Measured and predicted aerosol light scattering enhancement factors at the high alpine site Jungfraujoch R. Fierz-Schmidhauser, P. Zieger, M. Gysel, L. Kammermann, P. F. DeCarlo, U. Baltensperger, and E. Weingartner Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland
Abstract. 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 (σsp) is
therefore an important variable for climate forcing calculations. We
used a humidification system for a nephelometer which allows for the
measurement of σsp at a defined RH in the range of
20–95%. In this paper we present measurements of light scattering
enhancement factors f(RH)=σsp(RH)/σsp(dry)
from a 1-month campaign (May 2008) at the high alpine site
Jungfraujoch (3580 m a.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 simplified model, which uses the Ångström
exponent of σsp(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 σsp, backscatter fraction and
single scattering albedo have a distinct impact on the radiative
forcing of the Jungfraujoch aerosol.
Citation: Fierz-Schmidhauser, R., Zieger, P., Gysel, M., Kammermann, L., DeCarlo, P. F., Baltensperger, U., and Weingartner, E.: Measured and predicted aerosol light scattering enhancement factors at the high alpine site Jungfraujoch, Atmos. Chem. Phys., 10, 2319-2333, doi:10.5194/acp-10-2319-2010, 2010.