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).     

Parallel processing of a random-walk model of atmospheric dispersion

Algorithms for random-walk models of atmospheric dispersion take considerable processing time on sequential computers. Such algorithms are, however, more suited to parallel computers as they contain a high degree of parallelism. In this paper, to show this suitability and to investigate the improvement in the processing time with the use of a parallel computer, the model algorithm developed and applied in Luhar and Britter (1989, Atmospheric Environment23, 1911–1924) has been implemented on a Distributed Array of Processors (DAP). The results show a 14-fold improvement in the processing time over a sequential machine.     

Mathematical modeling of cloud chemistry in the Los Angeles basin

A mathematical model of cloud chemistry was evaluated with the cloud chemistry data collected by Richardset al. (1983,Atmospheric Environment17, 911–914) from 1981 to 1985 in stratus clouds in the Los Angeles Basin. The model simulates atmospheric chemistry including gas-phase chemistry, aqueous-phase chemistry and droplet mass transfer, vertical transport, rainfall and turbulent diffusion.Evaluation of the model entailed two major part. First, the equilibrium chemistry is evaluated with a total of 52 case studies. Then, the model with treatment of cloud chemistry, vertical transport and diffusion is evaluated with two case studies. The results of the model simulations show the importance of vertical transport in determining the chemical composition of stratiform clouds.The sensitivity of sulfate, nitrate, ammonium and hydrogen ion concentrations to precursor levels, various chemical kinetic and cloud microphysical parameters and various environmental conditions is discussed.     

Numerical simulation of transport from a point source: error analysis

The techniques used to represent point sources in numerical air pollution models have received little attention.However, an inadequate representation of these sources can lead to significant loss of solution accuracy. Kasibhatla et al. (1988, Atmospheric Environment22, 75–82) have performed a parametric error analysis for the two-dimensional transport of an inert species from an infinite line source. This paper extends the analysis to three-dimensional problems.Numerical results for an inert plume emanating from a single point source shown that the radio of the advection time scale to the turbulent time scale in the vertical direction, K_zΔx/uΔy², as well as the ratio of the advection time scale to the turbulent diffusion time scale in the cross-wind direction, K_yΔx/uδy², have an effect on solution accuracy. In addition, comparisons between the volume-averaged representation of a point source and the use of an irregular grid for point source representation demonstrate that, near the source, improved results can be obtained by placing a node at the source location. Numerical results for the case in which the wind velocity vector is not aligned with tghe grid lines indicate that the cross-derivative diffusion term can be negleted without loss of accuracy.     

A Lagrangian particle model of fumigation by breakdown of the nocturnal inversion

Several fumigation modelling studies are reviewed and an approach involving a coupled mesoscale grid point model and a Lagrangian particle dispersion model is chosen for further study. A number of particle model formulations are evaluated for their ability to maintain a well-mixed profile under steady-state convective conditions. The importance of using a formulation of the Langevin equation which is compatible with the specified turbulence parameterization is emphasised. Other considerations such as the form of the parameterizations and the magnitude of the timestep must also be taken into account.For prediction of hourly-averaged surface concentrations under fumigation conditions, it is found that a simple homogeneous turbulence formulation is equally effective as more sophisticated parameterizations involving inhomogeneity and skewness. Such a particle model, in conjunction with a mesoscale model for predicting the growth of the mixed layer, is able to reproduce well the results of the Deardorff and Willis (1982, Atmospheric Environment16, 1159–1170) laboratory experiments on fumigation.     

The removal of sulphur dioxide in a long-lived line echo system: A numerical study

The removal of SO₂ in precipitation systems is not only a major sink of SO₂ but is also directly related to the acidity of precipitation. However the physical and chemical processes are not well understood due to their complicated characteristics. The lack of measurements also hinders us from studying those processes. In order to investigate what mechanisms are important about the removal of SO₂ in precipitation systems, an alternative approach is to examine the simulation result from a numerical cloud model.A two-dimensional version of the Klemp-Wilhelmson numerical model (1978, J. atmos. Sci.35, 1070–1096) is used to simulate a fast-moving long-lived line echo precipitation system that occurred in the Taiwan area during Mei-Yu season. The simulation model was assumed to be perpendicular to the north-south orientated line echo. In this simulation an artificial source of SO₂ is placed in front of the line echo system in the low level. As the line echo system approaches the region of SO₂, some SO₂ is brought into the line echo system through advection and diffusion effects. A simple parameterization scheme similar to Fisher's (1982, Atmospheric Environment16, 775–783) is used to represent the removal of SO₂ by cloud and rain. The model results show that the major sink of SO₂ is removed by rain below the cloud base although some is removed by cloud and rain inside the precipitation system itself.     

