A full year of aerosol hygroscopicity measurements at the High Alpine Research Station Jungfraujoch (3580 m asl.)

Aerosol particles can absorb significant amounts of water at high relative humidity (RH). This hygroscopic growth influences both the aerosol direct and indirect effects on climate, therefore being highly relevant for climate models: Particles that have grown by water uptake to larger diameters scatter the incident sunlight to a greater extent. Furthermore, hygroscopic particles need a smaller minimum dry size to act as cloud condensation nuclei compared to non-hygroscopic particles. The water uptake of particles with dry diameters of 35, 50, 75, 110, 165, 265 nm was determined by a HTDMA (Hygroscopicity Tandem Differential Mobility Analyzer), which measures the diameter growth factor (GF) at elevated RH relative to the dry particle size. Here we present a whole year of HTDMA measurements taken at the remote High Alpine Research Station Jungfraujoch (3580 m asl.), which is representative of the aerosol in the lower free troposphere. Monthly mean GFs at RH=90% were between 1.29 and 1.54, whereas larger particles exhibited generally larger GFs (Fig. 1A). GFs depend on the chemical composition (Raoult term) and the dry size (Kelvin effect) of a particle. Petters and Kreidenweis (2007) introduced the hygroscopicity parameter κ, which captures the contribution of the Raoult term to the hygroscopic growth. Monthly mean κ values varied between 0.17 and 0.31 (Fig. 1B) and were similar for all investigated dry sizes, thus implying similar chemical composition. Furthermore, the observed size dependence of GFs can largely be explained by the Kelvin effect. More detailed results will be presented and compared to results of previous short-term studies at this site (Sjogren et al., 2008). Petters, M. D., and S. M. Kreidenweis (2007). Atmos. Chem. Phys., 7(8), 1961-1971. Sjogren, S., et al. (2008). Atmos. Chem. Phys., 8(18), 5715-5729. Figure 1: A) Mean values of the hygroscopic growth factor and B) hygroscopicity parameter κ for all months and all dry particle diameters measured.