Effect of humidity on aerosol light absorption and its implications for extinction and the single scattering albedo illustrated for a site in the lower free troposphere

The influence of relative humidity (RH) on the aerosol absorption coefficient σa was modeled for particles representative of the high-alpine site Jungfraujoch (JFJ, 3580 m asl). Based on a concentric core/coating particle model, RH enhancement factors for absorption, χ(RH)=σa(RH)/σa(RH=0%), were calculated for ten wavelengths between 370 and 950 nm for a summer and a winter case. This wavelength interval corresponds to long-term measurements of dry aerosol absorption coefficients at the JFJ. Depending on particle size and wavelength, the RH enhancement factors range from 0.94 to 1.78 in summer and from 0.84 to 1.53 in winter at RH between 0% and 99%. However, we demonstrate that even though the humidity effect on absorption is substantial, its maximum contribution (averaged over all considered size distributions) to the humidity effect on extinction and the single scattering albedo is only 0.2% within the wavelength range from 450 to 700 nm. There are two reasons for this: first, the humidity effect on scattering exceeds the humidity effect on absorption by far, and second, the JFJ aerosol particles scatter much more light than they absorb.