A non-local closure model for vertical mixing in the convective boundary layer

A simple non-local closure model for vertical mixing in Convective Boundary Layers (CBL) has been developed specifically for application in regional or mesoscale atmospheric chemistry models. The model, named the Asymmetrical Convective Model (ACM), is based on the concept that vertical transport within the CBL is inherently asymmetrical. Upward transport by buoyant plumes originating in the surface layer is simulated by mixing from the lowest model layer directly to all other layers in the CBL. Downward transport, however, proceeds only to the next layer in order to emulate gradual compensatory subsidence. The ACM is similar to the model developed by Blackadar (1978, 4th Symp. on Atmospheric Turbulence, Diffusion and Air Quality, pp. 443–447, Reno, Am. Meteorol. Soc.) but differs in its treatment of downward transport. The realism of the ACM is tested through comparisons to large-eddy simulations of several idealized test cases. These tests show that while the ACM shares the Blackadar model's ability to simulate rapid transport upward from the surface layer to all levels in the CBL, it is clearly superior in its treatment of material emitted from elevated sources either within or above the CBL. The ACM is also tested in the context of the Regional Acid Deposition Model (RADM) both to determine sensitivity to different CBL mixing schemes and to compare to vertically resolved aircraft measurements. These tests demonstrate quicker upward transport of ground-level emissions by the ACM as compared to the eddy diffusion scheme currently used in RADM. The ACM also affects ozone photochemistry in the boundary layer resulting in lower ozone concentrations in areas of high NO_x emissions.     

Estimates of maximum ground-level concentration in the convective boundary layer—the error in using The Gaussian distribution

This paper derives expressions for the maximum cross-wind integrated and ceterline concentrations at ground level for an elevated source in the convective boundary layer. These expressions account for the observation that the probability density function (pdf) of vertical velocities in the convective boundary is positively skewed with a negative mode. The estimates obtained with these formulae compare favorably with observations reported by Briggs (1993; Boundary-layer Met.62, 315–328).The paper also examines the error associated with using a symmetric Gaussian pdf instead of the skewed pdf to estimate the ground-level concentration. We show that the error is equivalent to an underestimation of a factor of almost 1.5 for the cross-wind integrated concentration and a factor of 1.3 for the ground-level centerline concentration.     

Dispersion modelling of nitrogen oxides in the United Kingdom

This paper describes the construction and application of an Eulerian-dispersion model for the calculation of NO₂ and NO_x concentrations in the U.K. Model results are compared with measurements from 44 U.K. sites. The sensitivity of the model results to chemical and dispersion parameterisations is assessed. The model is also used to verify the validity of ‘instantaneous mixing’ assumptions used in trajectory models. Finally, the distribution of ground-level NO_x concentrations from power stations, motor vehicles and other stationary sources is calculated across the U.K., including a preliminary assessment of the possible contribution from natural sources.     

燃煤电厂烟尘铅排放状况外场实测研究

选取30台燃煤电厂锅炉开展燃料铅含量及烟尘铅排放浓度的系列外场测试.结果表明,燃煤电厂燃料铅含量均值为8.50 mg·kg-1,烟尘铅平均排放浓度为0.0081 mg·m-3,排放因子为0.0643 g·t-1.不同机组容量及有无选择性催化还原(SCR)装置状况下烟尘铅排放因子无显著性差异(p＞0.1),不同除尘设施类型下烟尘铅排放因子有显著性差异(p＜0.1),布袋除尘(Fiber Filter,FF)电厂烟尘铅排放因子低于静电除尘(Electrostatic Precipitator,ESP)电厂.本研究中铅排放因子低于国内估算值,与AP 42燃煤电厂铅排放因子处于同一水平.基于本研究排放因子计算的全国2011年燃煤电厂烟尘铅排放量为126.76 t.     

