Distinct groundwater recharge sources and geochemical evolution of two adjacent sub-basins in the lower Shule River Basin,northwest China

Based on analysis of groundwater hydrogeochemical and isotopic data, this study aims to identify the recharge sources and understand geochemical evolution of groundwater along the downstream section of the Shule River, northwest China, including two sub-basins. Groundwater samples from the Tashi sub-basin show markedly depleted stable isotopes compared to those in the Guazhou sub-basin. This difference suggests that groundwater in the Tashi sub-basin mainly originates from meltwater in the Qilian Mountains, while the groundwater in the Guazhou sub-basin may be recharged by seepage of the Shule River water. During the groundwater flow process in the Tashi sub-basin, minerals within the aquifer material (e.g., halite, calcite, dolomite, gypsum) dissolve in groundwater. Mineral dissolution leads to strongly linear relationships between Na⁺ and Cl^? and between Mg²⁺+ Ca²⁺ and SO₄ ^2??+?HCO₃ ^?, with stoichiometry ratios of approximately 1:1 in both cases. The ion-exchange reaction plays a dominant role in hydrogeochemical evolution of groundwater in the Guazhou sub-basin and causes a good linear relationship between (Mg²⁺+ Ca²⁺)–(SO₄ ^2??+?HCO₃ ^?) and (Na⁺+ K⁺)–Cl^? with a slope of ?0.89 and also results in positive chloroalkaline indices CAI 1 and CAI 2. The scientific results have implications for groundwater management in the downstream section of Shule River. As an important irrigation district in Hexi Corridor, groundwater in the Guazhou sub-basin should be used sustainably and rationally because its recharge source is not as abundant as expected. It is recommended that the surface water should be used efficiently and routinely, while groundwater exploitation should be limited as much as possible.     

Mass Concentration and Mineralogical Characteristics of Aerosol Particles Collected at Dunhuang During ACE-Asia

Measurements were performed in spring 2001 and 2002 to determine the characteristics of soil dust in the Chinese desert region of Dunhuang, one of the ground sites of the Asia-Pacific Regional Aerosol Characterization Experiment (ACE-Asia). The mean mass concentrations of total suspended particle matter during the spring of 2001 and 2002 were 317 μg m?3 and 307 μg m?3, respectively. Eleven dust storm events were observed with a mean aerosol concentration of 1095 μg m?3, while the non-dusty days with calm or weak wind speed had a background aerosol loading of 196 μgm?3 on average in the springtime. The main minerals detected in the aerosol samples by X-ray diffraction were illite, kaolinite, chlorite, quartz, feldspar, calcite and dolomite. Gypsum, halite and amphibole were also detected in a few samples. The mineralogical data also show that Asian dust is characterized by a kaolinite to chlorite (K/C) ratio lower than 1 whereas Saharan dust exhibits a K/C ratio larger than 2. Air mass back-trajectory analysis show that three families of pathways are associated with the aerosol particle transport to Dunhuang, but these have similar K/C ratios, which further demonstrates that the mineralogical characteristics of Asian dust are different from African dust.     

大气本底站反应性气体观测数据处理系统功能设计及实现

基于反应性气体多年观测的较为成熟的数据处理和质量控制方法,采用模块化功能结构设计,通过应用Visual Studio开发平台并结合MAP WinGIS等空间和时间显示控件,对反应性气体观测数据处理系统进行了设计和开发.系统实现了观测数据在空间和时间上的可视化显示,实际业务应用表明,具有良好的人机交互和数据处理能力,具备一定的数据统计分析功能.实现了对大气本底站反应性气体观测数据的统一管理和综合处理,已成为大气成分观测数据质量控制业务系统的一个重要组成部分.     

