Abstract
Abstract. The North China Plain (NCP), which includes Beijing, is currently suffering from severe haze events due to a high pollution level of PM2.5. To mitigate the serious pollution problem, identification of the sources of PM2.5 is urgently needed for the effective control measures. Daily samples of PM2.5 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 PM2.5 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 PM2.5.
Highlights
The North China Plain (NCP) is frequently suffering from severe haze pollution in recent years (Chan and Yao, 2008; Liang et al, 2016), which has aroused great attention from the general public
The sampling site in Beijing city was on a rooftop in the Research Center for Eco-Environmental Sciences, which is located between the north fourth-ring road and the north fifth-ring road of Beijing and surrounded by some institutes, campuses, and residential areas (Pang and Mu, 2006)
It was possible that the mass concentration of PM2.5 measured by the TEOM 1405 Monitor was underestimated because the volatile even semi-volatile component in PM2.5 can be lost at 50 ◦C which is designed in the TEOM 1405 Monitor for avoiding water condensation on the filter
Summary
The North China Plain (NCP) is frequently suffering from severe haze pollution in recent years (Chan and Yao, 2008; Liang et al, 2016), which has aroused great attention from the general public J. Huang et al, 2014; Y.
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