325-m Meteorological Tower Research Articles

Analysis of urban turbulence intensity observed by Beijing 325-m tower and comparison with the IEC turbulence model for small wind turbines

The accurate assessment of turbulence intensity is crucial for the effective utilization of urban wind energy. However, the Normal Turbulence Model (NTM) prescribed by the IEC standard has not been adequately evaluated for its applicability in urban environments. To address this gap, we analyze one-year continuous turbulence observations collected at multiple levels on a 325-m meteorological tower in Beijing. The results show that: the turbulence intensity follows the Johnsonsb distribution instead of the Normal distribution in the IEC standard; it has significant diurnal variation and piecewise power-law vertical variation; the NTM underestimates turbulence intensity at most times and heights. A new turbulence intensity model with an exponential form is proposed, which significantly outperforms the NTM. Further, the exponential model fits the observations better than power-law model that outperforms the NTM. In terms of the fitting errors to the measurements, the full-year, monthly, and hourly RMSE (MAE) of this model are reduced on average by 51.81% (54.31%), 33.04% (34.38%) and 29.46% (28.24%) compared to that of the power-law model. These findings will contribute to the revision of the IEC standards governing the safe operation of small wind turbines in complex wind environments such as urban areas.

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.

Variability of turbulence dispersion characteristics during heavy haze process: A case study in Beijing

The turbulent standard deviations and the turbulent third-order and fourth-order moments are the key turbulence dispersion parameters in Lagrangian dispersion models. However, the characteristics of these parameters under heavy haze conditions in urban areas have not been fully investigated, and the commonly used similarity relations of these parameters in models were based on observations in highly flat and sparsely populated areas. In this paper, the vertical profiles of these parameters and their local similarity relations under heavy haze conditions in the wintertime of Beijing have been analyzed by using data collected at a 325-m meteorological tower. The heavy haze process has been divided into three stages: transport stage (TS), cumulative stage (CS), and dispersion stage (DS). Results show that the turbulent dispersion parameters behave differently during three stages. In the TS and DS, the maxima appear in the profiles of the turbulent standard deviations above the urban canopy; in the CS, the turbulent standard deviation are almost constant with height. The analysis of the third and fourth order moments shows that the wind velocities above the urban canopy in the TS deviate from the Gaussian distribution more significantly than those in the CS and DS. The local similarity relations of the turbulent dispersion parameters in the TS, especially for the longitudinal wind components, are normally different from those in the CS and DS. Thus, different from the common assumptions in Lagrangian models, the turbulence dispersion in horizontal directions is anisotropic and should be parameterized by multiple similarity relations under heavy haze conditions.

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.

Elucidating roles of near-surface vertical layer structure in different stages of PM2.5 pollution episodes over urban Beijing during 2004–2016

Local weather conditions play important roles in surface fine particulate matter (PM2.5) pollution episodes in metropolitan areas. Relative to surface meteorological factors, the roles of near-surface vertical structure of urban boundary layer in air pollution has yet to be fully understood, due to limitation of vertical observation. By using surface PM2.5 observations and multi-layer (15 levels) vertical meteorological elements from a 325-m meteorological tower during 2004–2016, we investigated the different stages of 456 PM2.5 Pollution Episodes (PPEs) and their associations with vertical urban boundary layer structure in Beijing. The average PM2.5 concentration during PPEs is 141.2 μg m−3, twice the 13-year average (70.3 μg m−3). PPEs are more frequent and severe in autumn and winter, showing significant seasonal variations. For vertical wind, southerly (southwesterly) winds prevailed from 80 m to 320 m in the early stage of pollution episodes, transporting pollutants to Beijing. While strong northerly (northwesterly) winds prevailed in near-surface layer in the dissipation stages (DSs) and after PPEs, effectively removing pollutants. Owing to different thermal dynamic conditions, the starting hours of PPEs are various, showing that PPEs with stable atmospheric stratification occur most frequently at 1800 LST. Particularly, for long-lasting PPEs, shortwave (longwave) radiation at near-surface layer is weakened (enhanced) due to the existence of clouds and pollutant aerosols, attenuating the momentum flux and sensible heat flux, which further aggravates the accumulation of pollutants. Our findings provide a better understanding of the physical formation mechanism of high concentrations of the PM2.5 pollution episodes and may aid in designing effective mitigation strategies for the urban environment and health.

