Beijing 325-m Meteorological Tower Research Articles

Submicron-scale aerosol above the city canopy in Beijing in spring based on in-situ meteorological tower measurements

Non-refractory submicron-scale aerosol (NR-PM1) chemical species above the city canopy in Beijing in spring were studied based on observations at the Beijing 325-m meteorological tower in combination with analyses of the meteorological field and mixing layer height (MLH). The parallel sampling showed that the NR-PM1 at 260 m (60 μg/m3) was ~70% of that at ground level. Organic aerosol (Org) was the species with the most significant vertical difference, as its average mass concentration at 260 m was half of that at ground level. In 91% of cases, the Org at ground level was higher than that at 260 m. Sulfate (SO4) and chloride had more regional transport characteristics. Nitrate (NO3) and Org were more enriched at ground level, as the online molar ratio of NO3 to SO4 and the mass ratio of Org to SNA (sulfate–nitrate–ammonium) were higher at ground level. Among all the five NR-PM1 chemical species, ammonium nitrate was the species with the most uniform vertical distribution. Pollution events in spring in urban Beijing were accompanied by a gradual increase in ground RH from 10% to more than 35%. Strong winds from the southwest polluted region could also maintain the NR-PM1 at a high level. The polluted air masses transported from the south accounted for 39%, which was about four times that of the clean air masses from the northwest (11%). Thus, atmospheric circulation in spring is generally conducive to the formation of pollution. The estimated dilution efficiency on NR-PM1 chemical species influenced by boundary layer variation in spring showed that, when the mix layer was elevated the dilution effect at higher altitude was stronger than that at ground level.

Multi-level CO2 fluxes over Beijing megacity with the eddy covariance method

Based on five years of eddy covariance measurements at multiple levels (47, 140, and 280 m) of Beijing's 325-m meteorological tower, the exchange process of CO2 fluxes between the atmosphere and urban surface were investigated. As a result of the total vehicle control policy from 2011 in Beijing, the growth rate of annual total CO2 flux at 140 m is 7.8% from 2008–2010 but 2.3% from 2010–2012. With the minimum vegetation cover and largest population density, the 5-yr average annual total CO2 flux at 140 m is largest (6.41 kg C m−2 yr−1), compared with that at 47 m (5.78 kg C m−2 yr−1) and 280 m (3.99 kg C m−2 yr−1). With regards to annual total CO2 fluxes in Beijing, vehicle numbers and population are the main controlling factors. The measured CO2 fluxes were highly dependent on land cover/use in the prevailing wind direction. The CO2 fluxes at three layers all correlated positively with road fraction, with the R2 values being 0.69, 0.57, and 0.54 (P < 0.05), respectively. The decreasing fraction of vegetation caused an increasing of the annual total CO2 flux, and there was an exponential relationship between them. The annual total CO2 fluxes were larger with higher population density.摘要本文基于北京325米气象塔在47, 140, 和280米三层高度的5年涡动相关观测资料, 研究了城市下垫面与大气间的CO2交换过程.由于北京市2011年开始实行工作日汽车尾号限行, 140米高度CO2通量的年增长率由2008–2010年的7.8%降低到2010–2012年的2.3%.140米高度通量源区内植被比例最小且人口密度最大, 因此140米高度的5年平均CO2通量年总量)6.41 kg C m−2 yr−1(大于47米)5.78 kg C m−2 yr−1(和280米)3.99 kg C m−2 yr−1(.在年尺度上, 北京汽车总保有量和总人口是最重要的CO2通量控制因子.CO2通量随风向的变化主要与风向对应的通量源区内下垫面土地利用方式有关.三层高度的夏季CO2通量均与道路的比例呈正相关关系.47, 140, 和280米的决定系数分别为0.69, 0.57, 和0.54 (P<0.05).植被比例的下降, 会导致CO2年总量上升, 两者存在近似于指数的关系.城市人口密度的上升会引起CO2年总量上升.

