Size-fractionated measurement of coarse black carbon particles in deposition samples

In a 1-year field study, particle deposition flux was measured by transparent collection plates. Particle concentration was simultaneously measured with a cascade impactor. Microscopic evaluation of deposition samples provided the discrimination of translucent (mineral or biological) and black carbon particles, i.e. soot agglomerates, fly-ash cenospheres and rubber fragments in the size range from 3 to 50 μm. The deposition samples were collected in two different sampling devices. A wind- and rain-shielded measurement was achieved in the Sigma-2 device. Dry deposition data from this device were used to calculate mass concentrations of the translucent and the black particle fraction separately, approximating particle deposition velocity by Stokes' settling velocity. In mass calculations an error up to 20% has to be considered due to assumed spherical shape and unit density for all particles. Within the limitations of these assumptions, deposition velocities of the distinguished coarse particles were calculated. The results for total particulate matter in this range are in good agreement with those from impactor measurement. The coarse black carbon fraction shows a reduced deposition velocity in comparison with translucent particles. The deviation depends on precipitation amount. Further measurements and structural investigations of black carbon particles are in preparation to verify these results.     

Dry deposition of atmospheric aerosols by dual tracer method — I. Area source

Aerosol deposition velocities can be estimated from measurements of the ratio of the concentration of a depositing species, such as Pb, to a non-depositing (conserved) species, such as CO, when both originate from the same source (dual tracer method). This method has been applied to size-segregated aerosol samples collected in Los Angeles, CA during the Southern California Air Quality Study, summer 1987. Lead size distributions were measured in ambient and tunnel samples using an eight-stage low pressure impactor. A new parameter, the fraction of aerosol remaining airborne, was directly determined. This fraction is of great importance receptor modelling. Dry deposition velocities calculated using a stirred atmosphere (box) model, varied over three orders of magnitude depending on particle size. The calculated deposition velocity for unit density particles > 0.5 μm, was approximately proportional to D_p² as expected for deposition by an interception mechanism (d_p = particle aerodynamic diameter).     

Removal of reactive gases at indoor surfaces: Combining mass transport and surface kinetics

The rate of deposition of reactive gaseous pollutants onto indoor surfaces is examined, taking into account mass transport processes and the kinetics of gas-surface interactions. A conceptual model for predicting indoor deposition velocities is proposed, and approximate analysis based on this model is used to obtain algebraic expressions for the deposition velocity of reactive gases under three model airflow conditions: (1) forced laminar convection parallel to a flat plate, (2) laminar natural convection flow along an isothermal vertical plate, and (3) homogeneous turbulence in an enclosure. Numerical simulations are used to refine the approximate analysis results and to predict reactive gas deposition under laminar natural convection flow in an enclosure. The kinetics of gas-surface interactions are modeled in terms of the reaction probability γ, defined as the fraction of pollutant molecular collisions with a surface that result in irreversible removal. Values of γ for the reaction of ozone with surfaces are obtained from published reaction chamber and tube penetration experiments. For common indoor materials, values range from as low as O(10⁻⁷) for glass and aluminium to O(10⁻⁵–10⁻⁴) for materials such as bricks, concrete and latex paint. Our results indicate that ozone deposition occurs at the transport-limited rate when γ > − 3 × 10⁻⁴ for typical indoor air flow conditions, and that ozone deposition can be predicted by surface kinetics alone if γ < ∼ 5 × 10⁻⁷.     

Computing heavy metals in Europe's atmosphere—I. Model development and testing

This paper presents the development of the TRACE model (TRace toxic Air Concentrations in Europe) which computers the air concentration and deposition of various heavy metals (As, Cd, Pb, Zn) on a European scale. TRACE is an improved climatological-type model in that (1) travel time is computed from an empirical function rather than from an assumed constant velocity, (2) the model tends to conserve mass, (3) the irregularity of spatial deposition patterns is captured, and (4) parameters are objectively determined. The dry deposition velocity is spatially varying, and is computed with a dry deposition model as a function of “local” u₁, z_o, together with an assumed characteristic particle size distribution.The model has been used to compute levels of heavy metals for 1978–1985 throughout Europe. Calculations agree with As and Pb observations with a factor of two, and underestimate Cd and Zn observations.Using the model it was estimated that wet deposition exceeds dry deposition in most of central Europe. The mean residence time of the mass of As, Cd, Pb and Zn in Europe's lower atmosphere is estimated to be about 64h and for Pb, 96 h.     

