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The North China Plain (NCP), which includes Beijing, is currently\udsuffering from severe haze events due to a high pollution level of PM2.5.\udTo mitigate the serious pollution problem, identification of the sources of\udPM2.5 is urgently needed for the effective control measures. Daily\udsamples of PM2.5 were collected in Beijing city and in a rural area in\udBaoding, Hebei Province through the year of 2014, and the seasonal variation\udof water-soluble ions (WSIs) in PM2.5 was comprehensively analysed to\uddetermine their possible sources. The results indicated that the periodic\udemissions from farmers' activities made a significant contribution to the\udatmospheric WSIs in Beijing. The relatively high concentration of K+ in\udwinter and autumn at the two sampling sites confirmed that crop straw\udburning contributed to atmospheric K+ in Beijing. The remarkable\udelevation of Cl− at the two sampling sites as well as the evident\udincrease of the Cl− ∕ K+ ratio and the Cl− proportion in WSIs\udduring the winter in Beijing could be ascribed to coal combustion for\udheating by farmers. The unusually high ratio of Cl− to Na+ in\udsummer, the obviously high concentrations of Cl− in the rural sampling\udsite and the elevation of Cl− proportion in WSIs in Beijing during the\udmaize fertilization could be explained by the use of the prevailing\udfertilizer of NH4Cl in the vast area of NCP. The abnormally high\udconcentrations of Ca2+ at the two sampling sites and the elevation of\udCa2+ proportion during the period of the maize harvest and soil\udploughing in Beijing provided convincing evidence that the intensive\udagricultural activities in autumn contributed to the regional mineral dust.\udThe most serious pollution episodes in autumn were coincident with\udsignificant elevation of Ca2+, indicating that the mineral dust\udemission from the harvest and soil ploughing not only increased the\udatmospheric concentrations of the primary pollutants, but also greatly\udaccelerated formation of sulfate and nitrate through heterogeneous reactions\udof NO2 and SO2 on the mineral dust. The backward trajectories also\udindicated that the highest concentrations of WSIs usually occurred in the\udair parcel from southwest–south regions, which have a high density of\udfarmers. In addition, the values of nitrogen oxidation ratio (NOR) and the\udsulfur oxidation ratio (SOR) were found to be much greater under haze days\udthan under non-haze days, implying that formation of sulfate and nitrate was\udgreatly accelerated through heterogeneous or multiphase reactions of\udNO2 and SO2 on PM2.5

The possible contribution of the periodic emissions from farmers' activities in the North China Plain to atmospheric water-soluble ions in Beijing