New processes to counteract sedimentation of coarse dust particles are required for climate models to agree with observations

Recent observations show that large mineral dust particles are more abundant in the atmosphere than expected and travel further than their mass and theoretical rapid deposition allow for. The presence of these large particles alters the impact of dust on Earth’s radiative budget, carbon and hydrological cycles, and human health. Research into the impacts of the mechanisms influencing large dust particle lifetime in models is vital in ascertaining how large dust particles travel thousands of kilometres further than expected. We employ a series of model simulations to better understand the long-range transport of large particles from the Sahara to the West Atlantic. We present results from two models—HadGEM3A and ICON-ART—which are run at differing resolutions and with different dust representations (size bins and lognormal modes). Observations are used to verify long-range transport in model simulations, including in-situ aircraft observations at the Sahara, Canary Islands, Cape Verde, and Caribbean. Coarse particle mass loading (validated against observations) is limited by excessively rapid deposition in both models, but is further limited in ICON-ART by a reduced size-range representation, with the coarsest mode having a mean diameter by mass of 14.2 µm, whereas the maximum dust size in HadGEM3A extends to 63.2 µm. The sensitivity of large particle long-range transport to sedimentation, convective and turbulent mixing, shortwave absorption, and impaction scavenging are tested in global HadGEM3A climate simulations. A reduction in sedimentation by 80% is required to bring the modelled large particle transport into agreement with aircraft observations. None of the other processes tested were able to make the multiple order of magnitude changes to long-range large particle concentration in the model required for agreement with the observations. Convective and turbulent mixing in the model have minimal impact on large particle long-range transport, but are key in controlling the vertical distribution in the Saharan air layer and marine boundary layer, respectively. This work adds to the growing body of evidence that points to processes involved in large mineral dust transport and deposition which are not represented accurately or at all in models, which counteract the sedimentation of large particles in the real-world.