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1.
Two levels of triple-hot-film and sonic anemometers were deployed on a 5.5-m towerduring the Cooperative Atmospheric Surface Exchange Study (CASES-99) in October1999. Each triple-hot-film probe was collocated 50 mm from the sonic sensing path ona common boom. Various problems with using triple-hot-films in the atmosphere toresolve wind components are addressed including the derivation of a yaw angle correction using the collocated sensors. It was found that output voltage drift due to changes in environmental temperature could be monitored and corrected using an automated system. Non-unique solutions to heat transfer equations can be resolved using a collocated sonic anemometer. Multi-resolution decomposition of the hot-film data was used to estimate appropriate day and night averaging periods for turbulent flux measurements in and near the roughness sub-layer. Finally, triple-hot-film measurements of mean wind magnitude (M), turbulent kinetic energy (TKE), sensible heat flux (H), and local friction velocity (u*) are compared to those of the collocated CSAT3 sonic anemometers. Overall, the mean wind magnitudes measured by the triple-hot-film and the collocated sonic sensorswere close, consistent and independent of stability or proximity to the ground. The turbulent statistics, TKE, u*, and H, measured by the two sensor systems were reasonably close together at z = 5 m. However, the ratio of sonic measurement/hot-film measurement decreased toward the ground surface, especially during stable conditions.  相似文献   

2.
A series of sixteen atmospheric tracer experiments using sulfur hexafluoride (SF6), chemical smoke and meteorological balloons was conducted to explore the transport of airborne contaminants in the boundary layer over the ocean surface and in the separating boundary layer over an isolated island cape. The immediate objective of the tests was to determine the impact of local pollutant sources on a background air quality sampling program conducted in the South Pacific from elevated towers on Tutuila Island, American Samoa. In addition to satisfying this objective, the tests are of interest in that they illustrate the local behavior of pollutants in a complex natural atmospheric flow.Offshore tracer tests indicated that the crosswind dispersion of pollutants over the ocean surface can be approximately modeled using the simple Gaussian plume model. The observed crosswind dispersion of the tracer corresponded to that expected under neutrally stable atmospheric conditions, consistent with the near equilibration of the ocean surface and air temperature in the South Pacific. Local, or near-field, tests indicated that tracer released into the wake downwind of the leading edge of the cape mixed rapidly to a height of about 8 m above the surface (i.e., 30–40% of the cape height). Due to decoupling between the boundary layer over the cape and the freestream flow, however, very little of the tracer was observed above this height. This suggests that the impact of local pollutant sources (i.e., on the cape) would be minimized if the proposed sampling towers were elevated significantly above an 8 m altitude (e.g., twice that height).  相似文献   

3.
Analysis of Urban Atmosphere Plume Concentration Fluctuations   总被引：1，自引：1，他引：0  
Concentration variability in the fast-response tracer dataset for continuous, near-surface, point source releases in the urban core from the Joint Urban 2003 field study is analyzed. Concentration variability for conditionally and unconditionally sampled time series is characterized by probability densities, concentration fluctuation intensity, skewness, and kurtosis. Significant day-night differences in plume dispersion are observed. Relative to daytime, nighttime plumes were more likely to have reduced concentration fluctuation intensities, higher normalized surface concentrations, suppressed vertical mixing, and a greater prevalence of Gaussian-like distributions rather than log-normal or mixed mode distributions. This was in spite of the similar stability and turbulence conditions in the urban core for day and night. The potential roles of flow meander and thermal stability in explaining these differences are examined. Probability densities of concentration are found to be a strong function of fluctuation intensity. There are few differences in probability densities between day and night when classified by fluctuation intensity. There are no appreciable differences between conditional and unconditional probability densities and only small differences between conditional and unconditional sampling statistics relative to the larger differences usually observed in more homogeneous settings. Fluctuation intensity, skewness, and kurtosis are higher for the daytime experiments, and closer to the source, but show little difference between conditional and unconditional results over most of their range of values. The log-normal distribution provides a better overall fit to a broader range of the dataset than the exponential or clipped-normal distributions.  相似文献   

