Harmonization of size distributions of dust at emission

Current climate models approximate dust aerosols as spherical particles and use the volume-equivalent (geometric) diameter to quantify dust size and abundance in emission, transport, and deposition. Although this idealization in dust representation simplifies simulations and reduces computational cost, dust aerosols have been observed to be highly aspherical. Treating dust as spherical particles thus introduces uncertainties in the calculated effects of the dust cycle on the Earth system, such as on the radiation budget, cloud microphysics, biogeochemistry, and even human health. Moreover, the asphericity of dust causes different methodologies for measuring dust size distributions to produce substantially different results, which could bias dust size distribution parameterizations based on these measurements. Specifically, measurements of aerodynamic, area-equivalent, optical, and geometric dust diameters are inconsistent with one another if particles are assumed to be spherical. Here we harmonize measured particle size distributions (PSDs) of dust at emission by accounting for the effects of dust asphericity on the different methodologies. Specifically, we approximate dust as tri-axial ellipsoidal particles and use the measured lognormal distributions of aspect ratio and height-to-width ratio to establish the link between area-equivalent and geometric diameters. We further use the single-scattering properties of tri-axial ellipsoidal dust to link optical and geometric diameters. Preliminary evaluation shows that the resulting harmonized PSDs of fresh (newly emitted) dust are finer than previously thought. Because our method accounts for dust asphericity and establishes the link between geometric, aerodynamic, area-equivalent, and optical diameters, it enables a more realistic representation of the dust cycle and its impacts on the Earth system.