Nontargeted Screening Nitrogen-Containing Organic Compounds in Frost and Wet Deposition in Rural Northeast China

Nitrogen-containing organic compounds (NOCs) in frost serve as a critical pathway for atmospheric nitrogen deposition, significantly impacting the biogeochemical cycles of nitrogen. However, the molecular characteristics of NOCs in frost and their deposition fluxes are scarcely studied. In this work, frost samples, collected in rural Northeast China in the winter of 2023, were analyzed using nontargeted ultrahigh performance liquid chromatography-orbitrap mass spectrometry (UHPLC-Orbitrap MS) to reveal their content in nitrogen-containing organic compounds (NOCs) and explore their wet deposition fluxes. The average number of assigned molecular formulas were lager on hazy days compared to nonhazy days for both water-soluble (WSOM) and water-insoluble organic matter (WISOM) in frost (3114 vs. 1934 for WSOM and 3042 vs. 2224 for WISOM in electrospray ionization (ESI−); 6921 vs. 5954 for WSOM and 6629 vs. 5547 in ESI+). Specifically, the number proportions of CHON were 35.6–49.9% (724–1517) and 47–51.1% (2686–3388) in the ESI– and ESI+ modes, respectively. Nitrophenol (C₆H₅NO₃) and methyl nitrophenol (C₇H₇NO₄) were the most abundant NOCs, with wet deposition fluxes (at maximum average concentrations) of 22.2 and 21.2 μg m^–2·h^–1, respectively. On hazy days, the deposition fluxes of nitrophenol compounds reached up to 1.73 times that of nonhazy days, indicating significant ambient nitrogen deposition during the haze episode. This deposition flux positively correlated with PM_2.5 concentration, implying the important role of atmospheric particulates in influencing NOC deposition through frost. These findings highlight the susceptibility of frost to capturing NOCs from the atmosphere, potentially impacting nitrogen cycling in ecosystems.