Salting out and nitrogen effects on cloud-nucleating ability of amino acid aerosol mixtures

Abstract

Atmospheric aerosols exist as complex mixtures containing three or more compounds. Ternary aerosol mixtures composed of organic/organic/inorganic can undergo liquid–liquid phase separation (LLPS) under supersaturated conditions, affecting phase morphology and water uptake propensity. Phase separation and water uptake in ternary systems has previously been parameterized by oxygen to carbon (O : C) ratio; however, nitrogen containing organics, such as amino acid aerosols, also exist within complex mixtures. Yet, amino acid mixture CCN activity is poorly understood. In this study, we study the supersaturated hygroscopicity of three systems of internal mixtures containing ammonium sulfate (AS), 2-methylglutaric acid (2-MGA), and an amino acid. The three systems are AS/2-MGA/proline (Pro), AS/2-MGA/valine (Val), and AS/2-MGA/leucine (Leu). The amino acids are similar in O : C ratios but vary in solubility. Water-uptake, across a range of aerosol compositions in the ternary space, is measured using a cloud condensation nuclei counter (CCNC) from 0.4 to 1.7% supersaturation (SS). The single hygroscopicity parameter, κ, was calculated from CCNC measurements. All three systems exhibit two regions; one of these regions is phase separated mixtures when the composition is dominated by AS and 2-MGA; 2-MGA partitions to the droplet surface due to its surface-active nature and has a negligible contribution to water uptake. The second region is a homogeneous aerosol mixture, where all three compounds contribute to hygroscopicity. However, well mixed aerosol hygroscopicity is dependent on the solubility of the amino acid. Mixed Pro aerosols are the most hygroscopic while Leu aerosols are the least hygroscopic. Theoretical κ values were calculated using established models, including traditional κ-Köhler, O : C solubility and O : C-LLPS models. To account for the possible influence of polar N–C bonds on solubility and water uptake, the X : C parameterization is introduced through the X : C solubility and X : C-LLPS models; X : C is obtained from the ratio of oxygen and nitrogen to carbon. The study demonstrates competing organic–inorganic interactions driven by salting out effects in the presence of AS. Traditional methods cannot further encapsulate the non-ideal thermodynamic interactions within nitrogen-containing organic aerosol mixtures thus predictions of LLPS and hygroscopicity in nitrogen containing ternary systems should incorporate surface activity, O–C, N–C bonds, and salting out effects.