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ABSTRACT: 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. IEPOXSOA 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. 

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: Isoprene has the highest atmospheric emissions of any nonmethane hydrocarbon, and isoprene epoxydiols (IEPOX) are well-established oxidation products and the primary contributors forming isoprene-derived secondary organic aerosol (SOA). Highly acidic particles (pH 0−3) widespread across the lower troposphere enable acid-driven multiphase chemistry of IEPOX, such as epoxide ring-opening reactions forming methyltetrol sulfates through nucleophilic attack of sulfate (SO4 2−). Herein, we systematically demonstrate an unexpected decrease in SOA formation from IEPOX on highly acidic particles (pH < 1). While IEPOX-SOA formation is commonly assumed to increase at low pH when more [H+] is available to protonate epoxides, we observe maximum SOA formation at pH 1 and less SOA formation at pH 0.0 and 0.4. This is attributed to limited availability of SO4 2− at pH values below the acid dissociation constant (pKa) of SO42− and bisulfate (HSO4−). The nucleophilicity of HSO4− is 100× lower than SO42−, decreasing SOA formation and shifting particulate products from low-volatility organosulfates to higher-volatility polyols. Current model parameterizations predicting SOA yields for IEPOX-SOA do not properly account for the SO42−/HSO4 − equilibrium, leading to overpredictions of SOA formation at low pH. Accounting for this underexplored acidity-dependent behavior is critical for accurately predicting SOA concentrations and resolving SOA impacts on air quality. 

Abstract Number: 327 Working Group: Aerosol Chemistry Abstract Low-pH aerosols comprise a large fraction of atmospheric fine particulate matter. The effects of pH on secondary organic aerosol (SOA) formation are not well understood, in part because of the difficulty of accurately measuring aerosol pH. Of particular interest are the atmospherically-abundant isoprene epoxydiols (IEPOX), which undergo acid-driven reactions to form SOA. Models have assumed no upper limit for IEPOX-SOA formation rates as acidity increases. However, recent work has shown that organosulfate formation from IEPOX slows as the equilibrium of inorganic sulfate (Sulfinorg) shifts from sulfate (SO42-) towards bisulfate (HSO4-), which is a weaker nucleophile. We performed a series of trans-ß-IEPOX uptake experiments with ammonium sulfate seed solutions acidified to between pH 0 and 3, and modelled time-resolved methyltetrol (MT) and methyltetrol sulfate (MTS) formation and Sulfinorg consumption (kMT = 0.018 M-2 s-1, kMTS = 0.28 M-2 s-1). We found an inflection point between pH 1 and 1.4, below which MT formation dominates and above which MTS formation dominates, consistent with a changing balance of protonated and deprotonated Sulfinorg. Modelled MT and MTS formation fit the experimental data well both above and below the inflection point except at pH 1.4, where it significantly underpredicted the data at low initial IEPOX/Sulfinorg ratios. This indicates multi-phase chemical dynamics beyond those represented in our model, leading to very efficient IEPOX-SOA formation at pH 1.4. Further investigation is warranted into the connection of IEPOX-SOA formation with initial IEPOX/Sulfinorg ratio and aerosol pH. 

Abstract Number: 381 Working Group: Instrumentation and Methods Abstract The phase state of atmospheric aerosol particles – solid, semi-solid, or liquid – influences their ability to take up water and participate in heterogeneous chemical reactions. Changes in phase state have been predicted by glass transition temperature (Tg) and viscosity; however, direct measurements of these properties is challenging for sub-micron particles. Historically, bulk measurements have been used, but this does not account for particle-to-particle variation or the impacts of particle size. Melting temperature (Tm) is the most significant predictor of Tg, and the two properties can be related through the Boyer-Beaman rule. Herein, we apply a recently developed method utilizing a nano-thermal analysis (nanoTA) module coupled to an atomic force microscope (AFM), to determine the Tm of individual secondary organic aerosol (SOA) particles generated from the reactive uptake of isoprene-derived epoxydiols (IEPOX) onto acidic ammonium sulfate aerosol particles. NanoTA works by using a specialized AFM probe which can be heated while in contact with a particle of interest. As the temperature increases, the probe deflection will first increase due to thermal expansion of the particle followed by a decrease at its Tm. The direct measurements are compared with model predictions based on molecular composition from hydrophilic interaction liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOF-MS) analysis. We compared the Tm of the SOA particles formed from IEPOX uptake onto acidic ammonium sulfate particles created at 30, 65, and 80% relative humidity (RH), and found that increasing RH from 30 to 80% led to an overall decrease in average Tm, indicating less viscous particles at higher RH conditions. Our measurements with this technique will allow for more accurate representations of the phase state of aerosols in the atmosphere. 

