Morphology and Viscosity Changes after Reactive Uptake of Isoprene Epoxydiols in Submicrometer Phase Separated Particles with Secondary Organic Aerosol Formed from Different Volatile Organic Compounds

Abstract

Secondary organic aerosol (SOA), formed from the oxidation of volatile organic compounds (VOCs), is a large contributor to atmospheric fine particulate matter (PM2.5) and is commonly present in mixed inorganic–organic submicron particles. SOA formed from varying biogenic and anthropogenic VOCs results in unique aerosol physicochemical properties that modify climate impacts (i.e., water uptake). Understanding reactive uptake of VOC-derived semivolatile oxidation products to inorganic-SOA mixed particles remains limited, particularly for particles at the most abundant sizes (by number) in the atmosphere (∼100 nm). These particles are challenging to study as SOA can be quite viscous (i.e., solid or semisolid), and mixed particles can have complex morphologies (e.g., core–shell). Herein, we show that the viscosity and morphology of initially core–shell (inorganic–organic) particles changed substantially after acid-catalyzed reactive uptake of isoprene epoxydiols (IEPOX), and that differences were highly dependent on VOC precursor (α-pinene, β-caryophyllene, isoprene, and toluene). SOA from two higher molecular weight precursors (α-pinene and β-caryophyllene) were less viscous after IEPOX uptake at 50% relative humidity (RH), while SOA viscosities from lower molecular weight precursors (isoprene and toluene) did not change appreciably, based on atomic force microscopy (AFM) measurements. The evolution of inorganic-SOA particle viscosity and morphology could alter the predicted impacts of SOA on air quality and climate.

Acidity-Dependent Atmospheric Organosulfate Structures and Spectra: Exploration of Protonation State Effects via Raman and Infrared Spectroscopies Combined with Density Functional Theory

“…The same organosulfates are listed in Table 4 along with their VOC precursors: isoprene, α-pinene, β-caryophyllene, and toluene. SOA from each of these precursors was generated in a series of laboratory chamber experiments described in Lei et al, 62 and the major Raman mode associated with the organosulfate products for each are also given. Experimental Raman measurements of the sulfur−oxygen stretches of the available synthetic organosulfate standards, measured in this work and previously by Bondy et al, 25 are given for the isoprenederived organosulfates.…”

Emerging investigator series: surfactants, films, and coatings on atmospheric aerosol particles: a review

“…Lei et al 388 employed a combination of AFM with photothermal infrared spectroscopy, Raman microspectroscopy, and SEM-EDX to directly show how reactive IEPOX uptake to acidified ammonium sulfate particles coated with α-pinene, β-caryophyllene, isoprene, and toluene SOA alters the physicochemical properties of the particles. IEPOX uptake significantly decreased the viscosity of the particles coated with α-pinene and β-caryophyllene SOA but did not significantly alter the viscosity of the particles coated with isoprene or toluene SOA, as shown in Fig.…”

“…18e shows that IEPOX uptake altered the chemical composition of both the shell and the core of a-pinene SOA. This work by Lei et al 388 is an example of the powerful insights into heterogeneous reactions on coated particles that can be obtained by combining measurements of morphology and composition for single particles.…”

“…More recent work has employed SOA coatings derived from a-pinene, toluene, and b-caryophyllene. [385][386][387][388] For example, Zhang et al 386 found a 30-50% decrease in the reactive uptake coefficient of IEPOX to particles coated with just $15 nm of a-pinene SOA (across a relative humidity range of 15-50%).…”

Mario Molina Memorial Special Issue

“…24 The acidity may differ between the bulk and interface of aqueous droplets. 25 Reactive uptake of volatile organics can lead to liquid−liquid phase separation 26 or the formation of highly viscous phases 27 in aerosols, while the salt content affects the size dependence of liquid−liquid phase separation 28 and the stability of reverse micelles when surfaceactive organics are present. 29 Ohno et al present a fluorescence-based aerosol flow tube technique for laboratory studies of liquid−liquid phase separation.…”