Lidar and Sun photometer observations of atmospheric boundary-layer characteristics over an urban area in a mountain valley

We present measurements of the vertical aerosol structure and the aerosol optical depth in the lower troposphere performed above the city of Sofia (an urban area situated in a mountain valley), western Bulgaria by means of a ground-based aerosol lidar operating continuously for a number of years. The lidar measurements were accompanied by measurements of the aerosol optical depth (AOD) in the visible and near infrared regions of the spectrum performed in October 2004 using Microtops II radiometers. The maximum values of the AOD were found to occur 1–2 h before the complete development of the atmospheric boundary layer, i.e. during the residual layer destruction, which confirms our hypothesis concerning the slope circulation effect on the processes taking place in the atmospheric boundary layer. The AOD values obtained by the lidar are lower than those taken by the sun photometer. Further, the AOD exhibits two different types of behaviour. In the case of a ‘clear atmosphere’ (i.e. in the absence of volcanic eruptions and/or dust transport from the Sahara) most of the aerosol accumulated within the atmospheric boundary layer over the urban area considered. The combined use of the two instruments allows the comparison between the optical characteristics of the atmospheric aerosol (e.g. aerosol extinction coefficient, etc.) obtained by the lidar and through an independent method (sun photometer).     

A Case Study of CO<Subscript>2</Subscript>, CO and Particles Content Evolution in the Suburban Atmospheric Boundary Layer Using a 2-μm Doppler DIAL,a 1-μm Backscatter Lidar and an Array of In-situ Sensors

A network of remote and in-situ sensors was deployed in a Paris suburb in order to evaluate the mesoscale evolution of the daily cycle of CO₂ and related tracers in the atmospheric boundary layer (ABL) and its relation to ABL dynamics and nearby natural and anthropogenic sources and sinks. A 2-μm heterodyne Doppler differential absorption lidar, which combines measurements of, (1) structure of the atmosphere, (2) radial velocity, and (3) CO₂ differential absorption was a particularly unique element of the observational array. We analyse the differences in the diurnal cycle of CO, CO₂, lidar reflectivity (a proxy for aerosol content) and H₂O using the lidar, airborne measurements in the free troposphere and ground-based measurements made at two sites located few kilometres apart. We demonstrate that vertical mixing dominates the early morning drawdown of CO and aerosol content trapped in the former nocturnal layer but not the H₂O and CO₂ mixing ratio variations. Surface fluxes, vertical mixing and advection all contribute to the ABL CO₂ mixing ratio decrease during the morning transition, with the relative importance depending on the rate and timing of ABL rise. We also show evidence that when the ABL is stable, small-scale (0.1-km vertical and 1-km horizontal) gradients of CO₂ and CO are large. The results illustrate the complexity of inferring surface fluxes of CO₂ from atmospheric budgets in the stable boundary layer.     

TROPOSPHERIC WATER VAPOR OBSERVATIONS USING A RAMAN LIDAR OVER HEFEI*

This paper presents a Raman lidar for measuring tropospheric water vapor profiles over Hefei(31.9°N,117.17°E),China.Intercomparisons of water vapor mixing ratio obtained by this Raman lidar and GZZ-59 type radiosonde observations show the good agreements when relative humidity is higher than 20%.Typical vertical profiles and seasonal variations of water vapor mixing ratio distribution are reported.Many observation eases indicate that the high moisture layer corresponds to large aerosol scattering ratios in the lower troposphere.     

Atmospheric Aerosol Characterization Over Naples During 2000–2003 EARLINET Project: Planetary Boundary-Layer Evolution and Layering

The evolution of the planetary boundary layer and the influence of local circulation phenomena over Naples (southern Italy, 40.838° N, 14.183° E, 118 m above sea level) have been studied by systematic lidar measurements of aerosol optical properties and vertical distributions carried out from May 2000 to August 2003, in the course of the EARLINET project. In particular, our data show the development of aerosol layers typically located in the range between 1,000 and 2,300 m, with variable thickness. The optical properties of the observed layers have been determined. In order to analyse the evolution of the planetary boundary layer, detailed observations of complete diurnal cycles have also been performed. The analysis of lidar measurements of vertical profiles of wind speed and wind direction and air mass back-trajectories allowed us to characterize the sea-breeze circulation influence on both the planetary boundary-layer evolution and the observed aerosol vertical distribution.     

