High levels of HONO have frequently been observed in Chinese haze periods and underestimated by current models due to some unknown sources and formation mechanisms. Combining lab-chamber simulations and field measurements in Xi'an and Beijing, China, we found that NH3 can significantly promote HONO formation via the reduction-oxidation of SO2 with NO2 in the aqueous phase of hygroscopic particles (e.g., NaCl). Concentrations of HONO formed in the aerosol phase showed an exponential increase (R2 = 0.91) with NH3 levels under the chamber conditions and a linear growth with NH3 levels in the two Chinese cities. The uptake coefficient of NO2 on NaCl particles ranged from 2.0 × 10-5 to 1.7 × 10-4, 3-4 orders of magnitude larger than that on water droplets. Our results further showed that HONO formed from the aerosol phase accounted for 4-33% of the total in the chamber, indicating that aerosol-phase formation is an important source of HONO in China, especially in haze periods. Since NH3, SO2, and NO2 abundantly coexist in China, the positive effect of NH3 on HONO formation could enhance the atmospheric oxidizing capacity in the country, causing severe secondary aerosol pollution. Our work suggests that NH3 emission control is imperative for mitigating air pollution in China.