Prediction of Phase State of Secondary Organic Aerosol Internally Mixed with Aqueous Inorganic Salts

Abstract

In the presence of inorganic salts, secondary organic aerosol (SOA) undergoes liquid–liquid phase separation (LLPS), liquid–solid phase separation, or a homogeneous phase in ambient air. In this study, a regression model was derived to predict aerosol phase separation relative humidity (SRH) for various organic and inorganic mixes. The model implemented organic physicochemical parameters (i.e., oxygen to carbon ratio, molecular weight, and hydrogen-bonding ability) and the parameters related to inorganic compositions (i.e., ammonium, sulfate, nitrate, and water). The aerosol phase data were observed using an optical microscope and also collected from the literature. The crystallization of aerosols at the effloresce RH (ERH) was semiempirically predicted with a neural network model. Overall, the greater SRH appeared for the organic compounds with the lower oxygen to carbon ratios or the greater molecular weight and the higher aerosol acidity or the larger fraction of inorganic nitrate led to the lower SRH. The resulting model has been demonstrated for three different chamber-generated SOA (originated from β-pinene, toluene, and 1,3,5-trimethylbenzene), which were internally mixed with the inorganic aqueous system of ammonium–sulfate–water. For all three SOA systems, both observations and model predictions showed LLPS at RH <80%. In the urban atmosphere, LLPS is likely a frequent occurrence for the typical anthropogenic SOA, which originates from aromatic and alkane hydrocarbon.

Emerging investigator series: surfactants, films, and coatings on atmospheric aerosol particles: a review

“…The organic species initially formed islands on the surface but then transitioned to a core–shell structure with increasing RH and/or organic mass fraction. Yu et al 187 applied machine learning and a neural network to predict phase separation in SOA generated from β-pinene, toluene, and mesitylene that was internally mixed with water and ammonium sulfate. Modeling studies, like laboratory experiments, are moving towards ever more complex systems representative of aerosols in the atmosphere.…”

Secondary organic aerosol formation via multiphase reaction of hydrocarbons in urban atmospheres using CAMx integrated with the UNIPAR model

“…In brief, the key components of the UNIPAR model are described as follows, 1. SOA formation via aqueous phase reactions of organic species is simulated based on the assumption of liquidliquid phase separation (LLPS) between the organic phase and the salted inorganic solution (Yu et al, 2021a;Im et al, 2014). SOA formation is processed via multiphase partitioning, organic phase oligomerization, and aqueous phase reactions in wet inorganic salted aerosol.…”

“…For the ISORROPIA model, mutual deliquescence relative humidity (MDRH) is predicted. In addition, the efflorescence relative humidity (ERH) is predicted using a pre-trained neural network model based on the inorganic composition (Yu et al, 2021a). With MDRH and ERH, the aerosol condition is then estimated to be wet (organic phase + inorganic aqueous phase) or dry (organic phase + solid-dry inorganic phase).…”

“…Therefore, the quantity of aerosol liquid water content is an important model parameter to simulate aqueous reactions. In the CAMx-UNIPAR modules, the phase state of inorganic aerosol is determined by using the calculated ERH based on the pre-trained neural network mathematical equation (Yu et al, 2021a) and the inorganic compositions (sulfate, ammonium, nitrate, and protons) predicted from the inorganic thermodynamic model, ISORROPIA (Fountoukis and Nenes, 2007). When inorganic aerosol is wet, the aerosol liquid water content is calculated by using the inorganic thermodynamic model.…”

Kinetics of the nitrate-mediated photooxidation of monocarboxylic acids in the aqueous phase

“…3 However, in aqueous aerosols, they are attributed to both aqueous processing (in the case of dicarboxylic acids) and the formation of nonvolatile carboxylate salts during long-range transport (in the case of monocarboxylic acids). 7,12,19 The hydroxyl radical ($OH) is widely recognized to be an important oxidant in atmospheric aqueous phases. 20 Many organic compounds have high reaction rates with $OH in the aqueous phase, most of which will lead to lifetimes close to those in the gas phase.…”