Abstract. The contribution of biogenic secondary organic aerosol (BSOA) to cirrus cloud formation remains unresolved, contributing to uncertainty in aerosol-cloud interactions in global climate models. Laboratory studies report highly variable ice nucleating efficiencies for BSOA, suggesting that these particles may act as either homogeneous or moderately effective heterogeneous ice nuclei. Here, we investigate the deposition ice nucleating properties of α-pinene- and limonene-derived BSOA, including both self-nucleated particles and BSOA coatings on ammonium sulfate and ammonium bisulfate seed particles. Deposition ice nucleation relevant to cirrus clouds (−45 °C, −40 °C, −35 °C; 1.0 ≤ Sice ≤ 1.6) was measured using the SPectrometer for Ice Nucleation (SPIN). Bulk physicochemical properties relevant to ice nucleation were characterized using aerosol mass spectrometry (AMS) and volatility distributions. Pre-cooling was applied to modulate phase state as inferred from glass transition temperature (Tg). BSOA ice nucleating properties were strongly precursor dependent (p < 0.001). Tg was an unreliable predictor of freezing behavior, correctly anticipating freezing mode for only two of eleven particle combinations. Limonene-derived BSOA nucleated ice almost exclusively via heterogeneous freezing, with Sice onsets as low as 1.27±0.07 at -39.8±0.3 °C. α-pinene-derived BSOA predominantly nucleated ice homogeneously. BSOA coatings on ammonium bisulfate shifted freezing from homogeneous to heterogeneous, while the role of acid-catalyzed multiphase chemistry in ice nucleation remained inconclusive due to experimental limitations. These results demonstrate that cirrus-relevant BSOA parameterizations must explicitly account for precursor specific chemistry and broad classifications of BSOA ice nucleating abilities are inappropriate.

This review aims to summarize the effects of per- and poly-fluoroalkyl substances (PFAS) exposures on the lung, emphasizing data coverage across steps of human health risk assessments. There is expansive literature characterizing PFAS contamination in water, but recent studies have identified PFAS as a component of air pollution, thus impacts on the lung have been an increasing point of inquiry. Mounting evidence from human clinical/epidemiological, animal, and in vitro investigations supports relationships between PFAS exposures and adverse pulmonary outcomes including asthma, allergies, infections, and cancer. Focusing on toxicology studies using animal and in vitro lung cell models, exposures to PFAS modulated inflammation/immune responses, oxidative stress, mucus production, surfactant properties, and epithelial barrier integrity, representing important mechanisms impacting pulmonary health. There are expanding datasets linking PFAS exposures to adverse pulmonary outcomes; however, these data originated from mostly oral/ingestion exposure and not from volatilized or aerosolized PFAS exposure designs. Furthermore, there is a general lack of data informing dose-response modeling and risk characterization, representing gaps needed to characterize pulmonary health risks.

Atmospheric aerosols in Ethiopian urban environments remain poorly characterized. This study investigated the spatiotemporal variability of equivalent black carbon (eBC) concentrations and the molecular composition of organics in total suspended particulate matter (TSPM) collected in urban Addis Ababa. The level of eBC was monitored at major traffic intersections from January 2024 to December 2025. eBC concentrations were measured using a portable MicroAeth AE51. TSPM samples were collected on Teflon filters using a Leland Legacy sampler and analyzed by reversed-phase liquid chromatography coupled to electrospray ionization interfaced to quadrupole time-of-flight mass spectrometry (RPLC-ESI-QTOFMS) operated in the negative ion mode. The study revealed substantial spatiotemporal variability in eBC concentration, with a mean of 26.24 ± 42.81 µg m-3. Episodic peaks exceeding 1000 µg m-3 were recorded at heavily trafficked locations such as Kality. Diurnal patterns exhibited distinct morning and evening rush-hour maxima. Non-targeted analysis identified 74 compounds (with molecular weights spanning from 102 to 370 g mol-1), 41 of which were structurally assigned with high confidence. The organic fraction of TSPM was dominated by oxygenated (CHO) and nitrogen-containing (CHON) species, including carboxylic acids, oxocarboxylic acids, phenolics, aromatic acids, and nitro-aromatic compounds. Spatial heterogeneity was evident, particularly at Awotobistera, where it showed enrichment in biomass-burning tracers and lignin-derived compounds. Twenty-three identified compounds functioned as brown carbon chromophores, including nitro-aromatic and oxygenated aromatic species. Their occurrence suggests contributions from both primary combustion sources and atmospheric processing, although source apportionment was beyond the scope of this study. These findings provide a baseline for future investigations of aerosol sources, atmospheric processing, and urban air quality dynamics in Addis Ababa.

