Real-Time Detection of Gas-Phase Organohalogens from Aqueous Photochemistry Using Orbitrap Mass Spectrometry

Abstract

Marine short-lived halogenated compounds, emitted from algae, phytoplankton and other marine biota, affect significantly both the troposphere and the stratosphere. Here we show that such compounds might also be photochemically produced through photosensitized reactions in surface water. Gas-phase products were detected and identified by high-resolution mass spectrometry, more particularly by means of an atmospheric pressure chemical ionization (APCI) source coupled to an Orbitrap mass spectrometer. Under simulated solar irradiation, halogenated organic compounds were produced and detected in the gas phase when a proxy of dissolved organic matter, i.e., 4-benzoylbenzoic acid, was excited into its triplet state. We present a mechanism explaining the formation of a variety of such halogenated compounds. These photochemical reactions take place at the air/sea interface and are, therefore, a potential source of short-lived halogenated compounds in the atmosphere, participating in the tropospheric halogen cycle.

Seawater Analysis by Ambient Mass Spectrometry-Based Seaomics and Implications on Secondary Organic Aerosol Formation

“…Particle formation was observed for CAT 8 and GP 10 SML samples. Results from the atmospheric simulation experiments conducted on SML samples were in agreement with previous laboratory studies that demonstrated air-sea interfacial driven chemistry as a source of marine secondary aerosol (Roveretto et al, 2019;Ciuraru et al, 2015;Fu et al, 2015).…”

“…21 and 34) that were identified as halogenated compounds were enriched in SML samples (Table S4). Possible sources of halogenated compounds in SML samples are photochemical reactions occurring at the water/air interface (Roveretto et al, 2019;Donaldson and George, 2012). It is worth noting that organic compounds identified in the discriminant panel may have derived both from the secreted (exometabolome) and/or intracellular metabolites (endometabolome) of biological organisms such as algal species and microorganisms present in seawater.…”

Halogen activation and radical cycling initiated by imidazole-2-carboxaldehyde photochemistry

“…The predicted maxima are shifted towards higher halide concentrations compared with our observations. This can be explained if CA derived radicals reacted with halogen radicals to produce halogen-containing organic compounds, as already observed in aquatic media (Roveretto et al, 2019), which could result in a partial scavenging of halogens. Another feature captured by our model is the fact that the downward slopes of observed HO 2 production are greater for films containing bromide than for those containing iodide.…”

“…De Laurentiis et al (2012) suggested that excited triplet states could oxidize bromide faster than OH radicals in seawater. Some modeling studies of aerosol chemistry consider inorganic halogen chemistry to be important (Sherwen et al, 2016b, a); however, Pechtl et al claimed that reactions with dissolved organic matter may be included as a relevant HOI deactivation pathway (Sarwar et al, 2016;Pechtl et al, 2007;Roveretto et al, 2019). Photosensitized halogen activation is less understood and has not been included in these models.…”

“…In combination with the synchronized behavior of both releases (HO 2 and I 2 ), this indicates that iodide is nearly completely depleted in our films after 100 min of irradiation and presumably most of the iodide is converted into molecular iodine, consistent with the lifetime estimate based on the observed maximum release rate. Alternatively, sinks of halides in the films could be the reaction of halide radicals (I q or I − 2 ) and of HOI or HOBr with organics producing Org-X (Abrahamsson et al, 2018 ;Gilbert et al, 1988;Roveretto et al, 2019) or further oxidation of iodine to iodate, which was beyond the scope of our study.…”