基于激光雷达资料的气溶胶辐射效应研究

利用新型激光雷达气溶胶探测资料及综合数值模式,以地形复杂的兰州市及周边地区冬季典型天气形势下的大气边界层为研究对象,通过理想试验模拟研究了城市气溶胶辐射效应与大气边界层的相互作用。结果表明:夜间,低空(50~600 m)气溶胶所在气层冷却效应明显,温度降低0.13~0.18℃,600 m高度以上,气溶胶浓度较低,其冷却效应较小,温度降低不足0.1℃;白天,受气溶胶短波辐射效应影响,边界层内增温明显,增温最大值位于低层脱地逆温层顶300 m高度附近,600 m以上由于气溶胶浓度减小,加热率亦降低,增温由0.2℃减至0.1℃。此外,气溶胶的存在使得所在层的风速降低。可见,激光雷达探测资料在边界层模式中有很好的应用价值,对于研究气溶胶辐射效应的大气边界层响应有重要意义。     

BOUNDARY LAYER GROWTH AND LAPSE RATE CHANGES DETERMINED BY LIDAR AND SURFACE HEAT FLUX IN SOFIA*

In this study the results from a boundary layer experiment,conducted in autumn 1991 over a flat,build-up urban area in Southeast Sofia,together with some models for mixed layer growth rates are used to investigate the layered structure of the vertical atmospheric stability distribution in the Sofia Valley.Lidar measurements of aerosol layer heights and morning boundary layer development are combined with surface eddy correlation measurements of kinematic heat and moisture fluxes,profiles of temperature and humidity,wind speed and wind direction.A diagnostic method is presented for determining vertical lapse rates using surface meteorological measurements and lidar returns observed during the transition from nighttime stable stratification to daytime convective boundary layer after the sunrise.     

BOUNDARY LAYER GROWTH AND LAPSE RATE CHANGES DETERMINED BY LIDAR AND SURFACE HEAT FLUX IN SOFIA

In this study the results from a boundary layer experiment,conducted in autumn 1991 over a flat,build-up urbanarea in Southeast Sofia,together with some models for mixed layer growth rates are used to investigate the layered struc-ture of the vertical atmospheric stability distribution in the Sofia Valley.Lidar measurements of aerosol layer heightsand morning boundary layer development are combined with surface eddy correlation measurements of kinematic heatand moisture fluxes,profiles of temperature and humidity,wind speed and wind direction.A diagnostic method is pres-ented for determining vertical lapse rates using surface meteorological measurements and lidar returns observed duringthe transition from nighttime stable stratification to daytime convective boundary layer after the sunrise.     

Atmospheric boundary-layer structure observed during a haze event due to forest-fire smoke

During a haze event in Baltimore, U.S.A. from July 6 to 8, 2002, smoke from forest fires in the Québec region (Canada), degraded air quality and impacted upon local climate, decreasing solar radiation and air temperature. The smoke particles in and above the atmospheric boundary layer (ABL) served as a tracer and provided a unique opportunity to investigate the ABL structure, especially entrainment. Elastic backscatter lidar measurements taken during the haze event distinctly reveal the downward sweeps (or wisps) of smoke-laden air from the free atmosphere into the ABL. Visualisations of mechanisms such as dry convection, the entrainment process, detrainment, coherent entrainment structures, and mixing inside the ABL, are presented. Thermals overshooting at the ABL top are shown to create disturbances in the form of gravity waves in the free atmosphere aloft, as evidenced by a corresponding ripple structure at the bottom of the smoke layer. Lidar data, aerosol ground-based measurements and supporting meteorological data are used to link free atmosphere, mixed-layer and ground-level aerosols. During the peak period of the haze event (July 7, 2002), the correlation between time series of elastic backscatter lidar data within the mixed layer and the scattering coefficient from a nephelometer at ground level was found to be high (R=0.96 for z =324 m, and R=0.89 for z=504 m). Ground-level aerosol concentration was at a maximum about 2 h after the smoke layer intersected with the growing ABL, confirming that the wisps do not initially reach the ground.     