4.
Single-tower eddy-covariance measurements represent the complete surface flux of a scalar only under idealized conditions. Therefore, we often find an underestimation of energy fluxes expressed as a lack of energy balance closure at many sites. In this study, a multi-tower approach to measure atmospheric energy fluxes based on spatial averaging is evaluated and possible mechanisms causing a lack of energy balance closure are analysed, focussing on daytime data only. It is shown that the multi-tower technique is also unable to measure the entire flux for our site, likely because the assumption of horizontal homogeneity is violated. Heterogeneity-induced and buoyancy-driven quasi-stationary circulations are probably the dominant processes causing the underestimation of energy fluxes. A dependence of the energy balance residual on stability is found, with residuals close to zero for stable stratification, a maximum under unstable to near-neutral conditions and still relatively large residuals for stronger instability. Assuming the processes transporting energy and CO2 are similar, the implications on long-term CO2 flux measurements are analysed. Accordingly, the resulting selective systematic error of cumulative net ecosystem exchange estimates for agricultural regions such as ours can be of the order of more than 100%, since mainly the fluxes during periods of net CO2 uptake are underestimated while periods of net CO2 release are much less affected by this bias. Further investigations about this issue are highly warranted.  相似文献   

5.
Horizontal and Vertical Co2 Advection In A Sloping Forest   总被引：7，自引：1，他引：7  
A system measuring the horizontal and vertical advection was devised and installed in a sloping forest at the Vielsalm site, Belgium. The measurements showed that under stable conditions a flow regime established below the canopy: air flowed horizontally along the slope and entrained the air above the canopy vertically. This movement occurs during stable nights characterised by strongly negative net radiation. It creates negative air concentration gradients in both the vertical and horizontal directions. The advection fluxes associated with these movements are opposite and of a similar order of magnitude. This implies that the horizontal advection cannot be ignored in the carbon budget equation at night. Unfortunately, the large variability of, and considerable uncertainty about, advection fluxes does not enable one to produce estimates of the source term from these equations. Advection measurement systems should be improved in order to enable such estimates to be made. Particular attention should be paid to the estimation of the vertical velocity above the canopy and to the vertical profiles of the horizontal velocity and horizontal CO2 gradient below the canopy.  相似文献   

6.
In this study, we used satellite data (GOME and MOPITT) together with a global chemical-transport-model of atmosphere (MOZART-2) to characterize the chemical/aerosol composition over eastern China. We then estimated the effects of local emissions in China on the chemical budgets in other regions of the world. Likewise, we also investigated the effects of air pollution from other regions on the chemical budget over eastern China. The study shows that the column CO and NO x concentrations are also high in eastern China. The high CO and NO x concentrations produce modest levels of O3 concentrations during summer (about 40 to 50 ppbv) and very low O3 during winter (about 10 to 20 ppbv) in eastern China. The calculated NO2 column is fairly consistent from the GOME measurement. The calculated CO column is underestimated from the MOPITT measurement. One of the reasons of the underestimation of the predicted CO is due to a fact that the CO emissions were taken without considering the rapid increase of emissions from 1990 to 2000. The calculated surface O3 is consistent with the measured values, with strong seasonal variations. However, the measurement is very limited, and more measurements in eastern China will be needed. The column NO2 has a very strong seasonal variation in eastern China, with the highest concentrations during winter and the lowest concentrations during summer. The cause of this seasonal variability is mainly due to the seasonal changes in the chemical loss of NO x , which is very high in summer and very low during winter. The effects of the local emissions in China and long-range transport from other regions on the chemical distributions in eastern China are studied. The results show that NO x concentrations in eastern China are mostly caused by the local emissions in China, especially during the winter. The CO concentration over eastern China is from both the local emissions (30% to 40%) and the transport from other regions. Likewise, the CO emissions in China have an important effect on the other regions of the world, but the effect is limited in the northern hemisphere. The local emissions in China also have an important effect on surface O3 concentrations. During winter, the local emissions reduce the surface O3 concentrations by 30 to 50%. During summer, the local emissions produce about 50 to 70% of the O3 concentration in eastern China.  相似文献   

