It is known that there are semiconductor oxides involved in mineral dust, which have photocatalytic properties. However, soot particles contained in carbonaceous aerosol and their photoactivity under sunlight are rarely realized. In this study, reactive oxygen species (ROS) such as superoxide anions and hydroxyl radicals were generated upon visible-light irradiation of soot particles, and the production activity was consistent with the carbonaceous core content, indicating that the atmospheric soot particles can serve as a potential photocatalyst. The increase of oxygen-containing functional groups, environmentally persistent free radicals, oxygenated polycyclic aromatic hydrocarbons, and the oxidative potential (OP) of soot after irradiation confirmed the occurrence of visible-light-triggered photocatalytic oxidation of the soot itself. The mechanism analyses suggested that the carbonaceous core caused the production of ROS, which subsequently oxidize the extractable organic species on the soot surface. It is oxidized organic extracts that are responsible for the enhancements of the OP, cell mortality, and intracellular ROS generation. These new findings shed light on both the photocatalytic role of the soot and the importance of ROS during the photochemical self-oxidation of soot triggered by visible light and will promote a more comprehensive understanding of both the atmospheric chemical behavior and health effects of soot particles.

The knowledge of the chemical composition of brown carbon (BrC) is limited to the categories of components or parts of specific organic components. In this paper, the light-absorbing properties and molecular compositions of lipid-soluble organic components in fine particulate matter of Beijing from 2016 to 2018, characterized by an ultraviolet−visible spectrometer and gas chromatography coupled with time-of-flight mass spectrometry, respectively, were combined to untargetedly screen the key BrC molecules by a partial least squares regression model for the first time. A total of 421 molecules were obtained, where 61 molecules were identified qualitatively and 22 molecules quantitatively. To the best of our knowledge, 11 molecules were newly identified BrC species. These qualitative molecules included polycyclic aromatic hydrocarbons with higher ambient concentrations and mass absorbing efficiencies (MAEs), as well as oxygen-and nitrogencontaining aromatic components with relatively lower concentrations and MAEs. The absorption contribution at 365 nm of quantified BrC species to lipid-soluble BrC during heating seasons was 39.1 ± 17.0%, which was about 5 times as high as previous studies. These results help establish a complete BrC molecular database and provide data support for better evaluating the climate effect of atmospheric carbonaceous aerosols and formulating air pollution control strategies.

Abstract. Renoxification is the process of recycling NO3- / HNO3 into NOx under illumination and is mostly ascribed to the photolysis of nitrate. TiO2, a typical mineral dust component, is able to play a photocatalytic role in the renoxification process due to the formation of NO3 radicals; we define this process as “photocatalytic renoxification”. Formaldehyde (HCHO), the most abundant carbonyl compound in the atmosphere, may participate in the renoxification of nitrate-doped TiO2 particles. In this study, we established a 400 L environmental chamber reaction system capable of controlling 0.8 %–70 % relative humidity at 293 K with the presence of 1 or 9 ppm HCHO and 4 wt % nitrate-doped TiO2. The direct photolyses of both nitrate and NO3 radicals were excluded by adjusting the illumination wavelength so as to explore the effect of HCHO on the “photocatalytic renoxification”. It was found that NOx concentrations can reach up to more than 100 ppb for nitrate-doped TiO2 particles, while almost no NOx was generated in the absence of HCHO. Nitrate type, relative humidity and HCHO concentration were found to influence NOx release. It was suggested that substantial amounts of NOx were produced via the NO3-–NO3⚫–HNO3–NOx pathway, where TiO2 worked for converting “NO3-” to “NO3⚫ ”, that HCHO participated in the transformation of “NO3⚫ ” to “HNO3” through hydrogen abstraction, and that “HNO3” photolysis answered for mass NOx release. So, HCHO played a significant role in this “photocatalytic renoxification” process. These results were found based on simplified mimics for atmospheric mineral dust under specific experimental conditions, which might deviate from the real situation but illustrated the potential of HCHO to influence nitrate renoxification in the atmosphere. Our proposed reaction mechanism by which HCHO promotes photocatalytic renoxification is helpful for deeply understanding atmospheric photochemical processes and nitrogen cycling and could be considered for better fitting atmospheric model simulations with field observations in some specific scenarios.