Abstract
Renoxification is the recycling of NO3−/HNO3 into NOx under illumination; it is promoted by the photocatalysis of TiO2. Formaldehyde (HCHO), the most abundant carbonyl compound in the atmosphere, may participate in the renoxification of nitrate-doped TiO2 (NO3−-TiO2) aerosols. In this study, we established an environmental chamber reaction system under different light sources, excluding direct photolysis of nitrate by adjusting the illumination wavelength, to explore the photocatalytic renoxification process. It is suggested that HCHO and TiO2 have a significant synergistic effect on photocatalytic renoxification via the NO3−-NO3•-HCHO-HNO3-NOx pathway. Adsorbed HCHO may react with nitrate radicals through hydrogen abstraction to form HNO3 on the surface, resulting in the mass generation of NOx. We found that for 4 wt% NO3−-TiO2 aerosols (e.g., KNO3-TiO2), the NOx concentration reached up to 110 ppb, and was 2 orders of magnitude higher than in the absence of HCHO. Nitrate type and contents, relative humidity, and HCHO concentration were found to influence NOx release. The significant synergistic enhancement effect of renoxification affects photochemical processes such as atmospheric oxidation and nitrogen cycling on the surfaces of particles containing semiconductor oxides, with the participation of hydrogen donor organics.
Highlights
Some researchers have suggested that deposited NO3− and HNO3 can be recycled back to gas phase NOx under illumination, via the renoxification process (Schuttlefield et al, 2008; Romer et al, 2018; Bao et al, 2020; Shi et al, 2021b)
Photolytic renoxification occurs under light with a wavelength of < 350 nm, through the photolysis of NO3−/HNO3 adsorbed on the solid surface to generate NOx
We investigated the photocatalytic role of TiO2 on renoxification
Summary
Photolytic renoxification occurs under light with a wavelength of < 350 nm, through the photolysis of NO3−/HNO3 adsorbed on the solid surface to generate NOx. Notably, the photolysis of NO3−HNO3 is reported to occur at least 2 orders of magnitude faster on different solid surfaces (natural or artificial) or aerosols than in the gas phase (Ye et al, 2016a; Zhou et al, 2003; Baergen and Donaldson, 2013). Nitrate ions adsorbed at the oxide surface react with the photogenerated holes (h+) to form nitrate radicals (NO3·), which are subsequently photolyzed to NOx, mainly under visible light illumination (Schuttlefield et al, 2008; George et al, 2015; Schwartz-Narbonne et al, 2019). We investigated the effects of various influential factors including nitrate type, nitrate content, RH, and initial HCHO concentration, to understand the atmospheric renoxification of nitrate in greater detail
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