Atmospheric photochemistry at a fatty acid–coated air-water interface

Abstract

Although fatty acids are believed to be photochemically inert in the actinic region, complex volatile organic compounds are produced during illumination of an air-water interface coated solely with a monolayer of carboxylic acid. When aqueous solutions containing nonanoic acid (NA) at bulk concentrations that give rise to just over a monolayer of NA coverage are illuminated with actinic radiation, saturated and unsaturated aldehydes are seen in the gas phase, and more highly oxygenated products appear in the aqueous phase. This chemistry is probably initiated by triplet-state NA molecules excited by direct absorption of actinic light at the water surface. Because fatty acids-covered interfaces are ubiquitous in the environment, such photochemical processing will have a substantial impact on local ozone and particle formation.

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 addition, Rossignol et al recently showed that photo-induced radical-chemistry occurs even in the absence of a classical photosensitizer from a pure fatty acid microlayer. 4 Therefore, it seems likely that similar 35 photosensitized processes at the air/water interface explain the observed VOC production from biofilm-containing solutions in this study. Furthermore, due to the ubiquity of microorganisms and biofilms in the ambient environment, it has to be assumed that similar processes occur globally on a large scale at air/water interfaces, e.g., of oceans, lakes, rivers, but even aerosol particles and cloud droplets.…”

“…All measurement were performed using the highest possible mass resolution (R=1.4·10 7 at m/z 200). Mass calibration was performed daily for the m/z range <500 using sodium acetate, as described in Rossignol et al 4 Data were processed and evaluated by XCalibur 2.2 (Thermo, USA). Further data analysis was 15 performed using the non-target screening approach of the MZmine 2.21 software package (http://mzmine.github.io).…”

“…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.…”