An intercomparison of semiempirical diffusion models under low wind speed,stable conditions

The performances of some diffusion models are analysed using concentration data measured at ground level up to 400 m from the emission point, in a series of diffusion tests conducted by U.S. National Oceanic and Atmospheric Administration (NOAA) under inversion conditions with light winds. All tested models are simple semiempirical formulae based on the Gaussian formulation, with different assumptions concerning dispersion parameters; each model utilizes a minimum set of information, i.e. vertical stability category, mean wind speed and standard deviation of the horizontal wind direction over the considered averaging time, σ_θ.Results show that for cases with very low wind speed and large plume spread, explicit consideration of diffusion along the mean wind direction, which is neglected in the standard plume model, significantly improves model results; moreover, when σ_θ is very large (greater than 50–60°), the analysis suggests that standard deviations of the horizontal wind speed may significantly differ from the estimates commonly found in the literature.     

Aerosol formation and growth in atmospheric organic/NOχ systems—II. Aerosol dynamics

Aerosol dynamics that were observed in the outdoor smog chamber experiments described in Wang et al. (1992, Atmospheric Environment, 26A, 403–420) aresimulated by numerical solution of the aerosol general dynamic equation. The vapor source generation rate was estimated directly from the experimental measurements assuming a single surrogate condensing species for each hydrocarbon studied. Sensitivity analysis of the simulated aerosol dynamics to various input parameters revealed that the physical properties of the condensing vapor are important in determining the interplay between nucleation and condensation while the vapor source generation rate is the only factor that determines the eventual total amount of vapor converted to aerosol. The simulations suggest that over 99% of the mass of condensible vapor is converted to aerosol by condensation even when a significant burst of nucleation occurs.     

Diurnal variation of OH radical and hydrocarbons in a polluted air mass during long-range transport in central Japan

Long-range transport (LRT) of photochemical air pollution from the coastal area, with large emission sources to the inland mountainous region, occurs frequently in central Japan on clear summer days. Hydrocarbons and other pollutants were measured together with meteorological parameters along this route, and the diurnal variations of OH radical and hydrocarbons in the polluted air mass were investigated in relation with these photochemical reactions.At inland sampling sites, hydrocarbon concentrations in a polluted air mass decreased during LRT in the daytime. In particular, olefins were destroyed significantly because of more active photochemical reactions. From these destruction rates and/or O₃ formation rates the OH radical concentration was estimated and found to increase from 0.5 × 10⁻⁷ in the morning to a maximum of 8.0 × 10⁻⁷ ppm at midday. These values, as well as the diurnal variation, compared well with the numerical result of the comprehensive transport/reaction/removal model (Chang et al., 1989, Atmospheric Environment23, 363–393, 1749–1773). The contribution of each hydrocarbon component to the total hydrocarbon destruction was also discussed.     

The use of Procrustes Target Analysis to discriminate dominant source regions of fine sulfur in the western U.S.A.

Receptor modeling, using Procrustes Target Analysis (PTA), was applied to a fine sulfur data set to discriminate dominant sources and areas of influence in the western U.S.A. The work presented in this note is closely linked to a paper published by Malm et al. (Atmospheric Environment24, 3047–3060, 1990), which used two different analyses, area of influence analysis (AIA) and rotated principal component analysis (RPCA), to show similar areas of influence for most source regions in the western U.S.A. A critical examination of RPCA revealed that two issues might confound interpretations. These are (i) eigenvalue degeneracy combined with trying to fit nine orthogonal dimensions pairwise in the PC space and (ii) statistical issues related to missing data and eigenvector truncation. In determining how much of the AIA results are present in the data spanned by the PC space, our findings indicated a 164% improvement in the variance overlap when PTA was substituted for RPCA.     

