Photochemical aging of atmospherically reactive organic compounds involving brown carbon at the air–aqueous interface

Abstract

Abstract. Water-soluble brown carbon in the aqueous core of aerosol may play a role in the photochemical aging of organic film on the aerosol surface. To better understand the reactivity and photochemical aging processes of organic coating on the aqueous aerosol surface, we have simulated the photosensitized reaction of organic films made of several long-chain fatty acids in a Langmuir trough in the presence or absence of irradiation. Several chemicals (imidazole-2-carboxaldehyde and humic acid), PM2.5 samples collected from the field, and secondary organic aerosol samples generated from a simulation chamber were used as photosensitizers to be involved in the photochemistry of the organic films. Stearic acid, elaidic acid, oleic acid, and two different phospholipids with the same carbon chain length and different degrees of saturation, i.e. 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dioleoylsn-glycero-3-phosphocholine (DOPC), were chosen as the common organic film-forming species in this analysis. The double bond (trans and cis) in unsaturated organic compounds has an effect on the surface area of the organic monolayer. The oleic acid (OA) monolayer possessing a cis double bond in an alkyl chain is more expanded than elaidic acid (EA) monolayers on artificial seawater that contain a photosensitizer. Monitoring the change in the relative area of DOPC monolayers has shown that DOPC does not react with photosensitizers under dark conditions. Instead, the photochemical reaction initiated by the excited photosensitizer and molecular oxygen can generate new unsaturated products in the DOPC monolayers, accompanied by an increase in the molecular area. The DSPC monolayers did not yield any photochemical oxidized products under the same conditions. The spectra measured with polarization modulation-infrared reflection–absorption spectroscopy (PM-IRRAS) were also consistent with the results of a surface pressure–area isotherm. Here, a reaction mechanism explaining these observations is presented and discussed. The results of PM2.5 and SOA samples will contribute to our understanding of the processing of organic aerosol aging that alters the aerosol composition.

The reaction of oleic acid monolayers with gas-phase ozone at the air water interface: the effect of sub-phase viscosity, and inert secondary components

“…82 The reaction of oleic acid with other atmospheric oxidants has been studied at the air-water interface with nitrate radical, 83 OH radical, 84 nitrogen dioxide, 49 sulfur dioxide 85 and UV radiation via photosensitisation. [86][87][88]…”

Photosensitization mechanisms at the air–water interface of aqueous aerosols

“…Imidazole derivatives such as IC are produced from glyoxal reactions in aqueous aerosol particles (see Refs 8,25 and references cited therein). IC has been the focus of many experimental studies 9,24,[26][27][28][29][30][31][32] but apart from some simple computations, 9 its photochemistry has not been investigated theoretically. Some measurements have revealed that IC can lead to significant SOA production, 29,30 although some others 31 suggest that more abundant photosensitizer species, such as humic-like substances, may contribute more significantly to SOA growth.…”

Impact of Environmental Conditions on Secondary Organic Aerosol Production from Photosensitized Humic Acid

“…One such mechanism that has been recently proposed is photosensitization, ,, which involves a particle-phase, light-absorbing species promoting the oxidation of a volatile organic compound (VOC), which adds to SOA mass. Previously investigated photosensitizers include imidazole-2-carboxaldehyde, ,− non-phenolic aromatic carbonyls, , and humic-like substances (HULIS). , Photosensitization has also been investigated for its role in particle aging − and sulfate formation …”