HONO along with other trace gases and actinic flux were continuously measured in selected months from December 2015 to November 2016 near a major industrial zone of the Yangtze River Delta (YRD) region of China. The seasonalities of HONO and its formation mechanisms were explored. A box model was developed to assess the role of HONO chemistry in the atmospheric oxidative capacity in the study area. The model was constrained by in-situ observations to minimize simulation uncertainties. Our measurement results demonstrate that HONO mean concentration and diurnal profile varied noticeably between different seasons with the maximum (1.32 ± 0.92 ppbv) observed in winter, coincided with high mass loading of fine particles (PM_2.5 = 114 ± 59.6 μg m⁻³). The averaged nighttime NO₂ to HONO conversion ratios (hr⁻¹) were (1.83 ± 0.73)% in spring, (2.91 ± 1.06)% in summer, (0.72 ± 0.44)% in autumn, and (0.84 ± 0.45)% in winter. The homogeneous reaction of OH + NO was responsible for a large portion of HONO production throughout the year but was relatively weaker compared to other mechanisms in winter. Heterogeneous reactions on ground surfaces were much stronger than those on aerosol surfaces, dominating nighttime HONO production, except in summer. Primary HONO emissions were only responsible for a small portion of the HONO budget. Particulate nitrate photolysis was a significant source of HONO and it showed very little variation in different seasons. OH budget analysis shows that O₃ photolysis was a major contributor to the OH production in the study area while HONO photolysis provided an extra boost to the photochemical reactivity in the early morning, particularly during summer. However, HONO chemistry played a more important role in autumn and winter by dominating OH production throughout the day. Accordingly, effective control of NO_x emissions dominated by industries can significantly reduce HONO production and thus weaken the atmospheric oxidative capacity in the study area.