We investigated the photodegradation of secondary organic aerosol (SOA) particles by near-UV radiation and photoproduction of oxygenated volatile organic compounds (OVOCs) from various types of SOA. We used a smog chamber to generate SOA from α-pinene, guaiacol, isoprene, tetradecane, and 1,3,5-trimethylbenzene under high-NOx, low-NOx, or ozone oxidation conditions. The SOA particles were collected on a substrate, and the resulting material was exposed to several mW of near-UV radiation (λ ∼ 300 nm) from a light-emitting diode. Various OVOCs, including acetic acid, formic acid, acetaldehyde, and acetone were observed during photodegradation, and their SOA-mass-normalized fluxes were estimated with a Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS). All the SOA, with the exception of guaiacol SOA, emitted OVOCs upon irradiation. Based on the measured OVOC emission rates, we estimate that SOA particles would lose at least ∼1% of their mass over a 24 h period during summertime conditions in Los Angeles, California. This condensed-phase photochemical process may produce a few Tg/year of gaseous formic acid, the amount comparable to its primary sources. The condensed-phase SOA photodegradation processes could therefore measurably affect the budgets of both particulate and gaseous atmospheric organic compounds on a global scale.

Organic aerosols are subjected to atmospheric processes driven by sunlight, including the production of reactive oxygen species (ROS) capable of transforming their physicochemical properties. In this study, secondary organic aerosols (SOA) generated from aromatic precursors were found to sensitize singlet oxygen ( 1 O 2 ), an arguably underappreciated atmospheric ROS. Specifically, we quantified 1 O 2 , OH radical, and H 2 O 2 quantum yields within photoirradiated solutions of laboratory-generated SOA from toluene, biphenyl, naphthalene, and 1,8-dimethylnaphthalene. At 5 mg C L −1 of SOA extracts, the average steady-state concentrations of 1 O 2 and of OH radicals in irradiated solutions were 3 ± 1 × 10 −14 M and 3.6 ± 0.9 × 10 −17 M, respectively. Furthermore, ROS quantum yields of irradiated ambient PM 10 extracts were comparable to those from laboratory-generated SOA, suggesting a similarity in ROS production from both types of samples. Finally, by using our measured ROS concentrations, we predict that certain organic compounds found in aerosols, such as amino acids, organonitrogen compounds, and phenolic compounds have shortened lifetimes by more than a factor of 2 when 1 O 2 is considered as an additional sink. Overall, our findings highlight the importance of SOA as a source of 1 O 2 and its potential as a competitive ROS species in photooxidation processes.

We used a quartz crystal microbalance (QCM) to quantify the mass loss resulting from exposure of secondary organic aerosol (SOA) particles deposited on the QCM crystal to 254, 305, and 365 nm radiation. We coupled the QCM setup to a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) to chemically resolve the photoproduced volatile organic compounds (VOCs) responsible for the mass loss. The photoproduced VOCs detected by the PTR-ToF-MS accounted for ∼50% of the mass loss rates measured with the QCM. Weakly absorbing SOA produced by ozonolysis of α-pinene or d-limonene exhibited a much larger mass loss rate in both the QCM and the PTR-ToF-MS data compared to that of strongly absorbing SOA produced by photooxidation of guaiacol. We predict that the fractional mass loss rate of α-pinene ozonolysis SOA should be as high as ∼1%/h on the summer solstice in Los Angeles in the lower troposphere and ∼4%/h in the stratosphere. The mass loss rates for SOA particles crossing a typical 254 nm oxidation flow reactor, which is routinely used for rapid aging of organic aerosol particles, are expected to be negligible because of the short residence time inside the reactor.

The ability of a complex mixture of organic compounds found in secondary organic aerosol (SOA) to act as a photosensitizer in the oxidation of volatile organic compounds (VOCs) was investigated. Different types of SOAs were produced in a smog chamber by oxidation of various biogenic and anthropogenic VOCs. The SOA particles were collected from the chamber onto an inert substrate, and the resulting material was exposed to 365 nm radiation in an air flow containing ∼200 ppbv of limonene vapor. The mixing ratio of limonene and other VOCs in the flow was observed with a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). The photosensitized uptake of limonene was observed for several SOA materials, with a lower limit for the reactive uptake coefficient on the scale of ∼10. The lower limit for the uptake coefficient under conditions of Los Angeles, California on the summer solstice at noon was estimated to be on the order of ∼10. Photoproduction of oxygenated VOCs (OVOCs) resulting from photodegradation of the SOA material also occurred in parallel with the photosensitized uptake of limonene. The estimated photosensitized limonene uptake rates by atmospheric SOA particles and vegetation surfaces appear to be too small to compete with the atmospheric oxidation of limonene by the hydroxyl radical or ozone. However, these processes could play a role in the leaf boundary layer where concentrations of oxidants are depleted and concentrations of VOCs are enhanced relative to the free atmosphere.

PurposeCataract results from the formation of light-scattering precipitates due to point mutations or accumulated damage in the structural crystallins of the eye lens. Although excised cataracts are predominantly amorphous, in vitro studies show that crystallins are capable of adopting a variety of morphologies depending on the preparation method. Here we characterize thermal, pH-dependent, and UV-irradiated aggregates from wild-type human γS-crystallin (γS-WT) and its aggregation-prone variant, γS-G18V.MethodsAggregates of γS-WT and γS-G18V were prepared under acidic, neutral, and basic pH conditions and held at 25°C or 37°C for 48 hours. UV-induced aggregates were produced by irradiation with a 355-nm laser. Aggregation and fibril formation were monitored via turbidity and thioflavin T (ThT) assays. Aggregates were characterized using intrinsic aromatic fluorescence, powder x-ray diffraction, and mass spectrometry.ResultsγS-crystallin aggregates displayed different characteristics depending on the preparation method. γS-G18V produced a larger amount of detectable aggregates than did γS-WT and at less-extreme conditions. Aggregates formed under basic and acidic conditions yielded elevated ThT fluorescence; however, aggregates formed at low pH did not produce strongly turbid solutions. UV-induced aggregates produced highly turbid solutions but displayed only moderate ThT fluorescence. X-ray diffraction confirms amyloid character in low-pH samples and UV-irradiated samples, although the relative amounts vary.ConclusionsγS-G18V demonstrates increased aggregation propensity compared to γS-WT when treated with heat, acid, or UV light. The resulting aggregates differ in their ThT fluorescence and turbidity, suggesting that at least two different aggregation pathways are accessible to both proteins under the conditions tested.