We report an investigation into the magnetoresistance(MR)of La_(0.8)Ba_(0.2)MnO_(3)ultrathin films with various thicknesses.While the 13 nm-thick film shows the commonly reported negative magnetoresistive effect,the 6...We report an investigation into the magnetoresistance(MR)of La_(0.8)Ba_(0.2)MnO_(3)ultrathin films with various thicknesses.While the 13 nm-thick film shows the commonly reported negative magnetoresistive effect,the 6 nm-and 4 nm-thick films display unconventional positive magnetoresistive(PMR)behavior under certain conditions.As well as the dependence on the film's thickness,it has been found that the electrical resistivity and the PMR effect of the thinner films are very dependent on the test current.For example,the magnetoresistive ratio of the 4 nm-thick film changes from+46%to-37%when the current is increased from 10 nA to 100 nA under 15 kOe at 40 K.In addition,the two thinner films present opposite changes in electrical resistivity with respect to the test current,i.e.,the electroresistive(ER)effect,at low temperatures.We discuss the complex magnetoresistive and ER behaviors by taking account of the weak contacts at grain boundaries between ferromagnetic metallic(FMM)grains.The PMR effect can be attributed to the breaking of the weak contacts due to the giant magnetostriction of the FMM grains under a magnetic field.Considering the competing effects of the conductive filament and local Joule self-heating at grain boundaries on the transport properties,the dissimilar ER effects in the two thinner films are also understandable.These experimental findings provide an additional approach for tuning the magnetoresistive effect in manganite films.展开更多
In this study,the characteristics of turbulence transport and intermittency and the evolutionary mechanisms were studied in different pollution stages of heavy haze weather from December 2016 to January 2017 in the Be...In this study,the characteristics of turbulence transport and intermittency and the evolutionary mechanisms were studied in different pollution stages of heavy haze weather from December 2016 to January 2017 in the Beijing area using the method developed by Ren et al.(2019)as the automatic identification of atmospheric spectral gaps and the reconstruction of atmospheric turbulence sequences.The results reveal that turbulence intermittency is the strongest in the cumulative stage(CS)of heavy haze weather,followed by in the transport stage(TS),and it is the weakest in the dissipation stage(DS).During the development and accumulation of haze pollution,buoyancy contributes negatively to turbulent kinetic energy(TKE),and horizontal wind speed is low.The classical turbulent motion is often affected by submesoscale motion.As a result,the calculation results of turbulence parameters are affected by submesoscale motion,which causes intensified turbulence intermittency.During the dissipation of pollution,the downward momentum transfer induced by low-level jets provides kinetic energy for turbulent motion in the near surface layer.The turbulent mixing effect is enhanced,and intermittency is weakened.Due to the intermittency of atmospheric turbulence,turbulence parameters calculated from the original fluctuation of meteorological elements may be overestimated.The overestimation of turbulence parameters in the CS is the strongest,followed by the TS,and the DS is the weakest.The overestimation of turbulent fluxes results in an overestimation of atmospheric dissipation capability that may cause an underestimation of pollutant concentrations in the numerical simulations of air quality.展开更多
Since the founding of the People’s Republic of China 70 years ago,the subject of atmospheric physics and atmospheric environment has developed rapidly in China,providing important support for the development of atmos...Since the founding of the People’s Republic of China 70 years ago,the subject of atmospheric physics and atmospheric environment has developed rapidly in China,providing important support for the development of atmospheric science and guarantee for the development of national economy.In this paper,the general advancement of atmospheric physics and atmospheric environment in last 70 years was described.The main research progress of atmospheric physics and atmospheric environment in the past 40 years of reform and opening-up was reviewed,the outstanding research achievements since the 21 st century were summarized,the major problems and challenges are pointed out,and the key directions and suggestions for future development are put forward.展开更多
