To understand the air pollution problem in megacities such as Beijing, field measurement investigating the variation of NH3 and its association with PM2.5 chemical property was conducted from 25 November to 24 December 2013. The results indicated that the daily concentration of wintertime NH3 tended to be high on the days with relatively high temperatures and low wind speeds. Affected by the synoptic condition, NH3 concentration showed a bimodal diurnal variation pattern, which tended to peak at around 09:00 and 22:00 of the day. As the sole precursor for NH4+, NH3 exerted a significant impact on the ion chemistry of PM2.5 through enhancing the nighttime NH4Cl formation and promoting both homogeneous and heterogeneous formation of NO3–. During heavy pollution episodes with PM2.5 concentrations over 200 µg m–3, the NH3 levels and NH4+/NH3 ratios grew simultaneously with the increase of PM2.5 levels, indicating that NH3 is one of the key reasons for heavy pollution events. Revealed by the features of measured ionic species in PM2.5, in conjunction with the acidity analysis using thermodynamic model, our results suggested that NH3 was frequently sufficient in wintertime atmosphere of urban Beijing and the fine particulates were neutralized nearly fully by NH3.