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

Abstract

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 NH 4 Cl in the vast area of NCP. The abnormally high concentrations of Ca 2+ at the two sampling sites and the elevation of Ca 2+ 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 Ca 2+ , 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 NO 2 and SO 2 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 NO 2 and SO 2 on PM 2.5 .

Characteristics and Sources of Heavy Metals in PM2.5 during a Typical Haze Episode in Rural and Urban Areas in Taiyuan, China

“…The residents in the rural area frequently and randomly increased coal burning for heating due to colder weather, while the burning amount of coal for central-heating and industries in the urban area remained stable during the short period, which was suspected to be the main reason for the significant differences between the two sites during the start-up and peak stages. Although residential coal consumption in the rural area only accounts for a small fraction of the total (e.g.,~4 tons raw coal per year used for cooking and heating in each household according to our previous survey), the emission factor of PM 2.5 from farmers' coal stoves (about 1054-12,910 mg kg −1 ) are usually about 1~3 orders of magnitude greater than those from coal power plants or industry boilers (about 16-100 mg kg −1 ) [3]. The activities of humans, such as coal burning for heating and cooking, were also suspected to make a significant contribution to the deterioration of the regional air quality in winter in other rural areas in the North China Plain [3].…”

“…Although residential coal consumption in the rural area only accounts for a small fraction of the total (e.g.,~4 tons raw coal per year used for cooking and heating in each household according to our previous survey), the emission factor of PM 2.5 from farmers' coal stoves (about 1054-12,910 mg kg −1 ) are usually about 1~3 orders of magnitude greater than those from coal power plants or industry boilers (about 16-100 mg kg −1 ) [3]. The activities of humans, such as coal burning for heating and cooking, were also suspected to make a significant contribution to the deterioration of the regional air quality in winter in other rural areas in the North China Plain [3]. The most serious pollution events (or haze days) in the North China Plain have usually been coincident with the activities of farmers in recent years [3,39,41].…”

“…After standing for 24 h, the solution was heated with a hotplate to a boil and the temperature was maintained until the solution had nearly evaporated. After cooling to room temperature, the residue was dissolved with 10 mL of 6.5% HNO 3 and diluted in a 25-mL flask with ultrapure water. Then, the diluted solutions were transferred to numbered polyethylene vials and stored in a refrigerator (4 • C) for later analysis.…”

“…The activities of humans, such as coal burning for heating and cooking, were also suspected to make a significant contribution to the deterioration of the regional air quality in winter in other rural areas in the North China Plain [3]. The most serious pollution events (or haze days) in the North China Plain have usually been coincident with the activities of farmers in recent years [3,39,41]. …”

“…Currently, large-scale regions in North China are suffering from severe haze episodes [1][2][3][4][5], which are essentially caused by fine particles with aerodynamic diameter less than 2.5 µm (PM 2.5 ) [3,6,7]. This has attracted great scientific and public attention [8][9][10][11][12][13].…”