Measured and predicted aerosol light scattering enhancement factors at the high alpine site Jungfraujoch

Ambient relative humidity (RH) determines the water content of atmospheric aerosol particles and thus has an important influence on the amount of visible light scattered by particles. The RH dependence of the particle light scattering coefficient ( sigma sp) is therefore an important variable for climate forcing calculations. We used a humidification system for a nephelometer which allows for the measurement of sigma sp at a defined RH in the range of 20–95%. In this paper we present measurements of light scattering enhancement factors f (RH)= Sigma sp(RH)/ Sigma sp(dry) from a 1-month campaign (May 2008) at the high alpine site Jungfraujoch (3580ma.s.l.), Switzerland. Measurements at the Jungfraujoch are representative for the lower free troposphere above Central Europe. For this aerosol type hardly any information about the f (RH) is available so far. At this site, f (RH=85%) varied between 1.2 and 3.3. Measured f (RH) agreed well with f (RH) calculated with Mie theory using measurements of the size distribution, chemical composition and hygroscopic diameter growth factors as input. Good f (RH) predictions at RH<85% were also obtained with a simplifiedmodel, which uses the Angström exponent of  Sigma sp(dry) as input. RH influences further intensive optical aerosol properties. The backscatter fraction decreased by about 30% from 0.128 to 0.089, and the single scattering albedo increased on average by 0.05 at 85% RH compared to dry conditions. These changes in  sp, backscatter fraction and single scattering albedo have a distinct impact on the radiative forcing of the Jungfraujoch aerosol.