<p>Co-occurrences of high concentrations of PM<sub>2.5</sub> and ozone (O<sub>3</sub>) have been frequently observed in haze aggravating processes in the North China Plain (NCP) over the past few years, and higher O<sub>3</sub> concentrations during hazy days were supposed to be related to nitrous acid (HONO), but the key sources of HONO enhancing O<sub>3</sub> during haze aggravating processes remain unclear, and will be explored in this study by using the WRF-Chem model, which is improved to include ground-based (traffic, soil, and indoor emissions, and the NO<sub>2</sub> heterogeneous reaction on ground surface (Het<sub>ground</sub>)) and aerosol-related (the NO<sub>2</sub> heterogeneous reaction on aerosol surfaces (Het<sub>aerosol</sub>) and nitrate photolysis (Phot<sub>nitrate</sub>)) potential HONO sources. The results indicate that ground-based HONO sources producing HONO enhancements showed a rapid decrease with height, while the NO+OH reaction and aerosol-related HONO sources decreased slowly with height. Phot<sub>nitrate</sub> contributions to HONO concentrations enhanced with aggravated pollution levels, the enhanced HONO due to Phot<sub>nitrate</sub> in hazy days was about one order of magnitude larger than in clean days and Phot<sub>nitrate</sub> dominated HONO sources (~30–70 % when the ratio of the photolysis frequency of nitrate (J<sub>nitrate</sub>) to gas nitric acid (J<sub>HNO3</sub>) equals 30) at higher layers (> 800 m). Compared with that in clean days, the Phot<sub>nitrate</sub> contribution to the enhanced daily maximum 8-h averaged O<sub>3</sub> was increased by over one magnitude during the haze aggravating process. Phot<sub>nitrate</sub> contributed only ~5 % of the surface HONO in daytime with a J<sub>nitrate</sub>/J<sub>HNO3</sub> ratio of 30 but contributed ~30–50 % of the enhanced O<sub>3</sub> near the surface in NCP in hazy days. Surface O<sub>3</sub> was dominated by volatile organic compounds-sensitive chemistry, while O<sub>3</sub> at higher altitude (> 800 m) was dominated by NO<sub>x</sub>-sensitive chemistry. Phot<sub>nitrate</sub> had a limited impact on nitrate concentrations (< 15 %) even with a J<sub>nitrate</sub>/J<sub>HNO3</sub> ratio of 120. The above results suggest that more field studies of J<sub>nitrate</sub> in the atmosphere are still needed.</p>