耦合颗粒受力解析的CFD模型及其在超滤膜污染机制中的应用

膜污染可以视为颗粒在膜表面的沉降过程。为深入了解在过滤通道内发生的颗粒迁移和沉降过程,建立了一种耦合颗粒受力的计算流体力学（CFD）模型。通过分析颗粒在超滤过程中的受力,将颗粒受力分析的用户自定义函数（UDF）与CFD中的离散相模型（DPM）耦合,对颗粒的迁移轨迹及沉降进行模拟,并利用过滤实验和微粒子图像测速技术（Micro-PIV）对CFD模拟结果进行了验证。结果表明:颗粒沉降概率与跨膜压差呈正相关,与错流速度呈负相关,超滤实验证明了CFD模拟颗粒沉降的准确性。Micro-PIV示踪粒子的运动轨迹记录膜腔内的速度场分布也验证了CFD模拟流场的准确性。CFD模型可视化地并直观地揭示了膜过程流场和颗粒运动情况,为理解膜污染机制提供了科学依据,对优化膜模块具有重要的指导作用。     

A routine model of chemical transformations and transport of nitrogen compounds,ozone and PAN within a regional scale

A routine model of chemical transformations and regional transport of nitrogen compounds for calculation of deposition and concentrations in Europe is considered. To simulate rapid chemical reactions, a quasi-equilibrium scheme is applied and a hybrid (Lagrangian-Eulerian) three-dimensional model is used for the advection-diffusion calculation. Model parameters, along with temperature, latitude and seasonal effects on the latter, are evaluated by data on ozone and NO_x concentration measurements, carried out at the EMEP network and by one of the authors. Both test and model results within the EMEP grid are presented.     

Comparison of three dry deposition models applied to field measurements in the Southern Bight of the North Sea

Dry deposition velocities have been calculated using three different approaches. Turbulent wind profile theory has been used to predict the drag coefficient, wind speed and friction velocity at 10 m height when the wind speed is measured at a higher altitude. The resulting parameters were introduced in a two-layer deposition model. The second approach was the well-known model of Slinn and Slinn (1980, Atmospheric Environment14, 1013–1016), whereas the third corresponded to the model published by Williams (1982, Atmospheric Environment16, 1933–1938). Results point to clear differences. However, in a field experiment carried out at the Southern Bight of the North Sea, all three approaches show relatively comparable results. The role played by the size distribution of atmospheric particulate matter is essential. In our case any of the three models could have given satisfactory outcomes taking into account the wide spread of the experimental results cited in the literature for the same airshed.     

Laboratory measurements of sulfur dioxide deposition velocity on marble and dolomite stone surfaces

The deposition velocity of SO₂ on marble and dolomite stone surfaces in a humid atmosphere was measured as a function of time in the laboratory using continuous monitoring techniques. The deposition velocity of SO₂ on marble varied between 0.02 and 0.23 cm s⁻¹, and was generally observed to decrease with time. The deposition velocity of SO₂ on dolomite varied between 0.02 and 0.10 cm s⁻¹, and gradually increased over the first 2000 ppm-h of exposure. For both types of stones, the deposition velocity increased significantly when condensed moisture was observed on the stone surface. Chemical analysis of the stone samples indicated that the SO₂ deposited reacted with the stone materials to form gypsum (CaSO₄·2H₂O) on the marble surfaces and gypsum and epsomite (MgSO₄·7H₂O) on the dolomite surfaces.     

明渠中有机质微粒沉降过程的随机游走模拟

基于明渠流动的主要特点,考虑紊动强度的垂向非均匀性,及颗粒运动的游荡效应（loitering effect）影响,对微粒运动基本方程进行改进,建立了明渠非均匀紊流中考虑游荡效应的随机游走模型,对明渠中微粒的沉降损失进行了模拟分析.模拟结果显示,不考虑游荡效应影响时,明渠中微粒的沉降损失率可用微粒沉速与水深之比计算;而在游荡效应影响下,微粒沉降损失率会发生约18%~27%的下降,下降百分比与均匀紊流中微粒沉速在游荡效应影响下的折减百分比渐近值（19~25%）基本一致.本文结论基于不考虑微粒重悬浮、游荡效应为紊流对微粒沉降的主导影响机理等前提,主要适用于对明渠中细小有机质微粒沉降至床底过程更准确的计算分析.     

