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Abstract: Incorporated into the WRF–Chem model are the three HONO sources, i.e., the reaction of photo-excited NO2 with water (NO2 * chemistry), heterogeneous reactions on aerosol surfaces, and HONO emissions. Four case simulations were performed in Beijing, Tianjin, and Hebei Province (BTH region) in August of 2007. Results indicate that the NO2 * chemistry yields 30–50 ppb enhancements in daily maximum 1-h surface O3 concentrations in major cities and 3–10 ppb increases in monthly mean daily maximum 8-h surface O3 concentrations in most areas of the BTH region. Heterogeneous reactions on aerosol surfaces lead to 5–20 ppb decreases in monthly mean NOy concentrations in major cities over the BTH region. Heterogeneous reactions and HONO emissions are the largest, and the second largest contributor, accounting for ∼59% and 26% of simulated HONO concentrations, respectively. The three HONO sources produce enhancements in monthly mean daytime (10:00–17:00) concentrations of NOy (total reactive N-containing compounds), NO2, HNO3, HONO, and HOx being −1 to −5 ppb, −1 to −3 ppb, −0.3 to 0.5 ppb, 20–50 ppt, and 1–3 ppt, respectively. Comparison with observations shows that inclusion of the three HONO sources into the WRF–Chem model can considerably improve HONO and O3 simulations in the BTH region. For HONO the mean, the normal mean bias, the root mean square error, and the correlation coefficient is greatly improved from 0.04 to 0.56 ppb, from −97% to −39%, from 1.10 ppb to 0.51 ppb, and from −0.56 to 0.86, respectively, by comparison with the gas chemical mechanism only considered in the WRF–Chem model. This suggests that the three important HONO sources be included in air quality models, particularly in the areas with high emissions of NOx and particulate matter. [Copyright &y& Elsevier]