Influence of Salting Out and Organic Nitrogen on Mixed Amino Acid Aerosol Cloud-Nucleating Ability

Aerosols are present as complex organic-inorganic mixtures within our atmosphere, resulting in particles presenting phase separated morphology. Mixed organic-inorganic aerosols can be predominantly found in nascent sea spray aerosols (SSA). When these aerosols are exposed to supersaturated conditions (>100% RH), the water uptake ability of the aerosols vary based on the composition of the mixture. Previous studies have characterized phase separated systems through the determination of an average oxygen to carbon (O/C) ratio where liquid-liquid phase separation (LLPS) reaches its limit. The hygroscopicity of complex mixtures presenting LLPS was previously studied through the measurement of CCN activity within a 2-methylglutaric (2-MGA)/ammonium sulfate (AS) binary system and a 2-MGA/AS/sucrose ternary system; both studies correlated water-uptake abilities to O/C and surface tension. However, little is known about the influence of solubility of the third component on phase separation of a ternary mixture containing 2-MGA/AS. Additionally, the water-uptake properties of mixtures containing nitrogen containing compounds, such as amino acids, are not well defined. Amino acids are a major component of SSA and can contribute to aerosol hygroscopicity. Therefore, it is undetermined if O/C alone is an acceptable parameter for the estimation of solubility and hygroscopicity of complex amino acid mixtures. To improve our understanding of LLPS within aerosol mixtures and factors influencing its presence, three ternary systems were studied – a leucine system (2-MGA/AS/leucine), valine system (2-MGA/AS/valine), and proline system (2-MGA/AS/proline). For each system, the CCN activity of mixture compositions with varying O/C ratios and compositions was measured using a Cloud Condensation Nuclei Counter (CCNC) at 0.375% to 1.667% SS. For all mixtures, the single hygroscopic parameter κ was calculated. Experimental κ-results demonstrated increased hygroscopic activity as the amino acid became more soluble in the order of leucine<valine<proline. Experimental κ results were compared against four theoretical models; three of the theoretical models included were Köhler theory, O/C LLPS with surface tension (O/C LLPS-ST) and a newly developed model, X/C LLPS with surface tension (X/C LLPS-ST). For this study, a new parameter considering O/C and nitrogen to carbon (N/C) X/C, was introduced as a parameterization for solubility. The O/C LLPS-ST model was adapted to consider X/C for subsequent estimations of κ. A fourth theoretical model took a weighted average of the O/C LLPS-ST and X/C LLPS-ST models. The study provides an improved understanding of amino acid aerosol mixtures’ water uptake abilities through the introduction of a new parameter and model. As a result, the study is able to show varied N/C contribution to the system based on the structure of the amino acid as well as a method to improve current abilities to predict hygroscopicity of these complex, nitrogen-containing aerosol mixtures.