A Wind Tunnel Study of Dust Emissions from Salt Crusts

Small particles of dust that are removed from desert surfaces by the wind can have a big and varied impact on climate. They may reflect sunlight and reduce temperature, or absorb heat and increase temperature and also act as cloud seeds, increasing rainfall. When they are deposited in oceans, they help to fertilise algae and increase the conversion of carbon dioxide to oxygen. However, we know very little about exactly how these particles leave the surface, particularly on dry lake (or playa) surfaces which consist of complicated salt and clay mixtures. These minerals typically form protective crusts on the surface of the dry lake. These crusts may prevent dust from being removed from the surface, but they also change their roughness (or shape) and form polygonal patterns which change the way the wind interacts with the surface. Typically dust is more easily emitted from the surface when the crust is first disturbed by sand grains which are carried by the wind and help to break up the crust. However, given different temperature and moisture conditions, chemical reactions occur which change the protectiveness of the crust, particularly in some of the biggest dust emitting dry lakes such as the Makgadikgadi Pans in Botswana and Owens Lake in USA. These chemical reactions may occur quickly and often change the pattern on the surface, opening cracks or enlarging the height of surface polygon ridges (by as much as a cm in a few hours) and in the process typically form more dust material which is later emitted to the atmosphere. We know very little about how these chemical processes may alter the amount of dust produced by a surface, and it is difficult to control temperature and moisture in field experiments to test this. The wind tunnel at Trent University offers an ideal setting where both temperature and humidity can be controlled and wind of variable strength can be applied over crusted surfaces. The state-of-the-art facility also lets us measure the different surface patterns with mm accuracy, examine the path single dust grains take when they are transported from the surface and see under which conditions dust may spontaneously leave the surface or when it may need help from sand grains to first disturb the crust. It will enable us for the first time to link crust dynamics and dust emission to changes in temperature and humidity. This will be of major interest to both climate modellers for improving their characterisation of dust emissions in computer simulations and air quality scientists as it will provide insight into possible mitigation methods for dust ‘hot spots’ close to human populations which are currently a health risk.