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 scat- tered by particles. The RH dependence of the particle light scattering coefficient (σsp) is therefore an important variable for climate forcing calculations. We used a humidification system for a nephelometer which allows for the measure- ment of σsp at a defined RH in the range of 20–95%. In this paper we present measurements of light scattering en- hancement factors f (RH)=σsp(RH)/σsp(dry) from a 1-month campaign (May 2008) at the high alpine site Jungfraujoch (3580ma.s.l.), Switzerland. Measurements at the Jungfrau- joch 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 hygro- scopic diameter growth factors as input. Good f (RH) pre- dictions at RH<85% were also obtained with a simplified model, which uses the ˚ Angstr¨ om exponent of σsp(dry) as in- put. RH influences further intensive optical aerosol proper- ties. 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 σsp, backscatter fraction and single scatter- ing albedo have a distinct impact on the radiative forcing of the Jungfraujoch aerosol.