|Alternative Title||The impact of atmospheric pollutant emissions from rural areas in the North China Plain on regional air quality|
|Place of Conferral||北京|
|Keyword||华北平原,灰霾,农民活动,散煤燃烧,So42- The North China Plain, Haze, Farmers’ Activities, Residential Coal Combustion, Sulfate|
The North China Plain (NCP) is frequently suffering from severe haze pollution in recent years, which has aroused the attention of the public. The severe haze pollution is mainly due to the high level of fine particulate matter with an aerodynamic diameter less than 2.5μm (PM2.5). PM2.5 can reduce atmospheric visibility by absorbing or scattering the incident light and increase morbidity and mortality by penetrating the human bronchi and lungs. Based on these features, understanding the physical/chemical properties, sources and evolution processes of PM2.5 is vital for the investigation of haze formation mechanism and the prevention of atmospheric particle pollution. Currently, most studies about PM2.5 mainly focused on cities, whereas the emissions from farmers’ activities in the NCP were often neglected. Due to the lack of control measures, the emissions of atmospheric pollutants from rural areas were suspected to make significant contribution to the regional air quality, e.g. the emission factors of the typical pollutants from residential coal stoves are usually about 1-3 orders of magnitude greater than those from industry boilers and coal power plants. Therefore, daily samples of PM2.5 were simultaneously collected in both the cities (Beijing city, Baoding city and Wangdu county) and the rural area (Dongbaituo countryside) of the NCP for three years (2014-2016) by using four medium volume PM2.5 samplers, and the possible influence of farmers’ activities on the regional air quality in the NCP was comprehensively investigated in this study. The main conclusions were obtained as follows: (1) Based on the water-soluble ions (WSIs) in PM2.5 at the sampling site of Beijing compared to the sampling site of Dongbaituo in 2014, the results indicated that the periodic emissions from farmers’ activities made an evident contribution to the WSIs in Beijing. The relatively high concentration of K+ in autumn and winter 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 residential coal combustion for heating. The obviously high ratio of Cl? to Na+ in summer, the unusually high concentrations of Cl- in Dongbaituo 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 the NCP. The distinctly 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 south/southwest regions, which have a high density of farmers.
(2) Based on the daily variations, composition characteristics and correlations among the key species in PM2.5 at the four sampling sites of Beijing, Baoding, Wangdu and Dongbaituo during the winter and spring of 2014-2015, the results indicated that residential coal combustion made a significant contribution to atmospheric PM2.5 in the NCP. The concentrations of the typical WSIs (such as Cl-, NO3-, SO42- and NH4+) at the sampling site of Dongbaituo were found to be remarkably higher than those at the sampling site of Beijing in the two winters, but almost the same as those at the sampling site of Beijing in the two springs. The evidently greater concentrations of OC, EC and secondary inorganic ions (NO3-, SO42-, NH4+ and Cl-) at the sampling site of Dongbaituo than at the three sampling sites of Wangdu, Baoding and Beijing during the winter of 2015 indicated that the pollutants in the rural area were not due to transportation from its neighbor cities but dominated by local emissions. As the distinct source for atmospheric OC and EC in the rural area, the residential coal combustion also made contribution to secondary inorganic ions through the emissions of their precursors (NOx, SO2, NH3 and HCl) as well as heterogeneous or multiphase reactions on the surface of OC and EC. The average mass proportions of OC, EC, NO3- and SO42- at the two sampling sites of Baoding and Wangdu were found to be very close to those at the sampling site of Dongbaituo, but evidently different from those at the sampling site of Beijing, implying that the pollutants in Wangdu and Baoding which are fully surrounded by the countryside were strongly affected by the residential coal combustion. The OC/EC ratios at the four sampling sites were almost the same value (4.8) when the concentrations of PM2.5 were greater than 150μg/m3, suggesting that the residential coal combustion could also make dominant contribution to atmospheric PM2.5 in Beijing during the severe pollution period. The evident increase of the number of the species involved in significant correlations (p<0.05) from the countryside to the cities further confirmed that residential coal combustion was the dominant source for the key species in the rural area and, however, the complex sources including local emissions and regional transportation were responsible for the atmospheric species in the cities. The strong correlations among OC, EC, Cl-, NO3-, and NH4+ were found at the four sampling sites but strong correlation between OC (or EC) and SO42- was only found at the sampling site of Beijing, implying that the formation rate of SO42- via heterogeneous or multiphase reactions might be relatively slower than those of NO3-, NH4+ and Cl-. Based on the chemical mass closure (CMC) method, the contributions of the primary particle emission from residential coal combustion to atmospheric PM2.5 at the four sampling sites of Beijing, Baoding, Wangdu and Dongbaituo were estimated to be 32%, 49%, 43% and 58%, respectively.
(3) Based on the hourly variations of the key species in PM2.5 and the pollution characteristics of secondary aerosols at the sampling site of Beijing in the winter of 2016, the results indicated that atmospheric H2O2 made a remarkable contribution to SO42- production rate during the severe pollution period. The obvious elevation of NO3-, SO42- and NH4+ during the most serious pollution episode implied that severe haze events could be mainly ascribed to secondary formation of atmospheric fine particles. When atmospheric RH was greater than 60%, the slow decrease of NOR, the evident increase of aerosol water contents (AWC) and the relatively constant NOR/NHR confirmed that the deliquescence of NH4NO3 was observed in this case. The significant elevation of SOR at 60% RH implied that aqueous-phase reactions could greatly accelerate the conversion of SO2 to SO42-. Based on the accurate concentrations of atmospheric H2O2 and the estimated pH of atmospheric particles during severe pollution period, the contribution of H2O2 to SO42- production rate was found to be evidently greater than those of NO2 and O3 during haze events in Beijing. In addition, the sum of the contribution of H2O2, NO2 and O3 to SO42- production rate could not offset the missing source of SO42- when the concentrations of PM2.5 were greater than 100μg/m3, suggesting that atmospheric sulfate was dominant by extra unknown formation mechanism during the most serious pollution episode in Beijing.
|刘鹏飞. 华北地区农村面源大气污染排放对区域空气质量的影响[D]. 北京. 中国科学院生态环境研究中心,2018.|
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