Interfacial Dimerization by Organic Radical Reactions during Heterogeneous Oxidative Aging of Oxygenated Organic Aerosols

Abstract

Oxidative aging of atmospheric organic aerosols (OA) substantially modifies their chemical compositions, physical properties, and hence the various environmental impacts. Here, we report observations of a previously unrecognized process leading to dimer formation during heterogeneous •OH-initiated oxidative aging of oxygenated OA. Isomer-resolved ion mobility mass spectrometry measurements and reaction-diffusion kinetic simulations are in good agreement, elucidating new mechanisms of dimerization by organic radical (i.e., peroxy and alkoxy radicals) cross reactions using glutaric acid as a surrogate oxygenated OA. These radical reactions are predicted to occur more prominently near the gas-particle interface following oxidation, especially in diffusion-limited viscous OA particles. Chemical structure analysis shows that esters dominate the detected dimers, followed by organic peroxides and ethers, highlighting the importance of acyl peroxy and acyloxy radicals. Simulations suggest that the reported dimer formation through the new interfacial mechanism could be appreciable under both laboratory and ambient conditions. Therefore, the dimers that are formed and enriched at the gas–particle interface are expected to play a crucial role in the effective reactivity, volatility, viscosity, and hygroscopicity of aged OA particles.

Structural Characterisation of Dimeric Esters in α-Pinene Secondary Organic Aerosol Using N2 and CO2 Ion Mobility Mass Spectrometry

“…This technique is increasingly used in the structural characterisation of small molecules since the introduction of the first commercial IMS-TOFMS in 2006 [9]. In the field of atmospheric science, IMS-TOFMS is successfully used for the molecular characterisation of β-pinene originating organosulfates [10], isoprene originating organosulfates [11], aerosol bound alkyl nitrates [12], classification of atmospherically relevant organic compounds [13], aqueous phase oligomerisation of α,β-unsaturated carbonyls and acids [14], separation of highly oxygenated molecules in laboratory-generated α-pinene SOA [15], particle-phase reaction products of α-pinene oxidation [16], and heterogeneous oxidation of organic aerosols [17,18].…”

“…These include particle-phase esterification of carboxylic acids such as cis-pinic acid and terpenylic acid leading to Mw 358 [21], a gas phase mechanism involving Criegee intermediates [23], and the formation of RO 3 R' from the reaction of RO 2 • and R'O•, and the subsequent decomposition of acyl hydroperoxide groups to form stable esters [19]. Other suggested mechanisms that can lead to dimeric SOA compounds that are not exclusive to Mw 358 and Mw 368 include heterogeneous reactions of aldehydes with hydroperoxides [27,28], and more recently a series of organic peroxy and alkoxy radical cross-reactions at the interface forming dimeric compounds [17]. Recent mass spectrometric studies on α-pinene SOA showed that these dimers consist of diaterpenylic acid and cis-pinic acid for Mw 358 [19][20][21] and of cis-hydroxyl pinonic acid and cis-pinic acid for Mw 368 [19].…”

Accurate identification of dimers from α‐pinene oxidation using high‐resolution collision‐induced dissociation mass spectrometry

“…Their fraction is estimated to be 96.4% from the sum of deprotonated monomers (70.2%) and product ions with a lower m/z value (26.2%). It is known that for API‐MS, HB clusters are easily generated as instrumental artifacts . In this study, ions and neutral species in the ambient discharge area, which are related to α‐pinene oxidation products and air constituents, are introduced into the first differential pumping region of the mass spectrometer at ca.…”

“…To accurately determine these properties, we need to know the fraction of ion signal that originates from the abovementioned mechanisms (a) or (b). Furthermore, when using API‐MS, the formation of hydrogen‐bonded dimers as instrumental artifacts should be taken into consideration owing to various ionization processes and/or adiabatic expansion caused by the pressure difference between the ambient ionization area and the vacuum region in MS . The fractions of these artifacts should be subtracted from the detected ion signals.…”

“…The fractions of these artifacts should be subtracted from the detected ion signals. Recent mass spectrometric study of dimers in organic aerosols has mentioned the coexistence of the hydrogen‐bonded artifact clusters with the actual dimers; however, they have not been distinguished quantitatively …”