| Sandstorm is a frequent meso-scale disastrous event in spring over extensive north China. Asevere event may cause great losses of crops, livestock and human lives. Sandstorm acts as the primary source of sand aerosols. It is estimated that as high as two billion tons of such particles comes into air and, influencing radiation properties, cloud physics and environment, they exert strong impacts on the earth - air system and, particularly, the atmospheric environment.To investigate the space/time distributions and occurrence and evolution of such events, the diffusion pattern over deserts, turbulent transfer features in sandstorm weather, the particle size distribution, mass concentration and its distribution, optic properties, chemical composition and physical factors responsible for the initiation of raising sands, we made integrative observation and sounding of sandstorms deep in the large-scale desert area, including Tengri, Badanjilin and Maowusu, with the items consisting of micrometeorological measurement, 3D wind observation, data from KB-120 and Anderson samplers of mass concentrations of sands with their spectrum, the distribution of aerodynamic particle sizes from the APS3310A, retrieval of aerosols' optic depth from sunphotometer data, assay of the chemical composition by means of Neutron Activiation Analysis (NAA) and integrated study of all related factors for causing sandstorm to occur, based on the observations of all kinds. And the "muddy rain" produced a single event of sandstorm is simulated in the context of a sand transfer model. From the above we come to the following conclusions.1) The number of sandstorm days from 12 weather stations in the Helanshan zone is documented, indicating that the events can happen on the east and west sides of the ranges all the year round, with high frequencies in April to May, averaging 3.2 and 3.1 days on the west side, respectively and with the eastern high frequency in April, averaging 2.0 days. Also, analysis reveals that the high frequency period for eastern station Yanchi and western Guaizihu is April, with 5.4 and 5.2 days, on average, respectively. Nine of the twelve stations have apparently reduced the number of sandstorm days, with the decreasing trend quite obvious, but an increasing trend is shown for three stations among them for the 1960s to the late 1980s, with severe and extremely severe events appreciably augmented during the 1960s to 1970s, a point that deserves our particular attention. Both on the east and west sides of the Helanshan Mountains the preferred frequency of such events happens between 14:00 to 20:00 (Beijing Time BT), with mean persistence depending strongly on the distance from station to source. And the shorter the distance, the longer the duration.2) From the analysis of the aerodynamic parameters and turbulent flux transfer under different synoptic conditions we achieve the conclusion that on sands raising and sandstorm days turbulent momentum and sensible-heat fluxes are important exchange factors in the boundary layer, with turbulent exchange stronger in sandstorm weather than both on fine and sand raising days. Prior tothe occurrence of such an event, in particular, the super-adiabatic instability in the near-surface boundary will reinforce sandstorm weather condition.3) Atmospheric turbulent diffusion parameters z and y have apparent daily variation. Under a clear sky, the vertical (horizontal) diffusion parameteroz (oy) peaks around 14:00 BT and valleys between 03:00 to 06:00 BT. Under a sandstorm sky great difference arises in their daily variation, depending on the starting time of a sandstorm. The zis smaller in diffusion speed compared to y, which shows a bigger range of daily variation. Mean wind, when smaller, has no distinct correspondence to the daily cycle of z and y.4) The number and mass concentration distributions of the background atmospheric sand particles of Ymchuan, Bayanhot and Tonggunur display a unimodal form, obeying a logarithmic normal distribution, greatly differing f...
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