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

Siyang Li, Xiaotong Jiang, Marie Roveretto, Christian George, Ling Liu, Wei Jiang, Qingzhu Zhang, Wenxing Wang, Maofa Ge, Lin Du
2019 Atmospheric Chemistry and Physics Discussions  
<p><strong>Abstract.</strong> Photosensitizing compounds containing brown carbon can absorb UV light and transfer that energy to low volatile organic compounds at the surface of aqueous particles. 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
more » ... ls (imidazole-2-carboxaldehyde and humic acid), PM<sub>2.5</sub> samples collected from the field and secondary organic aerosols 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 (<i>trans</i> and <i>cis</i>) in unsaturated organic compounds has an effect on the surface area of the organic monolayer. The OA monolayer possessing a <i>cis</i> double bond in an alkyl chain is more expanded than 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 hydroperoxidation 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 will contribute to our understanding of the processing of organic aerosol aging that controls the aerosol composition.</p>
doi:10.5194/acp-2019-96 fatcat:heb2dqatgjfrfdlc7vt5qasgwe