UV photochemistry of carboxylic acids at the air‐sea boundary: A relevant source of glyoxal and other oxygenated VOC in the marine atmosphere

Abstract

Photochemistry plays an important role in marine dissolved organic carbon (DOC) degradation, but the mechanisms that convert DOC into volatile organic compounds (VOCs) remain poorly understood. We irradiated carboxylic acids (C7–C9) on a simulated ocean surface with UV light (<320 nm) in a photochemical flow reactor and transferred the VOC products into a dark ozone reactor. Glyoxal was detected as a secondary product from heptanoic, octanoic, and nonanoic acid (NA) films, but not from octanol. Primary glyoxal emissions were not observed, nor was glyoxal formed in the absence of ozone. Addition of a photosensitizer had no noticeable effect. The concurrent detection of heptanal in the NA system suggests that the ozonolysis of 2‐nonenal is the primary chemical mechanism that produces glyoxal. This source can potentially sustain tens of parts per trillion by volume (pptv) glyoxal over oceans, and helps to explain why glyoxal fluxes in marine air are directed from the atmosphere into the ocean.

Interfacial photochemistry of biogenic surfactants: a major source of abiotic volatile organic compounds

“…[2][3][4][5]40 In agreement, the same VOC classes were observed for the biofilm samples investigated in this work (Table 1 and Supplementary Information). In particular, not only saturated 15 oxygenated VOCs, such as aldehydes and ketones, but also unsaturated and functionalized VOCs were observed (e.g., alkenes, dienes, unsaturated aldehydes or ketones).…”

“…3,42 It has been demonstrated that photosensitizers, e.g., humic acids, can initiate photochemical VOC production in the presence of surfactants by transition to a triplet state after UV/visabsorption. 2,3,5,10,40 Subsequently, such excited molecules can trigger radical chemistry through H-30 abstraction from other organics, such as fatty acids, followed by addition of molecular oxygen. 43,44 Furthermore, photosensitizers were shown to be enriched in SMLs on aqueous solutions, 5,45,46 thus,…”

Microlayer source of oxygenated volatile organic compounds in the summertime marine Arctic boundary layer

“…Although SML chemical composition is complex and not fully understood, it contains both saturated and unsaturated fatty acids (32,34,35). Gas-phase production of smaller and more oxygenated molecules from these model SML constituents can occur via three different mechanisms: direct photooxidation (17,26), photosensitized oxidation (27,28,31), or heterogeneous oxidation (29). Thus, OVOCs can be produced from the SML in the presence of light with or without the action of photosensitizers, or in the presence of gas-phase oxidants such as ozone or the hydroxyl radical.…”

“…Recent laboratory studies (17,(26)(27)(28)(29)(30)(31) have uncovered novel mechanisms for the emission of OVOCs to the MBL involving chemistry at the interface between the ocean and atmosphere, where the sea surface microlayer (SML) can be enriched in organic matter. The thickness of the SML is typically about 50 μm (32), and substantial SML enrichments (relative to underlying bulk seawater) in surfactants, saccharides, and plankton exudates have been observed globally in both eutrophic and oligotrophic waters (e.g., ref.…”

“…That known sources fail to account for observed levels of OVOCs demonstrates our incomplete understanding of the processes controlling the organic composition of the remote atmosphere and has led to speculation that a missing source of MBL OVOCs must exist (5,8,(14)(15)(16). The missing sources of formic acid and glyoxal, as examples, have been suggested to arise from heterogeneous oxidation processes (17). The missing source for formic acid is thought to be particularly pronounced in the Arctic (18).…”