明渠中有机质微粒沉降过程的随机游走模拟

基于明渠流动的主要特点,考虑紊动强度的垂向非均匀性,及颗粒运动的游荡效应（loitering effect）影响,对微粒运动基本方程进行改进,建立了明渠非均匀紊流中考虑游荡效应的随机游走模型,对明渠中微粒的沉降损失进行了模拟分析.模拟结果显示,不考虑游荡效应影响时,明渠中微粒的沉降损失率可用微粒沉速与水深之比计算;而在游荡效应影响下,微粒沉降损失率会发生约18%~27%的下降,下降百分比与均匀紊流中微粒沉速在游荡效应影响下的折减百分比渐近值（19~25%）基本一致.本文结论基于不考虑微粒重悬浮、游荡效应为紊流对微粒沉降的主导影响机理等前提,主要适用于对明渠中细小有机质微粒沉降至床底过程更准确的计算分析.     

气溶胶粒子在毛细管中沉积与再悬浮

采用亚微米单分散聚苯乙烯球形硬气溶胶粒子和脉冲进样技术,测定了气溶胶粒子在管道中的沉积率,研究了沉积率与流体速度、管道长度、管道直径和气溶胶粒子大小之间的关系.结果发现,在相同条件下,沉积率随管长增加而增大;随流体速度增加而减小.在相同体积流量下,沉积率与管径关系不大;而沉积率与气溶胶粒子大小之间的关系比较复杂.沉积的气溶胶粒子如果在更大流速冲击下,将发生反弹,部分气溶胶粒子从黏附表面上分离再次悬浮于流体中.     

Transport and deposition of indoor radon decay products—I. Model development and validation

Commonly used mathematical models of indoor radon decay product behavior are based on macroscopic mass-balances, often referred to as ‘uniformly-mixed models’. The uniformly-mixed model's applicability is limited by its inability to track the movement of pollutants from their sources to other areas within the enclosure, to permit spatial- or time-dependent sources, or to take proper account of interactions with macroscopic surfaces. Although the uniformly-mixed model parameterizes the deposition process as a constant volumetric removal rate, in reality the deposition process is actually a surface phenomenon and is strongly affected by environmental conditions.This paper describes the development of RADTRAN, a two-dimensional radon progeny transport model that begins with the differential conservation equations describing the motion of air and the transport of reactive pollutants, introduces appropriate boundary conditions to represent surface deposition, and then calculates the concentration distribution of radon progeny throughout the entire region of interest. Knowing the concentration gradient near the surface, a local mass-transfer coefficient (the deposition velocity) can be determined as a function of environmental conditions. RADTRAN simulations have been based on several flow conditions: buoyancy-driven recirculating enclosure flows, free and forced-convection boundary layer flows, and one-dimensional diffusion. Free progeny diffusivity, D^f, and attachment rate, X, were varied over representative ranges. For these conditions, RADTRAN calculated free deposition velocities of u^f = 0.014–0.079 cm s⁻¹, for ²¹⁸Po. RADTRAN predictions are compared to a range of experimental measurements. It was found that the predicted range of deposition velocities is in rough agreement with findings from experiments conducted in flow conditions similar to the simplified flows used in RADTRAN.     