7.
The influence of the local terrestrial environment on nocturnal atmospheric CO2 measurements at a 329-m television transmitter tower (and a component of a CO2 monitoring network) was estimated with a tracer release experiment and a subsequent simulation of the releases. This was done to characterize the vertical transport of emissions from the surface to the uppermost tower level and how it is affected by atmospheric stability. The tracer release experiment was conducted over two nights in May of 2009 near the Department of Energy’s Savannah River Site (SRS) in South Carolina. Tracer was released on two contrasting nights—slightly stable and moderately stable—from several upwind surface locations. Measurements at the 329-m level on both nights indicate that tracer was able to mix vertically within a relatively short (～24 km) distance, implying that nocturnal stable conditions do not necessarily prevent vertical dispersion in the boundary layer and that CO2 measurements at the tower are at least partly influenced by nearby emissions. A simulation of the tracer release is used to calculate the tower footprint on the two nights to estimate the degree to which the local domain affects the tower readings. The effect of the nocturnal boundary layer on the area sampled by the tower can be seen clearly, as the footprints were affected by changes in stability. The contribution of local sources to the measurements at the tower was minimal, however, suggesting that nocturnal concentrations at upper levels are contributed mostly by regional sources.  相似文献   

8.
A method to tighten the cloud screening thresholds based on local conditions is used to provide more stringent schemes for Orbiting Carbon Observatory-2(OCO-2)cloud screening algorithms.Cloud screening strategies are essential to remove scenes with significant cloud and/or aerosol contamination from OCO-2 observations,which helps to save on the data processing cost and ensure high quality retrievals of the column-averaged CO2 dry air mole fraction(XCO2).Based on the radiance measurements in the 0.76μm O2A band,1.61μm(weak),and 2.06μm(strong)CO2 bands,the current combination of the A-Band Preprocessor(ABP)algorithm and Iterative Maximum A Posteriori(IMAP)Differential Optical Absorption Spectroscopy(DOAS)Preprocessor(IDP)algorithm passes around 20%-25%of all soundings,which means that some contaminated scenes also pass the screening process.In this work,three independent pairs of threshold parameters used in the ABP and IDP algorithms are sufficiently tuned until the overall pass rate is close to the monthly clear-sky fraction from the MODIS cloud mask.The tightened thresholds are applied to observations over land surfaces in Europe and Japan in 2016.The results show improvement of agreement and positive predictive value compared to the collocated MODIS cloud mask,especially in summer and fall.In addition,analysis indicates that XCO2 retrievals with more stringent thresholds are in closer agreement with measurements from collocated Total Carbon Column Observing Network(TCCON)sites.  相似文献   

9.
The concentration of gas-phase peroxides has been measured almost continuously at the Cape Grim baseline station (41° S) over a period of 393 days (7702 h of on-line measurements) between February 1991 and March 1992. In unpolluted marine air a distinct seasonal cycle in concentration was evident, from a monthly mean value of>1.4 ppbv in summer (December) to <0.2 ppbv in winter (July). In the summer months a distinct diurnal cycle in peroxides was also observed in clean marine air, with a daytime build-up in concentration and decay overnight. Both the seasonal and diurnal cycles of peroxides concentration were anticorrelated with ozone concentration, and were largely explicable using a simple photochemical box model of the marine boundary layer in which the central processes were daytime photolytic destruction of ozone, transfer of reactive oxygen into the peroxides under the low-NOx ambient conditions that favour self-reaction between peroxy radicals, and continuous heterogeneous removal of peroxides at the ocean surface. Additional factors affecting peroxides concentrations at intermediate timescales (days to a week) were a dependence on air mass origin, with air masses arriving at Cape Grim from higher latitudes having lower peroxides concentrations, a dependence on local wind speed, with higher peroxides concentrations at lower wind speeds, and a systematic decrease in peroxides concentration during periods of rainfall. Possible physical mechanisms for these synoptic scale dependencies are discussed.  相似文献   

