Nitrate aerosols, formed via nitric acid (HNO) to balance inorganic cations in the particle phase, have constituted a major fraction of fine particulate matters (PM) during wintertime haze events in the North China Plain(NCP), with a progressively increasing contribution to PMmass. HNOis produced through homogeneous and heterogeneous pathways in the atmosphere, but the contribution of the two pathways to nitrate remains elusive. Simulations of a wintertime haze event in the NCP using a source-oriented WRF-Chem model reveal that the homogeneous and heterogeneous pathways contribute 48.4% and 51.6% of near-surface nitrate mass on average, respectively. The heterogeneous pathway dominates the nighttime HNOproduction in the planetary boundary layer, with an average contribution of 83%. Although NOis photolytically liable during daytime, the heterogeneous NOhydrolysis still contributes 10% of HNO. Our study highlights the significantly important role of NOheterogeneous hydrolysis in the nitrate formation during wintertime haze days.