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Heterogeneous reactions of trace gases with mineral dust aerosol not only impact the chemical balance of the atmosphere but also the physicochemical properties of the dust particle and the ability of the particle to act as a cloud condensation nuclei (CCN). Recent field studies have shown that carbonate minerals are preferentially associated with nitrates whereas aluminum silicates (i.e., clay minerals) are preferentially associated with sulfates. To better understand how this association can impact the climate effects of mineral dust particles, we have measured the CCN activity of a number of pure and internal mixtures of aerosols relevant to these recent field studies. The CCN activity of CaCO 3 -Ca(NO 3 ) 2 aerosol, simulating the activity of mineral dust aerosol that has been partially processed by nitrogen oxides in the atmosphere, is significantly enhanced relative to CaCO 3 aerosol of the same diameter. Similar results are obtained for a clay mineral, kaolinite, internally mixed with (NH 4 ) 2 SO 4 . For example, at 0.3% supersaturation, a 200 nm particle containing a soluble nitrate or sulfate component is 2 to 4 times more active than an unreacted particle. The results presented here show that when determining the contribution of mineral dust aerosol to the overall impact of the aerosol indirect effect on radiative forcing, changes in chemical composition due to atmospheric processing cannot be ignored.

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Environmental context. Humic and fulvic acids are macromolecular, multifunctional, polyacidic compounds that are important proxies for humic-like substances (HULIS), which are ubiquitous components of tropospheric particulate matter. The hygroscopic nature of these substances suggests that they can contribute to direct and indirect climate forcing. Thus, the effects of water uptake in humic-like particles in the atmosphere must be well understood. Abstract. The water uptake of humic and fulvic acid aerosols was determined by hygroscopic tandem differential mobility analysis (hTDMA) and extinction Fourier transform infrared (FTIR) spectroscopy. Water uptake on humic and fulvic acid thin films was also investigated using attenuated total reflectance (ATR) FTIR spectroscopy. The hygroscopic growth of monodisperse, 100-nm (dry) Suwannee River fulvic acid (SRFA) and humic acid sodium salt (NaHA) aerosols was determined and modelled using Köhler theory. A single parameter, the ionic density (ρion), which contains physical properties that are not well established for these substances, was determined for SRFA and NaHA to be 2.1 × 10–3 and 7.0 × 10–3 mol cm–3 respectively. The hygroscopic growth was then modelled using the ρion-Köhler equation and the critical parameters determined. The critical percent supersaturation of SRFA and NaHA was determined to be 0.60 and 0.33% respectively using the surface tension of water; and 0.35 and 0.19% respectively using the surface tension of aqueous HULIS. κ-Köhler theory, was also used to calculate the critical supersaturation and was found to be in good agreement with the ρion representation. Both extinction FTIR of aerosols and ATR-FTIR absorption measurements of thin films confirm enhanced water uptake with increasing relative humidity (RH).

Mineral dust aerosol generated from windblown soil can contribute to climate forcing either directly through scattering or absorbing solar radiation or indirectly through acting as cloud condensation nuclei (CCN). In recent field studies organic materials, such as oxalic acid and humic-like substances (HULIS), have 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 (CaCO 3) particles coated with humic and fulvic acids have been measured. Furthermore, the CCN activity of calcite aerosol reacted with oxalic acid (H 2 C 2 O 4) 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.