ERRATIC DUST CONCENTRATIONS – Cape Town

Erratic Dust Concentrations in Cape Town during the Winter of 2011

–        During a high rainfall season surfaces become saturated and the rain penetrates (roadways, yards and the farm lands etc.) resulting in these becoming more stable and less likely to become airborne (less dust from these sources).

–         Agricultural soils absorb moisture and are more likely to conglomerate making the material form into larger clumps, which will not become airborne to the same extent.  When dumps or mined fines are wet down the effect is far less pronounced as the rock dust is not porous and the moisture will results in a less stable conglomeration state.  This dries out quicker as well releasing the fines to be easily picked up by wind.

–         Rounded quartz fine material (the finer natural sand constituents) being hard and rounded does not get wetter (as we noted for quantities or hard rock material) and being loosely packed rounded material dries out very quickly and becomes airborne with gusty winds.

This means that with a rainy season you will almost always have a higher content of rounded quartz sandy material prevailing.

–         The next question to answer is why organic material is more prevalent during the rainy season! Well in the Western Cape the rainy season coincides with winter and with a high humidity the air density is lower. Deciduous plants and trees lose their leaves in late summer, autumn and when we have early rains the process is delayed. When we have an extended dry period during the winter as we have has recently (5 weeks with minimal rain) then the leaves dry out break up and the fibrous content is then blown around and has a disproportionate content to that encountered during the summer.

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