Dry deposition velocity of total suspended particles and meteorological influence in four locations in Guangzhou, China

Dry deposition velocity of total suspended particles (TSP) is an effective parameter that describes the speed of atmospheric particulate matter deposit to the natural surface. It is also an important indicator to the capacity of atmosphere self-depuration. However, the spatial and temporal variations in dry deposition velocity of TSP at different urban landscapes and the relationship between dry deposition velocity and the meteorological parameters are subject to large uncertainties. We concurrently investigated this relationship at four different landscapes of Guangzhou, from October to December of 2009. The result of the average dry deposition velocity is (1.49 ± 0.77), (1.44 ± 0.77), (1.13 ± 0.53) and (1.82 ± 0.82) cm/sec for urban commercial landscape, urban forest landscape, urban residential landscape and country landscape, respectively. This spatial variation can be explained by the difference of both particle size composition of TSP and meteorological parameters of sampling sites. Dry deposition velocity of TSP has a positive correlation with wind speed, and a negative correlation with temperature and relative humidity. Wind speed is the strongest factor that affects the magnitude of TSP dry deposition velocity, and the temperature is another considerable strong meteorological factor.We also find out that the relative humidity brings less impact, especially during the dry season. It is thus implied that the current global warming and urban heat island effect may lead to correlative changes in TSP dry deposition velocity, especially in the urban areas.     

饱和砂质土壤中不同流速条件下1, 2-二氯乙烷的运移及模拟

采用室内土柱试验方法,研究了孔隙水流速对1,2-二氯乙烷(1,2-DCA)在饱和砂质土壤中运移行为的影响.借助CXTFIT模型,通过示踪剂(KBr)穿透试验,获取了土柱内部条件参数,包括弥散系数D和孔隙水流速v,以及可动水和不可动水的分配比例.应用示踪剂试验获取的参数,分别使用平衡模型和非平衡两点模型对1,2-DCA在不同孔隙水流速下的穿透曲线进行了拟合.结果表明,平衡模型不能很好地模拟本文试验条件下1,2-DCA的运移,而非平衡两点模型拟合精度较高.试验条件下,流速对1,2-DCA运移影响显著.孔隙水流速较高时穿透时间较短,穿透曲线峰值更高.土壤对1,2-DCA的吸附作用是影响穿透特征的主要因素,高流速条件下,吸附作用的影响较小.     

应用稳态模型计算中国南方地表水的硫沉降临界负荷

为了解我国地表水体对酸沉降的响应情况,应用基于酸度平衡的稳态模型计算我国南方地表水的硫沉降临界负荷.文中根据我国实际情况获得了计算地表水背景硫酸盐浓度的经验公式.结果表明,我国南方绝大部分地表水的硫沉降临界负荷大于2.0 keq·(hm²·a)^-1,对酸沉降不敏感.临界负荷小于2.0 keq·(hm²·a)^-1的水体主要为部分山地水体.除了这部分山地水体外,其余水体的现状S沉降都没有超过临界负荷,表明它们在短期内不会发生酸化.超临界负荷的地区分布和临界负荷类似,均表现为东南沿海的福建、广东和江西3省低于西南和华中地区.对模型结果的不确定性分析表明,只要模型参数的取值在合适的范围内,参数的不确定性对结果造成的误差较小,可以接受.     

Transport and deposition of indoor radon decay products—II. Influence of environmental conditions

The effects of indoor radon decay product behavior on overall concentrations have generally been characterized using uniformly-mixed models, mathematical formulations based on steady-state macroscopic mass-balances, assuming uniform concentrations within the enclosure. The uniformly-mixed model parameterizes the deposition process as a constant volumetric removal rate, given different values for the free and attached progeny. The model requires prior knowledge of the deposition rates, and assumes them to be constant, independent of environmental conditions, and identical for all decay products. There has generally been little agreement regarding the actual values of the deposition rates, and the uncertainty in these required values presents an important limitation.In response to the limitations of existing mass-balance models, an indoor radon mass-transport model, RADTRAN, was developed using a microscopic mass-balance. Deposition by molecular diffusion is accounted for through boundary conditions, and deposition velocity is calculated based on the concentration distribution near the wall. Parametric sensitivity studies using RADTRAN examined the sensitivity of the deposition of radon decay products to several factors: the size of the free progeny (measured by its diffusivity, D^f), particle concentration (using the attachment rate, X), and air motion. Deposition is described in terms of the deposition velocities of the free and attached progeny, u^f and u^a. The development of RADTRAN is described in a companion paper. This paper presents the results of the parametric sensitivity studies examining the influence of environmental conditions on radon progeny deposition. Results primarily focus on the influence on the free mode of the first radon decay product, ²¹⁸Po. RADTRAN is also used to examine the variations of deposition velocity between the decay products.     

