Laboratory studies of the physicochemical properties of mixed organic/mineral dust atmospheric aerosols: hygroscopicity and cloud condensation nuclei activity

Mineral dust aerosol generated from windblown soil can participate in climate forcing either directly through scattering or absorbing solar radiation or indirectly through acting as cloud condensation nuclei (CCN). In recent field studies organic material, such as oxalic acid and humic-like substances (HULIS), has been shown to be present in mineral dust aerosol. The presence of these internally mixed organic compounds can alter the physicochemical properties of the dust particles in the Earth's atmosphere. Thus, in this dissertation research the hygroscopic growth and CCN activity of model humic and fulvic acids and of calcite (CaCO₃) particles coated with humic and fulvic acids has been measured. Furthermore, the CCN activity of calcite aerosol reacted with oxalic acid (H₂C₂O₄) has been measured and compared to that of the humic and fulvic acids. The CCN measurements indicate that humic- or fulvic acid-coated calcite particles are significantly more CCN active than uncoated calcite particles, whereas reacted oxalate/calcite particles are not significantly more CCN active than the unreacted calcite particles, because the enhancement in CCN activity is reduced due to the reaction of calcite with oxalic acid to yield calcium oxalate. These results show that atmospheric processing of mineral dust through surface adsorption and/or heterogeneous reactions can alter hygroscopicity and CCN activity to an extent which depends on mineralogy and chemical speciation.