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A new approach for detecting highly oxygenated products is presented in which products are scavenged to the surface of matrix particles, providing new insight into oxidation products of ethylbenzene and its chemical pathways. 

Oxidized organics on atmospheric particle surfaces strongly influence water uptake, toxicity, and heterogeneous reaction kinetics. However, the nature of surface species and their interaction with gas-phase radicals are not well-understood. Here, experiments probed the impact of gas-phase HO2 and RO2 radicals on the surface products formed in the reaction of gas-phase OH with solid glutaric acid (GA) particles at 298 K and 1 atm in air. Hydroxyl radicals were formed from the reaction of tetramethylethylene (TME) with ozone in a flow system and reacted with atomized, dried GA particles. Surface products including alcohols, carbonyls, hydroperoxides, and organic peroxides were detected using Matrix Assisted Ionization in Vacuum – Mass Spectrometry (MAIV-MS), an emerging surface-sensitive technique. A product tentatively identified as an ester from reactions between surface-bound GA and gas-phase radicals was also observed. Concentrations of gas-phase radicals (RO2 and HO2) were varied by altering TME concentrations or by adding methanol or acetone, significantly impacting the observed product distribution. Particle size was also varied to alter the surface density of RO2(surf) and explore the role of surface availability. The results show that the fates of surface-bound radicals are largely determined by reactions with gas-phase HO2 or RO2. This complex competition is central in determining the surface composition of organic particles, and therefore the chemistry and environmental impacts of oxidized airborne organic particles. 

Gas-particle partitioning is critical for the evolution of secondary organic aerosols (SOA) in the atmosphere. SOA particles evaporate more slowly than expected at nearly size-independent rates, but the underlying mechanism remains controversial. Here, we apply kinetic multilayer modeling to simulate evaporation of α-pinene SOA, demonstrating that surface crust formation, emerging from accumulation of low-volatility compounds at the particle surface, leads to slow evaporation and reduced size dependence of the evaporation rate. While evaporation induced by decomposition of oligomers would naturally lead to size-independent evaporation rates, we observe and simulate nearly size-independent slow evaporation of polyethylene glycol mixture particles containing polymeric species that do not decompose, confirming the relevance of composition-dependent diffusivity for size-independent, slow evaporation. Slow evaporation of limonene SOA was also observed in environmental chamber experiments, and model simulations demonstrate strong surface crust formation with bulk diffusivity being depressed by up to 5 orders of magnitude compared to the inner bulk. We present experimental evidence using a surface-based mass spectrometry technique that shows that the particle surface becomes enriched in high molecular weight compounds upon evaporation of monomers. Our findings imply that viscous surface crusts may also limit the growth and chemical transformation of SOA particles, influencing their impacts on air quality and climate. 

Description: Mechanistic analysis of ion desorption from glutaric acid particles used in the development of surface-sensitive mass spectroscopy ionization methods. Abstract: Ionization via desorption of charged analytes from the surface of solid amorphous glutaric acid particles, without the assistance of an external energy source, has been shown to be a promising method that can be coupled to mass spectrometry. We conduct mechanistic studies of the later stages of this ionization process using atomistic molecular dynamics. Our analysis focuses on the hydrogen bonding, diffusion, and ion desorption from nano-aggregates of glutaric acid. These nano-aggregates exhibit an extended H-bonded network, often comprising H-bonded chains, linear dimeric assemblies, and occasionally cyclic trimeric assemblies. These local structures serve as centers for proton transfer reactions. The intermediate hydrocarbon chain between the proton-carrying oxygen sites prevents proton diffusion over a long distance unless there is significant translational or rotational movement of the proton-carrying diacid molecule. Our calculations show that diffusion on the surface is an order of magnitude faster than in the core of the nano-aggregate, which aids effective proton transfer on the particle's exterior. We find that ionic species desorb from the aggregate's surface through independent evaporation events of small clusters, where the ion is coordinated by only a few glutaric acid molecules. Near the nano-aggregate's Rayleigh limit, jets capable of releasing multiple ions were not observed. These observations suggest a more general ion-evaporation mechanism that applies to low-dielectric particles of various sizes, complementing the original ion-evaporation mechanism proposed for aqueous droplets with an approximate radius of 10–15 nm. The combined evidence from molecular modeling presented here and the thermodynamic properties of solid and supercooled liquid glutaric acid indicates that the stronger signals of glutaric acid observed in the mass spectra, relative to other experimentally tested diacids, can be attributed to its significantly lower melting point and the reduced enthalpy of vaporization of its amorphous state compared to other tested diacids.