On the scaling law of ramp structures in scalar turbulence

Ramp structures widely exist in scalar turbulence, such as temperature, water vapor, and carbon dioxide (CO2), which refer to the phenomenon that the physical quantity increases slowly with time and then suddenly drops. ramp structures lead to large gradients on a small scale and result in intermittency and anisotropy of turbulent flows. in this paper, wavelet analysis is used to analyze observed data from the beijing 325-m meteorological tower to extract ramp structures in temperature, water vapor, and CO2 signals. ramp structures in CO2 signals are different from those in temperature and water vapor in terms of the averaged temporal scale and normalized amplitude, and the ramp duration almost equals the cliff duration, which means ramp structures in CO2 signals are not easy to generate and different physical mechanisms may exist. in addition, both the ascending and descending part of ramp structures are linearly fitted. it is found that a scaling law exists between the slope and duration in the ascending part in the three scalar signals. the corresponding power exponents are slightly different. furthermore, the same rule exists in the descending part of ramp structures, which indicates that self-similarity may be a universal law in scalar turbulence. moreover, the maxima of selected ramp structures show the same pattern, i.e. there are ramp structures in the maximum sequence, which proves that small-scale ramp structures are superimposed on large-scale ramp structures.

Characteristics and sources of submicron aerosols above the urban canopy (260 m) in Beijing, China, during the 2014 APEC summit

Abstract. The megacity of Beijing has experienced frequent severe fine particle pollution during the last decade. Although the sources and formation mechanisms of aerosol particles have been extensively investigated on the basis of ground measurements, real-time characterization of aerosol particle composition and sources above the urban canopy in Beijing is rare. In this study, we conducted real-time measurements of non-refractory submicron aerosol (NR-PM1) composition at 260 m at the Beijing 325 m meteorological tower (BMT) from 10 October to 12 November 2014, by using an aerosol chemical speciation monitor (ACSM) along with synchronous measurements of size-resolved NR-PM1 composition near ground level using a high-resolution time-of-flight aerosol mass spectrometer (HR–ToF–AMS). The NR-PM1 composition above the urban canopy was dominated by organics (46 %), followed by nitrate (27 %) and sulfate (13 %). The high contribution of nitrate and high NO3− / SO42− mass ratios illustrates an important role of nitrate in particulate matter (PM) pollution during the study period. The organic aerosol (OA) was mainly composed of secondary OA (SOA), accounting for 61 % on an average. Different from that measured at the ground site, primary OA (POA) correlated moderately with SOA, likely suggesting a high contribution from regional transport above the urban canopy. The Asia–Pacific Economic Cooperation (APEC) summit with strict emission controls provides a unique opportunity to study the impacts of emission controls on aerosol chemistry. All aerosol species were shown to have significant decreases of 40–80 % during APEC from those measured before APEC, suggesting that emission controls over regional scales substantially reduced PM levels. However, the bulk aerosol composition was relatively similar before and during APEC as a result of synergetic controls of aerosol precursors. In addition to emission controls, the routine circulations of mountain–valley breezes were also found to play an important role in alleviating PM levels and achieving the "APEC blue" effect. The evolution of vertical differences between 260 m and the ground level was also investigated. Our results show complex vertical differences during the formation and evolution of severe haze episodes that are closely related to aerosol sources and boundary-layer dynamics.

Gustiness and coherent structure under weak wind period in atmospheric boundary layer

Statistical analysis of turbulent and gusty characteristics in the atmospheric boundary layer under weak wind period has been carried out. The data used in the analysis were from the multilevel ultrasonic anemometer-thermometers at 47m, 120m, and 280m levels on Beijing 325m meteorological tower. The time series of 3D atmospheric velocity were analyzed by using conventional Fourier spectral analysis and decompose into three parts: basic mean flow (period > 10 min), gusty disturbances (1 min < period < 10 min) and turbulence fluctuations (period < 1 min). The results show that under weak mean wind condition: 1) the gusty disturbances are the most strong fluctuations, contribute about 60% kinetic energy of eddy kinetic energy and 80% downward flux of momentum, although both the eddy kinetic energy and momentum transport are small in comparison with those in strong mean wind condition; 2) the gusty wind disturbances are anisotropic; 3) the gusty wind disturbances have obviously coherent structure, and their horizontal and vertical component are negatively correlated and make downward transport of momentum more effectively; 4) the friction velocities related to turbulence and gusty wind are approximately constant with height in the surface layer.

Effects of Coordinate Rotation on Turbulent Flux Measurements during Wintertime Haze Pollution in Beijing, China

AbstractEddy-covariance observations from the Beijing 325-m meteorological tower are used to evaluate the effects of coordinate rotation on the turbulent exchange of momentum and scalars during wintertime haze pollution (January–February 2013). Two techniques are used in the present evaluation; namely, the natural wind coordinate (NWC) and the planar fit coordinate (PFC), with the latter being applied by means of two methods for linear regression (i.e., overall and sector-wise). The different techniques show a general agreement in both turbulent fluxes and transport efficiencies, especially evident at the lower, 140-m level above the ground (compared to the higher, 280-m level), perhaps implying that the selection of a technique for coordinate rotation (NWC or PFC) is less of a concern for a sufficiently low level, despite the complexities of urban terrain. Additionally, sector-wise regression is a recommended approach for practical application of the PFC in a complex urban environment subjected to partic...

