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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[thin space (1/6-em)]:[thin space (1/6-em)]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[thin space (1/6-em)]:[thin space (1/6-em)]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[thin space (1/6-em)]:[thin space (1/6-em)]C solubility and O[thin space (1/6-em)]:[thin space (1/6-em)]C-LLPS models. To account for the possible influence of polar N–C bonds on solubility and water uptake, the X[thin space (1/6-em)]:[thin space (1/6-em)]C parameterization is introduced through the X[thin space (1/6-em)]:[thin space (1/6-em)]C solubility and X[thin space (1/6-em)]:[thin space (1/6-em)]C-LLPS models; X[thin space (1/6-em)]:[thin space (1/6-em)]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.
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Resolving the mechanisms of hygroscopic growth and cloud condensation nuclei activity for organic particulate matter
Abstract Hygroscopic growth and cloud condensation nuclei activation are key processes for accurately modeling the climate impacts of organic particulate matter. Nevertheless, the microphysical mechanisms of these processes remain unresolved. Here we report complex thermodynamic behaviors, including humidity-dependent hygroscopicity, diameter-dependent cloud condensation nuclei activity, and liquid–liquid phase separation in the laboratory for biogenically derived secondary organic material representative of similar atmospheric organic particulate matter. These behaviors can be explained by the non-ideal mixing of water with hydrophobic and hydrophilic organic components. The non-ideality-driven liquid–liquid phase separation further enhances water uptake and induces lowered surface tension at high relative humidity, which result in a lower barrier to cloud condensation nuclei activation. By comparison, secondary organic material representing anthropogenic sources does not exhibit complex thermodynamic behavior. The combined results highlight the importance of detailed thermodynamic representations of the hygroscopicity and cloud condensation nuclei activity in models of the Earth’s climate system.
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- Award ID(s):
- 1640378
- PAR ID:
- 10154335
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 9
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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