Sources of mineral dust aerosol to the cirrus-forming regions of the upper troposphere

Laboratory studies implicate mineral dust aerosol as effective ice-nucleating particles, and limited field data confirm dust’s dominant role in cirrus formation in some upper tropospheric (UT) environments. However, the net climate impact of mineral dust is not well constrained due in part to large uncertainties in dust aerosol concentrations in cirrus-forming regions. The NASA ATom (Atmospheric Tomography) airborne sampling campaign helps fill critical measurement gaps for mineral dust and other potential ice-nucleating particles throughout the background global troposphere. Using this novel data set we compare measured dust concentrations to global model simulations with the NCAR Community Earth System Model (CESM) using a sectional aerosol scheme. Large model-measurement discrepancies initially observed in the upper troposphere (UT) were substantially reduced by revising the vertical transport and wet removal schemes.

Using this improved modeling framework, we explore the contributions of Earth’s major dust emission sources to the cold UT where dust aerosol can promote cirrus formation. The meteorological environment into which dust is emitted is just as important as emission source strength. For instance, although the Saharan desert dwarfs all other northern hemisphere (NH) dust sources, emission regions such as eastern Asia and the Middle East contribute more dust aerosol to the UT due to efficient vertical transport. Strong north-south latitude gradients are observed in the UT, as generally expected from emissions gradients. However, inter-hemispheric transport from the source-rich NH to the cleaner southern hemisphere (SH) becomes increasingly efficient at higher altitudes, and consequently UT dust gradients are weaker than NH-SH surface emissions would imply. In some seasons the Sahara contributes more dust to the SH UT than Australia.