Evaluation of Mixing-Height Retrievals from Automatic Profiling Lidars and Ceilometers in View of Future Integrated Networks in Europe

The determination of the depth of daytime and nighttime mixing layers must be known very accurately to relate boundary-layer concentrations of gases or particles to upstream fluxes. The mixing-height is parametrized in numerical weather prediction models, so improving the determination of the mixing height will improve the quality of the estimated gas and particle budgets. Datasets of mixing-height diurnal cycles with high temporal and spatial resolutions are sought by various end users. Lidars and ceilometers provide vertical profiles of backscatter from aerosol particles. As aerosols are predominantly concentrated in the mixing layer, lidar backscatter profiles can be used to trace the depth of the mixing layer. Large numbers of automatic profiling lidars and ceilometers are deployed by meteorological services and other agencies in several European countries providing systems to monitor the mixing height on temporal and spatial scales of unprecedented density. We investigate limitations and capabilities of existing mixing height retrieval algorithms by applying five different retrieval techniques to three different lidars and ceilometers deployed during two 1-month campaigns. We studied three important steps in the mixing height retrieval process, namely the lidar/ceilometer pre-processing to reach sufficient signal-to-noise ratio, gradient detection techniques to find the significant aerosol gradients, and finally quality control and layer attribution to identify the actual mixing height from multiple possible layer detections. We found that layer attribution is by far the most uncertain step. We tested different gradient detection techniques, and found no evidence that the first derivative, wavelet transform, and two-dimensional derivative techniques have different skills to detect one or multiple significant aerosol gradients from lidar and ceilometer attenuated backscatter. However, our study shows that, when mixing height retrievals from a ultraviolet lidar and a near-infrared ceilometer agreed, they were 25?C40% more likely to agree with an independent radiosonde mixing height retrieval than when each lidar or ceilometer was used alone. Furthermore, we point to directions that may assist the layer attribution step, for instance using commonly available surface measurements of radiation and temperature to derive surface sensible heat fluxes as a proxy for the intensity of convective mixing. It is a worthwhile effort to pursue such studies so that within a few years automatic profiling lidar and ceilometer networks can be utilized efficiently to monitor mixing heights at the European scale.     

Nephelometer derived and directly measured aerosol optical depth of the atmospheric boundary layer

The aerosol optical depth of the atmospheric boundary layer was determined both from direct solar irradiance measurements and from vertical extrapolation of ground-based nephelometry, during a period with cloudless skies and high aerosol mass loadings in the Netherlands. The vertical profile of the aerosol was obtained from lidar measurements. From humidity controlled nephelometry at the ground and humidity profiles from soundings, the scattering aerosol extinction as a function of height was assessed. Integration of the extinction over the aerosol layer gave the aerosol optical depth of the atmospheric boundary layer. This optical depth at the narrow band of the nephelometer was translated to a spectrally integrated value, assuming an Angstrom wavelength exponent of 1.5, a typical value for The Netherlands.It was found that scattering by the boundary layer aerosol contributed on average 80% to the total atmospheric aerosol optical depth. The uncertainty in this value is estimated to be of the order of 13%. Ammonium nitrate dominated the light scattering. This is an anthropogenic aerosol component.The radiative forcing caused by the light scattering of the anthropogenic aerosol was calculated assuming an upward scattered fraction of 0.3. An average value of − 12 W m ⁻² was found (with an estimated uncertainty of 20%). This corresponds to an absolute increase in the planetary albedo of 0.03, which is equivalent to a 15% increase in the local planetary albedo of 0.2.     

