Adaptation of dry nephelometer measurements to ambient conditions at the Jungfraujoch.

In a numerical study the influence of relative humidity (RH) on aerosol scattering coefficients sigma was investigated. Based on a core/coating aerosol model, RH enhancement factors for scattering, xi(RH) = sigma(RH)/sigma(RH = 0), were calculated for the wavelengths lambda = 450, 550, and 700 nm for a summer and a winter case. The investigation was adapted to the situation (e.g., chemical composition, particle size distributions, hygroscopic behavior) of the high-alpine site Jungfraujoch (JFJ, 3580 m asl), where long-term measurements of dry aerosol scattering coefficients are performed at these wavelengths. The presented results are therefore representative of the lower free troposphere above a continent. The RH enhancement factors at a specific RH strongly depend on the average particle size. For example, at RH = 85% they vary between approximately 1.2 and approximately 2.7 in summer and between approximately 1.4 and approximately 3.8 in winter. It is shown that there is a strong relationship between xi and the Angstrom exponent å (based on scattering only) of the dry aerosol, which is directly derived from the dry scattering measurements. This allows for parametrizing xi for a specific wavelength and season with å and RH. The parametrization is applicable for RH up to approximately 90%--for higher RH the underlying hygroscopic models become unreliable--and for å between approximately -0.25 and approximately 2.75, which covers the range observed at the JFJ. Also addressed is a systematic error in the dry scattering coefficients measured with a nephelometer previously discussed in the literature, which arises from nonidealities in the angular intensity distribution of the light inside the instrument. This effect also depends strongly on the particle size and can be described by a correction factor C that can be parametrized with å. The scattering coefficient corrected for measurement artifacts at ambient RH for specific wavelength and season therefore can be estimated from the uncorrected dry nephelometer scattering coefficient sigma(neph) as sigma(å, RH) = C(å) x xi(å, RH) x sigma(neph). As additional information only ambient RH data are needed. The 95% confidence bound of this total correction ranges from less than 5% for low RH and large å up to approximately 40% for high RH and small å.