Atmospheric Environment (1967)最新文献

Studies of plumes from natural draft cooling towers have indicated that there is an underlying shortcoming in many of the integral plume models in the literature, which prevents them from simulta- neously predicting both plume trajectory and dilution rate accurately. Typically, if the entrainment constant(s) is chosen to yield trajectory predictions in good agreement with measurement, the corresponding dilution rate is overestimated and hence the visible length of a cooling tower plume is often greatly underestimated. By following the approaches of Slawson and Csanady (1971, J. Fluid Mech. 47, 33–49) and Briggs (1975, AMS) which lead to analytical expressions for plume variables, it is demonstrated that the inclusion of the resistive force of the atmosphere opposing the motion of the plume has a significant effect on model performance. For buoyancy dominated sources, the inclusion of this resistive or dynamic pressure force in the momentum balance, either through an added mass factor or through a drag term, allows trajectory predictions to be brought into agreement with measurements while the corresponding growth rate prediction is reduced. For momentum dominated sources, a reformulation of the initial condition for momentum flux, consistent with the assumptions of the integral analysis including the dynamic pressure force, is presented, and is also shown to reduce the entrainment rate required to match trajectory predictions to measurements. When this dynamic pressure force and modified initial momentum flux are included, simultaneous predictions of plume trajectory and growth rate obtained from a simple integral analysis are brought more into line with experimental data.

A compilation of published data on the rates of mass loss from calcareous (carbonate) stones is presented Data from nine of these field exposure experiments are then used in statistical analyses to postulate cause-and-effect relationships with environmental variables, i.e. damage functions, based on a theoretical framework. The metric recommended to estimate rates of damage is ‘material lost per meter of precipitation’, as affected by three mechanisms: calcite dissolution in “clean” rain (pH = 5.6), additional dissolution due to acidic precipitation, and loss by conversion to soluble salts as a result of dry deposition of SO₂ or other acidic species. The working hypothesis for dry deposition of SO₂ is that rain is needed to maintain the activity of dry deposition sites, by washing away the gypsum which has been formed. Although consistency is shown for damage functions derived from theory and from several different experimental programs, some outliers are also shown, indicating the importance of specific stone properties, physical arrangements, and experimental protocols. In addition, these damage functions are not capable of predicting catastrophic damage to stone resulting from exfoliation of sulfate crusts that have accumulated over time.

The variability in performance of two brands of wet/dry atmospheric deposition samplers was compared for 1 year at a single site. A total of nine samplers was used. Samples were collected weekly and analyzed for pH, specific conductance, common chemical constituents and sample mass. The non-normal distribution within the data set and the non-normal distribution of residuals necessitated the application of the non-parametric Friedman test to assess the comparability of sample chemical composition and volume between and within brands of samplers. Statistically significant differences existed for most comparisons however, the test does not permit quantification of their magnitudes, except in general terms. Differences in analyzed concentrations between samplers were small.

A method is developed for recursive prediction of emissions and concentrations at various positions, which obey an atmospheric dispersion model, yet have a least squares deviation from observations at the same points. As a by-product, the technique yields a concentration distribution grid on each time-step. This robust procedure rationalizes data which are in dispute, and makes optimal use of incomplete source or receptor observation records. Thus several unknown source-rates may be estimated on-line as the procedure steps through the remaining observation records. An accurate advection-diffusion solution is formulated as a linear transformation for each time-step, using a sub-grid adaptation of the pseudospectral method. This is extended to the vertical dimension using the zeroth, first and second vertical moments of concentration, allowing only uniform wind profiles, but gradual wind-field and diffusivity variations in the horizontal. A discrete Kaiman filter then provides optimal estimates of all source rates, constituting the state vector, to minimize deviations from any source and receptor observations. The algorithm has been applied in a 90 km × 90 km region of the Eastern Transvaal Highveld, including nine SO₂ sources and eight detectors. Indications are that the method will be a valuable aid in interpreting such data sets.

