Atmospheric boundary layer properties

In summary, while most studies of atmospheric boundary layer flows have used local theories involving eddy transport coefficients, it is now recognized that turbulent transport coefficients are not strictly a local property of the mean motion but actually depend on the whole flow field and its time history. The importance of this realization in simulating mean properties of atmospheric flows depends on the particular situation. However, for mesoscale phenomena that display abrupt changes in boundary properties, as is often the case in an urban area, local models are not expected to be reliable. 

The Atmospheric Boundary Layer. The atmospheric boundary layer can be loosely defined as that portion of the lower atmosphere which manifests the effects of surface features in influencing wind flow. It often extends up to heights of the order of 1 km or to the height of the mixing layer, above which the thermal properties of the atmosphere may effectively insulate it from ground effects. The atmospheric boundary layer is the carrier for pollutants that affect corrosion. There are several properties of the atmospheric boundary layer of concern here ... 

At high concentrations (>50ppbv ppbv = parts per billion by volume), O3 in the atmospheric boundary layer becomes a toxic pollutant that also has important radiative transfer properties. The production of nitric acid from NO influences atmospheric pH, and contributes to acid rain formation. In addition, the oxidation of NO to the nitrate (NO3) radical at night influences the oxidizing capacity of the lower troposphere. Determination of the magnitude and location of NO sources is critical to modeling boundary layer and free tropospheric chemistry. 

TURBULENCE is chaotic fluid flow characterized by the appearance of three-dimensional, irregular swirls. These swirls are called eddies, and usually turbulence consists of many different sizes of eddies superimposed on each other. In the presence of turbulence, fluid masses with different properties are mixed rapidly. Atmospheric turbulence usually refers to the small-scale chaotic flow of air in the Earth s atmosphere. This type of turbulence results from vertical wind shear and convection and usually occurs in the atmospheric boundary layer and in clouds. On a horizontal scale of order 1000 km, the disturbances by synoptic weather systems are sometimes referred to as two-dimensional turbulence. Deterministic description of turbulence is difficult because of the chaotic character of turbulence and the large range of scales involved. Consequently, turbulence is treated in terms of statistical quantities. Insight in the physics of atmospheric turbulence is important, for instance, for the construction of buildings and structures, the mixing of air properties, and the dispersion of air pollution. Turbulence also plays an... 

The approach described above is by no means complete or exclusive. For example, Lamb et al. (1975) have proposed an alternative route to assess the adequacy of the atmospheric diffusion equation. Their approach is based on the Lagrangian description of the statistical properties of nonreacting particles released in a turbulent atmosphere. By employing the boundary layer model of Deardorff (1970), the transition probability density p x, y, z, t x, y, z, t ) is determined from the statistics of particles released into the computed flow field. Once p has been obtained, Eq. (3.1) can then be used to derive an estimate of the mean concentration field. Finally, the validity of the atmospheric diffusion equation is assessed by determining the profile of vertical dififiisivity that produced the best fit of the predicted mean concentration field. 

The simulations of volatilization were conducted using the complete model described by Jury et al. (33) where each chemical is present in the soil at a uniform concentration of 1 kg/ha to a depth, L, in the soil and is allowed to volatilize through a stagnant air boundary layer for a specified time period in the presence or absence of water evaporation. The standard conditions or common properties assumed in the simulations are the same as those indicated in Jury et al. (35, 36), i.e., air diffusion coefficient, 0.43 m /d water diffusion coefficient, 4.3 X 10-3 m /d atmospheric relative humidity, 50% temperature, 25°C soil porosity, 50% bulk density, 1.35 g/cm3 soil water content, 0.30 organic carbon fraction, 0.0125 amount of pesticide in soil, 1 kg/ha depth in soil, 1 or 10 cm water evaporation rate, 0, 0.25, or 0.50 cm/d. 

Wet removal processes are further controlled by precipitation types and rates. Dry deposition processes on surfaces are affected by atmospheric transport rates that mix fresh pollutant into the surface boundary layers and by the physical properties of particles. For the Eastern U.S., the approximate annual deposition rates of sulfate can be compared as follows (Table III), considering that deposition flux is the product of a concentration and a velocity of deposition (Vd) (20) ... 

Carlotti, P. (2002) Two point properties of atmospheric turbulence very close to the ground comparison of a high resolution LES with theoretical models, Boundary Layer Meteor. 104, 381 110. 

The Air-Sea Interaction Laboratory, Institute of Oceanology, PAS, Sopot conducts research on mass, energy, momentum and radiation fluxes across the sea surface the concentration and size distribution of marine aerosol in the boundary layer over the sea surface and in coastal zones air-sea interaction phenomena, including sea-atmosphere chemistry. Research into aerosol production and its properties in the marine boundary layer was made during the AREX campaigns of 2000-2003 in the open Baltic Sea. 

Simple dispersion behaviour - as described above - is scale-independent, so that, for example, Gaussian variances estimated from small-scale experiments in a wind tunnel are applicable in the prediction of the atmospheric dispersion of smoke from a large chimney. However, the specification of a dispersion environment (the planetary boundary layer (PBL), terrain, obstacles, closed spaces, etc.) and particular assets (i.e. people or property that we might wish to protect) introduces absolute spatial and temporal scales. 

Hoppel, W. A., Fitzgerald, J. W., Frick, G. M., Larson, R. E., and Mack, E. J. (1989) Atmospheric Aerosol Size Distributions and Optical Properties in the Marine Boundary Layer over the Atlantic Ocean, NRL Report 9188, Washington, DC. 

This can come from other solids or liquids with which the surface has been in contact, as well as from the atmosphere. Thus the surface of a metal, which, outside the realms of surface science might be described as clean, will be covered by complex layer including an oxide, probably partly hydrated, perhaps carbonated, adsorbed water, residual lubricant, and processing aids. Such a layer, in molecular terms, will be thick. It is unlikely to have good wetting properties and is likely to be cohesively weak. Unless removed by pretreatment, it is likely to form a weak boundary layer and to give poor practical adhesion. A successful pretreatment is very unlikely to produce a molecularly clean surface, but the surface produced will not suffer from the same cohesive weakness and lyophobicity of the original. Polymers are also likely to require pretreatment, because their surfaces usually carry complex layers, chemically different from the bulk material. The formation of these layers is now considered. 

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