Diurnal Evolution of the Wintertime Boundary Layer in Urban Beijing, China: Insights from Doppler Lidar and a 325-m Meteorological Tower

The diurnal evolution of the atmospheric boundary layer—the lowermost part of the atmosphere where the majority of human activity and meteorological phenomena take place—is described by its depth. Additionally, the boundary layer height (BLH) and the turbulence intensity strongly impact the pollutant diffusion, especially during transition periods. Based on integrated observations from a 325-m meteorological tower and a Doppler Wind lidar in the center of Beijing, the entire diurnal cycle of urban BLH in December 2016 was characterized. Results highlight that the Doppler lidar exhibited it is well suited for monitoring convective BLH while it trudges in monitoring stable BLH, while a 325-m meteorological tower provided an important supplement for Doppler lidar under nocturnal boundary layer and heavily polluted conditions. For the diurnal cycle, under light wind condition, the pattern of urban BLH was largely modulated by thermal forcing of solar radiation and may partly be affected by wind speed. While under strong wind condition, the pattern of urban BLH was largely modulated both by thermal forcing and dynamical forcing. The present work also presented evidence for several new features in the morning and afternoon transitions of the urban boundary layer, showing the duration of the morning transition varied between 1 and 5 h, with the largest value occurring under weak wind with high PM2.5 concentration; while the afternoon transition ranged from 3 to 6 h, which was positively (negatively) correlated to wind speed (PM2.5 concentration). Our work highlights that weak wind speed (weak dynamic motion) and heavy aerosol pollution (weak thermal forcing due to the effect of cooling) can dramatically affect the evolution of the boundary layer.

Persistent Nonagricultural and Periodic Agricultural Emissions Dominate Sources of Ammonia in Urban Beijing: Evidence from 15N Stable Isotope in Vertical Profiles.

Ammonia (NH3) emission reduction is key to limiting the deadly PM2.5 pollution globally. However, studies of long-term source apportionment of vertical NH3 are relatively limited. On the basis of the one-year measurements of weekly vertical profiles of δ15N-NH3 at 5 heights (2, 15, 102, 180, and 320 m) on a 325-m meteorological tower in urban Beijing, we found that vertical profiles of NH3 concentrations generally remained stable with height. δ15N-NH3 increased obviously as a function of height in cold seasons (with heating) and decreased in warm seasons (with fertilization), indicating a stronger human-induced seasonal variation via regional transport at higher altitudes. Relatively stable δ15N-NH3 near the ground surface suggested the strong local emission. The results of isotopic mixing model (SIAR) indicate that source apportionment using measured δ15N-NH3 only would overestimate the contribution of agricultural emissions to NH3. By using an estimation of initial δ15N-NH3, we found that nonagricultural sources contributed ∼72% of NH3 on average. Our study suggests that (i) both persistent nonagricultural and periodic agricultural emissions drive atmospheric NH3 concentration and its vertical distribution in urban Beijing; and (ii) source apportionment based on measured δ15N-NH3 only likely underestimates fossil fuel source contribution, if the combined NHx isotope effects are not considered.

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.

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.

Characteristics of the Drag Coefficient in the Roughness Sublayer over a Complex Urban Surface

The statistics of momentum exchange in the urban roughness sublayer are investigated. The analysis focuses on the characteristics of the dimensionless friction velocity, $${u_{*}}/U$$ , which is defined as the square root of the drag coefficient. The turbulence observations were made at a height of 47 m above the ground on the 325-m meteorological tower, which is located in a very inhomogeneous urban area in Beijing. Under neutral conditions, the dependence of the drag coefficient on wind speed varies with wind direction. When the airflow is from the area of densely built-up buildings, the drag coefficient does not vary with wind speed, while when the airflow is from the area covered by vegetation, the drag coefficient appears to decrease with increasing wind speed. Also, the drag coefficient does not vary monotonically with the atmospheric stability. Both increasing stability and increasing instability lead to the decrease of the drag coefficient, implying that the roughness length and zero-plane displacement may vary in urban areas.

