Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX), key isoprene oxidation products, with inorganic sulfate aerosol yields substantial amounts of secondary organic aerosol (SOA) through the formation of organosulfur compounds. The extent and implications of inorganic-to-organic sulfate conversion, however, are unknown. In this article, we demonstrate that extensive consumption of inorganic sulfate occurs, which increases with the IEPOX-to-inorganic sulfate concentration ratio (IEPOX/Sulfinorg), as determined by laboratory measurements. Characterization of the total sulfur aerosol observed at Look Rock, Tennessee, from 2007 to 2016 shows that organosulfur mass fractions will likely continue to increase with ongoing declines in anthropogenic Sulfinorg, consistent with our laboratory findings. We further demonstrate that organosulfur compounds greatly modify critical aerosol properties, such as acidity, morphology, viscosity, and phase state. These new mechanistic insights demonstrate that changes in SO2 emissions, especially in isoprene-dominated environments, will significantly alter biogenic SOA physicochemical properties. Consequently, IEPOX/Sulfinorg will play an important role in understanding the historical climate and determining future impacts of biogenic SOA on the global climate and air quality.

Acid-catalyzed reactions between gas-and particle-phase constituents are critical to atmospheric secondary organic aerosol (SOA) formation. The aerosol-phase state is thought to influence the reactive uptake of gas-phase precursors to aerosol particles by altering diffusion rates within particles. However, few experimental studies have explored the precise role of the aerosol-phase state on reactive uptake processes. This laboratory study systematically examines the reactive uptake coefficient (γ) of trans-β-isoprene epoxydiol (trans-β-IEPOX), the predominant IEPOX isomer, on acidic sulfate particles coated with SOA derived from α-pinene ozonolysis. γ IEPOX is obtained for core-shell particles, the morphology of which was confirmed by microscopy, as a function of SOA coating thickness and relative humidity. γ IEPOX is reduced, in some cases by half of the original value, when SOA coatings are present prior to uptake, especially when coating thicknesses are >15 nm. The diurnal trend of IEPOX lost to acid-catalyzed reactive uptake yielding SOA compared with other known atmospheric sinks (gas-phase oxidation or deposition) is derived by modeling the experimental coating effect with field data from the southeastern United States. IEPOX-derived SOA is estimated to be reduced by 16−27% due to preexisting organic coatings during the afternoon (12:00 to 7:00 p.m., local time), corresponding to the period with the highest level of production.

Isoprene-derived secondary organic aerosol (SOA) is mainly formed through acid-catalyzed reactive uptake of isoprene-derived epoxydiols (IEPOX) onto sulfate aerosol particles. The effect of IEPOX-derived SOA on the physicochemical properties of existing aerosols and resulting capacity for further SOA formation remains unclear. This study systematically examined the influences of IEPOX-derived SOA on the phase state, morphology, and acidity of pre-existing sulfate aerosol particles, as well as their implications on the reactivity and evolution of these particles. By combining aerosol thermodynamic and viscosity modeling, our predictions show that aerosol viscosity and acidity change drastically after IEPOX reactive uptake, with the aerosol becoming less acidic (increasing by up to 1.5 pH units) and more viscous by 7 orders of magnitude, thereby significantly reducing the diffusion time scale of the molecules inside the particles. Decreased aerosol acidity and increased viscosity co-contribute to a self-limiting effect where newly formed IEPOX-derived SOA inhibits additional multiphase chemical reactions of IEPOX. The relative contribution to the inhibitory effect of pH versus viscosity depends on the initial ratio of the IEPOX-to-inorganic sulfate aerosol, which differs between geographic regions. Moreover, reduced aerosol acidity and increased kinetic limitation to diffusion leading to lower hydronium ions and slower mixing times may impede other multiphase chemical processes after the formation of IEPOX-derived SOA. This study highlights important interconnections between physical and chemical properties of aerosol particles that come from interactions of inorganic and organic components, which jointly influence the evolution of atmospheric aerosols.

Aerosol phase state is critical for quantifying aerosol effects on climate and air quality. However, significant challenges remain in our ability to predict and quantify phase state during its evolution in the atmosphere. Herein, we demonstrate that aerosol phase (liquid, semisolid, solid) exhibits a diel cycle in a mixed forest environment, oscillating between a viscous, semisolid phase state at night and liquid phase state with phase separation during the day. The viscous nighttime particles existed despite higher relative humidity and were independently confirmed by bounce factor measurements and atomic force microscopy. High-resolution mass spectrometry shows the more viscous phase state at night is impacted by the formation of terpene-derived and higher molecular weight secondary organic aerosol (SOA) and smaller inorganic sulfate mass fractions. Larger daytime particulate sulfate mass fractions, as well as a predominance of lower molecular weight isoprene-derived SOA, lead to the liquid state of the daytime particles and phase separation after greater uptake of liquid water, despite the lower daytime relative humidity. The observed diel cycle of aerosol phase should provoke rethinking of the SOA atmospheric lifecycle, as it suggests diurnal variability in gas–particle partitioning and mixing time scales, which influence aerosol multiphase chemistry, lifetime, and climate impacts.

Methyltetrol sulfates are unique tracers for secondary organic aerosols (SOA) formed from acid-driven multiphase chemistry of isoprene-derived epoxydiols. 2-Methyltetrol sulfate diastereomers (2-MTSs) are the dominant isomers and single most-abundant SOA tracers in atmospheric fine particulate matter (PM 2.5 ), but their atmospheric sinks remain unknown. We investigated the oxidative aging of authentic 2-MTS aerosols by gas-phase hydroxyl radicals ( • OH) at a relative humidity of 61 ± 1%. The effective rate constant for this heterogeneous reaction was determined as 4.9 ± 0.6 × 10 −13 cm 3 molecules −1 s −1 , corresponding to an atmospheric lifetime of 16 ± 2 days (assuming an • OH concentration of 1.5 × 10 6 molecules cm −3 ). Chemical changes to 2-MTSs were monitored by hydrophilic interaction liquid chromatography interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). Plausible reaction mechanisms are proposed for previously unknown OSs detected in atmospheric PM 2.5 at mass-to-charge ratios (m/z) of 139 (C