Mechanistic Insights on the Photosensitized Chemistry of a Fatty Acid at the Air/Water Interface

Abstract

Interfaces are ubiquitous in the environment and many atmospheric key processes, such as gas deposition, aerosol, and cloud formation are, at one stage or another, strongly impacted by physical and chemical processes occurring at interfaces. Here, the photoinduced chemistry of an air/water interface coated with nonanoic acid—a fatty acid surfactant we use as a proxy for chemically complex natural aqueous surface microlayers—was investigated as a source of volatile and semivolatile reactive organic species. The carboxylic acid coating significantly increased the propensity of photosensitizers, chosen to mimic those observed in real environmental waters, to partition to the interface and enhance reactivity there. Photochemical formation of functionalized and unsaturated compounds was systematically observed upon irradiation of these coated surfaces. The role of a coated interface appears to be critical in providing a concentrated medium allowing radical–radical reactions to occur in parallel with molecular oxygen additions. Mechanistic insights are provided from extensive analysis of products observed in both gas and aqueous phases by online switchable reagent ion-time of flight-mass spectrometry and by off-line ultraperformance liquid chromatography coupled to a Q Exactive high resolution mass spectrometer through heated electrospray ionization, respectively.

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).…”

“…In the past, it was shown that unique photochemical reactions with significant implications for atmospheric processes can occur at such interfaces, leading to the formation of volatile organic 5 compounds (VOCs) [1][2][3][4][5] and secondary organic aerosols, 6 or acting as sinks for reactive species, such as NO 2 or ozone. [7][8][9][10] This interfacial photochemistry is exclusively due to the presence of surfactants which tend to concentrate in surface layers with respect to the underlying bulk water.…”

“…24-31 30 Although previous studies demonstrated photo-induced VOC production from surfactants on aqueous solutions, these experiments were typically conducted far from ambient conditions or for a very limited number of samples. [1][2][3][4][5] However, since biogenic surfactants are omnipresent in the environment, such photochemical processes possibly contribute to a significant extent to ambient VOC concentrations. To investigate this potentially underestimated VOC source under more realistic 35 conditions, in this study, biogenic surfactants were produced from authentic riverine biofilms.…”

“…Additionally, such surfactants also increase the propensity of less surface-active compounds to enrich there as well, creating a unique chemical environment, affecting not only chemistry but also trace-gas 10 exchange. 5,[10][11][12][13][14][15][16] A major source of biogenic surfactants in the ambient environment are so-called biofilms, loosely defined as a population of microorganisms (i.e., fungi, algae, archaea) that accumulate at an interface. In addition, such microorganisms can also form cellular aggregates not attached to a surface, sometimes also referred to as flocs or sludge, which have very similar characteristics as 15 biofilms.…”

“…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,…”