Averaging period effects on the turbulent flux and transport efficiency during haze pollution in Beijing, China

Based on observations at the heights of 140 and 280 m on the Beijing 325-m meteorological tower, this study presents an assessment of the averaging period effects on eddy-covariance measurements of the momentum/scalar flux and transport efficiency during wintertime haze pollution. The study period, namely from January 6 to February 28 2013, is divided into different episodes of particulate pollution, as featured by varied amounts of the turbulent exchange and conditions of the atmospheric stability. Overall, turbulent fluxes of the momentum and scalars (heat, water vapor, and CO2) increase with the averaging period, namely from 5, 15, and 30 up to 60 min, an outcome most evident during the ‘transient’ episodes (each lasting for 2–3 days, i.e., preceded and followed by clean-air days with mean concentrations of PM1 less than 40 μg m−3). The conventional choice of 30 min is deemed to be appropriate for calculating the momentum flux and its transport efficiency. By comparison, scalar fluxes and their transport efficiencies appear more sensitive to the choice of an averaging period, particularly at the upper level (i.e., 280 m). It is presupposed that, for urban environments, calculating the momentum and scalar fluxes could invoke separate averaging periods, rather than relying on a single prescription (e.g., 30 min). Furthermore, certain characteristics of urban turbulence are found less sensitive to the choice of an averaging period, such as the relationship between the heat-to-momentum transport efficiency and the local stability parameter.

Effects of Coordinate Rotation on Turbulent Flux Measurements during Wintertime Haze Pollution in Beijing, China

Eddy-covariance observations from the Beijing 325-m meteorological tower are used to evaluate the effects of coordinate rotation on the turbulent exchange of momentum and scalars during wintertime haze pollution(January-February 2013). Two techniques are used in the present evaluation; namely, the natural wind coordinate(NWC) and the planar fit coordinate(PFC), with the latter being applied by means of two methods for linear regression(i.e., overall and sector-wise). The different techniques show a general agreement in both turbulent fluxes and transport efficiencies, especially evident at the lower, 140-m level above the ground(compared to the higher, 280-m level), perhaps implying that the selection of a technique for coordinate rotation(NWC or PFC) is less of a concern for a sufficiently low level, despite the complexities of urban terrain. Additionally, sector-wise regression is a recommended approach for practical application of the PFC in a complex urban environment subjected to particulate pollution, because this method is found to produce a better correlation between the mean vertical velocity at the 140- and 280-m heights.

Urban Boundary-Layer Stability and Turbulent Exchange during Consecutive Episodes of Particle Air Pollution in Beijing, China

Based on measurements at the Beijing 325-m Meteorological Tower,this study reports an analysis of atmospheric stability conditions and turbulent exchange during consecutive episodes of particle air pollution in Beijing(China),primarily due to haze and dust events(15–30 April 2012).Of particular interest were relevant vertical variations within the lower urban boundary layer(UBL).First,the haze and dust events were characterized by different atmospheric conditions,as quite low wind speed and high humidity are typically observed during haze events.In addition,for the description of stability conditions,the bulk Richardson number(RiB) was calculated for three different height intervals: 8–47,47–140,and 140–280 m.The values of RiB indicated an apparent increase in the occurrence frequency of stably-stratified air layers in the upper height interval—for the 140–280-m height interval,positive values of RiB occurred for about 85% of the time.The downward turbulent exchange of sensible heat was observed at 280 m for the full diurnal cycle,which,by contrast,was rarely seen at 140 m during daytime.These results reinforce the importance of implementing high-resolution UBL profile observations and addressing issues related to stably-stratified flows.

