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

Abstract

<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 chemicals (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>

Naphthalene‐Derived Secondary Organic Aerosols Interfacial Photosensitizing Properties

“…Many previous studies showed that photosensitized chemistry may be an important pathway for promoting the oxidation of VOCs (Kaur et al., 2019; Li et al., 2019a) in atmospheric particles, which can increase the mass of SOA. For example, irradiation of imidazole‐2‐carboxaldehyde (IC) and humic acid in the presence of various gaseous VOCs, such as d‐limonene, has been shown to produce highly oxygenated compounds and thereby initiate aerosol growth by photosensitized mechanisms (Aregahegn et al., 2013; González Palacios et al., 2016; Monge et al., 2012; Rossignol et al., 2014; Tsui et al., 2017).…”

“…Both sulfate and secondary organic aerosol (SOA) significantly impact air quality, climate, as well as human and ecosystem health (Fuzzi et al, 2015;Nel, 2005). Despite extensive research spanning nearly a century, we still do not fully understand all possible pathways for their formation and transformation in the atmosphere (Wang et al, 2014;Zhang et al, 2007).Many previous studies showed that photosensitized chemistry may be an important pathway for promoting the oxidation of VOCs (Kaur et al, 2019;Li et al, 2019a) in atmospheric particles, which can increase the mass of SOA. For example, irradiation of imidazole-2-carboxaldehyde (IC) and humic acid in the presence of various gaseous VOCs, such as d-limonene, has been shown to produce highly oxygenated compounds and thereby initiate aerosol growth by photosensitized mechanisms (…”

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

“…Collins et al 441 observed the similar formation of oxygenated products when the lipid diacylglycerol phosphatidylcholine was exposed to UV radiation in the presence of docosahexaenoic acid as a photosensitizer. The work of Li et al 440 is unique because they not only used traditional model photosensitizers like IC and humic acid, they also used chambermade SOA and ambient PM 2.5 (particular matter with diameter <2.5 mm). SOA was generated from the photo-oxidation of limonene in the presence of OH, NO x , and ammonium sulfate seed particles.…”

“…Similar results were obtained with these more complex photosensitizers as has been observed with IC and humic acid. The Li et al 440 study marks the rst time SOA and PM 2.5 were tested as photosensitizers for reactions of organic Langmuir lms.…”

“…Photochemical reactions of phospholipids are of particular interest in the marine atmosphere because they are commonly found on SSA. Li et al 440 tested the photostability of two model phospholipids, DSPC (1,2-distearoyl-sn-glycero-3-phosphocholine) and DOPC (1,2-dioleoylsn-glycero-3-phosphocholine), at the airwater interface. No signicant changes to the DSPC monolayer were observed upon irradiation in the presence of photosensitizers.…”