Production of Peroxy Radicals from the Photochemical Reaction of Fatty Acids at the Air–Water Interface

Abstract

Peroxy radicals are known for their role in tropospheric photochemistry as intermediates in the oxidation of volatile organic compounds, leading to the formation of ozone and organic nitrate compounds. Similarly, in the particle phase, peroxy radicals, considered a type of reactive organic species (ROS), are also involved in many chemical transformations and produce a consequential fraction of aerosols with an impact on health. Here, we show that peroxy radicals are efficiently produced at the air/water interface upon irradiation of an organic film made of a simple fatty acid (i.e., nonanoic acid). This source of peroxy radicals was quantified as 0.27 ppbv after interfacial titration of the peroxy radicals by nitric oxide in a photochemical flow reactor. Using a combination of proton transfer reaction-time of flight-mass spectrometry (PTR-ToF-MS) and ultrahigh performance liquid chromatography-heated electrospray ionizationhigh resolution orbitrap-mass spectrometry (UHPLC-HESI-HR Orbitrap-MS), the products of 2 this photochemistry were identified in the presence and absence of NO. The amount of peroxy 19 radicals produced by this photochemistry was comparable to those measured in surface water, or the ROS bounded to ambient secondary organic aerosols. Accordingly, the photochemistry of surfactant at the air/water interface might play a significant role in the health impact of organic aerosols. KEYWORDS Peroxy radicals, Reactive oxygen species, nonanoic acid, photochemistry, airwater interface. ASSOCIATED CONTENT Supporting Information. Determination of the peroxy radicals produced by the photochemical reaction of the nonanoic acid monolayer, Figure S1-S4 and Scheme S1. This material is available 300 free of charge via the Internet at http://pubs.acs.org.

Measurement of gas-phase OH radical oxidation and film thickness of organic films at the air–water interface using material extracted from urban, remote and wood smoke aerosol

“…Oxidation kinetics of proxy aerosol samples have been well studied, 95 commonly using ozone, 24,38,96 OH radicals 55,64,97–100 photosensitization 53,101–103 or nitrate radicals 104–107 as the oxidants. In the study presented here, films of aerosol extract formed at the air–water interface were oxidized through exposure to gas-phase OH radicals.…”

Does Tert-Butyl Alcohol Really Terminate the Oxidative Activity of •OH in Inorganic Redox Chemistry?

“…In scenarios for investigating the degradation of these organic pollutants by direct photodegradation and by • OH oxidation, when TBA is used as the • OH scavenger, secondary ROO • will be generated and may participate in the reactions with organic pollutants. The role of secondary ROO • in the degradation and transformation of organic pollutants should not be ignored, because ROO • plays a significant role in the oxidation of volatile organic compounds . Thus, owing to the oxidation of organic pollutants by ROO • , the oxidative roles of • OH in aqueous redox reactions cannot be examined simply by comparison of the results with and without TBA.…”

“…The role of secondary ROO • in the degradation and transformation of organic pollutants should not be ignored, because ROO • plays a significant role in the oxidation of volatile organic compounds. 39 Thus, owing to the oxidation of organic pollutants by ROO • , the oxidative roles of • OH in aqueous redox reactions cannot be examined simply by comparison of the results with and without TBA. It is alarming that TBA cannot terminate the oxidative activity of • OH.…”

Emerging investigator series: surfactants, films, and coatings on atmospheric aerosol particles: a review

“…The reaction proceeded through excitation of nonanoic acid to a triplet state, 423 which can form at the surface but not in the bulk because of the high concentration of nonanoic acid molecules required. 422 Hayeck et al 424 later showed that peroxy radicals form as reactive intermediates at the air–water interface when nonanoic acid films are directly irradiated. The mechanism for the photochemical reactions of nonanoic acid is shown in Scheme 1.…”