We analyzed the structure and evolution of turbulent transfer and the wind profile in the atmospheric boundary layer in relation to aerosol concentrations during an episode of heavy haze pollution from 6 December 2016...We analyzed the structure and evolution of turbulent transfer and the wind profile in the atmospheric boundary layer in relation to aerosol concentrations during an episode of heavy haze pollution from 6 December 2016 to 9 January 2017. The turbulence data were recorded at Peking University＇s atmospheric science and environment observation station. The results showed a negative correlation between the wind speed and the PM2.5 concentration. The turbulence kinetic energy was large and showed obvious diurnal variations during unpolluted（clean） weather, but was small during episodes of heavy haze pollution. Under both clean and heavy haze conditions, the relation between the non-dimensional wind components and the stability parameter z/L followed a 1/3 power law, but the normalized standard deviations of the wind speed were smaller during heavy pollution events than during clean periods under near-neutral conditions. Under unstable conditions, the normalized standard deviation of the potential temperatureσθ/｜θ＊｜ was related to z/L, roughly following a –1/3 power law, and the ratio during pollution days was greater than that during clean days. The three-dimensional turbulence energy spectra satisfied a –2/3 power exponent rate in the high-frequency band. In the low-frequency band, the wind velocity spectrum curve was related to the stability parameters under clear conditions, but was not related to atmospheric stratification under polluted conditions. In the dissipation stage of the heavy pollution episode, the horizontal wind speed first started to increase at high altitudes and then gradually decreased at lower altitudes. The strong upward motion during this stage was an important dynamic factor in the dissipation of the heavy haze.展开更多
With rapid urbanization in recent years,severe air pollution has emerged as a major issue for many regions of China,especially in some metropolises.A persistent pollution case during 6 December 2016–8 January 2017 wa...With rapid urbanization in recent years,severe air pollution has emerged as a major issue for many regions of China,especially in some metropolises.A persistent pollution case during 6 December 2016–8 January 2017 was selected to investigate the relations between turbulent intermittency and frequent PM2.5(particulate matters with diameter less than 2.5μm)pollution events over the metropolitan region of Beijing,China.The accumulation of PM2.5 near the surface frequently occurred as a combined result of strong inversion layers,stagnant winds,high ambient humidity levels,and stable stratification during this case.Arbitrary-order Hilbert spectral analysis indicated that steep decreases in the PM2.5 concentration were simultaneous with the occurrence of intermittent turbulence and strong vertical mixing.A wind profiler observation revealed existence of low-level jets(LLJs)at the end of the polluted periods,suggesting that the upper-level turbulent mixing accompanied by the wind shear of LLJ was transported downward and enhanced the vertical mixing near the surface,which might have caused an abrupt reduction in PM2.5 and improvement in air conditions.展开更多
Atmospheric boundary layer height(ABLH)is an important parameter used to depict characteristics of the planetary boundary layer(PBL)in the lower troposphere.The ABLH is strongly associated with the vertical distributi...Atmospheric boundary layer height(ABLH)is an important parameter used to depict characteristics of the planetary boundary layer(PBL)in the lower troposphere.The ABLH is strongly associated with the vertical distributions of heat,mass,and energy in the PBL,and it is a key quantity in numerical simulation of the PBL and plays an essential role in atmospheric environmental assessment.In this paper,various definitions and methods for deriving and estimating the ABLH are summarized,from the perspectives of turbulent motion,PBL dynamics and thermodynamics,and distributions of various substances in the PBL.Different methods for determining the ABLH by means of direct observation and remote sensing retrieval are reviewed,and comparisons of the advantages and disadvantages of these methods are presented.The paper also summarizes the ABLH parameterization schemes,discusses current problems in the estimation of ABLH,and finally points out the directions for possible future breakthroughs in the ABLHrelated research and application.展开更多