Four-year (2006–2009) eddy covariance measurements of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; flux over an urban area in Beijing

Abstract. Long-term measurements of carbon dioxide flux (Fc) and the latent and sensible heat fluxes were performed using the eddy covariance (EC) method in Beijing, China over a 4-yr period in 2006–2009. The EC setup was installed at a height of 47 m on the Beijing 325-m meteorological tower in the northwest part of the city. Latent heat flux dominated the energy exchange between the urban surface and the atmosphere in summer, while sensible heat flux was the main component in the spring. Winter and autumn were two transition periods of the turbulent fluxes. The source area of Fc was highly heterogeneous, which consisted of buildings, parks, and highways. It was of interest to study of the temporal and spatial variability of Fc in this urban environment of a developing country. Both on diurnal and monthly scale, the urban surface acted as a net source for CO2 and downward fluxes were only occasionally observed. The diurnal pattern of Fc showed dependence on traffic and the typical two peak traffic patterns appeared in the diurnal cycle. Also Fc was higher on weekdays than on weekends due to the higher traffic volumes on weekdays. On seasonal scale, Fc was generally higher in winter than during other seasons likely due to domestic heating during colder months. Total annual average CO2 emissions from the neighborhood of the tower were estimated to be 4.90 kg C m−2 yr−1 over the 4-yr period. Total vehicle population was the most important factor controlling the inter-annual variability of Fc in this urban area.

Levels and Vertical Distributions of PCBs, PBDEs, and OCPs in the Atmospheric Boundary Layer: Observation from the Beijing 325-m Meteorological Tower

Polyurethane foam disk passive air sampling was carried out to investigate the levels, vertical distributions, and potential sources of polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and organochlorine pesticides (OCPs) in the atmospheric boundary layer of an urban site in Asia. Sampling was performed at nine heights (15, 47, 80, 120, 160, 200, 240, 280, 320 m) of the 325-m meteorological tower in Beijing, China over three 2-month periods between December 2006 and August 2007. This is the first study to report vertical variations of PBDEs in the ABL and one of only a few studies to investigate vertical distributions of persistent organic pollutants. The levels of sigma19PCBs and sigma8PBDEs were relatively low, ranging from 22 to 65 and from 2.3 to 18 pg m(-3), respectively. Air concentrations of gamma-HCH were high, with values in the range of 39-103 pg m(-3) in winter, 100-180 pg m(-3) in spring, and 115-242 pg m(-3) in summer, respectively. alpha-HCH concentrations ranged from 20 to 86 pg m(-3), p,p'-DDT between 7.3 and 78 pg m(-3), and HCB between 15 and 160 pg m(-3). The seasonal variations of PCBs, PBDEs, and OCPs may reflect different sources for these chemicals, such as those related with regional use (gamma-HCH), volatilization/re-emission (PBDEs, PCBs, alpha-HCH), and pesticide impurities (HCB). Although the performance reference compounds (PRCs) were spiked before deployment, the sampling rates showed strong dependency on wind speeds, resulting in large variations in uptake rates in the ABL, ranging from approximately 7.0 m3 day(-1) at ground level to 11 m3 day(-1) at 320 m. Levels of PCBs, PBDEs, and OCPs decreased with increasing ABL height indicating the potential of Beijing as the local sources.

Relationship between turbulent flux and variance in the urban canopy

The relationship between fluxes and variances has been widely studied in the frame of the similarity theory. However, the dynamics of urban morphology should be highlighted in the urban canopy. Therefore, the relationship between fluxes and variances of turbulent quantities (wind velocity, temperature, carbon dioxide, and water vapor concentration) without stability parameters has been investigated with data collected from the Beijing 325-m meteorological tower in the urban canopy. Though the similarity theory is available for the statistics of wind velocity and temperature, urban land use plays an important role in the linear relationship between momentum fluxes and velocity variances, while the linear relationship between sensible heat fluxes and temperature variances is influenced at large by stratification. As for carbon dioxide and water vapor, the linear relationship between their fluxes and variances is a better way compared to the similarity theory to describe their motion, though the plots are a little scattered owing to their fugitive source.

A primary study of the variations of vertical radiation with the Beijing 325-m Meteorological Tower

The Beijing 325-m Meteorological Tower (325MT) is used to observe the vertical variation of solar radiation. Results of the experiments indicate that the automatic radiation monitoring system, including a sun tracker and data collection system, works well and all the specifications meet WMO observation standards. The measurement data show that there is a significant radiation decrease from 320 m to the surface, where the difference is only about 30 W m−2 on light air-pollution days, while the maximum reaches about 110 W m−2 when heavy pollution appears near the ground. The global UV radiation decreases on heavy air-pollution days and under poor visibility conditions, and the difference between 300 m and 8 m is larger than on clear days.