10.
Continuous measurements of ozone and its precursors including NO, NO2, and CO at an urban site (32°03′N, 118°44′E) in Nanjing, China during the period from January 2000 to February 2003 are presented. The effects of local meteorological conditions and distant transports associated with seasonal changed Asian monsoons on the temporal variations of O3 and its precursors are studied by statistical, backward trajectory, and episode analyses. The diurnal variation in O3 shows high concentrations during daytime and low concentrations during late night and early morning, while the precursors show high concentrations during night and early morning and low concentrations during daytime. The diurnal variations in air pollutants are closely related to those in local meteorological conditions. Both temperature and wind speed have significant positive correlations with O3 and significant negative correlations with the precursors. Relative humidity has a significant negative correlation with O3 and significant positive correlations with the precursors. The seasonal variation in O3 shows low concentrations in late autumn and winter and high concentrations in late spring and early summer, while the precursors show high concentrations in late autumn and winter and low concentrations in summer. Local mobile and stationary sources make a great contribution to the precursors, but distant transports also play a very important role in the seasonal variations of the air pollutants. The distant transport associated with the southeastern maritime monsoon contributes substantially to the O3 because the originally clean maritime air mass is polluted when passing over the highly industrialized and urbanized areas in the Yangtze River Delta. The high frequency of this type of air mass in summer causes the fact that a common seasonal characteristic of surface O3 in East Asia, summer minimum, is not observed at this site. The distant transports associated with the northern continental monsoons that dominate in autumn and winter are related to the high concentrations of the precursors in these two seasons. This study can contribute to a better understanding of the O3 pollution in vast inland of China affected by meteorological conditions and the rapid urbanization and industrialization.  相似文献   

11.
Three surface-layer flux footprint models have been evaluated with the results of an SF6 tracer release experiment specifically designed to test such models. They are a Lagrangian stochastic model, an analytical model, and a simplified derivative of the analytical model. Vertical SF6 fluxes were measured by eddy correlation at four distances downwind of a near-surface crosswind line source in an area of homogeneous sagebrush. The mean fluxes were calculated for 136 half-hour test periods and compared to the fluxes predicted by the footprint models. All three models gave similar predictions and good characterizations of the footprint over the stability range -0.01 < z 0/L < 0.005. The predictions of the three models were within the limits of the uncertainty of the experimental measurements in all but a few cases within this stability range. All three models are unconditionally recommended for determining the area defined by the footprint over short vegetative canopies in this range. They are also generally appropriate for estimating flux magnitudes within the limits of experimental uncertainties. Most of the mean differences observed between the measured and predicted fluxes at each of the four towers reflect a tendency for the measured fluxes to be greater than those predicted by the three models. Rigorous verification of the models in strongly stable conditions was complicated by the need to obtain very accurate measurements of small fluxes in only marginally stationary conditions. Verification in strongly unstable conditions was hampered by the limited number of appropriate data.  相似文献   

12.
A large set of tower data was used to identify the gap that separates small-scale turbulence and mesoscale structures in the cospectra of surface fluxes. The cospectra were obtained using a multi-resolution decomposition algorithm. The gap time scale τ g was found by fitting a fifth-order polynomial to the cospectra and identifying special points occurring after the peak at small scales. In unstable conditions (day) τ g was found to fall as the mean wind speed increased, while no such dependence was observed in stable conditions (night). The gap scale was found to change very weakly with stability both in moderately stable and moderately unstable conditions, with a sharp drop from about 1100 to 250 s occurring in near-neutral conditions. The vertical fluxes computed at different averaging intervals were found to correlate exceptionally well with each other, the scatter being somewhat larger during the night. Although considerable discrepancy may occur for individual records, when averaged over 10 months, the difference in the flux estimated at 7 to 109 min intervals never exceeded 4%, which is comparable or less than the instrumental error.  相似文献   

13.
参考AP-42方法的采样规范（USEPA,2011）,对武汉市13个城区的不同类型道路采集了137个扬尘样,并记录采样面积、车流情况、车道状况、地理位置、周围环境以及气象数据要素信息,得到了不同类型道路的积尘负荷,估算了其扬尘排放因子和排放量.结果表明：武汉总城区尘负荷由大到小顺序为支路 > 次干道 > 主干道 > 快速路,其中支路平均尘负荷为2.396 g/m2,快速路为0.852 g/m2,远城区平均尘负荷是主城区平均尘负荷的2倍左右.各类型道路不同粒径范围的道路交通扬尘排放因子大小顺序为支路 > 次干路 > 主干路 > 高速路,与尘负荷大小趋势一致.2016年道路交通扬尘源TSP的年排放量为156 931.4 t,PM10的年排放量为39 868.7 t,PM2.5的年排放量为11 574.8 t,其不确定性范围分别为-24.7%~31.4%、-31.3%~32.9%、-31.8%~30.5%.其中主干道扬尘排放量最大,其TSP、PM10和PM2.5的年排放量分别为64 447.1、16 372.9和4 753.4 t.  相似文献   

