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ABSTRACT: Isoprene, the most abundant nonmethane volatile organic compound in the atmosphere, undergoes photochemical reactions with hydroxyl radical (•OH), a major sink for isoprene, leading to the formation of secondary organic aerosol (SOA). Using a Vocus Chemical Ionization Mass Spectrometer with ammonium-adduct ions (Vocus NH4+ CIMS), this study used the positive ion mode to quantify the yields and time-dependent reactiveuptake of oxidized volatile organic compounds (OVOCs) produced from •OH-initiated oxidation of isoprene under dry conditions. Molar gas-phase yields of key oxidation products were constrained using sensitivities derived from a voltage scan of the front and back end of the Vocus ion−molecule reactor region. Carefully designed chamber experiments measured uptake coefficients (γ) for key isoprene-derived oxidation products onto acidic sulfate particles. The γ values for both C5H10O3 isomers (IEPOX/ISOPOOH) and C5H8O4, another epoxy species from isoprene photo-oxidation, rapidly decreased as the SOA coating thickness increased, demonstrating a self-limiting effect. Despite ISOPOOH/IEPOX contributing around 80% to total reactive uptake, other oxidation products from isoprene photooxidation were estimated to contribute 20% of the total SOA formation. These findings highlight the importance for future models to consider the self-limiting effects of ISOPOOH/IEPOX and SOA formation through non-IEPOX pathways. 

Abstract. 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. 

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. 

he phase states and glass transition temperatures (Tg) of secondary organic aerosol (SOA) particles are important to resolve for understanding the formation, growth, and fate of SOA as well as their cloud formation properties. Currently, there is a limited understanding of how Tg changes with the composition of organic and inorganic components of atmospheric aerosol. Using broadband dielectric spectroscopy, we measured the Tg of organic mixtures containing isoprene epoxydiol (IEPOX)-derived SOA components, including 2-methyltetrols (2-MT), 2-methyltetrolsulfate(2-MTS), and 3-methyltetrol-sulfate (3-MTS). The results demonstrate that the Tg of mixtures depends on their composition. The Kwei equation, a modified Gordon−Taylor equation with an added quadratic term and a fitting parameter representing strong intermolecular interactions, provides a good fit for the Tg-composition relationship of complex mixtures. By combining Raman spectroscopy with geometry optimization simulations obtained using density functional theory, we demonstrate that the non-linear deviation of Tg as a function of composition may be caused by changes in the extent of hydrogen bonding in the mixture.