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Tile wind lift coefficient ce of dust from the ground is defined as the ascending current of dust, lifted by the wind, per unit density of surface contamination A [1]. There are three main methods of determining ce: the diffusion method, the gradient method, and using radionuclide ratios [2]. The gradient method is based on a calculation of the ascending current of radioactive dust from the measured vertical dust concentration gradient in the air. The simplicity and small number of input quantities are its advantages, but its drawback is its low accuracy. This method has been repeatedly used to determine oe both in the contaminated zone and on its periphery. Since measurements of the concentration of radioactive dust in air must be accompanied by a gradient determination of meteorological quantities, its use in a zone of high contamination without using automation becomes dangerous for the health of the servicing personnel. Hence, at the present time measurements are usually carried out on the periphery of the contaminated zone. Air-filtering equipment [3] or gauze cones through which the wind blows [4], are used to collect samples of radioactive dust from the air.Gradient measurements using gauze cones were carried out for the first time in Russia in the Southern Urals in the summer of 1959 [5] after an accident by the "Mayak" Industrial Association (1957). Simultaneously, oe was also determined with greater accuracy by the diffusion method. The results of the measurements were summarized in [2]. After the accident at the Chernobyl Nuclear Power Plant, gradient measurements of oe were made on the southern periphery of the contaminated region [6] and in the Novozybkov Bryansk region [7].The wind uplift coefficient of radioactive dust from the underlying surface is usually calculated in the gradient method from the formula (1) where K z is the mean vertical turbulent mixing coefficient in the terrestrial layer of the atmosphere between levels z I and z 2, qt.2 is the concentration of radioactive dust at these levels, and A is the density of radioactive contamination of the locality. Formula (l), strictly speaking, only holds for estimating the wind uplift factor of an unsettling impurity over an inf'mite horizontally uniform plane under fixed meterological conditions. However, in practice, these conditions can never be exactly realized. Hence, the difficulty involved in organizing natural spatial measurements to estimate advection, in calculating the washing-out of impurity by atmospheric precipitation, in determining the rate of deposition of polydisperse radioactive dust under varying meterological conditions, and the complex field of the density of radioactive contamination at the actual locality, usually necessitate using (1), while acknowledging that the values of oe obtained are approximate.A serious drawback of the gradient method when making measurements on the periphery of the contamination zone is its low sensitivity. This is due to the difficulty of choosing a prolonged period of steady meterological...

551.510.72+551.515.3 In recent years, radioactive dust raised from the surface of the soil by the wind has increased the t37Cs concentration in the air of regions that have been contaminated as a result of the Chernobyl disaster [1]. The distribution of that concentration in the atmosphere at the ground in Bryansk Oblast (Province) was calculated by using a digital electronic map of the contamination density with an interval of 0.5 x 0.5 km and a landscape map with the same resolution. The maps were drawn by the method of [2] from the results of a helicopter 7 -spectrometric survey by "Taifun" in 1992.A digital map of the 137Cs contamination of the territory of Bryansk Province has been obtained with such high resolution for the f'trst time. An idea of the possibilities of using the data as a starting point in various calculations is given by the map with a resolution of 0.5 x 0.5 km that shows the cumulative external 131I radiation dose received by the population in that region [3]. The high degree of spatial detailing made it possible to detect large fluctuations of the t3tI radiation dose, amounting to 2 sSv (rem).A digital landscape map of the district made it possible to take new approaches when calculating the 137Cs contamination of the air by secondary migration when 137Cs is raised from the surface of the soil. The rate at which dust is formed obviously depends on the type of underlying surface: field, grassland, forest, swamp, water surface, or village. During the helicopter -/-ray survey the operator pressed one of six buttons on the control panel to choose the type of underlying surface, which was determined visually inside the collimation detector scanning range with a radius of 100-150 m. The type of underlying surface was recorded to decrease the spread of data from one measurement to another because of the difference in the -/-ray attenuation factors. Possible errors were due to, e.g., the poor visual differences of the old and fresh deposits and the fact that the survey area could be classified as simultaneously having several types of underlying surface.The problem of constructing a field with the general characteristics of the underlying surface (degree of tillage, deforestation, watering, etc.) from the data obtained along the parallel paths flown by the helicopter was solved on the assumption that those landscape characteristics were isotropic and inhomogeneons over a distance smaller than the survey scale (i.e., up to 1 km). For points at the corners of a square grid with a cell size of 0.5 km, we calculated the relative frequency with which each type of underlying surface occurred during a flight and equated it to the probability of detection of a given type inside the cell. For a helicopter speed of about 150 km/h (about 40 km/sec), depending on the position of the grid node relative to the flight path we determined the type of underlying surface on the basis of roughly 30-40 observations at 1-sec intervals for every 40 m. For a strip about 200-300 m wide the observed sample at each gri...

The intensity of dust formation depends on a large number factors, such as the type of underlying surface (pasture, meadow, forest, swamp), the state of the soil surface (dry, moist, wet), wind speed, stratification of the atmosphere, and others. It is characterized by the coefficient of wind uplift ~x, which for radioactive dust is defined as the ratio of the intensity of wind uplift Q to the density of radioactive contamination of the soil A. If the Monin-Obukhov similarity theory is used for the expression of the resulting vertical flow of a weightless impurity over a horizontally uniform underlying surface, then for a thermally stratified ground layer of the atmosphere the coefficient ~x determined by the gradient method can be found according to the vertical concentration profiles q(z) of the radionuclide in air:(1) where x is the Karman constant, u, is the dynamic velocity, and 6(z/L) is the universal similarity function.In the case of neutral stratification of the atmosphere at the ground, Eq. (1) simplifies and assumes the formIn [1], conditions under which a gradient method can be used to determine the coefficient of wind uplift of radioactive dust from the earth's surface at a location in the zone of radioactive contamination were considered. The main condition is horizontal uniformity of the location with respect to its dust-formation capability as well as a high density of radioactive contamination, making it possible to measure vertical gradients of the concentration of dust lifted by wind from the earth's surface. After the Chernobyl accident the highest density of radioactive contamination on the territory of Russia over a large area is observed in the Bryansk Region and it is of a sporadic character. For (Ills reason, the condition of horizontal uniformity of A(x, y) is difficult to satisfy. Moreover, the diffusion method of determining ~x, based on the results of numerical calculations of the concentration fields q(x, y) in models of any type, imposes less stringent requirements and makes it possible to measure cr even under conditions of a nonuniform underlying surface.We shall compare different methods for f'mding a under the conditions of a real nonuniform underlying surface. For this, we employ the results of model calculations and in situ measurements performed in the Bryansk Region in July-August 1992. We chose the most appropriate region according to the form of vertical concentration profiles, calculated by the method of [2], of 137Cs, lifted by wind from the earth's surface, in air. The calculations were performed for different points of the region with a coefficient of wind uplift 7.5-10 -12 sec -1, determined by the diffusion method for Novozybkov during the summer taking account of the climatic data on wind speed and direction. Figure I displays on a logarithmic scale the 137Cs concentration distribution as a function of height above the soil surface in the air layer 1-200 m at seven typical points of the Bryansk Region with different soil contamination density (in Ci/km2): 27 (Verteb...

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