14.
Ozone Concentrations in Rural Regions of the Yangtze Delta in China   总被引：4，自引：0，他引：4  
Elevated concentrations of ozone have been observed at six non-urban, surface monitoring sites in the Yangtze Delta of China during a 16-month field experiment carried out in 1999 and 2000 as part of the joint Chinese-American China-MAP Project (the Yangtze Delta of china as an Evolving Metro-Agro-Plex). The average daytime (0900–1600 h) ozone levels for the monitoring period at sites ranged from 35 to 47 ppbv (parts per billion by volume) and the mean ozone levels from 26 to 35 ppbv. Observed data show seasonal variation obviously, with highest mixing ratios of ozone in May. Average daytime ozone levels in May at sites were between 60 and 79 ppbv. High ozone concentrations were most prevalent during the late spring. Frequency counts of hourly mean ozone concentration over 60 ppbv and 40 ppbv appeared peak values of 22–39% and 42–74% in May at sites. Even higher daytime ozone levels were observed during two regional episodes, in which average daytime (0900–1600 h) ozone concentrations during 10 May and 23 May 2000 were 68 to 81 ppbv, during Oct. 18 and Oct. 28, 1999 were 59 to 67 ppbv at sites. Peak value of ozone mixing ratio appearing in late spring, instead of in summer, was attributed to summer monsoon. Backward trajectories showed that ozone episodes associated with meteorological conditions. Also many high ozone levels associated with high CO levels and high CO to NO x ratios, which suggests a contribution from sources of emission involving incomplete combustion.  相似文献   

15.
利用蒙特卡罗不确定性分析方法,分析了嵌套网格空气质量模式系统(NAQPMS/IAP)中154个模式输入变量不确定性对臭氧模拟的影响,量化了模式在北京奥运会期间臭氧模拟的不确定性,并确定出了主要不确定性因子。结果表明:(1)在奥运会期间(2008年8月8日～2008年8月24日),北京城区臭氧模拟的平均不确定性为19ppb,不确定性存在明显的日变化特征,白天不确定性大,夜间不确定性小。(2)在白天,北京城区近地层臭氧模拟最重要的不确定性来源是局地前体物排放,其次是NO2光解系数、风向、北京周边前体物排放和垂直扩散系数。另外,地面约150m以上,对臭氧模拟影响最大的不确定性因子是风向和北京周边前体物排放;夜间,北京城区近地层臭氧模拟的最大不确定性来源是局地NOx排放和垂直扩散系数。  相似文献   

16.
The propagation of optical and electromagnetic waves is affected by small-scale atmospheric turbulence, quantified by the structure parameter of the refractive index. In the atmospheric surface layer, the mean structure parameter Cn2{C_{n}^{2}} , as averaged over the large-scale turbulence, relates to meteorological forcings through well-documented relationships. Present-day numerical weather forecast models routinely produce these forcings at the global scale. This study introduces a method where the products of such a model are used to calculate the mean optical turbulence near the surface. The method is evaluated against scintillometry measurements over climatologically distinct sites in Western Europe. The diurnal cycle modulation, and regional and seasonal contrasts, are all reproduced by our predictions. Hence, the present method explains and predicts some essential aspects of the meteorological variability of Cn2{C_{n}^{2}} near the surface. The noted discrepancies combine instrumental limitations, site peculiarities, differences related to distinct averaging procedures, and model errors, notably from weather forecasts. The minute-scale fluctuations of the measured scintillation rate are also analysed in the light of the forecast weather conditions. Fair-weather daytime periods consistently show a small short-term variability compared to the nighttime and perturbed weather periods. Thus, this short-term variability appears to have a predictable component.  相似文献   

