Abstract. This study presents a characterization of the hygroscopic growth behaviour and effects of different inorganic seed particles on the formation of secondary organic aerosols (SOAs) from the dark ozone-initiated oxidation of isoprene at low NOx conditions. We performed simulations of isoprene oxidation using a gas-phase chemical reaction mechanism based onthe Master Chemical Mechanism (MCM) in combination with an equilibriumgas–particle partitioning model to predict the SOA concentration. Theequilibrium model accounts for non-ideal mixing in liquid phases, includingliquid–liquid phase separation (LLPS), and is based on the AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients) model for mixture non-ideality and the EVAPORATION (Estimation of VApour Pressure of ORganics, Accounting for Temperature,Intramolecular, and Non-additivity effects) model for pure compound vapourpressures. Measurements from the Cosmics Leaving Outdoor Droplets (CLOUD)chamber experiments, conducted at the European Organization for NuclearResearch (CERN) for isoprene ozonolysis cases, were used to aid inparameterizing the SOA yields at different atmospherically relevanttemperatures, relative humidity (RH), and reacted isoprene concentrations. To represent the isoprene-ozonolysis-derived SOA, a selection of organicsurrogate species is introduced in the coupled modelling system. The modelpredicts a single, homogeneously mixed particle phase at all relativehumidity levels for SOA formation in the absence of any inorganic seedparticles. In the presence of aqueous sulfuric acid or ammonium bisulfateseed particles, the model predicts LLPS to occur below ∼ 80 % RH, where the particles consist of an inorganic-rich liquid phase andan organic-rich liquid phase; however, this includes significant amounts of bisulfate and water partitioned to the organic-rich phase. The measurements show an enhancement in the SOA amounts at 85 % RH, compared to 35 % RH, for both the seed-free and seeded cases. The model predictions of RH-dependent SOA yield enhancements at 85 % RH vs. 35 % RH are 1.80 for a seed-free case, 1.52 for the case with ammonium bisulfate seed, and 1.06 for the case with sulfuric acid seed. Predicted SOA yields are enhanced in the presence of an aqueous inorganic seed, regardless of the seed type (ammonium sulfate, ammonium bisulfate, or sulfuric acid) in comparison with seed-free conditions at the same RH level. We discuss the comparison of model-predicted SOA yields with a selection of other laboratory studies on isoprene SOA formation conducted at different temperatures and for a variety of reacted isoprene concentrations. Those studies were conducted at RH levels at or below 40 % with reported SOA mass yields ranging from 0.3 % up to 9.0 %, indicating considerable variations. A robust feature of our associated gas–particle partitioning calculations covering the whole RH range is the predicted enhancement of SOA yield at high RH (> 80 %) compared to low RH (dry) conditions, which is explained by the effect of particle water uptake and its impact on the equilibrium partitioning of all components.
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Single-particle experiments measuring humidity and inorganic salt effects on gas-particle partitioning of butenedial
Abstract. An improved understanding of the fate and properties of atmospheric aerosolparticles requires a detailed process-level understanding of fundamentalfactors influencing the aerosol, including partitioning of aerosolcomponents between the gas and particle phases. Laboratory experiments withlevitated particles provide a way to study fundamental aerosol processesover timescales relevant to the multiday lifetime of atmospheric aerosolparticles, in a controlled environment in which various characteristicsrelevant to atmospheric aerosol can be prepared (e.g., highsurface-to-volume ratio, highly concentrated or supersaturated solutions,changes to relative humidity). In this study, the four-carbon unsaturatedcompound butenedial, a dialdehyde produced by oxidation of aromaticcompounds that undergoes hydration in the presence of water, was used as amodel organic aerosol component to investigate different factors affectinggas–particle partitioning, including the role of lower-volatility“reservoir” species such as hydrates, timescales involved inequilibration between higher- and lower-volatility forms, and the effect ofinorganic salts. The experimental approach was to use a laboratory systemcoupling particle levitation in an electrodynamic balance (EDB) withparticle composition measurement via mass spectrometry (MS). In particular,by fitting measured evaporation rates to a kinetic model, the effectivevapor pressure was determined for butenedial and compared under differentexperimental conditions, including as a function of ambient relativehumidity and the presence of high concentrations of inorganic salts. Even underdry (RH<5 %) conditions, the evaporation rate of butenedial isorders of magnitude lower than what would be expected if butenedial existedpurely as a dialdehyde in the particle, implying an equilibrium stronglyfavoring hydrated forms and the strong preference of certain dialdehydecompounds to remain in a hydrated form even under lower water contentconditions. Butenedial exhibits a salting-out effect in the presence ofsodium chloride and sodium sulfate, in contrast to glyoxal. The outcomes ofthese experiments are also helpful in guiding the design of future EDB-MSexperiments.
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- Award ID(s):
- 1808084
- NSF-PAR ID:
- 10176966
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 19
- Issue:
- 22
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 14195 to 14209
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-19-14195-2019
