Direct Formation of Electronic Excited NO2 Contributes to the High Yield of HONO during Photosensitized Renoxification.

Photosensitized renoxification of HNO3 is found to produce HONO in an unexpectedly high yield, which has been considered an important source for atmospheric HONO. Conventionally, the production of HONO is ascribed to the secondary photolysis of the primarily formed NO2. In this study, by using humic acid (HA) as a model environmental photosensitizer, we provide evidence of the direct formation of NO2 in its electronic excited state (NO2*) as a key intermediate during the photosensitizing renoxification of HNO3. Moreover, the high HONO yield originates from the heterogeneous reaction of the primarily formed NO2* with the co-adsorbed water molecules on HA. Such a mechanism is supported by the increase of the product selectivity of HONO with relative humidity. Further luminescence measurements demonstrate clearly the occurrence of an electronic excited state (NO2*) from photolysis of adsorbed HNO3 on HA. This work deepens our understanding of the formation of atmospheric HONO and gives insight into the transformation of RNS.

Key Role of Equilibrium HONO Concentration over Soil in Quantifying Soil-Atmosphere HONO Fluxes.

Nitrous acid (HONO) is an important component of the global nitrogen cycle and can regulate the atmospheric oxidative capacity. Soil is an important source of HONO. [HONO]*, the equilibrium gas-phase concentration over the aqueous solution of nitrous acid in the soil, has been suggested as a key parameter for quantifying soil fluxes of HONO. However, [HONO]* has not yet been well-validated and quantified. Here, we present a method to retrieve [HONO]* by conducting controlled dynamic chamber experiments with soil samples applied with different HONO concentrations at the chamber inlet. We show a bi-directional soil-atmosphere exchange of HONO and confirm the existence of [HONO]* over soil: when [HONO]* is higher than the atmospheric HONO concentration, HONO will be released from soil; otherwise, HONO will be deposited. We demonstrate that [HONO]* is a soil characteristic, which is independent of HONO concentrations in the chamber but varies with different soil water contents. We illustrate the robustness of using [HONO]* for quantifying soil fluxes of HONO, whereas the laboratory-determined chamber HONO fluxes can largely deviate from those in the real world for the same soil sample. This work advances the understanding of the soil-atmosphere exchange of HONO and the evaluation of its impact on the atmospheric oxidizing capacity.

Photoinduced Uptake and Oxidation of SO2 on Beijing Urban PM2.5.

Sulfate, as a major component of aerosol particles, greatly contributes to haze formation and affects global climate change. Although formation pathways of sulfate aerosols from the conversion of SO2 have been extensively studied, the discrepancy between field observations and model simulations suggests that there are still unknown sulfate sources. Herein, we report for the first time a photoinduced SO2 uptake and oxidation pathway in Beijing urban PM2.5 aerosols. In comparison with the NO2- and O3-induced SO2 oxidation pathways, this SO2 photo-oxidation in Beijing urban PM2.5 could make an important contribution to the daytime sulfate formation. Reactive species, such as •OH radicals and H2O2, are the major oxidants leading to sulfate formation in PM2.5. The water-soluble matter (WSM) and water-insoluble organic matter (WISOM) in PM2.5 were identified as the main photo-oxidant producers. Our work highlights an important daytime sulfate source in the atmosphere and provides new insight into the photochemical aging of ambient aerosols.

Enhanced Photochemical Volatile Organic Compounds Release from Fatty Acids by Surface-Enriched Fe(III).

Both Fe(III) and fatty acids are ubiquitous and important species in environmental waters. Because they are amphipathic, many fatty acids are surface active and prone to enrichment at the air-water interface. Here, we report that by using nonanoic acid (NA) as a model fatty acid, coexisting Fe(III), even at concentrations as low as 1 µM, markedly enhanced the photochemical release of NA-derived volatile organic compounds (VOCs) such as octanal and octane into the air. Further studies indicated that the surface-enriched fatty acids dramatically increase the local concentration of Fe(III) at the water surface, which enables Fe(III)-mediated photochemical reactions to take place at the air-water interface, and the VOCs facilely produced by fatty acid photooxidation can then be released into the air. Moreover, the product distribution in the Fe(III)-mediated reactions was largely different from that in other photochemical systems, and a mechanism based on photochemical decarboxylation is proposed. Considering that the coexistence of fatty acids and Fe(III) in the environment is common, the enhanced photochemical release of VOCs by surface-enriched fatty acids and Fe(III) may be an important channel for the atmospheric emission of VOCs, which are known to play an essential role in the formation of ozone and secondary organic aerosols.

Development of Bisindole-Substituted Aminopyrazoles as Novel GSK-3ß Inhibitors with Suppressive Effects against Microglial Inflammation and Oxidative Neurotoxicity.