Ammonia (NH_(3)) can interact with other trace chemicals in the atmosphere,significantly impacting atmospheric chemistry and global climate change.China is a largely agricultural country with high consumption of nitro...Ammonia (NH_(3)) can interact with other trace chemicals in the atmosphere,significantly impacting atmospheric chemistry and global climate change.China is a largely agricultural country with high consumption of nitrogen fertilizer and large livestock herds,resulting in high NH_(3) emissions.In this study,a comprehensive county-level inventory of Chinese NH_(3) emissions from 2013 to 2018 was compiled.Based on previous research,an estimate of NH_(3) emissions from household coal combustion was added to the inventory.The estimation of emissions from open biomass burning was improved by using a method based on fire radiative energy (FRE).The total NH_(3) emissions in China increased from 2013 (9.64 Tg) to 2015 (9.75 Tg),and then decreased to 9.12 Tg in 2018.Emissions from fossil fuels reached a peak value in 2018,accounting for 8.4%of total emissions,while fertilizer application and livestock waste were responsible for fewer emissions than in previous years,accounting for 27.7%and 49.9%of the total from non-fossil fuel sources,respectively.The highest emission rates were in central and southwestern China.Seasonally,NH_(3) emissions peaked in spring and summer.The inventory had a 1-km spatial resolution and a monthly temporal resolution,which confirmed its suitability for global and regional air quality simulations.展开更多
The North China Plain(NCP)is troubled by severe haze pollution and the evolution of haze pollution is closely related to the atmospheric boundary layer(ABL).However,experimental and theoretical studies on the physical...The North China Plain(NCP)is troubled by severe haze pollution and the evolution of haze pollution is closely related to the atmospheric boundary layer(ABL).However,experimental and theoretical studies on the physical-chemical processes of the ABL in the NCP are lacking,with many scientific problems to be addressed.To solve these problems,the Comprehensive Observation on the Atmospheric boundary layer Three-dimensional Structure(COATS)during haze pollution was carried out in the NCP from 2016 to 2020.The COATS experiment adopted a"point-line-surface"spatial layout,obtaining both spatial-temporal profiles of the meteorological and environmental elements in the ABL and the turbulent transport data of fine particulate matter(PM_(2.5))in winter and summer.The research achievements are as follows.The spatial-temporal distribution characteristics of the ABL structure and PM_(2.5)concentrations in NCP were determined.The typical thermal structure of persistent heavy haze events and the pollutant removal mechanism by low-level jets were revealed.It was determined that the spatial structure of the ABL adjusted by the Taihang Mountains is responsible for the heterogeneous distribution of haze pollution in the NCP,and that mountain-induced vertical circulations can promote the formation of elevated pollution layers.The restraints of the atmospheric internal boundaries on horizontal diffusion of pollutants were emphasized.The contribution of the ABL to haze pollution in winter and summer was qualitatively compared and quantitatively estimated.The turbulent transport nature behind the relationship between the atmospheric boundary layer height(ABLH)and surface PM_(2.5)concentrations was analyzed.The concept of"aerosol accumulation layer"was defined,and the applicability of the material method in determining ABLH was clarified.A measurement system for obtaining the turbulent flux of PM_(2.5)concentrations was developed,and the turbulence characteristics of PM_(2.5)concentrations were demonstrated.The COATS experiment is of great theoretical significance for thoroughly understanding the physical mechanisms of the ABL during haze pollution and filling the knowledge gap on the impacts of the ABL three-dimensional structure on haze pollution.The results of this study are conducive to the improvement and development of ABL parameterization schemes and serve as a scientific basis for formulating regional pollution prevention and control measures.展开更多
The mechanisms of new particle formation(NPF)events that occurred under high aerosol loadings(“polluted”NPF)in the atmosphere have been unclear,which has inhibited the precision of particlepollution control.To deepe...The mechanisms of new particle formation(NPF)events that occurred under high aerosol loadings(“polluted”NPF)in the atmosphere have been unclear,which has inhibited the precision of particlepollution control.To deepen the understanding of how the“polluted”NPF events occur,a one-monthcomprehensive measurement was conducted in the atmosphere of Beijing during the summer of2016.The“clean”NPF events(frequency=22%)(condensation sink,CS<0.015 s^(-1))were found to becaused by local nucleation and growth.The“polluted”NPF events(frequency=28%)(CS>0.015 s^(-1))were influenced by both local nucleation-growth and regional transport,and the contributions from thetwo factors to 6e25 nm particle number concentration were 60%and 40%,respectively.This studyemphasized the importance of the transport for nanoparticles in relatively polluted atmospheres,and forthat the regional joint particle pollution control would be an essential policy.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant No.11674298)the National Key Research and Development Program of China(Grant No.2017YFA0403502)the Users with Excellence Project of Hefei Science Center CAS(Grant No.2018HSC-UE013).
