Enhanced Photochemical Volatile Organic Compounds Release from Fatty Acids by Surface-Enriched Fe(III)

Abstract

Both Fe­(III) and fatty acids are ubiquitous and important species in environmental waters. Because they are amphipathic, many fatty acids are surface active and prone to enrichment at the air–water interface. Here, we report that by using nonanoic acid (NA) as a model fatty acid, coexisting Fe­(III), even at concentrations as low as 1 μM, markedly enhanced the photochemical release of NA-derived volatile organic compounds (VOCs) such as octanal and octane into the air. Further studies indicated that the surface-enriched fatty acids dramatically increase the local concentration of Fe­(III) at the water surface, which enables Fe­(III)-mediated photochemical reactions to take place at the air–water interface, and the VOCs facilely produced by fatty acid photooxidation can then be released into the air. Moreover, the product distribution in the Fe­(III)-mediated reactions was largely different from that in other photochemical systems, and a mechanism based on photochemical decarboxylation is proposed. Considering that the coexistence of fatty acids and Fe­(III) in the environment is common, the enhanced photochemical release of VOCs by surface-enriched fatty acids and Fe­(III) may be an important channel for the atmospheric emission of VOCs, which are known to play an essential role in the formation of ozone and secondary organic aerosols.

Photoinduced Release of Volatile Organic Compounds from Fatty Alcohols at the Air–Water Interface: The Role of Singlet Oxygen Photosensitized by a Carbonyl Group

“…Taking Fe as an example, the third reaction pathway is electron transfer. With Fe as a photoactive transition metal, its coordinative interactions with fatty acids lead to a ligand-to-metal electron transfer process under photoirradiation, and then, further formation of radicals and the release of VOCs occur . To sum up, it can be found that previous studies have not mentioned the role of singlet oxygen ( 1 O 2 ) in the interfacial photoreaction.…”

“…Then, aldehydes or ketones produce alkenes through the photolytic cleavage via the Norrish reaction . The second reaction pathway of photoinduced release of VOCs is hydrogen abstraction by a photosensitizer from photochemically inert surfactants (such as fatty alcohols) − due to the widespread presence of photosensitizers at the air–water interface (such as humic substances, , aromatic carbonyls, , chlorophyll, or Fe 3+ species). It is mainly attributed to direct hydrogen abstraction of surfactants by excited-state photosensitizers, , which is a key step to initiate the photoreaction.…”

Pre-deliquescent water uptake in deposited nanoparticles observed with in situ ambient pressure X-ray photoelectron spectroscopy

“…Reaction rates in micrometer-scale droplets have been measured (Jacobs et al, 2017;Marsh et al, 2019;Zhang et al, 2021) and modeled (Benjamin, 2019;Mallick and Kumar, 2020) to be higher than those in bulk water, with some reactions even proceeding spontaneously (Lee et al, 2019). For cloud and fog systems where the interfacial region makes up a significant fraction of the condensed aqueous phase, the reaction rate at the surface can be the rate-limiting step in multi-phase OH oxidation involving surface-active organic species such as pinonic acid (Huang et al, 2018). Interfacial water molecules can promote reactions between organic acids and SO 3 , which are distinct from those that occur in the gas phase and important for heterogeneous formation of H 2 SO 4 and subsequent new particle formation (Zhong et al, 2019;Lv and Sun, 2020).…”

“…Organic material comprises a large fraction of the ambient aerosol mass (Kanakidou et al, 2005), and malonic acid and sucrose represent hygroscopic organic compounds with different chemical functionalities. Dicarboxylic acids such as malonic acid have been identified in ambient aerosol samples where they can dominate the water-soluble organic fraction (Khwaja, 1995;Yu et al, 2005;Decesari et al, 2000Decesari et al, , 2001 and are hygroscopic in both sub-and supersaturated conditions (Prenni et al, 2001;Hori et al, 2003;Rissman et al, 2007;Pope et al, 2010). Sucrose is studied as a model carbohydrate that can form a glassy state in response to changing relative humidity with effects on its ability to act as either cloud or ice nuclei (Zobrist et al, 2011;Estillore et al, 2017).…”

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

“…This would imply that at higher pHs, the charged headgroups on the dissociated nonanoic acid would reduce the presence of halides at the interface, which may have led to the reduction in photochemistry from seawater when oxygen is present. Huang et al, 90 in contrast, found that Fe(III) enhanced the formation of VOCs from nonanoic acid. This could be due to the fact that Fe(III) rapidly quenches 1 NOM*, [91][92][93] which could increase the importance of intersystem crossing, leading to a greater 3 NOM* concentration.…”