Evaporative sodium salt crust development and its wind tunnel derived transport dynamics under variable climatic conditions

Joanna M Nield,Robert G Bryant,Giles F.S Wiggs,Patrick O’Brien,Cheryl Mckenna Neuman

doi:10.1016/j.aeolia.2016.09.003

Joanna M Nield, Robert G Bryant + Show 3 more

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https://doi.org/10.1016/j.aeolia.2016.09.003

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Abstract

Abstract Playas (or ephemeral lakes) can be significant sources of dust, but they are typically covered by salt crusts of variable mineralogy and these introduce uncertainty into dust emission predictions. Despite the importance of crust mineralogy to emission potential, little is known about (i) the effect of short-term changes in temperature and relative humidity on the erodibility of these crusts, and (ii) the influence of crust degradation and mineralogy on wind speed threshold for dust emission. Our understanding of systems where emission is not driven by impacts from saltators is particularly poor. This paper describes a wind tunnel study in which dust emission in the absence of saltating particles was measured for a suite of climatic conditions and salt crust types commonly found on Sua Pan, Botswana. The crusts were found to be non-emissive under climate conditions characteristic of dawn and early morning, as compared to hot and dry daytime conditions when the wind speed threshold for dust emission appears to be highly variable, depending upon salt crust physicochemistry. Significantly, sodium sulphate rich crusts were found to be more emissive than crusts formed from sodium chloride, while degraded versions of both crusts had a lower emission threshold than fresh, continuous crusts. The results from this study are in agreement with in-situ field measurements and confirm that dust emission from salt crusted surfaces can occur without saltation, although the vertical fluxes are orders of magnitude lower (∼10 μg/m/s) than for aeolian systems where entrainment is driven by particle impact.

Highlights

Playas contain vast reserves of silt and clay which, periodically, can become susceptible to erosion by wind; and are considered to be major dust sources on a global scale (Gill, 1996; Baddock et al, 2009; 2016; Bullard et al, 2011; Prospero et al, 2002; Washington et al, 2003; 2006)
The exploratory laboratory experiments reported in this paper address dust emission potential from degraded and continuous evaporative salt crusts under varying climatic conditions
The results demonstrate considerable complexity in the relationships between the flux of dust emitted from a playa surface to the atmospheric boundary layer, the threshold wind speed required for this to happen, and the physicochemical characteristics of the surface, the type and form of the salt crust

Summary

Introduction

Playas (or ephemeral lakes) contain vast reserves of silt and clay which, periodically, can become susceptible to erosion by wind; and are considered to be major dust sources on a global scale (Gill, 1996; Baddock et al, 2009; 2016; Bullard et al, 2011; Prospero et al, 2002; Washington et al, 2003; 2006) Degraded crusts typically protect an underlying ‘fluffy’ layer of sediment (1-5 cm thick) which represents salty sediment of dust-size fraction with notably low bulk density that is readily eroded by wind (Nield et al, 2015; Reynolds et al, 2007), if the crust has been disturbed or broken due to animal activity, saltation or vehicular traffic (e.g. Gillette et al, 2001; Baddock et al, 2011)

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