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Abstract. 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 (Tg) as low as −70 °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 °C, −40 °C, −35 °C) and ice saturation ratios (1.0<1.6). Methyl, ethyl, and dodecyl sulfates did not nucleate ice, despite dodecyl sulfate possessing a Tg higher than ambient temperature. Citric acid nucleated ice heterogeneously at −45 and −40 °C (1.2<1.4) but required pre-cooling temperatures of −70 °C, notably colder than the lowest published Tg. 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. 

Abstract. Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic air pollutants. The dispersion of PAHs in the atmosphere is influenced by gas–particle partitioning and chemical loss. These processes are closely interlinked and may occur at vastly differing timescales, which complicates their mathematical description in chemical transport models. Here, we use a kinetic model that explicitly resolves mass transport and chemical reactions in the gas and particle phases to describe and explore the dynamic and non-equilibrium interplay of gas–particle partitioning and chemical losses of PAHs on soot particles. We define the equilibration timescale τeq of gas–particle partitioning as the e-folding time for relaxation of the system to the partitioning equilibrium. We find this metric to span from seconds to hours depending on temperature, particle surface area, and the type of PAH. The equilibration time can be approximated using a time-independent equation, τeq≈1kdes+kads, which depends on the desorption rate coefficient kdes and adsorption rate coefficient kads, both of which can be calculated from experimentally accessible parameters. The model reveals two regimes in which different physical processes control the equilibration timescale: a desorption-controlled and an adsorption-controlled regime. In a case study with the PAH pyrene, we illustrate how chemical loss can perturb the equilibrium particulate fraction at typical atmospheric concentrations of O3 and OH. For the surface reaction with O3, the perturbation is significant and increases with the gas-phase concentration of O3. Conversely, perturbations are smaller for reaction with the OH radical, which reacts with pyrene on both the surface of particles and in the gas phase. Global and regional chemical transport models typically approximate gas–particle partitioning with instantaneous-equilibration approaches. We highlight scenarios in which these approximations deviate from the explicitly coupled treatment of gas–particle partitioning and chemistry presented in this study. We find that the discrepancy between solutions depends on the operator-splitting time step and the choice of time step can help to minimize the discrepancy. The findings and techniques presented in this work not only are relevant for PAHs but can also be applied to other semi-volatile substances that undergo chemical reactions and mass transport between the gas and particle phase. 

Thirdhand smoke (THS) persists in locations where smoking previously occurred and can be transported into non-smoking environments, leading to non-smoker exposure. Laboratory experiments using high-resolution mass spectrometry demonstrate that deposited particulate matter (PM) and smoke-exposed surrogate lung lining fluid (LLF) are substantial, chemically-complex reservoirs of gas-phase THS emissions, including hazardous air pollutants, polycyclic aromatic compounds, and nitrogen/oxygen-containing species. Both PM and LLF are persistent real-world THS reservoirs that chemically evolve over time, and can act as vehicles for the transport and emission of reactive pollutants and their reaction byproducts (e.g., acrolein). Deposited PM on clothes, furnishings, bodies, and/or airways will emit volatile to semi-volatile gases over long lifetimes, which can re-partition to other indoor materials and increase their overall persistence. On the other hand, LLF off-gassing consists predominantly of volatile organic compounds in amounts influenced by their aqueous solubilities, and their persistence in breath will be prolonged by re-distribution across internal aqueous reservoirs, as corroborated by multicompartment modeling in this study. 

Abstract. In the aqueous phase, fine particulate matter can form reactive species (RS)that influence the aging, properties, and health effects of atmosphericaerosols. In this study, we explore the RS yields of aerosol samples froma remote forest (Hyytiälä, Finland) and polluted urban locations(Mainz, Germany; Beijing, China), and we relate the RS yields to differentchemical constituents and reaction mechanisms. Ultra-high-resolution massspectrometry was used to characterize organic aerosol composition, electronparamagnetic resonance (EPR) spectroscopy with a spin-trapping technique wasapplied to determine the concentrations of ⚫OH,O2⚫-, and carbon- or oxygen-centered organic radicals, anda fluorometric assay was used to quantify H2O2. The aqueousH2O2-forming potential per mass unit of ambient PM2.5(particle diameter < 2.5 µm) was roughly the same for allinvestigated samples, whereas the mass-specific yields of radicals werelower for sampling sites with higher concentrations of PM2.5. Theabundances of water-soluble transition metals and aromatics in ambientPM2.5 were positively correlated with the relative fraction of⚫OH and negatively correlated with the relative fraction ofcarbon-centered radicals. In contrast, highly oxygenated organic molecules(HOM) were positively correlated with the relative fraction ofcarbon-centered radicals and negatively correlated with the relativefraction of ⚫OH. Moreover, we found that the relative fractionsof different types of radicals formed by ambient PM2.5 were comparableto surrogate mixtures comprising transition metal ions, organichydroperoxide, H2O2, and humic or fulvic acids. The interplay oftransition metal ions (e.g., iron and copper ions), highly oxidized organicmolecules (e.g., hydroperoxides), and complexing or scavenging agents (e.g.,humic or fulvic acids) leads to nonlinear concentration dependencies inaqueous-phase RS production. A strong dependence on chemical compositionwas also observed for the aqueous-phase radical yields oflaboratory-generated secondary organic aerosols (SOA) from precursormixtures of naphthalene and β-pinene. Our findings show how thecomposition of PM2.5 can influence the amount and nature ofaqueous-phase RS, which may explain differences in the chemical reactivityand health effects of particulate matter in clean and polluted air. 

Atmospheric aerosol particles with a high viscosity may become inhomogeneously mixed during chemical processing. Models have predicted gradients in condensed phase reactant concentration throughout particles as the result of diffusion and chemical reaction limitations, termed chemical gradients. However, these have never been directly observed for atmospherically relevant particle diameters. We investigated the reaction between ozone and aerosol particles composed of xanthan gum and FeCl 2 and observed the in situ chemical reaction that oxidized Fe 2+ to Fe 3+ using X-ray spectromicroscopy. Iron oxidation state of particles as small as 0.2 μm in diameter were imaged over time with a spatial resolution of tens of nanometers. We found that the loss off Fe 2+ accelerated with increasing ozone concentration and relative humidity, RH. Concentric 2-D column integrated profiles of the Fe 2+ fraction, α , out of the total iron were derived and demonstrated that particle surfaces became oxidized while particle cores remained unreacted at RH = 0–20%. At higher RH, chemical gradients evolved over time, extended deeper from the particle surface, and Fe 2+ became more homogeneously distributed. We used the kinetic multi-layer model for aerosol surface and bulk chemistry (KM-SUB) to simulate ozone reaction constrained with our observations and inferred key parameters as a function of RH including Henry's Law constant for ozone, H O3 , and diffusion coefficients for ozone and iron, D O3 and D Fe , respectively. We found that H O3 is higher in our xanthan gum/FeCl 2 particles than for water and increases when RH decreased from about 80% to dry conditions. This coincided with a decrease in both D O3 and D Fe . In order to reproduce observed chemical gradients, our model predicted that ozone could not be present further than a few nanometers from a particle surface indicating near surface reactions were driving changes in iron oxidation state. However, the observed chemical gradients in α observed over hundreds of nanometers must have been the result of iron transport from the particle interior to the surface where ozone oxidation occurred. In the context of our results, we examine the applicability of the reacto-diffusive framework and discuss diffusion limitations for other reactive gas-aerosol systems of atmospheric importance. 

The ozonolysis kinetics of viscous aerosol particles containing maleic acid are studied. Kinetic fits are constrained by measured particle viscosities.