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.

Measuring the acidity of atmospheric aerosols is critical, as many key multiphase chemical reactions involving aerosols are highly pH-dependent. These reactions impact processes, such as secondary organic aerosol (SOA) formation, that impact climate and health. However, determining the pH of atmospheric particles, which have minute volumes (10-10 L), is an analytical challenge due to the nonconservative nature of the hydronium ion, particularly as most chemical aerosol measurements are made offline or under vacuum, where water can be lost and acid-base equilibria shifted. Because of these challenges, there have been no direct methods to probe atmospheric aerosol acidity, and pH has typically been determined by proxy/indirect methods, such as ion balance, or thermodynamic models. Herein, we present a novel and facile method for direct measurement of size-resolved aerosol acidity from pH 0 to 4.5 using quantitative colorimetric image processing of cellular phone images of (NH)SO-HSO aqueous aerosol particles impacted onto pH-indicator paper. A trend of increasing aerosol acidity with decreasing particle size was observed that is consistent with spectroscopic measurements of individual particle pH. These results indicate the potential for direct measurements of size-resolved atmospheric aerosol acidity, which is needed to improve fundamental understanding of pH-dependent atmospheric processes, such as SOA formation.

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

Atmospheric oxidation of volatile organic compounds, such as isoprene, and subsequent condensation or heterogeneous reactions lead to the formation of secondary organic aerosol (SOA), a ubiquitous component of submicron aerosol. Liquid–liquid phase-separated organic–inorganic aerosol particles have been observed in the laboratory and field; however, the impacts of multiphase reactions on aerosol viscosity are not well understood for phase-separated aerosol particles. In this study, phase-separated aerosol particles were reacted with gaseous isoprene epoxydiol (IEPOX), an abundant isoprene oxidation product. Acidic sulfate particles (H2SO4 + (NH4)2SO4 at pH = 1.4) were coated with laboratory-generated biogenic SOA (α-pinene + O3) and anthropogenic SOA (toluene + OH), resulting in a core–shell morphology. After reaction with IEPOX, the phase-separated aerosol particles no longer displayed characteristics of a liquid core. Instead, they became irregularly shaped, taller after impaction onto substrates, and had decreased spreading ratios for both types of SOA, implying an increase in particle viscosity. As the SOA from α-pinene and toluene was already viscous, this is indicative of a change in phase state for the core from liquid to viscous state. An example reaction that may be facilitating this phase change is IEPOX reaction with inorganic sulfate to produce organosulfates, especially after IEPOX diffuses through the organic coating. The modification of the aerosol physicochemical properties suggests that phase state is dynamic over the atmospheric lifetime of SOA-containing particles, with multiphase chemistry between aerosol particles and gaseous species leading to more viscous aerosol after uptake of isoprene oxidation products (e.g., IEPOX).

Physicochemical analysis of individual atmospheric aerosols at the most abundant sizes in the atmosphere (<1 μm) is analytically challenging, as hundreds to thousands of species are often present in femtoliter volumes. Vibrational spectroscopies, such as infrared (IR) and Raman, have great potential for probing functional groups in single particles at ambient pressure and temperature. However, the diffraction limit of IR radiation limits traditional IR microscopy to particles > ∼10 μm, which have less relevance to aerosol health and climate impacts. Optical photothermal infrared (O-PTIR) spectroscopy is a contactless method that circumvents diffraction limitations by using changes in the scattering intensity of a continuous wave visible laser (532 nm) to detect the photothermal expansion when a vibrational mode is excited by a tunable IR laser (QCL: 800–1800 cm–1 or OPO: 2600–3600 cm–1). Herein, we simultaneously collect O-PTIR spectra with Raman spectra at a single point for individual particles with aerodynamic diameters <400 nm (prior to impaction and spreading) at ambient temperature and pressure, by also collecting the inelastically scattered visible photons for Raman spectra. O-PTIR and Raman spectra were collected for submicrometer particles with different substrates, particle chemical compositions, and morphologies (i.e., core–shell), as well as IR mapping with submicron spatial resolution. Initial O-PTIR analysis of ambient atmospheric particles identified both inorganic and organic modes in individual sub- and supermicrometer particles. The simultaneous IR and Raman microscopy with submicrometer spatial resolution described herein has considerable potential both in atmospheric chemistry and numerous others fields (e.g., materials and biological research).