  Natural wind-eroded soils contain a mixture of particle sizes. However,
models for aeolian saltation are typically derived for sediment bed surfaces
containing only a single particle size. To nonetheless treat natural mixed
beds, models for saltation and associated dust aerosol emission have typically
simplified aeolian transport either as a series of non-interacting single
particle size beds or as a bed containing only the median or mean particle
size. Here, we test these common assumptions underpinning aeolian transport
models using measurements of size-resolved saltation fluxes at three natural
field sites. We find that a wide range of sand size classes experience "equal
susceptibility" to saltation at a single common threshold wind shear stress,
contrary to the "selective susceptibility" expected for treatment of a mixed
bed as multiple single particle size beds. Our observation of equal
susceptibility refutes the common simplification of saltation as a series of
non-interacting single particle sizes. Sand transport and dust emission models
that use this incorrect assumption can be both simplified and improved by
instead using a single particle size representative of the mixed bed.