Tropospheric water vapour soundings by lidar at high Arctic latitudes

Ground-based lidars can provide continuous observations of tropospheric humidity profiles using the Raman scattering of light by water vapour and nitrogen molecules. We will present specific humidity profiles obtained at the high Arctic location Ny-Ålesund (Spitsbergen, 79°N). Under nighttime conditions the observations cover a range from about 500 m altitude up to the upper troposphere. Daylight limits the observations to the lower troposphere, depending on atmospheric transmission and the water vapour content. In a case study on 29 January, simultaneous observations of humidity and aerosol extinction show distinct differences in the various altitudes during the advection of aerosol-rich air masses. In the boundary layer, the aerosol is less affected by the humidity. In the free troposphere, the lidar ratio was observed to be up to 60 sr with some evidence for the uptake of water vapour by the aerosol particles. In another case study from 28 February 2002, the influence of the mean wind direction and the orography on the water vapour concentration near the ground and in the free troposphere will be discussed. During wintertime, a humidity inversion up to about 1.5 km altitude with dry air near the ground has frequently been found with wind from the southeast. Such local effects and small-scale structures observed by stationary lidar mostly cannot be resolved by other sounding methods like passive satellite soundings.     

Peculiarities of seasonal variations of atmospheric aerosol concentration in the surface air of Moscow environs

The seasonal variations of the concentration of particles of different sizes in the atmospheric surface layer are studied on the basis of the data of daily measurements of atmospheric aerosol characteristics in the town of Dolgoprudny (20 km from the center of Moscow) carried out in 2006–2009. It is revealed that the steady variations of monthly mean aerosol concentration are observed within the particle diameter interval of 0.02–1 μm. The annual course of concentration of these particles has two maxima, in February-March and in September–October, and one minimum in June. The concentrations of particles with the size of 0.01–0.02 μm defined by the general atmospheric background and the concentrations of particles of >1 μm associated with the local sources do not have clearly pronounced seasonal variations. It is shown that the regularities of the annual concentration variations of particles with the size of 0.02–1 μm are mainly explained by the sign and value of the lapse rate in the layer up to 925 hPa that indicates the prevalence of the vertical mixing in the processes of aerosol scattering in the surface layer as compared with the horizontal transfer.     

复杂地形城市冬季边界层对气溶胶辐射效应的响应

作者着眼于城市气溶胶辐射效应与大气边界层的相互作用问题,针对地形复杂的兰州市及周边地区,开发应用了WRF(Weather Research and Forecasting,天气研究和预报)模式,使之与包含了大气气溶胶辐射效应和气溶胶粒子扩散的综合大气边界层数值模式嵌套起来.通过个例分析,揭示了冬季气溶胶辐射效应对边界层结构的定量影响.主要特征为夜间气溶胶的长波辐射效应使地面附近的气温增高,增温幅度为0.1～0.3 K/h,使低空(25～300 m)大气层冷却,降温幅度为0.08～0.15 K/h,风速在150 m以下减小;白天气溶胶的短波辐射效应使地面层内明显增温,1 h内升温约0.5 K,增温最大值在混合层顶500～600 m高度.受增温影响,垂直风场和水平风场随之调整,风速在450 m以下增大约0.1 m/s左右,而在450 m以上风速减小0.1 m/s左右.     

北京秋季一次降雪前污染天气的激光雷达观测研究

以2009年11月5~8日北京地区发生的一次特殊天气形势下的重污染天气过程为例,研究分析本次污染特点和大气边界层结构特征以及此天气过程的大气温度和相对湿度结构特点。激光雷达是探测大气边界层及气溶胶的一个高效工具,利用ALS300激光雷达系统测量信号,应用Fernald方法反演大气消光系数,根据反演的气溶胶消光系数的最大突变,即最大递减率的高度来确定大气边界层的高度。利用其观测的退偏比分析大气污染物特性。利用微波辐射计数据,确定大气温度和湿度时空特征。研究结果表明:在本次污染天气下,大气具有很强的逆温结构,逆温最大可达近1 K(100 m)-1,500 m以上的大气相对湿度很低,在这种天气特征下的大气边界层高度在400 m左右,非常稳定。污染结束降雪开始前,大气逆温结构消失,大气湿度大幅度增加,接近饱和。根据lidar(light detection and ranging)退偏比的分析,本次污染天气是一次典型的烟尘类颗粒物的污染,污染具有区域性特点。PM2.5(空气动力学当量直径小于等于2.5μm的颗粒物)与AOT(Aerosol Optical Thickness)之间有明显的线性关系,相关系数达到0.72。该lidar系统能够反演出秋季降雪前本次污染天气背景下北京城区上空的大气污染特性和大气边界层高度。     