Indoor air pollution by organic emissions from textile floor coverings. Climate chamber studies under dynamic conditions

The time dependence of the emission of organic compounds from a polyamide floor covering with styrene-butadiene-rubber (SBR) backing was studied in three climate chambers (0.03, 1.0 and 38 m³) at 23°C 5nd 45% RH. While volatile compounds such as toluene reach a maximum concentration in the gas phase within 1 h and decrease in concentration to less than 2% within 60 h, the concentration of less volatile compounds, such as 4-phenylcyclohexene, decreases slowly over a period of months.If the chamber is well mixed and a defined chamber loading is maintained the observed concentrations do not depend on the chamber size, the wall material and air velocity. The concentration of the observed emissions is roughly proportional to the chamber loading. Surprisingly it is not inversely proportional to the air exchange rate. Rather, at high air exchange rates mass transfer from the carpet to the gas phase is enhanced.The “decreasing source models” of Dunn and Tichenor (Atmospheric Environment22, 885–894, 1988) have been applied to the data. They allow the extrapolation of experimental data beyond the time available for measurement.The model calculations reveal the presence of sink effects. The role of the chamber walls as sinks can be determined more reliably if constant sources of an organic compound are placed into the chamber and their increase in concentration with time is compared with the theoretical predictions neglecting sink effects.     

Numerical analysis of the role of sea breeze fronts on air quality in coastal and inland polluted areas

The role of moving sea breeze fronts on air quality in coastal and inland polluted areas was investigated by the numerical simulation of the transport/chemistry of air pollutants, using detailed structures of eddy diffusivity, temperature and flow fields associated with the moving sea breeze fronts. The eddy diffusivity field used was the output of a two-equation turbulence model (i.e. the k-ϵ model) and reasonably well expressed dynamical nature of turbulence in sea breeze, at its front and in an inland mixed layer (Kitada et al., 1987, Proc. Envir. Sani. Eng. Res.23, 103–113). The transport/chemistry of pollutants was calculated using a comprehensive Eulerian model, which adopts more than twenty chemical species advected (Kitada et al., 1983, Proc. 3rd. Int. Symp. on Numer. Methods in Engng, pp. 223–233; Carmichael et al., 1986, Atmospheric Environment20, 173–188).Several cases of numerical simulations were performed, being characterized by the locations of emission sources (i.e. coastal or inland), and the temporal activities of those. Results showed the following. In the coastal-source cases, (1) a circulation behind the sea breeze front was responsible for maintaining high concentration zone of photochemical product, just behind the front; (2) the local maximum of the product's concentration appeared at the upper part of the circulation (i.e. around a height of 500 m), where the temperature profile was stably stratified; (3) the concentration profile was vertically-uniform in the lower part of the circulation, where the thermal internal boundary layer extended over; in the inland-source cases, (4) updraft at the front swept away pollutants into the upper layer. Thus the vertical profile of the pollutant, having a peak at a higher level of 900–1000 m above ground, was formed; (5) this polluted zone at the higher layer remained for several hours after the front passed over. Finally, these indicate that the passage of the sea breeze front can result in complex layering of pollutants.     

A comparison of measurements of atmospheric ammonia by filter packs,transition-flow reactors,simple and annular denuders and fourier transform infrared spectroscopy

Using data obtained during the 1985 Nitrogen Species Methods Comparison Study (1988,Atmospheric Environment22, 1517), several measurement methods for sampling ambient NH₃ are compared. Eight days of continuous measurements at Pomona College, a smog receptor site in Los Angeles, provided an extensive data base for comparing the following methods: Fourier transform i.r. spectroscopy (FTIR), three filter pack configurations, a simple and an annular denuder, and the transition flow reactor. FTIR was defined as the reference method and it reported hourly NH₃ concentrations ranging from > 60 to 2280 nmol m⁻³ (1.5−57ppb) during the course of the study, the highest values coming from the influence of nearby livestock operations.Although only limited quality assurance procedures were carried out, the following conclusions can, nevertheless, be drawn: most of the methods correlated highly with the FTIR method (correlation coefficientr > 0.96); generally, the linear regression slopes were close to unity and the intercepts were insignificantly different from zero at the 95% confidence level); relative to the FTIR average values, (1) for 4–6 h sampling periods, the averages of the three filter packs from three research groups were 83–130% and the annular denuder average was 87%, and (2) for 10–12 h sampling periods, the simple denuder averaged 90% and the two transition flow reactors were 77–98%. Possible reasons for the reported systematic biases are presented, but these are not able to fully explain the large range of differences reported by the various methods.     

