002), and data are inverted according to Gysel et al. (2009). The aerosol at the remote High Alpine Research Station Jungfraujoch (altitude 3580 m asl.) is representative of the aerosol found in the lower free troposphere. In summer, diurnal variations are found in most aerosol variables due to the injection of the planetary boundary layer (PBL) air, while in winter the site is in the undisturbed free troposphere most of the time. GF data from previous intense campaigns in different years and seasons at the site are summarized in (Sjogren et al., 2008). Here we present the first extended data set comprising hygroscopicity measurements of different particle dry sizes (D0) at the Jungfraujoch, lasting from Mai 2008 to April 2009. Such long-term data sets are scarce to date (Swietlicki et al., 2008). Preliminary results confirm that in summer diurnal variations of the GF are found with minima at night (GF~1.45) for particles with D0 = 265 nm during the selected period (Figure 1). In the afternoon, GFs up to GF~1.62 are found for this size class, giving evidence to the site being influenced by injection of PBL air. Smaller particles show a similar behaviour while GFs are in general lower. This large dataset is currently being analysed in more detail.