Development of glycogen synthase kinase-3ß (GSK-3ß) inactivation-centric agents with polypharmacological profiles is increasingly recognized as a promising therapeutic strategy against the multifactorial etiopathology of Alzheimer's disease (AD). In this respect, a series of disubstituted aminopyrazole derivatives were designed and synthesized as a new class of GSK-3ß inhibitors. Most of these derivatives possess GSK-3ß inhibitory activities with IC50 values in the micromolar ranges, among which bisindole-substituted aminopyrazole derivative 6h displayed moderate GSK-3ß inhibition (IC50 = 1.76 ± 0.19 µM), and alleviative effects against lipopolysaccharide (LPS)-induced glial inflammation in BV-2 cells and glutamate-induced oxidative neurotoxicity in HT-22 cells. Further in vivo studies indicated that compound 6h had potent anti-inflammatory effect, by showing markedly reduced microglial activation and astrocyte proliferation in the brain of LPS-injected mice. Overall, the simultaneous modulation of 6h on multiple dysfunctions of disease network highlights this structural distinctively bisindole-substituted aminopyrazole could be a useful prototype for the discovery of novel therapeutic agents to tackle AD and other GSK-3ß associated complex neurological syndromes.

Photochemical aging of Beijing urban PM2.5: Production of oxygenated volatile organic compounds.

PM2.5 has become the dominant atmospheric pollutant in many countries. Many components of PM2.5 are highly photoactive. However, the photochemical aging of PM2.5 remains poorly understood. In this study, the photoaging of real PM2.5 samples collected from 2017 to 2018 in Beijing under simulated solar radiation (λ ~ 340-850 nm) was investigated. Our study showed that large amounts of oxygenated volatile organic compounds (OVOCs), such as acetaldehyde, formic acid, acetone and acetic acid, were released during the photochemical aging of PM2.5. Furthermore, although a positive correlation between the OVOCs yield and the organic matter (OM) in PM2.5 was observed, the product distribution from the photoaging of PM2.5 was different from that in the direct photolysis of artificially synthesized SOA. Because of the release of OVOCs, the PM2.5 mass loss was evaluated to be ~1.80% per day under typical atmospheric conditions. The OVOCs released during the photoaging of PM2.5 may contribute substantially to the OVOCs sources omitted from troposphere chemistry models and may have a significant effect on the OVOCs distribution and oxidation capacity of the atmosphere.

The Key Role of Sulfate in the Photochemical Renoxification on Real PM2.5.

The active nitrogen species (HONO, NO, and NO2) have important impacts on the atmospheric oxidative capacity and the transformation of many atmospheric species. In this study, a fast photochemical renoxification rate of adsorbed HNO3/NO3- to active nitrogen species (HONO, NO, and NO2) was detected on real urban PM2.5, and sulfate was found to play a key role in this process. Different from the reported direct photolysis pathway, the photochemical reaction of HNO3/NO3- on PM2.5 is dominated by a photosensitizing mechanism. Acidic protons are proved to be essential for this pathway. The role of sulfate, because of the nonvolatility of its conjugated acid, is to conserve the necessary acidic protons when interacting with HNO3 and thus maintain its photoreactivity. This work implies that sulfate will have important implications in atmospheric nitrogen cycling by accelerating the release of nitrogen oxides from photochemical renoxification of HNO3/NO3- adsorbed on ambient particulates and thus can cause major environmental problems.

Development of Novel N-hydroxypyridone Derivatives as Potential Anti-Ischemic Stroke Agents.

Our previous study had identified ciclopirox (CPX) as a promising lead compound for treatment of ischemic stroke. To find better neuroprotective agents, a series of N-hydroxypyridone derivatives based on CPX were designed, synthesized, and evaluated in this study. Among these derivatives, compound 11 exhibits significant neuroprotection against oxygen glucose deprivation and oxidative stress-induced injuries in neuronal cells. Moreover, compound 11 possesses good blood-brain barrier permeability and superior antioxidant capability. In addition, a complex of compound 11 with olamine-11·Ola possesses good water solubility, negligible hERG inhibition, and superior metabolic stability. The in vivo experiment demonstrates that 11·Ola significantly reduces brain infarction and alleviates neurological deficits in middle cerebral artery occlusion rats. Hence, compound 11·Ola is identified in our research as a prospective prototype in the innovation of stroke treatment.

Photochemical Aging of Soot in the Aqueous Phase: Release of Dissolved Black Carbon and the Formation of 1O2.

The photochemical aging of soot in the aqueous phase could have an important influence on water environments such as fog water and wet aerosols in the atmosphere, as well as lakes and oceans. In this study, we systematically investigated the photochemistry of soot in the aqueous phase. Soot releases dissolved black carbon into the aqueous phase during photoreactions, which is attributed to the phototransformation of the nonpolar unsaturated C-H species in soot to polar carbonyl-containing species. More importantly, we found that soot suspensions, particularly those of the dissolved part of soot, were effective photosensitizers for the generation of singlet oxygen (1O2). The 1O2 apparent quantum yield of the dissolved part reached 33 ± 2% under 377 nm irradiation, which is an order of magnitude higher than those of most types of well-studied dissolved organic matter in water. As a result, soot could impact the environmental fate of coexisting organic contaminants, such as the photodegradation of bisphenol A. This study will not only give insight into the photochemistry of soot in the liquid phase but also reveal the significant implications of soot photoaging in the aqueous phase by the release and degradation of organic matter.

