Abstract
The sea-surface microlayer (SML) has different physical, chemical and biological properties compared to the subsurface water, with an enrichment of organic matter i.e., dissolved organic matter including UV absorbing humic substances, fatty acids and many others. Here we present experimental evidence that dissolved organic matter, such as humic acids, when exposed to sunlight, can photosensitize the chemical conversion of linear saturated fatty acids at the air-water interface into unsaturated functionalized gas phase products (i.e. saturated and unsaturated aldehydes and acids, alkenes and dienes,…) which are known precursors of secondary organic aerosols. These functionalized molecules have previously been thought to be of biological origin, but here we demonstrate that abiotic interfacial photochemistry has the potential to produce such molecules. As the ocean is widely covered by the SML, this new understanding will impact on our ability to describe atmospheric chemistry in the marine environment.
Highlights
Actinic radiation, throughout the entire near-UV and visible spectrum
Dissolved organic matter (DOM) concentrations in surface water typically range from 0.1 mg L−1 to 20 mg L−1 and is mainly composed of humic substances; the sea surface microlayer contains up to ten times the concentration of dissolved organic material (DOM) compared to its bulk water concentration[13]
The main difference in chemical composition between terrestrial and marine DOM is the level of H/C, N/C and O/C and it has been shown that the ratios for the humic acids used in our study are ranging within the values reported for marine humic acids[14]
Summary
Actinic radiation, throughout the entire near-UV and visible spectrum. Similar observations were made by Jammoul et al.[8] with simple aromatic ketones, present at the air sea interface, instead of chlorophyll. The chromophores contained in the humic acids will be excited They initiate the degradation of the organic surfactant at the air-water interface from their triplet state. Such processes in the bulk water are well documented[13] and are known to initiate a series of potential oxidative pathways such H abstraction, charge transfer reactions, or the production of hydrated electrons, HOx or singlet oxygen, all of which have the potential to chemically degrade aqueous organic compounds, leading to the formation of alcohols, aldehydes or ketones the structure of which will depend on their respective precursor molecules. The gas phase products arising from the photoinduced reactions were analysed by means of high resolution proton transfer mass spectrometry (PTR-ToF-MS), using different ionization agents, in order to exclude any possible interference in the mass attribution
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