In order to cope with heavy haze pollution in Beijing, the Air Pollution Prevention and Control Action Plan was issued in 2013. In this study, we took January observations from 2014 to 2018 as a case study to assess wintertime air quality improvement in Beijing after five years of continuous emission control measures and to investigate the causes of haze formation. SO₂ and NO₂ concentrations in January 2018 decreased by 84% and 41% from January 2014, while NH₃ concentrations remained relatively stable during these years with an average of 7.28 μg m⁻³. The mean concentrations of PM_2.5 and water-soluble inorganic ions declined over the study period, while compared with 2014, decreased SO₄²⁻ ratio and increased NO₃⁻ contribution in PM_2.5 were observed, highlighting the importance of reactive nitrogen (Nr) pollution control. High concentrations of NH₃, NO₂, as well as NH₄⁺ and NO₃⁻ in PM_2.5 at roadside monitoring sites, indicated that vehicle sources played a vital role in local urban Nr emissions. Finally, we found most NH₃ existed in the gas phase in the Beijing atmosphere, even on polluted winter days, suggesting a considerable risk for additional secondary aerosol formation. Our findings demonstrate the effectiveness of the emissions control in alleviating Beijing's air pollution. Significant reductions of precursor emissions are needed to achieve the goals set by air quality standards in the future, especially for NH₃. It is especially crucial to jointly consider and control multiple pollution sources to effectively reduce the high levels of air pollution that remain in Beijing.