Humidity-Dependent Growth of Size-Segregated Aerosol Samples

This paper presents first results obtained with a measurement system designed to measure the humidity-dependent water uptake by deposited aerosol samples. Aerosol in the size range 0.06-16 μm aerodynamic equivalent diameter is sampled with a nine stage cascade impactor. The individual impaction foils are exposed to elevated relative humidities in a chamber containing an aqueous solution of CaCl2 of specified concentration. The mass increase of the deposits is measured with a balance above the chamber. Only equilibrium values of m/m0 (i.e., wet mass/dry mass) for increasing humidities can be obtained. The two samples discussed here showed a strongly size-selective growth pattern. Deposits with accumulation range particles have larger m/m0 than those with coarse mode particles (e.g., sample 1: 2.6 vs 1.33 at 90% humidity; sample 2: 2.35 vs 1.16). Although the two samples had been obtained at the same time of the year, the growth patterns are different, which might have been caused by the meteorological situation. Calculations of optical parameters (i.e., extinction coefficient and single scattering albedo) showed that in both cases, extinction (and therefore also the optical depth) and single scattering albedo increased with humidity. From the changes of the single scattering albedo it could be deduced that for the dry aerosol heating effects probably dominated over cooling effects, while for high humidities cooling predominated.