Site-Specific Mechanisms in OH-Initiated Organic Aerosol Heterogeneous Oxidation Revealed by Isomer-Resolved Molecular Characterization

Abstract

Organic aerosols (OA) in the atmosphere are composed of molecules with highly diverse chemical structures and functionalities. The OH-initiated heterogeneous aging of these organic molecules could occur in multiple generations, exponentially increasing the complexity of the oxidation products. Furthermore, the detailed reaction mechanisms are likely different and site-specific as the OA's chemical structures vary. To systematically study these mechanisms, in this work, heterogeneous OH oxidation of five surrogate diacid OA with different positions and numbers of branched methyl groups was examined in a flow tube reactor. The oxidation products were characterized by an ion mobility mass spectrometer, which could resolve isomers and unambiguously identify individual products. Through detailed analysis of the molecular compositions of the oxidized OA, we suggest that the site-specific oxidation mechanisms largely modulate the oxidation products from the functionalization, fragmentation, and oligomerization pathways. Depending on the molecular structures and adjacent functional group(s), oxidation on primary carbons could play a key role, while that on secondary and tertiary carbons could be less important if hindered by the mesomeric effect. Our results also suggest that fragmentation products are most likely formed from later-generation reactions, while functionalization occurs only at certain carbon sites in later generations. Finally, oligomers are also largely affected by the molecular structures of the parent OA, evidenced by the distinct carbon oxidation state patterns and distributions across carbon numbers. This work provides new insights into the detailed reaction mechanisms from heterogeneous OH oxidation for diverse organic molecules.

Isolating α-Pinene Ozonolysis Pathways Reveals New Insights into Peroxy Radical Chemistry and Secondary Organic Aerosol Formation

“…Thus, different drift times usually suggest different structures. In our previous studies, we have demonstrated that the IMS-TOF is able to clearly separate structural isomers. ,,, Using the ESI in both the positive (+) and negative (−) ion modes allows for detections of the majority of the SOA constituents. An overview of the SOA molecular compositions is shown in Figures S10, S11 …”

“…The SOA samples were then characterized offline by the IMS-TOF with electrospray ionization (ESI) (Tofwerk Inc. and Aerodyne Research Inc., t/Δt ∼ 100) through direct infusion or analyzed by the GC-EI-MS (Agilent Inc., 7890 GC and 5975 MSD) after derivatization (by additions of 25 μL N,O-Bis­(trimethylsilyl)­trifluoroacetamide (BSTFA) and 25 μL pyridine, both reagents from Sigma-Aldrich). Both analytical methods have been described in detail in our prior studies. ,− …”

T- and pH-Dependent Kinetics of the Reactions of ·OH(aq) with Glutaric and Adipic Acid for Atmospheric Aqueous-Phase Chemistry

“…Hydroxyl radicals (·OH) are important oxidants in the atmospheric environment. − In the aqueous phase, OH radicals can be formed from various sources including uptake from the gas phase, the Fenton reaction of H 2 O 2 with Fe 2+ , photolysis of H 2 O 2 , etc. , In the aqueous phases of aerosols and cloud droplets, the OH radicals can react with glutaric acid (HOOC­(CH 2 ) 3 COOH) and adipic acid (HOOC­(CH 2 ) 4 COOH) by H atom abstraction . The alkyl radicals formed can then generate higher level oxidation products in the presence of oxygen. , In addition, an electron transfer reaction also can occur between the deprotonated carboxyl group and ·OH. , Based on the speciation calculation, glutaric acid and adipic acid are fully protonated (H 2 A) when the pH value is lower than 2 and are fully deprotonated (A 2– ) when the pH value is higher than 8. , In previous studies, the OH radical reaction rate constants of the two DCAs in the H 2 A and A 2– forms have been determined at room temperature at pH values of 2 and 9 by Scholes and Willson and Herrmann .…”

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].…”