文摘We report an investigation into the magnetoresistance(MR)of La_(0.8)Ba_(0.2)MnO_(3)ultrathin films with various thicknesses.While the 13 nm-thick film shows the commonly reported negative magnetoresistive effect,the 6 nm-and 4 nm-thick films display unconventional positive magnetoresistive(PMR)behavior under certain conditions.As well as the dependence on the film's thickness,it has been found that the electrical resistivity and the PMR effect of the thinner films are very dependent on the test current.For example,the magnetoresistive ratio of the 4 nm-thick film changes from+46%to-37%when the current is increased from 10 nA to 100 nA under 15 kOe at 40 K.In addition,the two thinner films present opposite changes in electrical resistivity with respect to the test current,i.e.,the electroresistive(ER)effect,at low temperatures.We discuss the complex magnetoresistive and ER behaviors by taking account of the weak contacts at grain boundaries between ferromagnetic metallic(FMM)grains.The PMR effect can be attributed to the breaking of the weak contacts due to the giant magnetostriction of the FMM grains under a magnetic field.Considering the competing effects of the conductive filament and local Joule self-heating at grain boundaries on the transport properties,the dissimilar ER effects in the two thinner films are also understandable.These experimental findings provide an additional approach for tuning the magnetoresistive effect in manganite films.
基金supported by the National Natural Science Foundation of China (Grant No. 41544216)the National Key R & D Program of China (Grant Nos. 2017YFC0209600 & 2016YFC0203300)
文摘In this study,the characteristics of turbulence transport and intermittency and the evolutionary mechanisms were studied in different pollution stages of heavy haze weather from December 2016 to January 2017 in the Beijing area using the method developed by Ren et al.(2019)as the automatic identification of atmospheric spectral gaps and the reconstruction of atmospheric turbulence sequences.The results reveal that turbulence intermittency is the strongest in the cumulative stage(CS)of heavy haze weather,followed by in the transport stage(TS),and it is the weakest in the dissipation stage(DS).During the development and accumulation of haze pollution,buoyancy contributes negatively to turbulent kinetic energy(TKE),and horizontal wind speed is low.The classical turbulent motion is often affected by submesoscale motion.As a result,the calculation results of turbulence parameters are affected by submesoscale motion,which causes intensified turbulence intermittency.During the dissipation of pollution,the downward momentum transfer induced by low-level jets provides kinetic energy for turbulent motion in the near surface layer.The turbulent mixing effect is enhanced,and intermittency is weakened.Due to the intermittency of atmospheric turbulence,turbulence parameters calculated from the original fluctuation of meteorological elements may be overestimated.The overestimation of turbulence parameters in the CS is the strongest,followed by the TS,and the DS is the weakest.The overestimation of turbulent fluxes results in an overestimation of atmospheric dissipation capability that may cause an underestimation of pollutant concentrations in the numerical simulations of air quality.
文摘Since the founding of the People’s Republic of China 70 years ago,the subject of atmospheric physics and atmospheric environment has developed rapidly in China,providing important support for the development of atmospheric science and guarantee for the development of national economy.In this paper,the general advancement of atmospheric physics and atmospheric environment in last 70 years was described.The main research progress of atmospheric physics and atmospheric environment in the past 40 years of reform and opening-up was reviewed,the outstanding research achievements since the 21 st century were summarized,the major problems and challenges are pointed out,and the key directions and suggestions for future development are put forward.