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.

Validation of the Institute of Atmospheric Physics emergency response model with the meteorological towers measurements and SF6 diffusion and pool fire experiments

The urban canopy layer parameterization (UCP), a successive bias correction method (SBC), an atmospheric dispersion module for denser-than-air releases, and the emission intensity of chemicals monitored by a Fourier-transform-infrared remote sensor (EM27) were incorporated into the Institute of Atmospheric Physics emergency response model (IAPERM). IAPERM's performance was tested in Beijing using the field data collected from a 325-m meteorological tower and sulfur hexafluoride (SF6) diffusion and pool fire experiments. The results show that the IAPERM simulations of the vertical wind speeds in the urban canopy layer (UCL) with the UCP perform much better than those with the Monin–Obukhov similarity parameterization scheme. The IAPERM forecasts for air temperature and relative humidity are more accurate than those for wind speed and direction, which require correction. When the SBC with the local terrain effect is adopted, the wind speed and direction and the maximum concentrations of black carbon near the ground are well forecasted. IAPERM reproduces the spatial distributions of the SF6 observations more accurately near the release source (≤500 m) than at locations far away from the release source with the use of the observed meteorological parameters. These results suggest that IAPERM could be a promising tool for passive and dense gas diffusion simulations or forecasts.

Vertical ozone characteristics in urban boundary layer in Beijing

Vertical ozone and meteorological parameters were measured by tethered balloon in the boundary layer in the summer of 2009 in Beijing, China. A total of 77 tethersonde soundings were taken during the 27-day campaign. The surface ozone concentrations measured by ozonesondes and TEI 49C showed good agreement, albeit with temporal difference between the two instruments. Two case studies of nocturnal secondary ozone maxima are discussed in detail. The development of the low-level jet played a critical role leading to the observed ozone peak concentrations in nocturnal boundary layer (NBL). The maximum of surface ozone was 161.7ppbv during the campaign, which could be attributed to abundant precursors storage near surface layer at nighttime. Vertical distribution of ozone was also measured utilizing conventional continuous analyzers on 325-m meteorological observation tower. The results showed the NBL height was between 47 and 280m, which were consistent with the balloon data. Southerly air flow could bring ozone-rich air to Beijing, and the ozone concentrations exceeded the China's hourly ozone standard (approximately 100ppb) above 600m for more than 12h.

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.

Impact of vertical atmospheric structure on Beijing aerosol distribution

In August 2004, original informations on Beijing aerosol layers were obtained downtown from experiments achieved during clear and polluted days along the IAP 325-m meteorological tower and a micro-pulse LIDAR operating at BMEMC. Beijing urban boundary layer (UBL) is seen to follow the expected diurnal pattern with low height values at night (80±50 m) and much higher values (up to 3000 m) at daytime. An original process of temperature profiles is shown to closely reconstruct the UBL and sub-layer evolution. UBL vertical extension becomes significant after a short transition period at 10:00 during which at all the tower levels, wind speed increases and wind direction changes dramatically from the synoptic northerly flow to almost the opposite direction (SSW). Additional data from background stations show pollution episodes that may be related to horizontal transport at low altitude during the morning or evening transition periods. Vertical profiles of black carbon (BC) and particle number concentration (N) and LIDAR signal evidence the formation of accumulation layers at 60 m and 90 m probably related to the urban canopy. Using the N/BC ratio, an important chemical reactivity may be observed within the accumulation layers and is still distinguishable during cloudy weather. Reactivity in the vertical column appears to increase with altitude, which might point to the significance of solar radiation shielding by particles. Chemical analyses confirm, in the morning, the presence of a very distinct layer at the top of the tower and show, in the afternoon, a significant mixing (17%) between the UBL and the regional flow above.