Abstract

Abstract. Data from measurements of hygroscopic growth of submicrometer aerosol with a hygroscopicity tandem differential mobility analyzer (HTDMA) during four campaigns at the high alpine research station Jungfraujoch, Switzerland, are presented. The campaigns took place during the years 2000, 2002, 2004 and 2005, each lasting approximately one month. Hygroscopic growth factors (GF, i.e. the relative change in particle diameter from dry diameter, D0, to diameter measured at higher relative humidity, RH) are presented for three distinct air mass types, namely for: 1) free tropospheric winter conditions, 2) planetary boundary layer influenced air masses (during a summer period) and 3) Saharan dust events (SDE). The GF values at 85% RH (D0=100 nm) were 1.40±0.11 and 1.29±0.08 for the first two situations while for SDE a bimodal GF distribution was often found. No phase changes were observed when the RH was varied between 10–90%, and the continuous water uptake could be well described with a single-parameter empirical model. The frequency distributions of the average hygroscopic growth factors and the width of the retrieved growth factor distributions (indicating whether the aerosol is internally or externally mixed) are presented, which can be used for modeling purposes. Measurements of size resolved chemical composition were performed with an aerosol mass spectrometer in parallel to the GF measurements. This made it possible to estimate the apparent ensemble mean GF of the organics (GForg) using inverse ZSR (Zdanovskii-Stokes-Robinson) modeling. GForg was found to be ~1.20 at aw=0.85, which is at the upper end of previous laboratory and field data though still in agreement with the highly aged and oxidized nature of the Jungfraujoch aerosol.

Highlights

  • Aerosol particles in the atmosphere affect the earth’s radiation balance in various ways (e.g. Solomon et al, 2007)

  • The RH-dependence of growth factor (GF) was measured by variation of the RH in the HTDMA between 10 and 85%

  • The growth curves were fitted with an empirical power law fit GF=(1−aw)γ (Swietlicki et al, 2000), dashed lines in Fig. 2, but for this model we found consistently larger χ 2-residuals than with the former model

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Summary

Introduction

Aerosol particles in the atmosphere affect the earth’s radiation balance in various ways (e.g. Solomon et al, 2007). This direct aerosol effect is influenced by the hygroscopicity of the aerosol particles, which is determined mainly by their chemical composition. The tendency for cloud formation and resulting cloud properties depend on chemical composition as well as on size distribution of the aerosol particles The cloud albedo and the radiative properties of cloud droplets are influenced; this is termed the indirect aerosol effect. The presence of particulate water allows for physical processes (e.g., shape modification) or heterogeneous chemical reactions, which in turn influences the chemical composition. These processes are commonly referred to as the aging of aerosols

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