基金Supported by the National Key Project of Ministry of Science and Technology of China(2016YFC0203300)National Natural Science Foundation of China(91544216,41475007,and 41675018)
文摘We analyzed the structure and evolution of turbulent transfer and the wind profile in the atmospheric boundary layer in relation to aerosol concentrations during an episode of heavy haze pollution from 6 December 2016 to 9 January 2017. The turbulence data were recorded at Peking University＇s atmospheric science and environment observation station. The results showed a negative correlation between the wind speed and the PM2.5 concentration. The turbulence kinetic energy was large and showed obvious diurnal variations during unpolluted（clean） weather, but was small during episodes of heavy haze pollution. Under both clean and heavy haze conditions, the relation between the non-dimensional wind components and the stability parameter z/L followed a 1/3 power law, but the normalized standard deviations of the wind speed were smaller during heavy pollution events than during clean periods under near-neutral conditions. Under unstable conditions, the normalized standard deviation of the potential temperatureσθ/｜θ＊｜ was related to z/L, roughly following a –1/3 power law, and the ratio during pollution days was greater than that during clean days. The three-dimensional turbulence energy spectra satisfied a –2/3 power exponent rate in the high-frequency band. In the low-frequency band, the wind velocity spectrum curve was related to the stability parameters under clear conditions, but was not related to atmospheric stratification under polluted conditions. In the dissipation stage of the heavy pollution episode, the horizontal wind speed first started to increase at high altitudes and then gradually decreased at lower altitudes. The strong upward motion during this stage was an important dynamic factor in the dissipation of the heavy haze.
基金Supported by the National Key Research and Development Program of China(2016YFC0203300)National Natural Science Foundation of China(91544216 and 41705003).
文摘With rapid urbanization in recent years,severe air pollution has emerged as a major issue for many regions of China,especially in some metropolises.A persistent pollution case during 6 December 2016–8 January 2017 was selected to investigate the relations between turbulent intermittency and frequent PM2.5(particulate matters with diameter less than 2.5μm)pollution events over the metropolitan region of Beijing,China.The accumulation of PM2.5 near the surface frequently occurred as a combined result of strong inversion layers,stagnant winds,high ambient humidity levels,and stable stratification during this case.Arbitrary-order Hilbert spectral analysis indicated that steep decreases in the PM2.5 concentration were simultaneous with the occurrence of intermittent turbulence and strong vertical mixing.A wind profiler observation revealed existence of low-level jets(LLJs)at the end of the polluted periods,suggesting that the upper-level turbulent mixing accompanied by the wind shear of LLJ was transported downward and enhanced the vertical mixing near the surface,which might have caused an abrupt reduction in PM2.5 and improvement in air conditions.
基金Supported by the National Key Research and Development Program of China(2016YFC0203300 and 2017YFC0209600)National Research Program for Key Issues in Air Pollution Control(DQGG0104 and DQGG0106)National Natural Science Foundation of China(91544216).
文摘Atmospheric boundary layer height(ABLH)is an important parameter used to depict characteristics of the planetary boundary layer(PBL)in the lower troposphere.The ABLH is strongly associated with the vertical distributions of heat,mass,and energy in the PBL,and it is a key quantity in numerical simulation of the PBL and plays an essential role in atmospheric environmental assessment.In this paper,various definitions and methods for deriving and estimating the ABLH are summarized,from the perspectives of turbulent motion,PBL dynamics and thermodynamics,and distributions of various substances in the PBL.Different methods for determining the ABLH by means of direct observation and remote sensing retrieval are reviewed,and comparisons of the advantages and disadvantages of these methods are presented.The paper also summarizes the ABLH parameterization schemes,discusses current problems in the estimation of ABLH,and finally points out the directions for possible future breakthroughs in the ABLHrelated research and application.
文摘Ammonia (NH_(3)) can interact with other trace chemicals in the atmosphere,significantly impacting atmospheric chemistry and global climate change.China is a largely agricultural country with high consumption of nitrogen fertilizer and large livestock herds,resulting in high NH_(3) emissions.In this study,a comprehensive county-level inventory of Chinese NH_(3) emissions from 2013 to 2018 was compiled.Based on previous research,an estimate of NH_(3) emissions from household coal combustion was added to the inventory.The estimation of emissions from open biomass burning was improved by using a method based on fire radiative energy (FRE).The total NH_(3) emissions in China increased from 2013 (9.64 Tg) to 2015 (9.75 Tg),and then decreased to 9.12 Tg in 2018.Emissions from fossil fuels reached a peak value in 2018,accounting for 8.4%of total emissions,while fertilizer application and livestock waste were responsible for fewer emissions than in previous years,accounting for 27.7%and 49.9%of the total from non-fossil fuel sources,respectively.The highest emission rates were in central and southwestern China.Seasonally,NH_(3) emissions peaked in spring and summer.The inventory had a 1-km spatial resolution and a monthly temporal resolution,which confirmed its suitability for global and regional air quality simulations.