Doppler Lidar Measurements of Vertical Velocity Spectra in the Convective Planetary Boundary Layer

We utilized a Doppler lidar to measure spectra of vertical velocity w from 390m above the surface to the top of the daytime convective boundary layer (CBL). The high resolution 2μm wavelength Doppler lidar developed by the NOAA Environmental Technology Laboratory was used to detect the mean radial velocity of aerosol particles. It operated continuously during the daytime in the zenith-pointing mode for several days in summer 1996 during the Lidars-in-Flat-Terrain experiment over level farmland in central Illinois, U.S.A. The temporal resolution of the lidar was about 1 s, and the range-gate resolution was about 30m. The vertical cross-sections were used to calculate spectra as a function of height with unprecedented vertical resolution throughout much of the CBL, and, in general, we find continuity of the spectral peaks throughout the depth of the CBL. We compare the observed spectra with previous formulations based on both measurements and numerical simulations, and discuss the considerable differences, both on an averaged and a case-by-case basis. We fit the observed spectra to a model that takes into account the wavelength of the spectral peak and the curvature of the spectra across the transition from low wavenumbers to the inertial subrange. The curvature generally is as large or larger than the von Kármán spectra. There is large case-to-case variability, some of which can be linked to the mean structure of the CBL, especially the mean wind and the convective instability. We also find a large case-to-case variability in our estimates of normalized turbulent kinetic energy dissipation deduced from the spectra, likely due for the most part to a varying ratio of entrainment flux to surface flux. Finally, we find a relatively larger contribution to the low wavenumber region of the spectra in cases with smaller shear across the capping inversion, and suggest that this may be due partly to gravity waves in the inversion and overlying free atmosphere.     

激光雷达反演边界层高度方法评估及在北京的应用

边界层高度是影响大气边界层发展和空气污染程度的重要因子,是环境和气候研究的重要参数。本文利用激光雷达对北京地区2011年5月至2012年4月的边界层高度进行探测分析,采用小波协方差方法反演边界层高度,评估了该方法的适用性。得到基于小波协方差方法自动判断边界层高度的最优参数组合,激光雷达与飞机探测结果对比一致性较好;与探空结果相关系数0.88,激光雷达反演的边界层高度略偏高。当激光雷达的垂直分辨率为30 m时,更加适合北京地区的步长和阈值分别为210 m和0.05;当激光雷达的垂直分辨率为15 m时,步长和阈值分别为135 m和0.05。分析期间,不同季节边界层高度日变化有明显的不同,夏季14:00（北京时）左右达到最高,较高的边界层高度可维持3~4 h,平均可达1.30 km;冬季较高边界层高度只能维持2 h左右,平均为1.08 km。有云与无云天气边界层日变化特征以及边界层高度存在显著的差异,云的存在减少了到达地面的直接辐射,抑制了湍流的发展,进一步抑制了边界层的发展;本文也将激光雷达反演边界层高度结果应用于观测时期边界层高度与地面污染的关系研究中,统计得到边界层高度与PM2.5浓度的相关系数为－0.340。     

北京等地对流层低层晴天气溶胶微物理特征

1982年4—7月对北京等地对流层低层晴天气溶胶数浓度、尺度谱分布进行了飞机观测。结果表明:来自西、西北和北部气团的气溶胶数浓度相当低（平均为10-1—100 /厘米3）。尺度谱分布用dn/dD=C/DA拟合，相关显著水平α≤0.01，幂指数A值多小于3（占总数73%以上），比Junge给出的A=4值小些。鉴于直径D=0.5—2微米粒子在大气中的重要性，也按dn/dD=C′/DB进行拟合，多数测值α≤0.01，少数α≤0.05。幂指数A、B和D=0.5—1微米粒子相对数浓度f1在水平、垂直方向上是相当均匀稳定的。受城市影响时，混合层内气溶胶数浓度明显增大，尺度谱分布斜率变陡（A、B增大），特别是和“气-粒”转化有关的D=1微米左右粒子数浓度增加非常显著，比不受城市影响时，增加了1—2个量级，最大可增加3个量级。可见，城市影响是造成混合层气溶胶数浓度增大、尺度谱分布变陡、局地大气污染加重、低层能见度变坏的重要原因。     

