Search

Introduction: Oxidative stress mediated by reactive oxygen species (ROS) is a key process for adverse aerosol health effects. Secondary organic aerosols (SOA) account for a major fraction of particulate matter with aerodynamic diameter ≤2.5 µm (PM 2.5 ). PM 2.5 inhalation and deposition into the respiratory tract causes the formation of ROS by chemical reactions and phagocytosis of macrophages in the epithelial lining fluid (ELF), but their relative contributions are not well quantified and their link to oxidative stress remains uncertain. The specific aims of this project were (1) elucidating the chemical mechanism and quantifying the formation kinetics of ROS in the ELF by SOA; (2) quantifying the relative importance of ROS formation by chemical reactions and macrophages in the ELF., Methods: SOA particles were generated using reaction chambers from oxidation of various precursors including isoprene, terpenes, and aromatic compounds with or without nitrogen oxides (NO x ). We collected size-segregated PM at two highway sites in Anaheim, CA, and Long Beach, CA, and at an urban site in Irvine, CA, during two wildfire events. The collected particles were extracted into water or surrogate ELF that contained lung antioxidants. ROS generation was quantified using electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping technique. PM oxidative potential (OP) was also quantified using the dithiothreitol assay. In addition, kinetic modeling was applied for analysis and interpretation of experimental data. Finally, we quantified cellular superoxide release by RAW264.7 macrophage cells upon exposure to quinones and isoprene SOA using a chemiluminescence assay as calibrated with an EPR spin-probing technique. We also applied cellular imaging techniques to study the cellular mechanism of superoxide release and oxidative damage on cell membranes., Results: Superoxide radicals (·O 2 - ) were formed from aqueous reactions of biogenic SOA generated by hydroxy radical (·OH) photooxidation of isoprene, β-pinene, α-terpineol, and d-limonene. The temporal evolution of ·OH and ·O 2 - formation was elucidated by kinetic modeling with a cascade of aqueous reactions, including the decomposition of organic hydroperoxides (ROOH), ·OH oxidation of primary or secondary alcohols, and unimolecular decomposition of α-hydroxyperoxyl radicals. Relative yields of various types of ROS reflected the relative abundance of ROOH and alcohols contained in SOA, which generated under high NO x conditions, exhibited lower ROS yields. ROS formation by SOA was also affected by pH. Isoprene SOA had higher ·OH and organic radical yields at neutral than at acidic pH. At low pH ·O 2 - was the dominant species generated by all types of SOA. At neutral pH, α-terpineol SOA exhibited a substantial yield of carbon-centered organic radicals (R·), while no radical formation was observed by aromatic SOA., Organic radicals in the ELF were formed by mixtures of Fe 2+ and SOA generated from photooxidation of isoprene, α-terpineol, and toluene. The molar yields of organic radicals by SOA were 5-10 times higher in ELF than in water. Fe 2+ enhanced organic radical yields by a factor of 20-80. Ascorbate mediated redox cycling of iron ions and sustained organic peroxide decomposition, as supported by kinetic modeling reproducing time- and concentration-dependence of organic radical formation, as well as by additional experiments observing the formation of Fe 2+ and ascorbate radicals in mixtures of ascorbate and Fe 3+ . ·OH and superoxide were found to be efficiently scavenged by antioxidants., Wildfire PM mainly generated ·OH and R· with minor contributions from superoxide and oxygen-centered organic radicals (RO·). PM OP was high in wildfire PM, exhibiting very weak correlation with radical forms of ROS. These results were in stark contrast with PM collected at highway and urban sites, which generated much higher amounts of radicals dominated by ·OH radicals that correlated well with OP. By combining field measurements of size-segregated chemical composition, a human respiratory tract model, and kinetic modeling, we quantified production rates and concentrations of different types of ROS in different regions of the ELF by considering particle-size-dependent respiratory deposition. While hydrogen peroxide (H 2 O 2 ) and ·O 2 - production were governed by Fe and Cu ions, ·OH radicals were mainly generated by organic compounds and Fenton-like reactions of metal ions. We obtained mixed results for correlations between PM OP and ROS formation, providing rationale and limitations of the use of oxidative potential as an indicator for PM toxicity in epidemiological and toxicological studies., Quinones and isoprene SOA activated nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in macrophages, releasing massive amounts of superoxide via respiratory burst and overwhelming the superoxide formation by aqueous chemical reactions in the ELF. The threshold dose for macrophage activation was much smaller for quinones compared with isoprene SOA. The released ROS caused lipid peroxidation to increase cell membrane fluidity, inducing oxidative damage and stress. Further increases of doses led to the activation of antioxidant response elements, reducing the net cellular superoxide production. At very high doses and long exposure times, chemical production became comparably important or dominant if the escalation of oxidative stress led to cell death., Conclusions: The mechanistic understandings and quantitative information on ROS generation by SOA particles provided a basis for further elucidation of adverse aerosol health effects and oxidative stress by PM 2.5 . For a comprehensive assessment of PM toxicity and health effects via oxidative stress, it is important to consider both chemical reactions and cellular processes for the formation of ROS in the ELF. Chemical composition of PM strongly influences ROS formation; further investigations are required to study ROS formation from various PM sources. Such research will provide critical information to environmental agencies and policymakers for the development of air quality policy and regulation., (© 2023 Health Effects Institute. All rights reserved.)

