Differences in Photosensitized Release of VOCs from Illuminated Seawater versus Freshwater Surfaces

Abstract

Recent studies have shown that photochemical reactions occurring at the air-water interface are a source of volatile organic compounds (VOCs) to the atmosphere. We report here the photosensitized formation of VOCs from illuminated freshwater and seawater mimics containing nonanoic acid (NA) and/or Suwannee River natural organic matter (SRNOM). Under an atmosphere of air, the total blank-corrected steady-state concentration of VOCs formed from illuminated seawater coated with nonanoic acid is somewhat smaller than that formed from freshwater, suggesting some differences in photochemical pathways for the two substrates. The total blank-corrected steady-state concentration of VOCs more than doubles from both freshwater and seawater NA-coated surfaces under nitrogen compared to air. The addition of SRNOM as a photosensitizer induces some photochemistry from the seawater sample under air, but no chemistry is seen with freshwater, or under nitrogen for either substrate. Adding SRNOM to the nonanoic acid-containing solutions roughly doubles the total steady-state concentration of VOCs emitted from both freshwater and seawater surfaces under air. The small differences in product formation for the two substrates implies some difference in the photochemical mechanisms operating in freshwater versus seawater, which may be due to the presence of halides and metals as well as pH differences between the two aqueous systems. importance and interest. 1,2 The presence of an organic coating, even as little as a sub-monolayer, at a water surface may alter the kinetics of heterogeneous reactions there, with the exact effect depending upon the composition of the monolayer. [3][4][5][6] The air-water interface is also exposed to sunlight for large portions of time, suggesting the importance of heterogeneous photochemistry processes that may occur there. 7 Recent studies have focused on understanding how the air-water interface influences the formation of volatile organic compounds (VOCs) in the aqueous phase and their release into the atmosphere. Field studies found evidence to suggest that the air-water interface is a source of VOCs, such as isoprene and formic acid, [8][9][10][11] which suggests that a marine source of VOCs is more important than previously thought for the global VOC budget. [12][13][14] In addition, it has been suggested that the type of aquatic environment (fresh versus saline) may influence the production of VOCs. 15 Recent laboratory studies, 16-24 using simplified aquatic environments, have shown that photochemistry at the air-water interface serves as a major abiotic source of functionalized VOCs, [25][26][27] which has further implications towards the oxidative capacity of the atmosphere and the formation of secondary aerosols.These laboratory studies have also highlighted the importance of a microlayer in the formation of the gas-phase products. The sea surface microlayer (SML) has been shown to play a crucial role as a boundary in the physical and chemical exchange between the ocean and the atmosphere. 28,2...

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“…The authors propose three possible reasons: (1) differences in solubility of the products, (2) a decrease in the surface coverage of nonanoic acid on saltwater because of the elevated pH, and (3) interactions between nonanoic acid and the ions in saltwater. 425 These results are relevant to photochemical reactions at the surface of SSA with minimal organic content.…”

“…Although George and coworkers 422,424,425 have shown that it is possible for fatty acids to undergo direct photochemical reactions in highly concentrated surface lms, reactions are far more likely to proceed in the presence of a photosensitizer. 421 A good photosensitizer efficiently absorbs solar radiation and transfers energy to a less photochemically active species.…”

“…The mechanism for the photochemical reactions of nonanoic acid is shown in Scheme 1. 422 Stirchak et al 425 compared VOC release from photochemical reactions of 2 mM nonanoic acid on simulated saltwater vs. freshwater solutions. The yield was actually slightly lower on saltwater.…”

“…The reactions of nonanoic acid described so far have focused on relatively simple model systems. When Stirchak et al 425 added Suwannee River natural organic matter to nonanoic acid solutions, they found a doubling in VOC yield from photochemical reactions. Trueblood et al 427 further increased complexity by using genuine marine dissolved organic matter (DOM) as a photosensitizer for nonanoic acid.…”

Mario Molina Memorial Special Issue

“…organic matter in seawater is implicated in HONO formation 33 and the release of VOCs. 34 The presence of surface-active organics can impact the oxidation of bromide by ozone. 35 Heterogeneous reactions involving chloride can change the speciation of gaseous oxidized mercury.…”