Aerosol Hygroscopicity during the Haze Red-Alert Period in December 2016 at a Rural Site of the North China Plain

A humidification system was deployed to measure aerosol hygroscopicity at a rural site of the North China Plain during the haze red-alert period 17–22 December 2016. The aerosol scattering coefficients under dry [relative humidity (RH) < 30%] and wet (RH in the range of 40%–85%) conditions were simultaneously measured at wavelengths of 450, 550, and 700 nm. It is found that the aerosol scattering coefficient and backscattering coefficient increased by only 29% and 10%, respectively when RH went up from 40% to 80%, while the hemispheric backscatter fraction went down by 14%, implying that the aerosol hygroscopicity represented by the aerosol scattering enhancement factor f(RH) is relatively low and RH exerted little effects on the aerosol light scattering in this case. The scattering enhancement factors do not show significant differences at the three wavelengths, only with an approximate 2% variation, suggesting that the aerosol hygroscopicity is independent of the wavelength. Aerosol hygroscopicity is highly dependent on the aerosol chemical composition. When there is a large mass fraction of inorganics and a small mass fraction of organic matter, f(RH) reaches a high value. The fraction of NO₃^– was strongly correlated with the aerosol scattering coefficient at RH = 80%, which suggests that NO₃^– played an important role in aerosol hygroscopic growth during the heavy pollution period.     

Chemical Components,Variation, and Source Identification of PM1 during the Heavy Air Pollution Episodes in Beijing in December 2016

Air pollution is a current global concern. The heavy air pollution episodes(HPEs) in Beijing in December 2016 severely influenced visibility and public health. This study aims to survey the chemical compositions, sources, and formation processes of the HPEs. An aerodyne quadruple aerosol mass spectrometer(Q-AMS) was utilized to measure the non-refractory PM1(NR-PM1) mass concentration and size distributions of the main chemical components including organics, sulfate, nitrate, ammonium, and chloride in situ during 15–23 December 2016. The NR-PM1 mass concentration was found to increase from 6 to 188 μg m–3 within 5 days. During the most serious polluted episode, the PM1 mass concentration was about 2.6 times that during the first pollution stage and even 40 times that of the clean days. The formation rates of PM2.5 in the five pollution stages were 26, 22, 22, 32, and 67 μg m~(–3) h–1, respectively. Organics and nitrate occupied the largest proportion in the polluted episodes, whereas organics and sulfate dominated the submicron aerosol during the clean days. The size distribution of organics is always broader than those of other species, especially in the clean episodes. The peak sizes of the interested species grew gradually during different HPEs. Aqueous reaction might be important in forming sulfate and chloride, and nitrate was formed via oxidization and condensation processes. PMF(positive matrix factorization) analysis on AMS mass spectra was employed to separate the organics into different subtypes. Two types of secondary organic aerosol with different degrees of oxidation consisted of 43% of total organics. By contrast, primary organics from cooking, coal combustion, and traffic emissions comprised 57% of the organic aerosols during the HPEs.     

在二维和三维CT图象重建理论方法的研究工作及新进展

本文主要介绍近几年清华大学工程物理系“粒子技术与辐射成象”国家专业实验室在二维和三维ＣＴ图象重建领域的研究成果，这些成果基本上代表了我国在核领域的研究水平。     

The strong El Niño of 2015/16 and its dominant impacts on global and China's climate

The oceanic and atmospheric conditions and the related climate impacts of the 2015/16 ENSO cycle were analyzed, based on the latest global climate observational data, especially that of China. The results show that this strong El Niño event fully established in spring 2015 and has been rapidly developing into one of the three strongest El Niño episodes in recorded history. Meanwhile, it is also expected to be the longest event recorded, attributable to the stable maintenance of the abnormally warm conditions in the equatorial Pacific Ocean since spring 2014. Owing to the impacts of this strong event, along with climate warming background, the global surface temperature and the surface air temperature over Chinese mainland reached record highs in 2015. Disastrous weather in various places worldwide have occurred in association with this severe El Niño episode, and summer precipitation has reduced significantly in North China, especially over the bend of the Yellow River, central Inner Mongolia, and the coastal areas surrounding Bohai Bay. Serious drought has occurred in some of the above areas. The El Niño episode reached its peak strength during November-December 2015, when a lower-troposphere anomalous anticyclonic circulation prevailed over the Philippines, bringing about abnormal southerlies and substantially increased precipitation in southeastern China. At the same time, a negative phase of the Eurasia-Pacific teleconnection pattern dominated over the mid-high latitudes, which suppressed northerly winds in North China. These two factors together resulted in high concentrations of fine particulate matter (PM_2.5) and frequent haze weather in this region. Currently, this strong El Niño is weakening very rapidly, but its impact on climate will continue in the coming months in some regions, especially in China.     