During their atmospheric lifetime, organic compounds within aerosols are exposed to sunlight and undergo photochemical processing. This atmospheric aging process changes the ability of organic aerosols to form cloud droplets and consequently impacts aerosol-cloud interactions. We recently reported changes in the cloud forming properties of aerosolized dissolved organic matter (DOM) due to a photomineralization mechanism, transforming high-molecular weight compounds in DOM into organic acids, CO and CO₂. To strengthen the implications of this mechanism to atmospheric aerosols, we now extend our previous dataset and report identical cloud activation experiments with laboratory-generated secondary organic aerosol (SOA) extracts. The SOA was produced from the oxidation of α-pinene and naphthalene, a representative biogenic and anthropogenic source of SOA, respectively. Exposure of aqueous solutions of SOA to UVB irradiation increased the dried organic material's hygroscopicity and thus its ability to form cloud droplets, consistent with our previous observations for DOM. We propose that a photomineralization mechanism is also at play in these SOA extracts. These results help to bridge the gap between DOM and SOA photochemistry by submitting two differently-sourced organic matter materials to identical experimental conditions for optimal comparison.

Direct aqueous photolysis of cis-pinonic acid (PA; 2-(3-acetyl-2,2-dimethylcyclobutyl)acetic acid; CAS Registry No. 473-72-3) by 280-400 nm radiation was investigated. The photolysis resulted in Norrish type II isomerization of PA leading to 3-isopropenyl-6-oxoheptanoic acid (CAS Registry No. 4436-82-2), also known as limononic acid, as the major product, confirmed by (1)H and (13)C NMR analysis, chemical ionization mass spectrometry, and electrospray ionization mass spectrometry. Several minor products resulting from Norrish type I splitting of PA were also detected. The molar extinction coefficients of aqueous PA were measured and used to calculate the photolysis quantum yield of aqueous PA as 0.5 ± 0.3 (effective average value over the 280-400 nm range). The gas-phase photolysis quantum yield of 0.53 ± 0.06 for PA methyl ester (PAMe; CAS Registry No. 16978-11-3) was also measured for comparison. These results indicate that photolysis of PA is not significantly suppressed by the presence of water. This fact was confirmed by photodissociation dynamics simulations of bare PA and of PAMe hydrated with one or five water molecules using on-the-fly dynamics simulations on a semiempirical potential energy surface. The calculations correctly predicted the occurrence of both Norrish type I and Norrish type II photolysis pathways, both driven by the dynamics on the lowest triplet excited state of PA and PAMe. The rate of removal of PA by direct aqueous photolysis in cloudwater and in aerosol water was calculated for a range of solar zenith angles and compared with rates of other removal processes such as gas-phase oxidation by OH, aqueous-phase oxidation by OH, and gas-phase photolysis. Although the direct photolysis mechanism was not the most significant sink for PA in cloud and fog droplets, direct photolysis can be expected to contribute to removal of PA and more soluble/less volatile biogenic oxidation products in wet particulate matter.

Introduction: The U.S. Consumer Product Safety Commission announced a recall of Aqua Dots (Spin Master Ltd.; Toronto, Canada) on November 7, 2007 due to children becoming ill after swallowing beads from these toy craft kits. Reports suggested that the beads contained 1,4-butanediol (1,4-BD), a precursor to gamma-hydroxybutyrate (GHB), rather than the intended, but more expensive 1,5-pentanediol (1,5-PD). We measured the 1,4-BD and 1,5-PD content of Aqua Dots beads to determine if 1,5-PD had been completely substituted with 1,4-BD by the manufacturer, and if the reported clinical effects from swallowing Aqua Dots beads were consistent with the estimated ingested 1,4-BD dose., Methods: In vitro bench research using gas chromatography-mass spectroscopy (GC-MS) was performed. Dilute samples of pure 1,4-BD and 1,5-PD in water were used for the calibration of the GC-MS instrument. We then soaked Aqua Dots beads in water for varying durations, and the resultant solutions were analyzed for 1,4-BD and 1,5-PD content., Results: Aqua Dots beads weighed 79.3 mg each (+/- 0.6 mg, SD), and contained 13.7% (+/- 2.4%, SD) 1,4-BD by weight; this corresponds to a 1,4-BD content of 10.8 mg (+/- 1.9 mg, SD) per bead. No 1,5-PD was detected in any beads., Conclusions: Aqua Dots beads contained a surprisingly high amount (nearly 14%) of extractable 1,4-BD. No 1,5-PD was detected, corroborating reports that this chemical had been completely replaced with a substitute that is metabolized into GHB after ingestion. Reports of ataxia, vomiting, seizure activity, and self-limited coma in children are consistent with the ingestion of several dozen Aqua Dots beads.