Abstract/Summary

With more than 400-3000 Mt of mineral dust lofted into the atmosphere annually, dust affects the Earth’s radiation budget, hydrological and carbon cycles, human health, energy production, aviation and more. Recent observations revealed that coarse (5-10 µm) and super-coarse (10-62.5 µm) dust particles are more abundant after long-range transport than expected. Having different impacts on the Earth system than fine particles, it is vital that we understand how these particles travel so far to fully comprehend the impacts of dust globally. In this thesis, I analyse the dust size distribution evolution from the Sahara to the western Caribbean in aircraft observations and compare to a climate model, then use the model to understand the sensitivity of coarse particle lifetime to transport and deposition processes. I show that the model deposits coarse particles too quickly, resulting in an underestimation of dust mass of increasing orders of magnitude with westwards transport. Processes in the model which may increase coarse transport are tested. Coarse dust is shown to be most sensitive to sedimentation, with reductions in sedimentation beyond 80% increasing the volume size distribution by up to 7 orders of magnitude, bringing the model into agreement with observations. Convective and turbulent mixing, impaction scavenging, and shortwave absorption are found to have minimal impact on long-range transport of coarse particles. Raising the dust in the model to 5km at the Sahara with the hope to increase long-range transport is shown to increase particle lifetime, though the coarsest dust is still deposited within 24 hours. Findings in this thesis suggest the presence and importance of processes not in the model which could counteract sedimentation, such as asphericity and electric charging, and suggest that explicit convection representation could improve model transport. This work demonstrates the need for thorough research of these undetermined processes to accurately model size distribution evolution.