Black carbon in the atmosphere of Ljubljana during military action by the Yugoslav army in Slovenia

Aerosol black carbon (BC) is monitored on a timebase of 30 min at a location in the city centre of Ljubljana, the capital city of the Republic of Slovenia, during military action by the Yugoslav army. Methods and instrumentation are described in another paper (Bizjak et al., 1992, submitted to Atmospheric Environment).     

Multivariate geostatistical trend detection and network evaluation of space-time acid deposition data—II. Application to NADP/NTN data

A multivariate geostatistical technique was presented in the first part of this paper (Rouhani et al., 1992, Atmospheric Environment 26A, 2603–2614) with the objective of addressing two key issues of trend detection and network evaluation of the acid deposition data. The investigated data include weekly reported SO₄²⁻ concentrations and depositions from 34 level-1 stations of the NADP/NTN. The duration of the available data ranges from 6 to 10 years. In order to extract the maximum amount of information from these relatively short time series, it is imperative to avoid any data reduction, such as integrating the data into seasonal or annual series. Direct quadratic spectrum estimation is applied to the data, which clearly indicates the presence of annual cycles in a large number of stations. Based on these preliminary results, measurement stations are grouped into three main eco-regions, including the Northeast/central, the West and the Midwest/Gulf regions. While the first two regions exhibit significant annual periodicities, the latter region does not show any significant cyclic characteristics. This analysis is followed by multi-scale temporal variography that further confirms the presence of periodic trends. Two types of time series are generated by co-kriging: (1) the non-periodic components at each station, and (2) the non-periodic regionalized factors for each region. Kendall's test for trend detection is applied to all generated time series, as well as to the original data. The results indicate that by geostatistical filtering of periodic components, the proposed procedure offers an efficient technique for trend detection. Using the estimated coregionalization matrices, spatial correlograms are generated for various temporal scales and regions. Viewing the estimated integral scale of each spatial correlogram as an indicator of the radius of information-influence of each measurement station, the NADP/NTN network is evaluated for its adequacy of coverage under different temporal scales.     

Numerical modeling of long-range transport of acidic species in association with mesoβ-convective-clouds across the Japan sea resulting in acid snow over coastal Japan—II. Results and discussion

The acid snow/rain model [describedin Part I, Kitada et al., Atmospheric Environment27A, 1061–1076, 1993] was applied to investigate transport/transformation/deposition of acidic species in association with snow-precipitating cloud over the Japan Sea in winter. The model results showed: (1) The snow-precipitating clouds generated by relatively weak convective motions tend to trap aerosols of sulfate and nitrate and soluble gases such as SO₂ and HNO₃ below cloud levels, thus keeping their concentrations at higher levels than those for no-cloud situations. The mechanisms involved are: transfer of gas- and aerosol-phase species to cloud-phase through absorption and nucleation scavenging, then their transfer from cloud to snow through riming, and subsequent release from sublimating snow back to gas- and aerosol-phases below cloud base. (2) In-cloud oxidation enhanced the overall conversion of SO₂ to SO₄²⁻ by some 25% with respect to no-cloud situation after 12 h. Furthermore, contributions to the oxidation were 77.4%, 21.1% and 1.5% for S(IV)H₂O₂, S(IV)O₂ with catalysts of Fe³⁺ + Mn²⁺ and S(IV)O₃ reactions, respectively. (3) The sulfate wet deposited by precipitating snow for 12 h was due mostly to in-cloud scavenging and in-cloud oxidation, i.e. 66% by nucleation scavenging and the remaining by in-cloud oxidation of S(IV), while the contribution of below-cloud scavenging was negligible. (4) The adsorption process of HNO₃ onto the surface of falling snow was found to account for major below-cloud scavenging of snow, and thus in contrast to SO₄²⁻, the below-cloud scavenging contributed very significantly to the nitrate wet deposition. Throughout the stimulation, below-cloud scavenging was responsible for 75% of the snow-NO₃⁻ formation. Therefore, taking account of this process in acid snow models is important.     