Multi wavelength measurements of atmospheric turbidity and determination of the fluctuations in total ozone over Antarctica

Sun photometer measurements have been made at λ = 310, 368 and 500 nm during a number of Indian expeditions to Antarctica. The measurements have also been made at λ = 368 and 500 nm over the ocean on a cruise to one of the expeditions. Both the optical depth (a_H) and the turbidity (T) due to atmospheric haze aerosols have been computed over the ocean and over Antarctica. In general, the values of the optical depth are higher at λ = 368 nm that at λ = 500 nm. The values of the optical depth (and turbidity) are also higher over the ocean than over Antarctica. Based on the measurements made at λ = 368 and 500 nm, the exponent α = 2 describes the wavelength dependence of the optical depth. This wavelength dependence of the optical depth has been used to obtain the optical depth at λ = 310 nm. The corresponding value of the selective absorption due to ozone (kx ≈ 1.1, total ozone x = 0.3 atm-cm) is much greater than the aerosol optical depth a_H (≈ 0.11). The optical depth due to Rayleigh scattering (a_R ≈ 1.03) being constant. The relative variability (rms/mean), due to fluctuations on time-scales of a few hours to daily variations, in the computed values of the optical depth a_H has been observed to be about 6% during January 1988. The relative variability in the measured intensity at λ = 310 nm, at a zenith angle of 60°, during this period is observed to be about 14%. The relative variability in the total ozone obtained from in situ measurements with the Total Ozone Mapping Spectrometer (TOMS) for the same period is about 3%. It is shown that the wavelength λ = 310 nm, which is towards the upper limits of the u.v.-B band that is highly absorbed, is perhaps best suited to monitor fluctuations in the total ozone (x). This technique is likely to be more effective during the local spring period of September–November over Antarctica, where the phenomenon of the ozone ‘hole’ occurs.     

珠三角秋季典型气象条件对空气污染过程的影响分析

利用空气质量指数(AQI)、主要大气污染物浓度和气象要素、天气图等数据资料,结合中尺度数值天气预报模式WRF,对2014年10月珠三角地区污染期间的天气形势及气象特征进行了分析.结果表明,WRF模式可以较好地反映珠三角地区主要城市地面和高空气象要素的时空变化,9个城市平均地表的温度、相对湿度和风速的模拟值与观测值的相关系数分别为0.90、0.87和0.78.对2014年10月3次污染过程的分析表明,造成该时段珠三角地区空气污染的天气形势主要是高压底部型和均压场型.静风或小风(2 m·s~(-1))及稳定的大气层结均不利于污染物的扩散,同时由于偏北气流输送周边污染物到珠三角地区,导致污染物浓度不断增加.相对湿度低于65%时,珠三角地区首要污染物以O_3为主;相对湿度高于70%时,PM_(2.5)浓度逐渐增加,成为主要污染物.高温等气象条件会影响光化学反应,加重珠江三角洲的空气污染,表现了该地区大气复合污染的特性.     

A case study of ozone production in a rural area of central ontario

An O₃ episode observed at Dorset, a rural site in central Ontario, during a stagnant high pressure period of the intensive Eulerian Model Evaluation Field Study (EMEFS), in the summer of 1988, is simulated using a photochemical box model with a two-layer treatment. In the model analysis, natural hydrocarbon chemistry is simulated based on an isoprene-only scenario. Sensitivity tests indicate that local isoprene emissions are an important contributor to local O₃ production, relative to anthropogenic hydrocarbons (AHCs), during the event.The model calculated isoprene contribution to the local O₃ production, defined as the ratio of the O₃ amount formed in the absence of AHCs to that in the presence of AHCs, is characterized by a strong NO_x dependence. A minimum value (∼50%) of the contribution was found at a NO_x level of ∼ 6 ppbv for the representative hydrocarbon composition during the episode. At this NO_x level, O₃ production was strongly influenced by the presence of AHCs. At significantly higher or lower NO_x levels, isoprene is more important than AHCs in the local O₃ production.     

