Nitrous acid (HONO), as a key reservoir of hydroxyl radical (OH), plays a significant role in atmospheric chemistry. To study the sources and atmospheric influence of HONO, continuous observations of HONO and relevant air pollutants were performed from June to July 2019 at an urban site in Beijing. A comparative study on the ambient levels, diurnal variation, the sources in daytime and nighttime, and the formation mechanisms were investigated for a haze pollution period (Period I) and an ozone pollution period (Period II). The average hourly HONO concentrations during Period I, Period II, and the whole observation period, were 0.58 ± 0.23, 0.54 ± 0.19 and 0.44 ± 0.24 ppb, respectively. The emission from vehicle exhaust was an important source of nocturnal HONO formation. During the nighttime, compared with the homogeneous reaction of NO with OH, the heterogeneous conversion from NO2 was the dominant pathway for HONO formation. The heterogeneous conversion frequency was 0.0075 h−1 during Period I, higher than that during Period II (0.0028 h−1), suggesting a higher conversion potential to HONO formation during the haze episode. Based on the analysis of HONO budget, it was found that the daytime unknown source Punknown during Period II was higher than that during Period I. Correlation analysis implied that the photo-enhanced NO2 conversion on the aerosol surface might be a potential source for daytime HONO. Without HONO constraint, the Observation-Based Model (OBM) would largely underestimate the averaged daytime atmospheric oxidative capacity (24%), OH production rate (57%), and net O3 production rate (20%). The study results further demonstrated the necessity for clarifying the formation mechanism of HONO to improve the understanding of the influence of HONO to atmospheric chemical processes.