Measurement report: Distinct urban and rural organonitrate formation regimes in a mixed anthropogenic-biogenic subtropical atmosphere

Abstract. Particulate organonitrates (pON) are important components of secondary organic aerosols and key reservoir of reactive nitrogen, yet their sources and formation pathways in different atmospheric environments remain poorly understood. Here, we present pON measured in field observations at urban and rural sites in Chinese subtropical region utilizing a soot-particle aerosol mass spectrometer (SP-AMS) combined with positive matrix factorization for source- resolved quantification. With comparable fractions of pON (in total nitrates and organics) and similar average concentrations at both sites, contrasting formation regimes for pON were revealed: at the urban site, daytime photochemistry (16 %) and regional transport (33 %) dominated, whereas nighttime formation was more pronounced (29 %) in rural areas. Elevated volatile organic compounds (VOCs) availability may enhance photochemical organonitrate formation, while nighttime formation was favored under NOx‑rich conditions, highlighting the distinct sensitivities of the OH- and NO₃-initiated pathways. Evaluation of organonitrate production potentials showed monoterpenes, isoprene, alkanes, and aromatics dominated the OH-initiated pathway, while styrene and phenols were important NO₃-initiated precursors. Despite overall dominance of biogenic VOCs, anthropogenic VOCs contributed remarkably (59 %) to rural organonitrate formation. Refractory pON associated with biomass burning and nighttime formation appeared at the rural site, representing a fraction largely missed by conventional AMS. Our results demonstrate a pronounced urban-rural shift in pON formation regimes, where anthropogenic precursors unexpectedly influence nocturnal chemistry at the rural site, highlighting the complexity of organonitrate formation in mixed anthropogenic–biogenic subtropical environments and the need to consider refractory pON in future studies.