Isoprene, the largest nonmethane volatile hydrocarbon emitted into Earth's atmosphere, is rapidly oxidized by ambient hydroxyl radical (•OH) to yield secondary organic aerosol (SOA). Early generation gas-phase products of isoprene oxidation remain to be fully characterized. •OH addition at terminal C1/C4 followed by the addition of O2, yields β- and δ-peroxy (ISOPO2•) isomers. Under low-nitric oxide conditions, δ-ISOPO2• isomers branch between bimolecular reaction with HO2• and intramolecular H-shifts. Initial C5H8O3 products of H-shifts reported in chamber experiments were assigned δ-hydroperoxyalkenal (HPALD) structures and predicted to photolyze rapidly. We observe by ion mobility spectrometry that a putative δ-HPALD isomer from •OH addition at C4 is the cyclic peroxyhemiacetal, 5-methyl dioxinol, and may be the relevant product in atmospheric chemistry. 5-Methyl dioxinol is robust to photolysis at tropospheric UV-vis wavelengths and is thus potentially a significant previously unrecognized source of SOA. In chamber experiments, we found that 5-methyl dioxinol forms SOA and determined that the steady-state ratio of the proximal major unimolecular/bimolecular products downstream of ISOPO2• is ∼0.26. Using the total steady-state dioxinol level formed in the isoprene oxidation experiment, dioxinols may contribute ∼3.2 × 109 g yr-1, or 4% to isoprene-derived atmospheric carbon.

The Earth's atmosphere during the Archean eon (4.0-2.5 billion years ago) likely contained an intermittent organic haze. This haze, formed through photochemical reactions in the atmosphere, is mostly composed of molecular nitrogen (N2), carbon dioxide (CO2), and methane (CH4) and could have served as a protective layer shielding Earth's surface from harmful ultraviolet (UV) radiation. However, the chemical composition of these organic haze particles remains poorly understood, due in part to the lack of knowledge of how trace atmospheric constituents, such as sulfur gases from volcanic activity, affect haze chemistry. Determining organic/inorganic S speciation from atmospheric chemistry is important for interpreting Earth's geologic record. Here, we chemically characterized organic haze particles that form under Archean-relevant atmospheric conditions (0.1% CH4, 5 ppm of H2S) with either 0.5 or 0.1% CO2 in a N2 background by using hydrophilic interaction liquid chromatography coupled to electrospray ionization and high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). HILIC/ESI-HR-QTOFMS was able to achieve chromatographic resolution of isomers and provide accurate molecular formula determinations of organic haze particulate constituents. We show how the relative abundances of 121 compounds vary between these two CO2 concentrations. The observed molecules fall into 6 categories of elemental composition (i.e., CHN, CHS, CHNS, CHNO, CHOS, and CHNOS). Compounds composed of CHN, CHS, and CHNS formed preferentially under low-CO2 conditions and CHNOS molecules under high-CO2 conditions. Important biomolecules such as urea were detected, highlighting the fact that atmospheric photochemistry may have been an important source of complex organic molecules that could help the proliferation of early life.

