Interfacial Ozone Oxidation Chemistry at a Riverine Surface Microlayer as a Source of Nitrogen Organic Compounds

Abstract

Nitrogen (N)-containing organic compounds, including “brown carbon” (BrC), represent an important fraction of organic aerosols. However, little is known about the processes of formation of the secondarily formed N-containing organics in the atmosphere. Here, we investigated the formation of gas-phase organic compounds, including N-containing organics, through interfacial oxidation chemistry of gaseous O3 with an authentic riverine surface microlayer (SML) by using a high-resolution quadrupole Orbitrap mass spectrometer coupled to a commercial secondary electrospray ionization source. The resulting hierarchical cluster diagram obtained for real-time observation for 60 min shows the occurrence of 677 ions in positive mode. The level of N-containing organics, including BrC compounds (e.g., imidazoles), formed during the heterogeneous processing of O3 on the SML in the dark and under ultraviolet–visible light irradiation, was on average 20.7% among all samples. Many of the detected N-containing compounds comprise a CN bond, suggesting that they are potentially toxic compounds that also affect urban air quality. Overall, this study provides evidence that interfacial ozone oxidation chemistry at the riverine SML plays an important role as an additional source of air pollution in urban environments, which can affect both human health and the absorption properties of urban aerosols.

Molecular Characterization of the Product Compounds Formed Upon Heterogeneous Chemistry of Ozone With Riverine Surface Microlayer

“…Most of them are with 3–11 oxygens, similar to organic nitrogen compounds detected in particle phase (Lee et al., 2016). Compounds with one N‐atom are dominant (131) whereas other 40 compounds contain two N‐atoms which is a similar trend with the observed gas‐phase products (Wang et al., 2022) indicating that these N‐containing compounds can likely be released in the gas phase and affect the air quality. Furthermore, 42 homologous series of common N‐containing organic products have been separated by KM and KMD calculation as shown in Figure 6b and listed in Table S4 in Supporting Information .…”

“…Figure 1 shows the overview of the condensed‐phase compounds detected before and after ozonolysis of the three SML samples. It has to be noted that one fraction of the observed products in the aqueous phase by FT‐ICR MS were also observed in the gas phase by SESI‐UHR‐MS in our previous study (Wang et al., 2022) indicating the importance of this chemistry for the atmospheric composition. The distributions of the peak intensities prior to ozonolysis were similar among the three observed SML samples.…”

“…Another interesting feature is that the rate of ozone deposition is not affected in the presence of biological organic material (Thalassiosira pseudonana) (Schneider, Lakey, et al., 2022). Photochemistry (Brüggemann, 2020; Chiu et al., 2017; Ciuraru, Fine, Pinxteren et al., 2015, Ciuraru, Fine, van Pinxteren et al., 2015; Fu et al., 2015; Lin et al., 2021; Rossignol et al., 2016; Stirchak et al., 2021; Tinel et al., 2016; Wang et al., 2020) and heterogeneous ozone chemistry (Wang et al., 2022; Zhou et al., 2014) occurring at the sea surface, result in formation of unsaturated oxygenated VOCs (organic acids, carbonyls, aldehydes, etc.) released to the gas phase, and highly oxygenated organic compounds which remain in the aqueous phase (Mungall et al., 2017).…”

Production of Volatile Organic Compounds by Ozone Oxidation Chemistry at the South China Sea Surface Microlayer

“…With regard to detected m/z 69 by PTR−TOF−MS, the possibility that it could be isoprene was ruled out because it was absent in the mass spectra of TD− GC−MS. Again, with reference to our previous study using U-HR-MS, 25 The chemical composition of SML is a very complex and intriguing issue, but it is known that it contains fatty acids, which are highly reactive with ozone. 2 Indeed, previous studies reported the formation of 3-nonenal (m/z 141, [C 9 H 16 O]H + ) and hexanal (m/z 101, [C 6 H 12 O]H + ) as major products when the monolayer of linoleic acid was oxidized by ozone.…”

“…3 For example, the deposition of ozone (O 3 ) to the sea surface is mainly controlled by the chemical reactivity at the SML. 4−13 Photochemical and photosensitized processes 14−23 and heterogeneous ozone chemistry 24,25 at the SML lead to the production of oxygenated volatile organic compounds (OVOCs) (acids, ketones, aldehydes, etc.). Mungall et al observed OVOCs above sea waters and suggested that they may stem from photosensitized chemistry and heterogeneous reactions of the hydroxyl radical (OH) and O 3 with the organics enriched at the SML.…”