Across North America tracer experiment (ANATEX): Sampling and analysis

Between 5 January 1987 and 29 March 1987, there were 33 releases of different tracers from each of two sites: Glasgow, MT and St. Cloud, MN. The perfluorocarbon tracers were routinely released in a 3-h period every 2.5 days, alternating between daytime and night-time tracer releases. Ground-level air samples of 24-h duration were taken at 77 sites mostly located near rawinsonde stations east of 105°W and between 26°N and 55°N. Weekly air samples were taken at 12 remote sites between San Diego, CA and Pt. Barrow, AK and between Norway and the Canary Islands. Short-term 6-h samples were collected at ground level and 200 m AGL along an arc of five towers between Tulsa, OK and Green Bay, WI. Aircraft sampling within several hundred kilometers of both tracer release sites was used to establish the initial tracer path.Experimental design required improved sampler performance, new tracers with lower atmospheric backgrounds, and improvements in analytic precision. The advances to the perfluorocarbon tracer system are discussed in detail.Results from the tracer sampling showed that the average and peak concentrations measured over the daily ground-level sampling network were consistent with what would be calculated using mass conservative approaches. however, ground-level samples from individual tracer patterns showed considerable complexity due to vertical stability or the interaction of the tracer plumes with low pressure and frontal systems. These systems could pass right through the tracer plume without appreciable effect. Aircraft tracer measurements are used to confirm the initial tracer trajectory when the narrow plume may miss the coarser spaced ground-level sampling network. Tower tracer measurements showed a more complex temporal structure than evident from the longer duration ground-level sampling sites. Few above background plume measurements were evident in the more distant remote sampling network due to larger than expected uncertainties in the ambient background concentrations.     

Reactive uptake of NO2 on volcanic particles: A possible source of HONO in the atmosphere

The heterogeneous degradation of nitrogen dioxide (NO₂) on five samples of natural Icelandic volcanic particles has been investigated. Laboratory experiments were carried out under simulated atmospheric conditions using a coated wall flow tube (CWFT). The CWFT reactor was coupled to a blue light nitrogen oxides analyzer (NO_x analyzer), and a long path absorption photometer (LOPAP) to monitor in real time the concentrations of NO₂, NO and HONO, respectively. Under dark and ambient relative humidity conditions, the steady state uptake coefficients of NO₂ varied significantly between the volcanic samples probably due to differences in magma composition and morphological variation related with the density of surface OH groups. The irradiation of the surface with simulated sunlight enhanced the uptake coefficients by a factor of three indicating that photo-induced processes on the surface of the dust occur. Furthermore, the product yields of NO and HONO were determined under both dark and simulated sunlight conditions. The relative humidity was found to influence the distribution of gaseous products, promoting the formation of gaseous HONO. A detailed reaction mechanism is proposed that supports our experimental observations. Regarding the atmospheric implications, our results suggest that the NO₂ degradation on volcanic particles and the corresponding formation of HONO is expected to be significant during volcanic dust storms or after a volcanic eruption.     

An improved method for predicting seasonal and annual shadowing from cooling tower plumes

An improved model developed at Argonne National Laboratory and the University of Illinois (ANL/UI) for predicting long-term shadowing due to cooling tower plumes is presented, and its assumption are compared with those used in previous models. The model is based on a method for the selection of representative categories of similar plumes developed by Dunn and Policastro (Dunn, 1980, Proc. IAHR Cooling Tower Workshop, San Francisco; Policastro et al., 1984, Report EPRI CS-3403-CCM). At a given site this method reduces the large number of meteorological data cases in a season or year to a much smaller number (≈35) of representative cases, each of which will have a predicted plume substantially different from the others. Plume predictions for the reduced set of category representative cases are then made with the validated ANL/UI plume model. With category representative plume shape, wind speed, wind direction and sun angles available for each hour, full effects of sun angles for the latitude and longitude of the site to be studied are included. The ANL/UI model yields seasonal and annual isopleths of hours of additional shadowing or of percentage reduction in total and direct solar energy arriving at the ground on a horizontal surface. Results for two hypothetical sites with 500 MWe generating capacity are presented and contrasted, one at Syracuse, NY, and the other at Spokane, WA.     

