The Role of Organic Nitrogen for Aerosol Solubility, Phase Separation and CCN Activity

NAHIN FERDOUSI, Kotiba A. Malek, Kanishk Gohil, Kiran Reddy Pitta, Tim Raymond, Dabrina Dutcher, Miriam Freedman, Akua Asa-Awuku, University of Maryland, College Park

     Abstract Number: 85
     Working Group: Aerosols, Clouds and Climate

Abstract
Aerosols are present as complex organic-inorganic mixtures within our atmosphere; and under certain conditions can exist with phase separated morphology. When submicron aerosols are exposed to supersaturated conditions (>100% RH), the water uptake ability of the aerosols vary based on the composition of the mixture. The hygroscopicity of complex mixtures presenting liquid -liquid phase separation 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. The water-uptake properties of mixtures containing nitrogen containing compounds, such as amino acids, are not well defined. 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/Leu), valine system (2-MGA/AS/Val), and proline system (2-MGA/AS/Pro). 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) from 0.3 to 1.5% supersaturation (SS). For all mixtures, the single hygroscopic parameter κ was calculated. 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. Each model provides insight into the complex nature of aerosol mixtures and subsequent droplet growth. Thus, this provides an improved understanding of organic nitrogen-containing aerosol mixtures’ water uptake abilities through the introduction of new coupled parameter models. 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.