Stereoisomers in aqueous atmospheric aerosols are assumed to be present in equal concentrations, as aerosols are most commonly mixtures of simple inorganic salts with a range of organic species that lack obvious pathways to induce stereoselectivity. Further, since submicron aerosols are highly acidic (pH ∼ 0-3), acid-driven nucleophilic reactions are a crucial pathway for forming key condensed-phase products. Herein, we show strong diastereoselectivity during the ring-opening reaction of β-isoprene epoxydiol (β-IEPOX) isomers by sulfate to form methyltetrol sulfates (MTS, e.g., 1,3,4-trihydroxy-2-methylbutan-2-yl sulfate) in acidic aqueous particles. These key epoxides are formed from isoprene (2-methyl-1,3-butadiene) oxidation and subsequent reactive uptake to aerosols, forming condensed-phase species (i.e., secondary organic aerosol, SOA). Diastereomers and regioisomers were quantified using hydrophilic interaction liquid chromatography coupled with high-resolution mass spectrometry. Acid-driven concerted attack of sulfate at the epoxide-ring tertiary site was the dominant pathway (∼93%) for MTS formation, while other pathways─concerted substitution at the secondary carbon and acid-driven stepwise substitution─contributed <10%. Computational chemistry simulations revealed a ∼15 kcal/mol lower energy barrier for concerted backside attack of sulfate to the epoxide compared with attack from the opposite face, in line with observed stereoselectivity. Major MTS diastereomers formed from trans-β-IEPOX and cis-β-IEPOX isomers were racemic mixtures of (2R,3S)/(2S,3R)-2-MTS and (2R,3R)/(2S,3S)-2-MTS, respectively, with similar diastereoselectivity. Analysis of MTS-derived products indicates that stereochemical differences are likely propagated through continued condensed-phase secondary chemistry, which may have significant implications for climate-relevant aerosol properties. Overall, this stereoselectivity within seemingly simple liquid aerosols has important implications for condensed-phase reactions in aqueous aerosols.

Per- and polyfluoroalkyl substances (PFAS) are highly persistent pollutants with known adverse impacts on environmental and public health. Traditional gas-phase PFAS detection approaches often involve labor-intensive sample collection and preparation, while offering low temporal resolution. Alternatively, chemical ionization mass spectrometry (CIMS) allows for real-time airborne PFAS detection at sub-pptv sensitivity. While reagent ion generation often requires hazardous chemicals (e.g., nitric acid, methyl iodide (I-), and acetic anhydride), superoxide (O2-) CIMS provides a safer alternative and is better suited for mobile platforms where ventilation, space, and weight are constrained. O2- CIMS has five main reagent ions (i.e., O2-, (H2O)O2-, CO3-, (CO2)O2-, and CO2(H2O)O2-) and low-background mass spectra above m/z 200. Thus, it is well suited to the relatively high molecular weights of airborne PFAS. Mass calibration was performed with a 5 : 1 fluorotelomer alcohol (FTOH) permeation tube. Ionization was found to occur mainly through deprotonation or adduct formation. Calibrations for fourteen environmentally- and industrially-relevant PFAS compounds are presented, including FTOHs, fluorotelomer diols, fluorinated sulfonamides, epoxides, and glycol ethers. While perfluoroalkyl carboxylic acids (PFCAs) were not detected, O2- CIMS offered higher sensitivity and lower detection/quantification limits than I- CIMS for FTOHs; however, it remains a complementary PFAS measurement technique to I- CIMS. Moreover, it yielded distinct fingerprint signals for FTOHs, confirming compound identification. This study demonstrates the utility of O2- CIMS for real-time airborne PFAS analysis in commonly encountered environments by capturing 6 : 2 FTOH gaseous emissions from fast-food packaging at room temperature, underscoring its strong promise for future development and applications.

