Measurement report: Long-term 2D MAX-DOAS observations of HONO vertical distributions in a rice–wheat rotation agricultural region in Shouxian, China

Abstract. Nitrous acid (HONO) photolysis is an important source of hydroxyl radicals (OH) in the atmospheric boundary layer, but long-term observations of HONO vertical distributions and their responses to agricultural activities remain limited. We conducted nearly 2 years (2022–2023) of two-dimensional multi-axis differential optical absorption spectroscopy (2D MAX-DOAS) observations in a rice–wheat rotation region in Shouxian, China. Retrieval reliability and stability were assessed through an intercomparison between two synchronized systems, spectral fit quality, degrees of freedom for signal (DOFS), and averaging-kernel analysis. Meteorological data, farming records, backward trajectories, potential source contribution function (PSCF) analysis, and Tropospheric Ultraviolet and Visible (TUV) calculations were integrated to investigate HONO variability, fertilization effects, and potential OH production. HONO was concentrated below 0.5 km, with morning and late-afternoon peaks and a noon minimum. Seasonal means followed the order winter > autumn > spring > summer. In spring and summer, near-surface HONO decreased toward midday, whereas HONO increased aloft, suggesting possible radiation-associated replenishment in elevated layers. During the 10 d following fertilization on 15 March 2023, HONO concentrations at 0.05, 0.20, and 0.50 km increased by factors of 5.3, 3.7, and 2.5, respectively, relative to the preceding 10 d. Near-site PSCF values suggested local and short-range agricultural influences. At 0.05 km, potential OH production from HONO photolysis increased after fertilization by 710 % under clear-sky conditions and by 86 % under cloudy conditions. These findings demonstrate the importance of resolving HONO vertical structure and agricultural-event responses when evaluating its photochemical relevance in agricultural regions.