A numerical simulation of airflows and SO2 concentration distributions in an arid south-western Valley

Three-dimensional numerical models were used to simulate wind and turbulence distributions in the Mohave Valley and plume transport from the Mohave Power Project, located at approximately 120 km south-south-east of Las Vegas, NV. The Modeled meteorological variables were input to a three-dimensional random-puff diffusion model to simulate observed tetroon trajectories and ground-level concentrations of SO₂ in the Valley. Numerical simulations indicated that hourly averaged surface concentrations could differ considerably, even when the mean and turbulence variables of the airflows were kept statistically identical. The concentration variations resulted from the differences in initial turbulence velocity and inhomogeneous distributions of airflows that each puff encountered during transport. Because the range of variations is, in general, unknown for both measurements and simulations, it is difficult to assess the representativeness of the observed and simulated values, and therefore to evaluate quantitatively the modeled concentrations against observations.     

Sodar in air pollution meteorology

Information concerning atmospheric stability and turbulence is basic in studies related to air pollution meteorology. In this context a monostatic sodar has been operating at the National Physical Laboratory, New Delhi, for many years to probe the thermal structure of the lower atmosphere in real time and space.Various observed thermal structures viz. nocturnal inversion, thermal plumes, formation and break-up of inversion, orographical mixing, multilayered structures and waves etc., are the result of the different atmospheric stabilities and turbulence conditions. These structures have been studied in the light of surface measurements of wind speed and direction for organized indexing of the structural details to infer meteorological conditions concerning Pasquill stability classifications.The height of the ground-based thermal structure has been correlated with the Richardson number, Ri, for quantitative estimation of the turbulence parameter. Estimated values of Ri have been further used to determine the cross wind dispersion coefficient, σ_y. The results obtained are realistic and can be used as input parameters for air pollution modelling.     

Improvement of the modelling of daytime nitrogen oxide oxidation in plumes by using instantaneous plume dispersion parameters

The effect of concentration fluctuations on the modelling of non-linear chemical reactions in the atmosphere will be considerable when hourly mean dispersion parameters are used. This effect can be reduced by introducing instantaneous dispersion parameters, as will be shown in this paper for the modelling of the oxidation of NO emited into an O₃ containing atmosphere.We used the results of atmospheric tracer gas experiments to derive the dimensions of the instanteneous plume. With these results model calculations were carried out for the NOO₃ reaction. Results were compared with airborne measurements of NO oxidation in the plumes of Dutch power plants.The introduction of the instantaneous plume leads to better agreement between model calculations and measurements. It is also found that the deviation from photochemical equilibrium in the plume is considerably smaller when instantaneous, instead of hourly, mean dispersion parameters are used. The dimension of the instantaneous plume did not show a dependence on atmospheric stability.     

烟雾箱与数值模拟研究苯和乙苯的臭氧生成潜势

结合光化学烟雾箱实验与数值模拟研究了苯和乙苯在NO x存在条件下的光氧化臭氧生成潜势.重复实验表明,在乙苯-NO x反应体系中,反应物初始浓度、温度、湿度和光照强度接近的条件下,整个反应过程中臭氧的最大偏差仅为4%,证明了烟雾箱的可重复性较高.在烟雾箱实验的基础上,使用MCM(master chemical mechanism)模拟了苯和乙苯的光氧化O3生成,并将其结果与实验数据进行了比对分析.干燥(≤5%)时MCM对苯和乙苯的模拟结果与实验结果较接近,如在苯-NO x反应体系中,MCM模拟的O3峰值比实验值偏大20%;在湿度为5%~70%时,MCM模拟的乙苯光氧化O3峰值与实验值偏高约(10%~25%).用MCM模拟了太阳光照条件下苯和乙苯的臭氧生成等值线,得到在它们浓度为(10~50)×10-9,NO x在(10~100)×10-9时,苯和乙苯的6 h臭氧贡献值分别为(3.1~33)×10-9和(2.6~122)×10-9,臭氧峰值范围分别是(3.5~54)×10-9和(3.8~164)×10-9.此外,模拟得到苯和乙苯的最大增量反应活性(maximum incremental reactivity,MIR)值分别为0.25/C和0.97/C(每单位碳).该结果与Carter通过SAPRC机制得到的MIR值趋势一致.模拟得到苯和乙苯的最大臭氧反应活性(maximum ozone reactivity,MOR)分别为0.73/C和1.03/C.苯的MOR值远高于Carter使用SAPRC得到的结果,说明根据Carter得到的苯MOR会低估苯的O3潜势.     

