This work aims to build a better understanding of AS/SA aerosols and work towards developing a complete mechanistic model for the appearance of the bimodal aerosol size distribution. Our working hypothesis, based on other work on AS/SA mixtures, is that SA experiences enhanced volatility in the presence of AS. Crystallization of different SA polymorphs may also be a factor, as polymorphism of another dicarboxylic acid, glutaric acid, has explained a similar bimodal size distribution.
To test these possibilities, aqueous mixtures of AS/SA of varying concentrations and mixing ratios were atomized into micronscale droplets, dried to nanoscale sizes, and analyzed using a Scanning Mobility Particle Sizer (SMPS) system to produce particle size distributions. The presence of the bimodal distribution was observed to depend on the AS/SA ratio. Further characterization techniques required particle collection, where particles were first collected on an impactor prior to analysis.
Collected aerosols were studied for their particle morphology and thermal properties. Powder X-ray diffraction (PXRD) was used to find the relative amounts of two different SA polymorphs (α and β) in aerosols containing varying ratios of AS/SA. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to investigate the evaporation of SA in different aerosols and aqueous mixtures. The PXRD and TGA/DSC data were analyzed to determine the relationship, if any, between aerosols exhibiting a bimodal size distribution in the SMPS data and their thermal and crystalline properties.
This combination of SMPS, PXRD, and TGA/DSC data was used to better characterize the AS/SA aerosol system and relate aerosol properties to possible mechanisms of formation. Understanding this phenomenon will improve how laboratory aerosols are used to model climate and may offer new insight into atmospheric processes involving mixed organic/inorganic aerosols.