Nitrate-Enhanced Oxidation of SO2 on Mineral Dust: A Vital Role of a Proton.

Heterogeneous oxidation of SO2 on mineral dust is a significant source of sulfate in the atmosphere. Given that a large fraction of nitrate is deposited on the mineral aerosols, the determination of the effect of nitrate on the SO2 oxidation on mineral dust and its in-depth mechanism are much desired. In this work, we report nitrate-enhanced SO2 oxidation on authentic mineral dust. By comparing the SO2 uptake behaviors on Arizona test dust (ATD, a typical proxy of mineral dust) with or without nitrate, we found that although nitrate hinders the initial SO2 uptake, it substantially accelerates SO2 uptake and oxidation after a pronounced induction period. In other words, a hindering-then-accelerating feature in the SO2 uptake profile was observed on nitrate-containing ATD (N-ATD) particles. In addition, HONO was released in the accelerating period as the reduction product of nitrate. The accumulation of protons (H+) from SO2 oxidation during the induction period plays a key role in the acceleration of SO2 oxidation. Our work suggests that the nitrate-participating SO2 oxidation on mineral dust can be one of the important contributions of the sulfate source in the atmosphere.

Neuroprotective Effect of Natural Alkaloid Fangchinoline Against Oxidative Glutamate Toxicity: Involvement of Keap1-Nrf2 Axis Regulation.

Oxidative glutamate toxicity plays a vital role in the neurodegeneration diseases, including Alzheimer's diseases (AD). This study set out with the aim to investigate the beneficial effects of fangchinoline (FAN), a natural alkaloid, against glutamate-induced oxidative damage, and to clarify the underlying cellular and biochemical mechanisms. FAN prevented HT22 cells death from oxidative glutamate cytotoxicity in a dose-dependent manner, and significantly attenuated the overproduction of intracellular reactive oxygen species (ROS) and reversed the reduction of superoxide dismutase (SOD) activity induced by glutamate. Further investigations on the underlying mechanisms demonstrated that FAN potently up-regulated the protein level of nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase (HO-1), in glutamate-exposed HT22 cells. The protective effects of FAN were almost completely antagonized by inhibitor of Nrf2. Subsequent studies revealed that FAN could down-regulate Kelch-like ECH-associated protein 1 (Keap1) in both mRNA level and protein level. To sum up, our result demonstrated the protective effects of FAN against glutamate-induced oxidative neuronal damage, and for the first time clarified the anti-oxidative mechanisms of FAN involve activating endogenous antioxidant defense system including enhancing SOD activity and regulating Keap1/Nrf-2 antioxidation signaling through modulation of Keap1 expression. Above results shed more light on the molecular mechanisms of FAN's neuroprotective effects, and may provide important clues for the drug development in preventing oxidative stress-associated neurodegenerative diseases.

Role of elemental carbon in the photochemical aging of soot.

Soot, which consists of organic carbon (OC) and elemental carbon (EC), is a significant component of the total aerosol mass in the atmosphere. Photochemical oxidation is an important aging pathway for soot. It is commonly believed that OC is photoactive but EC, albeit its strong light absorption, is photochemically inert. Here, by taking advantage of the different light absorption properties of OC and EC, we provide direct experimental evidence that EC also plays an important role in the photochemical aging of soot by initiating the oxidation of OC, even under red light irradiation. We show that nascent soot, in addition to undergoing photochemical oxidation under blue light with a wavelength of 440 nm, undergoes similar oxidation under red light irradiation of λ = 648 nm (L648). However, separated OC (extracted from soot by n-hexane) and EC exhibit little reactivity under L648 These observations indicate that EC plays a pivotal role in photoaging of soot by adsorbing light to initiate the oxidation of OC. Comparison of in situ IR spectra and photoelectrochemical behaviors suggests that EC-initiated photooxidation of OC proceeds through an electron transfer pathway, which is distinct from the photoaging induced by light absorption of OC. Since the absorption spectra of EC have a much larger overlap with the solar spectra than those of OC, our results provide insight into the chemical mechanism leading to rapid soot aging by organic species observed from atmospheric field measurements.

Photochemical Aging of Beijing Urban PM2.5: HONO Production.

Photochemical aging represents an important transformation process of aerosol particles in the atmosphere, which greatly influences the physicochemical properties and the environmental impact of aerosols. In this work, we find that Beijing urban PM2.5 aerosol particles release substantial HONO, a significant precursor of •OH radicals, into the gas phase during the photochemical aging process. The generation of HONO exhibits a high correlation with the amount of nitrate in PM2.5. The formation rate of HONO becomes gradually decreased with the irradiation time, but can be restored by introducing the acidic proton, indicative of the essential role of the acidic proton in the HONO production. Other environmental factors such as relative humidity, light intensity, and reaction temperature also possess important influences on HONO production. The normalized photolysis rate constant for HONO ( JHNO3→HONO) is in the range of 1.22 × 10-5 s-1 ∼ 4.84 × 10-4 s-1, which is 1-3 orders of magnitude higher than the reported photolysis rate constant of gaseous HNO3. The present study implies that the photochemical aging of Beijing PM2.5 is an important atmospheric HONO production source.