Large-eddy simulation of dispersion in the convective boundary layer

Large-eddy simulation (LES) is used to study the dispersion of a passive scalar downwind of a localized source in a convective boundary layer. The LES turbulent velocity statistics are compared with laboratory data and other LES studies. Two scalar source heights at 0.25 z_i and 0.50 z_i are considered, where z_i is the inversion height, and the mean concentration fields are compared with the experimental data of Willis and Deardorf (1978, Atmospheric Environment12, 1305–1312; 1981, Atmospheric Environment15, 109–117). Emphasis is placed on the fluctuating component of the concentration field due to the random turbulent velocity fluctuations, and amplitudes, temporal and spatial scales, and probability distributions are examined. Concentration fluctuation intensity continually decreases downstream, suggesting zero intensity as the asymptotic limit. Vertical profiles of both mean concentration and fluctuation variance become well mixed downstream. Dissipation and correlation scales increase nearly in proportion to the plume width, so that time- and space-averaging the concentration is less effective in reducing the fluctuations further downstream. Concentration probability distributions show intermittency near the source but become nearly normal as the plume moves downstream. Results are compared and contrasted with the neutral flow study of Sykes and Henn (1992a, Atmospheric Environment26A, 3127–3144).     

上海市城区VOCs的年变化特征及其关键活性组分

2010年在上海市城区开展了为期一年的连续观测,采用自动在线GC-FID方法定量测试了大气中56个VOCs物种的浓度.结果发现,上海市城区大气VOCs的全年小时体积分数为(2.47~301.48)×10^-9,平均体积分数为(26.45±23.36)×10^-9,其中,烷烃占46.72%,芳香烃占33.18%,烯烃占11.33%,乙炔占8.76%.T/B(甲苯/苯)为3.51±2.40,表明气团除受机动车影响外,受溶剂、油气和LPG挥发等其他VOCs排放的影响也比较突出;E/E(乙烷/乙炔)为0.98±0.68,表明气团存在老化现象,且春冬季节气团光化学年龄相对较短,夏秋季节光化学年龄相对较长.VOCs的浓度水平和组成在不同风向风速影响下存在一定差异,西南部石化和化工企业排放的VOCs对城区的影响明显,其主要物种为芳香烃和烯烃;该地区气团的OH消耗速率常数(K^OH)为8.05×10^-12 cm³·分子^-1·s^-1,平均VOCs最大O₃增量反应活性(4.00 mol·mol^-1)与乙烯相当,平均反应活性较强;对OH消耗速率(L^OH)贡献率最大的是烯烃(42.21%)和芳香烃(40.83%),对臭氧生成潜势(OFP)贡献率最大的是芳香烃(62.75%)和烯烃(21.70%),VOCs的关键活性组分是二甲苯、甲苯、乙苯、乙烯、丙烯、反-2-丁烯及异戊二烯.     

