Ruwim Berkowicz

Transformation of size distributions of emitted particles in streets

Abstract Dispersion and transformation of particulate matter in streets are studied by using a combination of a street pollution dispersion model, the Operational Street Pollution Model (OSPM) and a particle transformation model. The particle model implements a very fast implicit method for solving the changes in particle size distribution due to coagulation, condensation and dilution processes. Using available measurements of emitted particle size distributions, impact analysis of the various transformation processes affecting the size distribution of particles was performed. For this purpose, a plume model simulating dilution of diesel exhaust has been developed and coupled with the particle coagulation model. The results show that due to the rapid dilution of the exhaust plume, the coagulation is not significant. Growth of the emitted particles due to condensation of water vapour appears also to be marginal, but this conclusion depends critically on the assumption of particle hygroscopicity. The freshly emitted diesel particles are believed to be less hygroscopic. Analysis of recent street measurements of particle size distributions in the range 0.2–20 μm reveals in general very poor correlation with the street traffic, but strong dependence on the relative humidity. This indicates that the particles measured are not freshly emitted and aged aerosols are dominant. Calculations with the street pollution dispersion model OSPM, using the diesel exhaust emissions as the only source, show that the mass concentrations of emitted particles in the street are predicted to be significantly lower than the measured concentrations.

Effects of reduction of NOx on the NO2 levels in urban streets

Nitrogen dioxide is a major problem in urban areas. Persons suffering from respiratory diseases, especially asthma patients, are sensitive to NO 2 at high concentrations. Nitrogen oxide emissions from private cars have decreased due to the introduction of catalysts. However, the catalyst technique is today limited to petrol vehicles. Since heavy diesel vehicles, e.g. buses, contribute significantly to the NO x emissions, this will limit the reduction in emissions in many urban streets. Another factor is the continued increase in the car fleet in all countries. Most of the NO x from traffic is emitted as NO, which is thought to be harmless. NO is rapidly transformed to NO 2 by a reaction with ozone in the air. Tropospheric ozone levels have been increasing in Europe for many years as a consequence of increasing pollution. Ozone in Danish streets is seen to be the limiting factor for the production of NO 2 . Thereby O 3 is also the limiting factor for the NO 2 levels in the streets. In this paper the above will be supported both by observations from the Danish air quality measurement programmes and by model simulations with the Danish Operational Street Pollution Model.

Generalization of K Theory for Turbulent Diffusion. Part I: Spectral Turbulent Diffusivity Concept

Abstract The gradient transfer theory for turbulent diffusion is reformulated in order to obtain an improved method for applied dispersion studies. The basic innovation is that diffusivity of single Fourier components of the concentration field is treated separately, i.e., spectral turbulent diffusivity coefficients are introduced. The value of the diffusivity decreases with increasing wave vector k of the concentration spectrum. The rate of growth of an expanding cloud of material thus becomes dependent on the stage of growth. This is in qualitative agreement with the statistical dispersion theory. It is shown that the assumption of k-dependent diffusivity leads to a nonlocal flux-gradient relation. A new function, the turbulent diffusivity transfer function, is introduced. The turbulent diffusive flux depends on concentration gradients at all points in the space. The diffusion equation is written in terms of the turbulent diffusivity transfer function. The width of the turbulent diffusivity transfer fun...

Pseudospectral simulation of dry deposition from a point source

Abstract A pseudospectral, two-dimensional model for dispersion and dry deposition of atmospheric pollutants is developed on the basis of gradient-transfer theory (K-theory). A symmetrical transform is developed for the vertical direction satisfying the condition of mass conservation. The deposition to the ground is represented by a sink term at the surface and according to this the model is called the surface depletion model. Comparison with analytical solutions is performed in the case of constant wind and diffusivity profiles. Agreement between numerical and analytical results is within 2–5% even with only 17 grid points in the vertical direction. The error can be further reduced by application of more grid points. The pseudospectral method is more accurate than finite difference methods, especially with respect to advection. Multiple sources and time-dependent physically realistic, e.g. measured wind and diffusivity profiles can easily be treated. Sources between gridpoints can be accurately represented. The pseudospectral model is used for calculation of deposition rates with diffusivity and wind profiles for different atmospheric stability conditions. Comparison is made with the conventional. Gaussian source depletion method for estimates of dry deposition from a point source. The discrepancy between the two models increases with increasing atmospheric stability. In the stable case with a deposition velocity of 1 cms −1 and a point source at a height of 25m, the Gaussian source depletion model underestimates the suspension ratio by a factor of 1.5 at a downwind distance of 22 km from the source. The surface concentration is overestimated by nearly a factor of 2 at the same distance. Contrary to other, more simple surface depletion models which do not take wind and diffusivity profiles into account, it is found that the suspension ratio is smaller for the surface depletion than the source depletion model at short distances, while the opposite relation occurs only at larger distances. This effect is ascribed to the low wind velocity at the surface which results in stronger deposition close to the source, while at larger distances the vertical diffusive transport becomes more important for the rate of dry deposition. The present results are especially relevant for dispersion and deposition in cases of low diffusivities, where the difference between the two-dimensional pseudospectral surface depletion model and other less sophisticated deposition models is most pronounced.

On the spectral turbulent diffusivity theory for homogeneous turbulence

The spectral turbulent diffusivity (STD) theory, originally deduced from a spectral generalization of the gradient-transfer theory (Berkowicz & Prahm 1979), is here derived from a basic concept of turbulent mixing for the case of homogeneous turbulence. The turbulent mixing is treated in a way similar to Prandtls mixing-length concept. The contribution to the turbulent flux from eddies of different length is represented by a linear superposition. The spatial variation of the concentration distribution is described in terms of Fourier series. This procedure results in the spectral diffusivity formulation, which is Eulerian and scale dependent. If the concentration distribution is approximated by a truncated Taylor expansion instead of an exact representation by the Fourier series, the gradient-transfer approximation is retrieved. The turbulent energy density, as function of the eddy length, is related to the eddy transport velocity and a probability of the occurrence of the eddies. The eddy transport velocity, derived from the relation between the energy spectrum and the Lagrangian correlation function, is used for computation of the spectral turbulent diffusivity. The turbulent energy spectrum is approximated by the inertial sub-range form

Generalization of K Theory for Turbulent Diffusion. Part II: Spectral Diffusivity Model for Plume Dispersion

(-frac{5}{3}

Deposition of Nitrogen Compounds to the Danish Coastal Waters

law). The STD coefficient obtained here has, for large wavenumbers, a slope of

Determination Of Traffic Emissions From Pollution Measurements And Dispersion Modelling

k^{-frac{4}{3}}

Deposition of Gases and Particles in the PBL: Evaluation of the Influence of a Vertical Resolution in Atmospheric Transport Models

as predicted previously.

Impact of wood combustion on particle levels in a residential area in Denmark

Abstract Further development of the spectral turbulent diffusivity concept is presented with the aim of obtaining an Eulerian dispersion model applicable for multiple interacting sources. The theory is applied for studies of plume dispersion in a field of a homogeneous and stationary turbulence. A continuous plume is considered as consisting of an infinite number of expanding puffs. The puffs center of mass fluctuates following the long-wave range of the turbulent velocity fluctuation spectrum. The center-of-mass fluctuations are assigned to phases of the Fourier coefficients of the concentration distribution. The standard deviation of the velocity of the phase fluctuations is dependent on the wave vector of the Fourier coefficient. Time-averaging results in a spectral phase diffusivity coefficient. It is shown that the rate of growth and the center-line concentration obtained by the spectral diffusivity model are in agreement with results predicted by the Lagrangian statistical theory. For a narrow plum...