Relative contributions of boundary-layer meteorological factors to the explosive growth of PM<Subscript>2.5</Subscript> during the red-alert heavy pollution episodes in Beijing in December 2016

Based on observations of urban mass concentration of fine particulate matter smaller than 2.5 μm in diameter (PM_2.5), ground meteorological data, vertical measurements of winds, temperature, and relative humidity (RH), and ECMWF reanalysis data, the major changes in the vertical structures of meteorological factors in the boundary layer (BL) during the heavy aerosol pollution episodes (HPEs) that occurred in winter 2016 in the urban Beijing area were analyzed. The HPEs are divided into two stages: the transport of pollutants under prevailing southerly winds, known as the transport stage (TS), and the PM_2.5 explosive growth and pollution accumulation period characterized by a temperature inversion with low winds and high RH in the lower BL, known as the cumulative stage (CS). During the TS, a surface high lies south of Beijing, and pollutants are transported northwards. During the CS, a stable BL forms and is characterized by weak winds, temperature inversion, and moisture accumulation. Stable atmospheric stratification featured with light/calm winds and accumulated moisture (RH > 80%) below 250 m at the beginning of the CS is closely associated with the inversion, which is strengthened by the considerable decrease in near-surface air temperature due to the interaction between aerosols and radiation after the aerosol pollution occurs. A significant increase in the PLAM (Parameter Linking Aerosol Pollution and Meteorological Elements) index is found, which is linearly related to PM mass change. During the first 10 h of the CS, the more stable BL contributes approximately 84% of the explosive growth of PM_2.5 mass. Additional accumulated near-surface moisture caused by the ground temperature decrease, weak turbulent diffusion, low BL height, and inhibited vertical mixing of water vapor is conducive to the secondary aerosol formation through chemical reactions, including liquid phase and heterogeneous reactions, which further increases the PM_2.5 concentration levels. The contribution of these reaction mechanisms to the explosive growth of PM_2.5 mass during the early CS and subsequent pollution accumulation requires further investigation.     

Temporal variation and source identification of black carbon at Lin’an and Longfengshan regional background stations in China

Black carbon (BC) is a component of fine particulate matter (PM_2.5), associated with climate, weather, air quality, and people’s health. However, studies on temporal variation of atmospheric BC concentration at background stations in China and its source area identification are lacking. In this paper, we use 2-yr BC observations from two background stations, Lin’an (LAN) and Longfengshan (LFS), to perform the investigation. The results show that the mean diurnal variation of BC has two significant peaks at LAN while different characteristics are found in the BC variation at LFS, which are probably caused by the difference in emission source contributions. Seasonal variation of monthly BC shows double peaks at LAN but a single peak at LFS. The annual mean concentrations of BC at LAN and LFS decrease by 1.63 and 0.26 μg m^–3 from 2009 to 2010, respectively. The annual background concentration of BC at LAN is twice higher than that at LFS. The major source of the LAN BC is industrial emission while the source of the LFS BC is residential emission. Based on transport climatology on a 7-day timescale, LAN and LFS stations are sensitive to surface emissions respectively in belt or approximately circular area, which are dominated by summer monsoon or colder land air flows in Northwest China. In addition, we statistically analyze the BC source regions by using BC observation and FLEXible PARTicle dispersion model (FLEXPART) simulation. In summer, the source regions of BC are distributed in the northwest and south of LAN and the southwest of LFS. Low BC concentration is closely related to air mass from the sea. In winter, the source regions of BC are concentrated in the west and south of LAN and the northeast of the threshold area of s_tot at LFS. The cold air mass in the northwest plays an important role in the purification of atmospheric BC. On a yearly scale, sources of BC are approximately from five provinces in the northwest/southeast of LAN and the west of LFS. These findings are helpful in reducing BC emission and controlling air pollution.