Dry and ambient aerosol properties at the Jungfraujoch

Atmospheric aerosol is defined as a suspension of solid or liquid particles in air. The major concerns about aerosol particles are their adverse health effects and their role in the Earth's climate system. Atmospheric particles influence the Earth's radiation budget in two ways: either directly by scattering and absorbing incoming solar radiation (and to a far less extent longwave radiation emitted from the Earth) or indirectly by their ability to act as cloud condensation nuclei (CCN). With increasing particle concentrations, caused by, e.g., anthropogenic emissions, more CCN are available to form cloud droplets. For a fixed water content in the atmosphere this leads to more but smaller droplets and therefore to enhanced cloud albedos as well as to rain suppression. Both effects have been recognized in recent years to be of great importance for understanding the observed climate change. Nevertheless, they remain poorly understood and quantified. The Jungfraujoch (JFJ, 3580 m asl; 46.548°N, 7.984°E) High Alpine Research Station is equipped with instruments that have for decades performed important Global Climate Observations. A major part of the aerosol instruments perform in-situ measurements. However, due to the harsh weather conditions, they usually cannot be run outdoors. The ambient air has to be inducted into a housing, such that the aerosol is sampled at a temperature (T) and relative humidity (RH) different from the ambient values. Therefore, the measured aerosol properties may considerably differ from the ambient — the climate-relevant — ones. During the first Cloud and Aerosol Characterization Experiment (CLACE) at the JFJ in-situ aerosol size distributions were measured simultaneously indoor at dry conditions (T ≈ 25 °C and RH < 10%) and, for the first time in such an exposed place, outdoor at ambient conditions (T < –5 °C) by means of two scanning mobility particle sizers (SMPS). The data set was completed by measurements of hygroscopic growth factors using a hygroscopicity tandem differential mobility analyzer (H-TDMA). A comparison of dry and ambient size distributions shows two main features: First, the dry total number concentration is often considerably smaller (on average 28%) than the ambient total number concentration. This is most likely due to the evaporation of volatile material at the higher indoor temperature. These particle losses mainly concern small particles (dry diameter Ddry ≤ 100 nm), and therefore have only a minimal affect on the surface and volume concentrations. Second, the dry number size distribution is shifted towards smaller particles, reflecting the hygroscopic behavior of the aerosols. This shift was modeled using a modified Köhler equation adapted to the measured hygroscopic growth factors. The corrected dry surface and volume concentrations are in good agreement with the ambient measurements for the whole RH range, but the correction works best for RH < 80%. The results indicate that size distribution d