| Atmospheric aerosol particles change in size due to water uptake which is determined by
their chemical composition and the ambient relative humidity (RH). As a result also their
optical properties - especially the aerosol light scattering - strongly depend on RH. Therefore,
long-term measurements of aerosol physical and optical properties are generally
recommended at dry conditions in order to keep measurements comparable (e.g. RH < 30 -
40%). However, for the comparison of such ground-based measurements with other
optical aerosol measurements, for the purpose of aerosol correction of satellite
retrievals, or for the use in climate models, accurate knowledge of the RH effect is very
important.
The key parameter to describe the influence of RH on the aerosol light scattering is the
scattering enhancement factor f(λ,RH), which is defined as the scattering coefficient
Ïăsp(λ,RH) at a certain RH divided by the dry scattering coefficient Ïăsp(λ,RHdry):
-Ïăsp(RH,-λ)–
f(RH, λ) = Ïăsp(RHdry, λ),
(1)
where λ denotes the wavelength.
Here, we will present measurement and modeling results of the aerosol scattering
enhancement, which are based on our field experiments throughout Europe in the years
2008-2010. During these campaigns different aerosol optical and micro-physical properties
were measured in-situ and by various remote sensing techniques (e.g., LIDAR, MAX-DOAS
and satellite retrieval), which allowed comprehensive closure studies for the specific
sites.
Results will be shown for aerosol found at the high alpine site Jungfraujoch, Switzerland
(Fierz-Schmidhauser et al., 2010a), for Arctic aerosol measured at Ny-Ålesund,
Spitsbergen (Zieger et al., 2010), for maritime aerosol measured at Mace Head, Ireland
(Fierz-Schmidhauser et al., 2010b), and for aerosol typically found at Cabauw, The
Netherlands (Zieger et al., 2011) and Melpitz, Germany.
References:
Zieger, P., Weingartner, E., Henzing, J., Moerman, M., de Leeuw, G., Mikkilä, J., Ehn,
M., Petäjä, T., Clémer, K., van Roozendael, M., Yilmaz, S., Frieß, U., Irie, H.,
Wagner, T., Shaiganfar, R., Beirle, S., Apituley, A., Wilson, K., and Baltensperger,
U., Comparison of ambient aerosol extinction coefficients obtained from in-situ,
MAX-DOAS and LIDAR measurements at Cabauw, Atmos. Chem. Phys., 11, 2603-2624,
2011.
Fierz-Schmidhauser, R., Zieger, P., Vaishya, A., Monahan, C., Bialek, J., O’Dowd, C.D.,
Jennings, S.G., Baltensperger, U., and Weingartner, E.: Light scattering enhancement factors
in the marine boundary layer (Mace Head, Ireland), J. Geophys. Res., 115, D20204,
2010b.
Zieger, P., Fierz-Schmidhauser, R., Gysel, M., Ström, J., Henne, S., Yttri, K. E.,
Baltensperger, U., and Weingartner, E.: Effects of relative humidity on aerosol light scattering
in the Arctic, Atmos. Chem. Phys., 10, 3875-3890, 2010.
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, 2010a. |