17.
One-dimensional Lagrangian dispersion models, frequently used to relate in-canopy source/sink distributions of energy, water and trace gases to vertical concentration profiles, require estimates of the standard deviation of the vertical wind speed, which can be measured, and the Lagrangian time scale, T L , which cannot. In this work we use non-linear parameter estimation to determine the vertical profile of the Lagrangian time scale that simultaneously optimises agreement between modelled and measured vertical profiles of temperature, water vapour and carbon dioxide concentrations within a 40-m tall temperate Eucalyptus forest in south-eastern Australia. Modelled temperature and concentration profiles are generated using Lagrangian dispersion theory combined with source/sink distributions of sensible heat, water vapour and CO2. These distributions are derived from a multilayer Soil-Vegetation-Atmospheric-Transfer model subject to multiple constraints: (1) daytime eddy flux measurements of sensible heat, latent heat, and CO2 above the canopy, (2) in-canopy lidar measurements of leaf area density distribution, and (3) chamber measurements of CO2 ground fluxes. The resulting estimate of Lagrangian time scale within the canopy under near-neutral conditions is about 1.7 times higher than previous estimates and decreases towards zero at the ground. It represents an advance over previous estimates of T L , which are largely unconstrained by measurements.  相似文献   

18.
A set of tracer experiments studying concentration fluctuations in a pollutant plume dispersing near the surface in a stably stratified nocturnal boundary layer is described, and the results are compared with those obtained in near-neutral stability conditions by Mylne and Mason (1991). The results highlight the importance of slow meandering of the plume which is characteristic of stable conditions. This meandering makes it impossible to conduct experiments under near-stationary conditions, resulting in considerable statistical variability in the results, but is important in reducing time-averaged concentrations. Spectral characteristics of the plume and general fluctuation statistics are qualitatively similar to those in near-neutral stability, but there are significant quantitative differences. Fluctuation time scales are shown to be substantially longer under stable conditions. This difference cannot be fully explained by the reduced windspeed alone, indicating that the length scale of plume elements is also longer. Some of the differences observed in stable conditions, particularly the longer time scales, are shown to substantially increase the potential hazard due to fluctuations in practical applications. A conceptual model of plume dispersion is described, which explains the observed plume structure under different conditions by relating it to the turbulent velocity spectra.  相似文献   

19.
Measurements of surface ozone (O3), nitric oxide (NO), nitrogen dioxide (NO2), oxides of nitrogen (NOx=NO+NO2) and meteorological parameters have been made at Agra (North Central India, 27°10??N, 78°05??E) in post monsoon and winter season. The diurnal variation in O3 concentration shows daytime in situ photochemical production with diurnal maximum in noon hours ranging from 51 to 54 ppb in post monsoon and from 76 to 82 ppb in winter, while minimum (16?C24 ppb) during nighttime and early morning hours. Average 8-h O3 concentration varied from 12.4 to 83.9 ppb. The relationship between meteorological parameters (solar radiation intensity, temperature, relative humidity, wind speed and wind direction) and surface O3 variability was studied using principal component analysis (PCA), multiple linear regression (MLR) and correlation analysis (CA). PCA and MLR of daily mean O3 concentrations on meteorological parameters explain up to 80 % of day to day ozone variability. Correlation with meteorology is strongly emphasized on days having strong solar radiation intensity and longer sunshine time.  相似文献   

20.
Scintillation measurements with a HeNe and a CO2 laser were used to derive turbulent fluxes of heat and momentum in the surface layer. This was achieved by the structure constant or dissipation technique, i.e., by relating the measured structure constants and inner scales of refractive index fluctuations to structure constants of temperature fluctuations and dissipation rates of turbulent kinetic energy, respectively, and then assuming Monin-Obukhov similarity.The resulting heat fluxes agree well with measurements using the eddy correlation technique but for averaging periods of 10 min, the optical data show a much smoother and physically more plausible behaviour. The optically derived friction velocities are in good agreement with estimates derived from wind velocity and surface roughness. It was also observed that for stationary conditions, 1-min averaged optical measurements already provide good estimates for longer averaged heat and momentum fluxes.Even though some uncertainty remains about the empirical constants and Monin-Obukhov similarity expressions used, the method clearly proves to be of great value for monitoring surface-layer turbulence.  相似文献   

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