2-Methyltetrol sulfate diastereomers (2-MTS) form from acid-driven multiphase chemical conversion of inorganic sulfate (Sulfinorg) aerosol by isoprene-derived epoxydiols (IEPOX). Studies have explored 2-MTS sinks within aerosols, but observations of 2-MTS in atmospheric waters indicate a need to understand the fate of 2-MTS in clouds and fogs. In batch reactor experiments, we oxidized 2-MTS in 30, 300, and 3000 μM solutions to understand 2-MTS fate in cloud, fog, and aerosol water. Ion chromatography (IC) revealed production of acetate, oxalate, and Sulfinorg with 2-MTS oxidation. Our 30, 300, and 3000 μM experiments revealed a 5%, 20%, and 32% gap in the sulfur mass balance, respectively, suggesting organosulfates (OSs) formed from 2-MTS oxidation. OSs produced by 2-MTS oxidation were detected by hydrophilic liquid interaction chromatography interfaced to electrospray ionization high-resolution tandem mass spectrometry at mass-to-charge ratios 139 (C2H3O5S-), 169 (C3H5O6S-), 185 (C3H5O7S-), 199 (C4H7O7S-), 211 (C5H7O7S-), 213 (C5H9O7S-), 227 (C5H7O8S-), and 229 (C5H9O8S-). 2-MTS oxidation was then accurately predicted with a rate constant of 1.5×109 M-1s-1. Results emphasize the utility of 2-MTS for sulfur cycling in the atmosphere, and the potential for IC to measure 2-MTS in atmospheric waters.

Per- and polyfluoroalkyl substances (PFAS) are emerging pollutants of concern, primarily due to their terminal degradation products, which exhibit environmental persistence and mobility. Several groups of PFAS, including hydrofluoroolefins (HFOs), perfluoro olefins (PFOs), perfluoro vinyl ethers (PVEs), and hydrofluoroalkanes (HF-alkanes), are volatile and reside predominantly in the gas phase. PFAS such as HFOs, PFOs, and PVEs are considered reactive and may generate short-chain degradation products that persist in the environment. Despite the importance of these gaseous PFAS, there is a lack of analytical techniques capable of providing high-resolution temporal measurements of potential precursors to terminal degradation products. This study presents the first real-time method for detecting and quantifying atmospheric HFOs, PFOs, PVEs, and HF-alkanes using a high-resolution chemical ionization mass spectrometer (HR-CIMS). Using NO+ mixed with O2+ (NO+/O2+), and O2+ as reagent ions, the CIMS was able to identify and quantify PFAS via fluoride abstraction (M - F)+, hydride abstraction (M - H)+, or charge transfer (M+) mechanisms. The method achieves 10-s limits of detection (LOD) ranging from 2 to 40 ppt, enabling online monitoring in ambient air, especially near emission sources or in indoor environments. The use of NO+/O2+ and O2+ reagent ions with HR-CIMS provides a novel and sensitive approach for real-time detection of PFAS via positive reagent ion modes, especially for emerging gas-phase PFAS that currently lack suitable online measurement techniques to better constrain their atmospheric emissions and concentrations.

Volatility and viscosity are important parameters affecting the formation, reaction, and fate of atmospheric organic aerosols. In this study, a Vaporization Inlet for Aerosol (VIA) coupled with a Vocus chemical ionization mass spectrometer (Vocus-CIMS) using NH4 + adduct ionization is employed to simultaneously detect and quantify the molecular composition and volatility of organic aerosols through a program-controlled temperature ramp, thereby providing viscosity information. Volatility calibration was conducted with a series of reference aerosol particles with different chemical compositions, covering a vapor pressure range from 10-1 to 10-8 Pa. Secondary organic aerosols (SOA) produced from the potential aerosol mass reactor were analyzed by the VIA-CIMS. Chemical species ranging from semivolatile to low-volatility, including highly oxygenated dimers, were identified. Individual ions from the collected mass spectra were fitted and grouped by volatility basis sets to yield the volatility distribution of the SOA, allowing for the quantification of the glass transition temperatures and viscosities. Results show that β-caryophyllene ozonolysis SOA has lower volatility and is more viscous than the α-pinene SOA. This approach enables the online quantification of SOA particle chemical composition and volatility distribution, while simultaneously characterizing particle phase state, such as viscosity and water diffusion time, providing crucial insights into their chemical processes and climate impacts.