A computational model for the rise and dispersion of wind-blown,buoyancy-driven plumes—I. Neutrally stratified atmosphere

A multi-dimensional computational model for the rise and dispersion of a wind-blown, buoyancy-driven plume in a calm, neutrally stratified atmosphere is presented. Lagrangian numerical techniques, based on the extension of the vortex method to variable density flows, are used to solve the governing equations. The plume rise trajectory and the dispersion of its material in the crosswind plane are predicted. It is found that the computed trajectory agrees well with the two-thirds power law of a buoyancy-dominated plume, modified to include the effect of the initial plume size. The effect of small-scale atmospheric turbulence, modeled in terms of eddy viscosity, on the plume trajectory is found to be negligible. For all values of buoyancy Reynolds number, the plume cross-section exhibits a kidney-shaped pattern, as observed in laboratory and field experiments. This pattern is due to the formation of two counter-rotating vortices which develop as baroclinically generated vorticity rolls up on both sides of the plume cross-section. Results show that the plume rise can be described in terms of three distinct stages: a short acceleration stage, a long double-vortex stage, and a breakup stage. The induced velocity field and engulfment are dominated by the two large vortices. The effect of a flat terrain on the plume trajectory and dispersion is found to be very small. The equivalent radii of plumes with different initial cross-sectional aspect ratios increase at almost the same rate. A large aspect-ratio plume rises slower initially and then catches up with smaller aspect-ratio plumes in the breakup stage. The Boussinesq approximation is found to be valid if the ratio of the density perturation to the reference density is less than 0.1.     

基于船舶实时排放和烟羽在线监测的燃油硫含量识别方法研究

船用燃油超标识别方法的建立和技术发展是进行船舶排放控制区（Domestic Emission Control Areas,DECA）政策执行的重要保障.本研究建立了基于船舶自动识别系统（Automatic Identification System,AIS）数据的船舶排放实时计算模型和岸基环境观测相结合的技术方法,选取上海吴淞口航道水域开展实地外场观测实验,实现了对观测船只排放烟羽中SO₂和NO₂浓度的在线观测和对燃油硫含量（Fuel Sulfur Content,FSC）进行同步识别和反算.观测期间,通过差分吸收光谱（Differential Optical Absorption Spectroscopy,DOAS）技术共捕捉到1505艘次船舶的烟羽.经过观测截面的船舶总吨位为30~14308 t,船舶排放烟羽浓度峰值的平均持续时间为3~10 min.受船舶烟羽影响期间,SO₂和NO₂的浓度增量分别在0.03~35.51 ppb和0.02~39.26 ppb之间,实时排放模型估算出SO₂和NO₂的排放强度分别为1.32~28.06 g·min^-1和2.89~123.80 g·min^-1.结合在线观测和实时排放模型基于硫氮比对船用燃油硫含量进行反算识别,并与实测燃油硫含量数据样本进行对比验证,结果表明,实际燃油硫含量在0.05%以上时,反算硫含量数值误差在10%以内.本研究可为船舶燃油超标识别提供新的技术思路,并为船舶排放控制区政策落实提供科学基础.     

Urban climate studies in Johannesburg,A sub-tropical city located on a ridge—A review

This paper summarizes three decades of urban climate studies in Johannesburg, South Africa, which is situated on a series of ridges, at an altitude of 1700 m. The first research phase, Near-ground data collection, started with an intensive mobile unit survey measuring wet- and dry-bulb temperatures at midday and near dawn. The results showed that during strong-inversion winter (dry season) nights, the strong heat island and humidity island situated in the city center is more than 11°C warmer than northern suburban valleys. With multiple regression methods the heat island magnitude was estimated to be about 5 K and the relative humidity was 43% lower than the rural areas (but the humidity mixing ratio island was 0.33 g kg⁻¹ higher than the rural areas).The second phase, Upper air studies, included helicopter, pibal and tethered balloons. Near-ground observations (temperature and wind) were constricted to valleys where anomalies were previously found. The main findings at this stage relate to the interaction between mountain/valley winds with country breezes their connection with cold and warm plumes over the ridges which are dominated by the vertical nocturnal wind shear.In the third phase, Remote sensing, in situ and mobile acoustic soundings were combined with the other upper air measurement. In addition, ground temperature variations in Johannesburg were estimated from airborne infrared scanner images. The spatial structure of the ground heat-island core shows a steep thermal gradient of about 600–700 m from the city center, comparable to the screen level temperature distribution obtained previously using a meteorological mobile unit.