Abstract. The North China Plain (NCP), which includes Beijing, is currently suffering from severe haze events due to a high pollution level of PM_2.5. To mitigate the serious pollution problem, identification of the sources of PM_2.5 is urgently needed for the effective control measures. Daily samples of PM_2.5 were collected in Beijing city and in a rural area in Baoding, Hebei Province through the year of 2014, and the seasonal variation of water-soluble ions (WSIs) in PM_2.5 was comprehensively analysed to determine their possible sources. The results indicated that the periodic emissions from farmers' activities made a significant contribution to the atmospheric WSIs in Beijing. The relatively high concentration of K⁺ in winter and autumn at the two sampling sites confirmed that crop straw burning contributed to atmospheric K⁺ in Beijing. The remarkable elevation of Cl⁻ at the two sampling sites as well as the evident increase of the Cl⁻ ∕ K⁺ ratio and the Cl⁻ proportion in WSIs during the winter in Beijing could be ascribed to coal combustion for heating by farmers. The unusually high ratio of Cl⁻ to Na⁺ in summer, the obviously high concentrations of Cl⁻ in the rural sampling site and the elevation of Cl⁻ proportion in WSIs in Beijing during the maize fertilization could be explained by the use of the prevailing fertilizer of NH₄Cl in the vast area of NCP. The abnormally high concentrations of Ca²⁺ at the two sampling sites and the elevation of Ca²⁺ proportion during the period of the maize harvest and soil ploughing in Beijing provided convincing evidence that the intensive agricultural activities in autumn contributed to the regional mineral dust. The most serious pollution episodes in autumn were coincident with significant elevation of Ca²⁺, indicating that the mineral dust emission from the harvest and soil ploughing not only increased the atmospheric concentrations of the primary pollutants, but also greatly accelerated formation of sulfate and nitrate through heterogeneous reactions of NO₂ and SO₂ on the mineral dust. The backward trajectories also indicated that the highest concentrations of WSIs usually occurred in the air parcel from southwest–south regions, which have a high density of farmers. In addition, the values of nitrogen oxidation ratio (NOR) and the sulfur oxidation ratio (SOR) were found to be much greater under haze days than under non-haze days, implying that formation of sulfate and nitrate was greatly accelerated through heterogeneous or multiphase reactions of NO₂ and SO₂ on PM_2.5.