Isoprene-derived secondary organic aerosol (SOA) constituents, such as the 2-methyltetrols (2-MT) and 2-methyltetrol sulfates (2-MTS), have been readily detected in atmospheric aerosols (PM2.5) and within mixtures containing ammonium sulfate (AS). Despite its prevalence, the water uptake of 2-MT, 2-MTS, and their mixtures is not well understood. In this study, we determine the physicochemical properties (e.g., surface activity, diffusivity, phase morphology) of synthesized 2-MT and 2-MTS samples and their mixtures with AS. 2-MT and 2-MTS have been identified as surface active and viscous. Thus, dynamic surface tension (σs/a) measurements were taken to determine organic diffusion coefficients (Ds). The droplet growth of organic / AS mixtures was measured under subsaturated conditions using a humidified tandem differential mobility analyzer (H-TDMA) at 88.2 % RH ±1.5 %. Droplet activation was measured under supersaturated (>100 % RH) conditions using a cloud condensation nuclei counter (CCNC); supersaturation (SS) ranged from 0.3 %–1.4 %. Hygroscopicity in both regimes was parameterized by the single hygroscopicity parameter κ. This study demonstrates how diffusion and salting-in effects influence the water uptake of synthesized, isoprene-derived SOA mixtures. Results show that when mixed with AS, organic diffusion for 2-MTS / AS becomes an order of magnitude faster, while 2-MT diffusivity remains unchanged. Both 2-MT / AS and 2-MTS aerosols present a plateau in subsaturated κ values close to pure AS. However, under supersaturated conditions, 2-MTS / AS behaves ideally and well mixed and can be characterized by κ-Köhler theory. Isoprene-derived SOAs like 2-MT and 2-MTS samples are ubiquitous, and thus, the impact from biogenic sources and its non-ideal thermodynamic properties must be considered in aerosol–cloud interactions.

Isoprene is an abundant volatile organic compound emitted from broadleaf forests. Under low nitric oxide concentrations, isoprene is photochemically oxidized to form gas-phase isoprene epoxydiols (IEPOX). In the presence of acidified sulfate aerosols, IEPOX enhances the secondary organic aerosol (SOA) formation. Predictions of IEPOX-SOA in regional-scale models, e.g., the Community Multiscale Air Quality Model (CMAQ), are uncertain due to homogeneous aerosol assumptions, underpredictions of water uptake (hygroscopicity), and aerosol surface area. We used experimental measurements of IEPOX-SOA tracers, 2-methyltetrols (2-MT) and 2-methyltetrol sulfates (2-MTS), formed at initial IEPOX-to-inorganic sulfate ratios ranging from 1-10.5, at similar to 50% relative humidity to constrain key IEPOX-SOA parameters: phase separation, organic shell diffusivity (D org), acidity, hygroscopic growth, mass accommodation, and kinetics. The base CMAQ parametrization overpredicted experimental IEPOX-SOA with an average normalized mean bias (NMBaverage) of 1.63. CMAQ with phase separation underpredicted IEPOX-SOA (NMBaverage = -0.71). Using the phase-separated model, CMAQ model performance was optimized (NMBaverage = 0.077) with an increased D org = 2 & times; 10-16 m2s-1 and increased rate constants (k 2-MT = 1 & times; 10-3 M2 s-1, k 2-MTS = 8.83 & times; 10-3 M2 s-1). The optimized model explicitly accounted for hygroscopic growth by utilizing experimentally derived growth rates, improving aerosol surface area predictions. Our model highlights the importance of the aerosol mixing state (homogeneous versus phase-separated), aerosol size dynamics, and hygroscopic growth in modeling heterogeneous reactive uptake of IEPOX. Regional-scale modeling of isoprene epoxydiol secondary organic aerosol formation was constrained using chamber experiments and optimized with phase separation and explicit hygroscopic growth.