The ability of an atmospheric aerosol to take up water or to participate in heterogeneous reactions is highly influenced by its phase state – solid, semi-solid, or liquid. These changes in phase state can be predicted by glass transition temperature (Tg), as particles at temperatures below their Tg will show solid properties, while increasing the temperature above their Tg will allow for semi-solid and eventually liquid properties. Historically, measurements of the Tg of bulk materials have been studied in order to model the phase states of aerosols in the atmosphere; however, these methods only permit an estimation of aerosol Tg based on their bulk chemical composition. Determining the Tg of individual particles will allow for more accurate model predictions of aerosol phase state. Herein, we apply a recently developed method utilizing a nano-thermal analysis (nanoTA) module coupled to an atomic force microscope (AFM), to determine the Tg of individual secondary organic aerosol (SOA) particles generated from the reactive uptake of isoprene epoxydiol (IEPOX) onto acidic ammonium sulfate aerosol particles. NanoTA works by using a specialized AFM probe which can be heated while in contact with a particle of interest. As the temperature increases, the probe deflection will first increase due to thermal expansion of the particle followed by a decrease at its melting temperature (Tm). The Tg of the particle can then be determined from Tm using the Boyer–Beaman rule. We compared the Tg of IEPOX-derived SOA particles generated at relative humidity (RH) of 30, 65, and 80%, and found that increasing RH from 30 to 80% led to a decrease in average Tg of 22 K, indicating less viscous particles at higher RH conditions. Our measurements with this technique will allow for more accurate representations of the phase state of aerosols in the atmosphere. 

Abstract Number: 453 Working Group: Aerosol Chemistry Abstract Secondary organic aerosol (SOA) is composed of a significant fraction of low-volatility high-molecular-weight oligomer products. These species can affect particle viscosity, morphology, and mixing timescales, yet they are not very well understood. While strides have been made in elucidating oligomer formation mechanisms, their degradation is less studied. Previous work suggests that the presence of oligomers may suppress particle mass loss during atmospheric aging by slowing the production high-volatility fragments from monomers. Our work investigates the effects of relative humidity (RH) on oligomer formation in SOA and the effects of hydroxyl radical (·OH) exposure on oligomer degradation. To probe these questions, SOA is generated by the reactive uptake of isoprene epoxydiols (IEPOX) onto acidic ammonium sulfate aerosol in a 2-m3 steady-state chamber, followed by exposure to ·OH in an oxidation flow reactor. We investigate SOA formation at 30-80% RH, which is above and below the deliquescence point of ammonium sulfate. We examine the evolution of SOA bulk chemical composition as well as single-particle physicochemical properties over the course of aging using mass spectrometry-, spectroscopy-, and microscopy-based techniques. An optimized matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) method is used to identify and track the presence of oligomers in SOA over the course of aging. Our research will provide insight about the formation and degradation of oligomers in the atmosphere, which will allow better modeling of their impact on climate. 

Heterogeneous hydroxyl radical (•OH) oxidation is an important aging process for isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) that alters its chemical composition. It was recently demonstrated that heterogeneous •OH oxidation can age single-component particulate methyltetrol sulfates (MTSs), causing ∼55% of the SOA mass loss. However, our most recent study of freshly generated IEPOX-SOA particulate mixtures suggests that the lifetime of the complete IEPOX-SOA mixture against heterogeneous •OH oxidation can be prolonged through the fragmentation of higher-order oligomers. Published studies suggest that the heterogeneous •OH oxidation of IEPOX SOA could affect the organic atmospheric aerosol budget at varying rates, depending on aerosol chemical composition. However, heterogeneous •OH oxidation kinetics for the full IEPOX-SOA particulate mixture have not been reported. Here, we exposed freshly generated IEPOX-SOA particles to heterogeneous oxidation by •OH under humid conditions (relative humidity ∼57%) for 0−15 atmospheric-equivalent days of aging and derived an effective heterogeneous •OH rate coefficient (kOH) of 2.64 ± 0.4 × 10−13 cm^3 molecules−1 s−1. While ∼44% of particulate organic mass of nonoxidized IEPOX-SOA was consumed over the entire 15 day aging period, only <7% was consumed during the initial 10 aging days. By molecular-level chemical analysis, we determined oligomers were consumed at a faster rate (by a factor of 2−4) than monomers. Analysis of aerosol physicochemical properties shows that IEPOX-SOA has a core−shell morphology, and the shell becomes thinner with •OH oxidation. In summary, this study demonstrates that heterogeneous •OH oxidation of IEPOX-SOA particles is a dynamic process in which aerosol chemical composition and physicochemical properties play important roles.