Proportionality between SO2 emissions and wet SO42− concentrations: The effect of area of averaging

An earlier analysis of empirical associations between SO₂ emissions and wet SO₄²⁻ concentrations in central North America (Hilst and Chapman, 1990, Atmospheric Environment 24A, 1889–1901) showed that local wet SO₄²⁻ concentrations were not proportional to SO₂ emissions averaged over areas up to ∼10⁶ km². Because it is axiomatic that at a global level of averaging a proportionality between total S emissions and S deposition should exist, we have extended these analyses in an attempt to determine whether there is proportionality between S emissions and wet SO₄²⁻ deposition. We have found that for the eastern half of central North America (an area of about 4.3 × 10⁶ km²), the annual average wet SO₄²⁻ concentration exhibits a linear-proportional dependence on anthropogenic SO₂ emissions. However, the internal structure of this association for subareas of the eastern half of central North America suggests the “global” proportionality is achieved by a combination of imported SO₂ from major source areas and an oxidant-limited conversion of SO₂ to SO₄²⁻ within the major source areas. If this inference is even approximately correct, a rollback SO₂ control strategy for the eastern United States and southeast Canada should result in an immediate proportional decrease in wet SO₄²⁻ concentrations in minor SO₂ source areas, but no appreciable reduction of wet SO₄²⁻ concentrations in major SO₂ source areas until the oxidant limitation has been overcome.     

Building wake dispersion at an Arctic industrial site: Field tracer observations and plume model evaluations

Ten multi-hour atmospheric dispersion SF₆ tracer experiments were conducted during October and November of 1987 near a large oil gathering facility in the Prudhoe Bay, Alaska, oilfield reservation. The purpose of this study was to investigate dispersion under arctic conditions and in situations where building-generated airflow disturbances dominate downwind distributions of ground level pollutant concentrations. This was accomplished with a network of micrometeorological instruments, portable syringe tracer samplers, continuous tracer analyzers, and infrared visualization of near source plume behavior.Atmospheric stability and wind speed profiles at this arctic site are influenced by the smooth (surface roughness = 0.03 cm), snow covered tundra surface which receives negligible levels of solar isolation in winter. The dispersion of pollutants emitted from sources within the oil gathering facility, however, is dominated by the influence of nearby buildings when high winds generate elevated ground level concentrations. An order of magnitude increase in maximum ground level concentration was observed as wind speeds increased from 5 to 8 m s⁻¹ and another order of magnitude increase was observed as winds increased from 8 to 16 m s⁻¹. Variation in maximum concentrations was also observed with changes in wind direction. Vertical plume diffusion (σ_z) near the buildings was a factor of 2–3 greater than that observed in open terrain and was dependent on both wind speed and the projected building width and location of nearby buildings. Wind tunnel tracer distributions for east winds agree with field observations but also indicate that a significant increase in plume downwash occurs with other wind directions. Concentration distributions were calculated using several versions of the Industrial Source Complex (ISC) model. Model estimates of ground level concentrations were within a factor of three depending on wind direction. The model predictions are extremely sensitive to the ratio of plume height to vertical plume diffusion which is significantly influenced by a complex aerodynamic wake in the field.     

Tests of models of cloudwater deposition to forest canopies using artificial and living collectors

Mechanistic cloud deposition models are very useful in the routine quantification of cloudwater deposition to forest canopies. In order to test, in a natural field situation, several assumptions in these models, a passive string cloudwater collector, a small artificial tree, and a living Norway spruce were exposed to cloudwater on a raised platform at the summit (elevation, 1686 m) of Whitetop Mountain, Virginia over a 5 month period. Cloudwater collection rates by these three collectors were used to examine relationships between these rates and measured values for two important meteorological variables in the models, liquid water content and wind speed, the product of which is the horizontal cloudwater flux. Collection rates for all three collectors were predicted moderately well by horizontal cloudwater flux (R² ranged from 0.54 to 0.73; p<0.0001) across all hours of observation, but were least strongly related when liquid water content was low, probably because of various measurement uncertainties under this condition. For all three collectors, simple linear regressions using the horizontal water flux to predict collection rates were not appreciably improved by inclusion of a cloudwater collection efficiency term or by conversion to binomial or curvilinear models. Cloudwater collection efficiency for all three collectors was related to the logarithm of horizontal water flux, as predicted by the models, only when this relationship was analyzed within individual cloud events. Between individual cloud events, collection efficiency varied across a wide range (0.12–0.50 for the spruce tree), with efficiencies much higher during events of short duration. Cloudwater collection efficiency was often lower than predicted by cloud deposition models, possibly because the models use wind speed measurements which do not take into account reductions in wind speed occurring within needle clusters on branches. Collection rates for all three collectors correlated highly with each other (R² ranged from 0.72 to 0.88; p<0.0001), as well as with a mature red spruce canopy. It was concluded that either the string collector or an artificial tree such as the one used in this study would serve as a good surrogate collector for living spruce tree crowns.