Abstract
Abstract. A hygroscopicity tandem differential mobility analyzer (HTDMA) was operated at the high-alpine site Jungfraujoch in order to characterize the hygroscopic diameter growth factors of the free tropospheric Aitken and accumulation mode aerosol. More than ~5000 h of valid data were collected for the dry diameters D0 = 35, 50, 75, 110, 165, and 265 nm during the 13-month measurement period from 1 May 2008 through 31 May 2009. No distinct seasonal variability of the hygroscopic properties was observed. Annual mean hygroscopic diameter growth factors (D/D0) at 90% relative humidity were found to be 1.34, 1.43, and 1.46 for D0 = 50, 110, and 265 nm, respectively. This size dependence can largely be attributed to the Kelvin effect because corresponding values of the hygroscopicity parameter κ are nearly independent of size. The mean hygroscopicity of the Aitken and accumulation mode aerosol at the free tropospheric site Jungfraujoch was found to be κ≈0.24 with little variability throughout the year. The impact of Saharan dust events, a frequent phenomenon at the Jungfraujoch, on aerosol hygroscopicity was shown to be negligible for D0<265 nm. Thermally driven injections of planetary boundary layer (PBL) air, particularly observed in the early afternoon of summer days with convective anticyclonic weather conditions, lead to a decrease of aerosol hygroscopicity. However, the effect of PBL influence is not seen in the annual mean hygroscopicity data because the effect is small and those conditions (weather class, season and time of day) with PBL influence are relatively rare. Aerosol hygroscopicity was found to be virtually independent of synoptic wind direction during advective weather situations, i.e. when horizontal motion of the atmosphere dominates over thermally driven convection. This indicates that the hygroscopic behavior of the aerosol observed at the Jungfraujoch can be considered representative of the lower free troposphere on at least a regional if not continental scale.
Highlights
Atmospheric aerosol particles can scatter and absorb the incident sunlight, thereby changing the radiation budget of the earth
Chemical composition and particle diameter influence the ability for particles to take up water and to be activated to cloud condensation nuclei (CCN; McFiggans et al, 2006), which determines their indirect effect on climate, caused through modification of cloud properties (Lohmann and Feichter, 2005)
It has to be emphasized that mean growth factor probability density functions (GF-PDFs) represent the mean distribution of growth factors, where the appearance of e.g. a broad mode or two distinct modes in the mean GF-PDF does not imply that particles of distinctly different hygroscopicity and composition were simultaneously observed
Summary
Atmospheric aerosol particles can scatter and absorb the incident sunlight, thereby changing the radiation budget of the earth. The direct and indirect climate effects of anthropogenic aerosols are complex, associated with nonlinear feedbacks. They are still among the highest uncertainties in current climate models (IPCC, 2007). In this study we present a 13-month climatology of the aerosol hygroscopicity at the High Alpine Research Station Jungfraujoch (JFJ, 3580 m a.s.l.). This is, to our knowledge, the first study which investigated the hygroscopicity of the free tropospheric submicron aerosol at subsaturated RH over the course of a whole year. A detailed analysis of the seasonal cycle, diurnal patterns and synoptic scale weather classes was done in order to characterize the influence from e.g. injections of planetary boundary layer (PBL) air or different source regions on aerosol hygroscopicity
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