The abiotic production of sulfur-containing biomolecules under mild and globally relevant conditions has been an elusive endeavor in prebiotic chemistry experiments. As a result, a disconnect has emerged between understanding the origins of life and the later stages of biological evolution; the former potentially occurred independent of sulfur while the latter is universally dependent on it. Here, we demonstrate that planetary organic haze chemistry produces a suite of sulfur biomolecules including cysteine, coenzyme M, taurine, and potentially methionine and homocysteine. These compounds may form high in the atmosphere and subsequently deposit to early surface environments in sufficient amounts to support a budding global biosphere. Our findings thus challenge long-standing assumptions that sulfur biomolecules such as cysteine must have been biological "inventions."

Abstract. Aerosol particles contain complex mixtures of polar and non-polar species that can undergo organic-inorganic phase separations. In phase-separated aerosol particles, the phase state of the outer organic coating can modulate heterogeneous chemistry. Heterogeneous chemistry leading to isoprene epoxydiol (IEPOX)-derived secondary organic aerosol (IEPOX-SOA) is encoded in the Community Multiscale Air Quality (CMAQ) model and has been the focus of previous aerosol phase separation and phase state work. In a previous study, a constant ratio of water in the organic coating (ws) was assumed in modeling phase separation and state. Recent studies, however, have highlighted ws as an important modulator of phase state. This work uses CMAQ version (version 5.3) with capabilities to model dynamic water uptake to the organic coating to better predict ws and its impact on the organic coating phase state. In addition, new parameterizations for estimating organic aerosol phase state were implemented in CMAQ, and the subsequent model predictions were used to compare their impacts on phase state and IEPOX-SOA predictions. These evaluations were completed simulating a summertime episode over the continental United States. Simulated diurnal profiles of aerosol phase state agreed within one standard deviation of observationally-derived field measurements. The implementation of phase separation and phase state parameterizations, on average, decreased IEPOX reactive uptake by up to 99.99 % compared to Base CMAQ, resulting in mixed model performance. While 2-methyltetrol performance improved with phase separation and phase state updates, methyltetrol sulfates and total IEPOX-SOA concentrations further underpredicted field observations in comparison to Base CMAQ.

At fixed aerosol acidity, we recently demonstrated that dimers in isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) can heterogeneously react with hydroxyl radical (·OH) at faster rates than monomers. Aerosol acidity influences this aging process by enhancing the formation of oligomers in freshly generated IEPOX-SOA. Therefore, we systematically examined the role of aerosol acidity on kinetics and products resulting from heterogeneous ·OH oxidation of freshly generated IEPOX-SOA. IEPOX reacted with inorganic sulfate aerosol of varying initial pH (0.5, 1.5, and 2.5) in a steady-state smog chamber to yield a constant source of freshly generated IEPOX-SOA, which was aged in an oxidation flow reactor for 0-22 equiv days of atmospheric ·OH exposure. Molecular-level chemical analyses revealed that the most acidic sulfate aerosol (pH 0.5) formed the largest oligomeric mass fraction, causing the slowest IEPOX-SOA mass decay with aging. Reactive uptake coefficients of ·OH (γOH) were 0.24 ± 0.06, 0.40 ± 0.05, and 0.49 ± 0.20 for IEPOX-SOA generated at pH 0.5, 1.5, and 2.5, respectively. IEPOX-SOA became more liquid-like for pH 1.5 and 2.5, while exhibiting an irregular pattern for pH 0.5 with aging. Using kinetic and physicochemical data derived for a single aerosol pH in atmospheric models could inaccurately predict the fate of the IEPOX-SOA.

Sea spray aerosol (SSA) contributes significantly to global aerosol budgets and the NaCl present often heterogeneously reacts with H2SO4 to form Na2SO4 within particles of varying acidity. The acidic particles can subsequently undergo reactive uptake of isoprene-derived oxidation products, such as isoprene epoxydiols (IEPOX), to form significant amounts of secondary organic aerosol (SOA). While the complex pH-dependence of IEPOX-SOA formation with ammonium sulfate aerosol has been widely explored, there has been minimal research on pH-dependent SOA formation with sodium sulfate. Herein, aerosolized solutions of Na2SO4 mixed with different amounts of H2SO4 (pH = 1, 1.3, and 3) were used as seed aerosol to take up IEPOX and form SOA. We unexpectedly observed a distinct crystal structure (phase III) in more acidic particles using Raman spectroscopy, based on characteristic peaks for νas(SO42-) at 1077, 1130, and 1199 cm-1 versus 1101, 1131, and 1152 cm-1 for νas(SO42-) in typical crystalline Na2SO4 (phase V). The presence of phase III in iSOA indicates a more acidic environment. Our results show that reactive uptake of IEPOX to seed particles increases acidity (i.e., pH decreases) over time. This has important implications for IEPOX-SOA formation, particularly on aged SSA in marine and coastal environments.