基金supported by the National Natural Science Foundation of China(Grant Nos.42090031,42175092,92044301&91544216)the National Key R&D Program of China(Grant Nos.2016YFC0203306&2017YFC0209600)the National Research Program for Key Issues in Air Pollution Control(Grant Nos.DQGG0104&DQGG0106)。
文摘The North China Plain(NCP)is troubled by severe haze pollution and the evolution of haze pollution is closely related to the atmospheric boundary layer(ABL).However,experimental and theoretical studies on the physical-chemical processes of the ABL in the NCP are lacking,with many scientific problems to be addressed.To solve these problems,the Comprehensive Observation on the Atmospheric boundary layer Three-dimensional Structure(COATS)during haze pollution was carried out in the NCP from 2016 to 2020.The COATS experiment adopted a"point-line-surface"spatial layout,obtaining both spatial-temporal profiles of the meteorological and environmental elements in the ABL and the turbulent transport data of fine particulate matter(PM_(2.5))in winter and summer.The research achievements are as follows.The spatial-temporal distribution characteristics of the ABL structure and PM_(2.5)concentrations in NCP were determined.The typical thermal structure of persistent heavy haze events and the pollutant removal mechanism by low-level jets were revealed.It was determined that the spatial structure of the ABL adjusted by the Taihang Mountains is responsible for the heterogeneous distribution of haze pollution in the NCP,and that mountain-induced vertical circulations can promote the formation of elevated pollution layers.The restraints of the atmospheric internal boundaries on horizontal diffusion of pollutants were emphasized.The contribution of the ABL to haze pollution in winter and summer was qualitatively compared and quantitatively estimated.The turbulent transport nature behind the relationship between the atmospheric boundary layer height(ABLH)and surface PM_(2.5)concentrations was analyzed.The concept of"aerosol accumulation layer"was defined,and the applicability of the material method in determining ABLH was clarified.A measurement system for obtaining the turbulent flux of PM_(2.5)concentrations was developed,and the turbulence characteristics of PM_(2.5)concentrations were demonstrated.The COATS experiment is of great theoretical significance for thoroughly understanding the physical mechanisms of the ABL during haze pollution and filling the knowledge gap on the impacts of the ABL three-dimensional structure on haze pollution.The results of this study are conducive to the improvement and development of ABL parameterization schemes and serve as a scientific basis for formulating regional pollution prevention and control measures.
基金This study is funded by the National Natural Science Foundationof China(NSFC)(grant No.91844301)the NSFC e Creative ResearchGroup Fund(grant No.22221004)+1 种基金the National Key Research andDevelopment Program of China(grant No.2022YFC3701000,Task1)the bilateral SwedeneChina framework program“Photochemical smog in China:formation,transformation,impactand abatement strategies”(grant No.639-2013-6917).
文摘The mechanisms of new particle formation(NPF)events that occurred under high aerosol loadings(“polluted”NPF)in the atmosphere have been unclear,which has inhibited the precision of particlepollution control.To deepen the understanding of how the“polluted”NPF events occur,a one-monthcomprehensive measurement was conducted in the atmosphere of Beijing during the summer of2016.The“clean”NPF events(frequency=22%)(condensation sink,CS<0.015 s^(-1))were found to becaused by local nucleation and growth.The“polluted”NPF events(frequency=28%)(CS>0.015 s^(-1))were influenced by both local nucleation-growth and regional transport,and the contributions from thetwo factors to 6e25 nm particle number concentration were 60%and 40%,respectively.This studyemphasized the importance of the transport for nanoparticles in relatively polluted atmospheres,and forthat the regional joint particle pollution control would be an essential policy.