边界层方案对华北低层O3垂直分布模拟的影响

利用WRF-Chem模式,采用3种边界层参数化方案 (YSU, MYJ和ACM2),针对1个晴空、静稳日 (2013年8月26日20:00—27日20:00(北京时)) 进行模拟,着重分析不同边界层参数化方案对夜间残留层形成及日出前后O₃浓度垂直分布形式的模拟效果,并与固城站地面及垂直同步观测资料进行对比。结果表明:3种边界层参数化方案均能够模拟出温度及风速的区域分布形式以及风温垂直结构的变化特征;相比之下,MYJ方案模拟的夜间边界层高度较YSU方案和ACM2方案明显偏高,该对比结果可能是导致近地面污染物浓度模拟差异的重要原因;在夜间稳定层结至日出后稳定状态打破的边界层结构演变过程中,采用YSU方案和ACM2方案模拟的温度和风速垂直扩线形式与观测结果更为接近;同样采用非局地闭合的YSU方案和同时考虑局地和非局地闭合的ACM2方案,对于边界层高度内O₃浓度垂直分布形式的模拟效果具有明显优势。     

Diurnal variation of atmospheric CO<Subscript>2</Subscript> concentration and δ<Superscript>13</Superscript>C in an urban atmosphere during winter—role of the Nocturnal Boundary Layer

The paper presents the importance of the Nocturnal Boundary Layer in driving the diurnal variability of the atmospheric CO₂ mixing ratio and the carbon isotope ratio at ground level from an urban station in India. Our observations are the first of their kind from this region. The atmospheric CO₂ mixing ratio and the carbon isotopic ratio were measured for both the morning (05:30–07:30 IST) and afternoon time (16:00–18:00 IST) air samples at 5 m above ground level in Bangalore city, Karnataka State (12° 58′ N, 77° 38′ E, masl = 920 m) for a 10 day period during the winter of 2008. We observed a change of ~7% the in CO₂ mixing ratio between the morning and afternoon time air samples. A stable isotope analysis of CO₂ from morning samples showed a depletion in the carbon isotope ratio by ~2‰ compared to the afternoon samples. Along with the ground-based measurement of air samples, data of radiosonde measurements were also obtained from the Indian Meteorological Department to identify the vertical atmospheric structure at different time in a day. We proposed the presence or absence of the NBL as a controlling factor for the observed variability in the mixing ratio as well as its isotopic composition. Here we used the Keeling model approach to find out the carbon isotope ratio for the local sources. The local sources have further been characterized as anthropogenic and biological respiration (in %) using a two-component mixing model. We also used a vertical mixing model based on the concept of the mixing of isotopically depleted (carbon isotope) “polluted air” (PA) with isotopically enriched “free atmospheric air” (FA) above. Using this modeling approach, the contribution of FA at ground level is being estimated for both the morning and afternoon time air samples.     

On the stable stratification of the nocturnal lower troposphere inferred from lidar observations over Pune,India

The nocturnal structure of the lower troposphere is studied using aerosol profile data (50–2800 m AGL) obtained with a bistatic, continuous wave, Argon ion lidar system during October 1986–September 1989 at Pune (18°32 N, 73°51 E, 559m AMSL), India. The top of the nocturnal groundbased inversion is taken as the height above ground where the negative vertical gradient in aerosol concentration first reaches a maximum. During the post-sunset period over this station, this height is as low as 160m and frequently lies around 550m. Greater heights are observed in pre-monsoon months and smaller ones during the southwest monsoon season. Positive vertical gradients in aerosol concentration, indicative of stable/elevated layers, appear frequently around 750m. Temporal variations of aerosol concentration gradients in two adjacent air layers, 920–1000m and 100–1100m, provide evidence that stability increases downward in the early night hours.