<p>Although stringent emission mitigation strategies have been carried out since 2013 in Beijing-Tianjin-Hebei (BTH), China, heavy haze with high levels of fine particulate matters (PM<sub>2.5</sub>) still frequently engulfs the region during wintertime and the nitrate contribution to PM<sub>2.5</sub> mass has progressively increased. The N<sub>2</sub>O<sub>5</sub> heterogeneous hydrolysis is referred to as the most important pathway of the nitrate formation at nighttime. In the present study, the WRF-CHEM model is applied to simulate a heavy haze episode from 10 to 27 February 2014 in BTH to evaluate the contribution of the N<sub>2</sub>O<sub>5</sub> heterogeneous hydrolysis and the effect of organic coating to the nitrate formation. The model generally performs reasonably well in simulating meteorological parameters, air pollutants and aerosol species against observations in BTH. The N<sub>2</sub>O<sub>5</sub> heterogeneous hydrolysis with all the secondary organic aerosol assumed to be involved in coating considerably improves the nitrate simulations compared to the measurements in Beijing. On average, organic coating decreases nitrate concentrations by 8.4 % in BTH during the episode, and the N<sub>2</sub>O<sub>5</sub> heterogeneous hydrolysis with organic coating contributes about 30.1 % of nitrate concentrations. Additionally, the reaction also plays a considerable role in the heavy haze formation, with a PM<sub>2.5</sub> contribution of about 11.6 % in BTH. Sensitivity studies also reveal that future studies need to be conducted to predict the organic aerosol hygroscopicity for accurately representing the organic coating effect on the N<sub>2</sub>O<sub>5</sub> heterogeneous hydrolysis.</p>