Deposition velocity and the collision model of atmospheric dispersion—II. Extension to cases with variable turbulent velocity scale

The collision model of turbulent dispersion has been extended to deal with situations where the turbulent velocity scale is dependent on position, which requires that the particles of the Monte Carlo simulation accelerate between collisions.The theory has been applied to an idealized vegetative canopy in order to illustrate the differences from the predictions of eddy-diffusivity (K)-theory arising from the finite decorrelation length scale associated with vertical turbulent velocity. For cases of high deposition to canopy elements, the collision-model results do deviate from K-theory results, although the differences are not practically significant.An extension to the model enables gravitational settling to be handled in a more realistic manner than in K-theory. By this means, it is found that the simple expedient of adding the settling velocity to the deposition velocity due to other processes is more closely valid than appears from K-theory results.     

Statistical comparison of the AGDISP model with deposit data

An aerial spray Agricultural Dispersal (AGDISP) model was tested against quantitative field data. The microbial pesticide Bacillus thuringiensis (Bt) was sprayed as fine spray from a helicopted over a flat site in various meteorological conditions. Droplet deposition on evenly spaced Kromekote cards, 0.15 m above the ground, was measured with image analysis equipment. Six complete data sets out of the 12 trials were selected for data comparison. A set of statistical parameters suggested by the American Meteorological Society and other authors was applied for comparisons of the model prediction with the ground deposit data. The results indicated that AGDISP tended to overpredict the average volume deposition by a factor of two. The sensitivity test of the AGDISP model to the input wind direction showed that the model may not be sensitive to variations in wind direction within 10 degrees relative to aircraft flight path.     

Mexico City and the biogeochemistry of global urbanization

Mexico City is far advanced in its urban evolution, and cities in currently developing nations may soon follow a similar course. This paper investigates the strengths and weaknesses of infrastructures for the emerging megacities. The major driving force for infrastructure change in Mexico City is concern over air quality. Air chemistry data from recent field campaigns have been used to calculate fluxes in the atmosphere of the Valley of Mexico, for compounds that are important to biogeochemistry including methane (CH₄), carbon monoxide (CO), nonmethane hydrocarbons (NMHCs), ammonia (NH₃), sulfur dioxide (SO₂), nitrogen oxides (NO_x and NO_y), soot, and dust. Leakage of liquified petroleum gas approached 10% during sampling periods, and automotive pollutant sources in Mexico City were found to match those in developed cities, despite a lower vehicle-to-person ratio of 0.1. Ammonia is released primarily from residential areas, at levels sufficient to titrate pollutant acids into particles across the entire basin. Enhancements of reduced nitrogen and hydrocarbons in the vapor phase skew the distribution of NO_y species towards lower average deposition velocities. Partly as a result, downwind nutrient deposition occurs on a similar scale as nitrogen fixation across Central America, and augments marine nitrate upwelling. Dust suspension from unpaved roads and from the bed of Lake Texcoco was found to be comparable to that occurring on the periphery of the Sahara, Arabian, and Gobi deserts. In addition, sodium chloride (NaCl) in the dust may support heterogeneous chlorine oxide (ClO_x) chemistry. The insights from our Mexico City analysis have been tentatively applied to the upcoming urbanization of Asia.     

Measurements of ammonia flux to a spruce stand in Denmark

This work demonstrates the existence of a linear relation between the deposition velocity of ammonia and the friction velocity measured above a spruce stand in the western part of Denmark. In order to estimate the ammonia deposition velocity and flux to a Norway spruce forest, concentration gradients of ammonia and several meteorological parameters were measured in a meteorology tower during two periods, 1 week in spring and 1 week in late summer 1991. The estimated deposition velocities lie in the range −0.125 to 0.201 m s⁻¹, with a mean of 0.026 m s⁻¹. The deposition velocity and the flux were generally largest in the afternoon. On the basis of 24-h measurements of ammonia and routine meteorological measurements the relation between deposition velocity and friction velocity is extrapolated to an estimate of the average flux for the growing season May to September 1991. The estimate gave an average flux of 87 μg NH₃N m⁻² h⁻¹ (=0.02 μg NH₃N m⁻² s⁻¹). The average deposition velocity for the period was 0.045 m s⁻¹.