A Lagrangian statistical (Monte Carlo) model for airborne pollutant dispersion is presented. Its ability to simulate the atmospheric dispersion both in homogeneous and inhomogeneous turbulence by comparison with an analytical solution and with the Willis and Deardorff water tank experiments, respectively, has been stated in previous papers. In the present paper the model is used to simulate dispersion in the real atmospheric PBL. The numerical results obtained are verified against experimental data from the Karlsruhe Nuclear Research Center tracer experiments. The model is applied to the problem of predicting the ground level concentration of two different tracers simultaneously released from two heights (160 and 195 m) at the Karlsruhe meteorological tower. Convectively unstable and neutral conditions were prevailing during the two tracer experiments which have been simulated. Model performance was evaluated through two statistical indexes: relative mean bias and normalized mean square error. The cumulative frequency distribution of the point-by-point ratio between observed and predicted ground level concentrations (glcs) was also computed. The simulated concentrations agree very well with observations. The tracer data were also compared to the simulations of 10 Gaussian models. They differed one another for the choice of dispersion sigma curves and for the way to insert the wind speed and direction. None of them proved to perform better than our particle model in all the exercises.

The present paper deals with numerical effects occurring in the application of the implicit (“backward Euler”) method to solve the diffusion equation in the case of a point source (i.e. singular initial data). The numerical over-estimation of the concentration at the source level as well as conditions for an over- or under-estimation of the ground-level concentration are investigated. To improve the results, a specific filtering of the initial concentration distribution is suggested. All theoretical results are illustrated by numerical examples; for this, an approach of constructing analytical ‘reference’ solutions, for special profiles of the diffusion coefficient, is presented.

The number distribution as a function of sodium mass has been determined for individual particles in ambient air at an urban location about 40 km inland from the Pacific ocean. The measurements were made in real-time by single particle mass spectrometry. The aerosol is ‘externally mixed’ with respect to sodium, that is, a small fraction of the total number of particles are highly enriched in Na. Size distributions calculated for Na mass equivalent spherical NaCl particles over the size range 0.1–0.4 μm show a consistent peak at 0.19 μm. Samples taken after foggy mornings had number densities for Na containing particles around 9 cm⁻³ compared with 0.9 cm⁻³ for samples taken after clear mornings. Comparison of the size distribution over this size range with marine particle size distributions indicates that sub-μm Na containing particles at the inland sampling site are of marine origin. Sodium mass density for particles smaller than 0.4 μm was found to be ∼ 0.02 μg m⁻³ and for particles of diameter ⩽2.5 μm, it was estimated to be ∼ 1.4 μg m⁻³.

In order to study carbonyl sulfide sources and sinks at ground level, two experiments were conducted in 1986 during temperature inversion events. In the first experiment, the samples were collected in a coastal area during land-breeze events. In the second experiment, COS vertical profiles were carried out in an agricultural area, within and above an inversion layer near the ground. Both stable atmospheric situations resulted in a deficit of COS near the ground which is attributed to the existence of a sink of COS at this level. Deposition onto vegetation seems to be the most likely mechanism for this COS uptake, a conclusion in agreement with the results of laboratory and soil flux chambers experiments.

Factor analysis receptor models attempt to estimate both the source composition and the source intensity from a series of observations. The factor analysis solution resulting from Principle Component Analysis (PCA) has no real physically interpretable meaning. Only an appropriate transformation enables a realistic interpretation. Any realistic transformation solution must obey certain natural and physical constraints, such as non-negative source elemental composition and non-negative source intensity, which are not explicitly examined in the existing receptor models. If these natural constraints are violated the results will be uninterpretable.

All observed data sets contain more or less information about the sources. This paper presents a receptor model, which extracts source information from the observed data set to deduce the source profiles, and respects the important natural constraints. This receptor model was tested with a simulated test data set, which was generated with the source profiles and intensities used in the Quail Roost II Workshop. It has also been applied to an ambient data set sampled in Berlin (West) during January and February 1984.