Evolution of aqSOA from the Air–Liquid Interfacial Photochemistry of Glyoxal and Hydroxyl Radicals

Abstract

The effect of photochemical reaction time on glyoxal and hydrogen peroxide at the air−liquid (a−l) interface is investigated using in situ time-of-flight secondary ion mass spectrometry (ToF−SIMS) enabled by a system for analysis at the liquid vacuum interface (SALVI) microreactor. Carboxylic acids are formed mainly by reaction with hydroxyl radicals in the initial reactions. Oligomers, cluster ions, and water clusters formed due to longer photochemistry. Our results provide direct molecular evidence that water clusters are associated with proton transfer and the formation of oligomers and cluster ions at the a−l interface. The oligomer formation is facilitated by water cluster and cluster ion formation over time. Formation of higher m/z oligomers and cluster ions indicates the possibility of highly oxygenated organic components formation at the a−l interface. Furthermore, new chemical reaction pathways, such as surface organic cluster, hydration shell, and water cluster formation, are proposed based on SIMS spectral observations, and the existing understanding of glyoxal photochemistry is expanded. Our in situ findings verify that the a−l interfacial reactions are important pathways for aqueous secondary organic aerosol (aqSOA) formation.

Studying Interfacial Dark Reactions of Glyoxal and Hydrogen Peroxide Using Vacuum Ultraviolet Single Photon Ionization Mass Spectrometry

“…VUV SPI-MS and ToF-SIMS use different ionization methods and those differences were reflected in the observed products [24,25,39]. ToF-SIMS can detect cluster ions including water clusters and water and organic cluster ions to study environmental and biological surfaces and interfaces [23,24,28,30,40,41]. VUV SPI-MS, deemed as a soft ionization technique, has demonstrated its ability to capture more oligomers in this work.…”

“…The expanded reaction flow chart showing glyoxal dark reactions is illustrated in Figure 4 with representative new products marked in blue based on the comparison results with those of recent liquid ToF-SIMS. The cluster ions and water clusters observed in the liquid ToF-SIMS spectra were not detected in VUV SPI-MS [24,28]. In contrast, VUV SPI-MS detected some new oligomers that were not observed in liquid ToF-SIMS, and these products are highlighted in gray in Table S3.…”

“…Excess hydrogen peroxide as the oxidant ensures that the glyoxal oxidation reactions meet the pseudo-first-order condition. Polymers and oligomers can also form at the a-l interface using this concentration [24,28]. We covered the devices in clean aluminum foils after introducing glyoxal and H 2 O 2 liquid mixture into the channel at different times to avoid any possible irradiation during sample preparation.…”

“…In addition, these products observed by VUV SPI-MS are compared with results from in situ liquid ToF-SIMS shown in Table S3. The latter is a novel approach to study a-l interfacial reactions [23,24,28]. Interestingly, the two techniques show different kinds of oligomer products.…”

Single-particle Raman spectroscopy for studying physical and chemical processes of atmospheric particles

“…The production of sulfate via nitrate photolysis was discussed earlier. Recently, R. reported a prominent formate production by glyoxal oxidation during particulate nitrate photolysis, instead of the more commonly found oxalic acids in the oxidation of glyoxal in bulk solutions (R. Tan et al, 2009;Zhang et al, 2019). Furthermore, Liang et al (2021) studied nitrate photolysis in particles containing sucrose, which was used as a surrogate for atmospheric viscous organics.…”