1. Introduction
This brief review aims to contribute to the understanding of the mechanisms of dust propagation and transport originated by open pit mining which occurs on clear nights (no clouds) and calm(no breeze).
In order to understand the characteristics is required to examine the particular conditions presented by the atmosphere on clear and calm nights, and how, on these nights, it is possible the propagation of fine dust to large distances (tens of kilometers) from its origin
2. The atmosphere on clear and calm nights
In the following section, we present the concepts of “atmosphere stratification” and "downslote wind", a phenomenon of displacement of large masses of air in calm and clear nights over complex topography, and then apply these concepts to the displacement of air with dust in the case of mining.
2.1- Atmosphere stratification
During clear nights (no clouds or fog) the ground loses heat by infrared radiation to the sky and therefore cools. If the night is also calm (windless) then the lower strata of the atmosphere (those near the ground) will be cooler. In these conditions, it develops a strong thermal inversion (air temperature increasing with height) in the lower layers of the atmosphere, keeping the air cooler (and therefore denser) in the layers near the ground, as shown in Figure 1. As the density decreasing with height, the atmosphere organization described is very stable and, on a clear night, only the breezes are able to destroy it. This peculiar condition of the atmosphere that is common in clear and calm nights, is called "stratification of the atmosphere." (see Fig.1) .
Fig. 2.1–Stratified atmosphere (stability condition) at the end of a calm, clear night.
These events occur at night during stratification and throughout the year as long as conditions are favorable for it (clear sky and no breeze). In case of winds and / or cloud stratification is altered in several ways: the breezes produce a mixture of the higher layers and warm with the lowest and cold (thus increasing the temperature of the lower layers) On the other hand the presence of clouds reduces the energy loss of the ground and blocking their radiant emission and stratification disappears if clouds prevent any radiant exchange between the ground and the sky.
The necessary conditions for the development of an event of stratification are generated by climatic phenomenon of medium and large scale, so these events are spread over the terrain surface and in areas of tens of square kilometers.
2.2- Downslope winds
Under the conditions of atmosphere stratification described before and if there are slopes in the ground, the cold and dense air, which is located in the lower layers will flow down-slope due to the action of gravity and tend to move through the valleys to the lower areas.
The cold air that flows down-slope is called in the literature usually as "downslope wind" or "katabatic wind" to differentiate the "synoptic wind" caused by pressure differences at scales of hundreds of kilometers. Figure 2 shows a schematic of the slope and wind velocity distribution inside. Both the flow and height of the "wind" increase in the flow direction (downhill), while the air temperature at the same height above ground level, decreases in the direction of flow. Higher speeds on the slope winds are near the ground, unlike the synoptic wind speeds which grow with distance from the ground.
Ground with slope + stratification of the atmosphere = DOWNSLOPE WIND
Fig. 2.2 –Representation of the downslope wind indicating the wind velocity distribution inside.
Regarding the "stratification of the atmosphere" and "downslope winds" consult the book "Micrometeorology" of O. G. Sutton, McGraw-Hill, 1953.
3. Application to open pit mining
3.1- Day and night dust concentrations en populated areas near the pit (less than 25 km)
Introduced the concepts of "stratification of the atmosphere" and "downslope wind" these apply to the interpretation of what happens in an open pit mine during clear, and calm nights.
In the aforementioned nights the ground cools by heat loss radiated to the sky, the atmosphere stratifies and downslope winds emerge. Those downslope winds go downslope and enter the mine pits because they’re cooler (and therefore more dense) than the air of the atmosphere inside the pit. Upon entering downslope into wells, move these dust-contaminated air (due to the usual night work at the mine) that flows into existing channels in the topography surrounding the pit. These channels transported the dust downstream into possibly populated areas.
Figure 3.1 shows the outline of a mine pit, the surrounding area and a populated area. The contour lines (every 50 m) of the area indicate the height in meters. The populated area could be several kilometers away from the mine pit. On this scheme slopewinds have been drawn that occur after the sunset in a topography that channel the flow toward a populated area.
Fig. 3.1 Downslope winds during the night. Fresh Air in blue and polluted air in orange.
Figure 3.1 shows, outlined in blue, clean downslope winds (Point A) and moving downstream towards the pit (Point B).These cold and dense flows enter the pit and dragging the dust in there. The orange line shows the downslope winds (now contaminated by dust) (Point C) flowing downstream from the pit and now reaching populated areas (Point D)
Fig. 3.2 shows a classic downslope wind pattern (clean air–in blue- upstream the pit) and polluted air (driven and channeled by downslope wind) downstream the pit, affecting a town, city or village.
Fig. 3.2-Flow of clean cold air in the upper area and polluted at the exit of the mine pit. Shown are sectors A, B, C and D.
Figure 3.2 shows the sections A, B, C and D which shows the flow of clean or polluted air as appropriate.
Note that from the minepit an equal entering flow is discharged downstream to the surroundingsl. If it doesn’t occur the pressure inside the well would increase or decrease during the night.
On the other hand, it should be noted that while incoming and outgoing flows from the minepit are equal (in volume and weight of air per unit oftime), the inflow air is clean and the outflow takes the generated MP10 inside the minepit and then flows through the channels to sensitive areas.
If there is a mechanism of pollution propagation as the one illustrated in Figure 5.2, in the populated area in clear and calm nights a significant increase oin the average concentration over the average daytime concentration.
3.2- Natural ways of pollution cleaning on the valley
A very important thing to be noted and that is common to the vast majority of open pit mining is the full-time job, working day and night every day of the year. This means that the dust emitted into the atmosphere occur day and night.
If the atmosphere did not have natural cleaning mechanisms capable of moving up the dust that continually enter from mining, the average concentration (day and night) should increase continuously until an exceptional event (for example a rain or a strong long time wind) and then the cycle would begin again buildup.
If there is accumulation in the course of days, this accumulation would be expressed in the systematic increase of the daily average concentration (for example) of dust, as a representative point. If not, this proves that although the daily intake of issuers not stop, the atmosphere (through natural convection and diurnal high winds) is able to extract pollution from the area occupied by mining in such exploitation.
Therefore, it can be concluded that the dynamics of daily and nightly along with the natural convection of the valley, are capable (while missing a systematic process of increasing the daily average concentration) of extracting from the valley atmosphere, a daily flow of pollution less than what we now add the sum of issuers that exist within the valley.
Thus the problem aims to control the spread of night dust reaching to the populated area. This doesn’t mean that will prevent the emission of dust pollution at night but that will prevent that the pollution reaches the populated area to be protected.