The role of secondary organic aerosol (SOA) in atmospheric ice nucleation is not well understood, limiting accurate predictions of aerosol indirect effects in global climate simulations. This article details experiments performed to characterize the ice-nucleating properties of proxy SOA. Experimental techniques in conditioning aerosol to glass transition temperatures (T-g) as low as-70 degrees C using a pre-cooling unit are described. Ice nucleation measurements of proxy organosulfates (i.e., methyl, ethyl, and dodecyl sulfates) and citric acid were performed using the SPectrometer for ice nucleation (SPIN), operating at conditions relevant to upper-tropospheric cirrus temperatures (-45 degrees C, -40 degrees C, -35 degrees C) and ice saturation ratios (1.0 < S-ice < 1.6). Methyl, ethyl, and dodecyl sulfates did not nucleate ice, despite dodecyl sulfate possessing a T(g )higher than ambient temperature. Citric acid nucleated ice heterogeneously at-45 and-40 degrees C (1.2 < S-ice < 1.4) but required pre-cooling temperatures of-70 degrees C, notably colder than the lowest published T-g. A kinetic flux model was used to numerically estimate water diffusion timescales to verify experimental observations and predict aerosol phase state. Diffusion modeling showed rapid liquefaction of glassy methyl and ethyl sulfates due to high hygroscopicity, preventing heterogeneous ice nucleation. The modeling results suggest that citric acid nucleated ice heterogeneously via deposition freezing or immersion freezing after surface liquefaction. We conclude that Tg alone is not sufficient for predicting heterogeneous ice formation for proxy SOA using the SPIN.

Per and polyfluoroalkyl substances (PFAS) are ubiquitous in the indoor environment, resulting in indoor exposure. However, a dearth of concurrent indoor multi-compartment PFAS measurements, including air, has limited our understanding of the contributions of each exposure pathway to residential PFAS exposure. As part of the Indoor PFAS Assessment (IPA) Campaign, we measured 35 neutral and ionic PFAS in air, settled dust, drinking water, clothing, and on surfaces in 11 North Carolina homes. Ionic and neutral PFAS measurements reported previously and ionic PFAS measurements reported herein for drinking water (1.4-34.1 ng L-1), dust (202-1036 ng g-1), and surfaces (4.1 × 10-4-1.7 × 10-2 ng cm-2) were used to conduct a residential indoor PFAS exposure assessment. We considered inhalation of air, ingestion of drinking water and dust, mouthing of clothing (children only), and transdermal uptake from contact with dust, air, and surfaces. Average intake rates were estimated to be 3.6 ng kg-1 per day (adults) and 12.4 ng kg-1 per day (2 year-old), with neutral PFAS contributing over 80% total PFAS intake. Excluding dietary ingestion, which was not measured, inhalation contributed over 65% of PFAS intake and was dominated by neutral PFAS because fluorotelomer alcohol (FTOH) concentrations in air were several orders of magnitude greater than ionic PFAS concentrations. Perfluorooctanoic acid (PFOA) intake was 6.1 × 10-2 ng kg-1 per day (adults) and 1.5 × 10-1 ng kg-1 per day (2 year-old), and biotransformation of 8 : 2 FTOH to PFOA increased this PFOA body burden by 14% (adults) and 17% (2 year-old), suggesting inhalation may also be a meaningful contributor to ionic PFAS exposure through biotransformation.