An optimized particle size bin scheme for modeling mineral dust aerosol

[1] During transport, atmospheric aerosol particles experience nonlinear size-dependent deposition processes which affect their size distribution. This implies that the aerosol particle size distribution (PSD) should be well represented in three-dimensional transport models that simulate their transport and their radiative or biogeochemical impacts. In particular, mineral dust aerosols exhibit a broad range of particle sizes but the dust PSD is generally described in such models by a limited number of particle size bins. A common approach in the literature consists in defining such bins following a geometric progression of the particle diameter (‘‘isolog’’ bins). Here we propose to define size bins following the size dependence in dry deposition velocity (Vd), by satisfying an ‘‘isogradient’’ in Vd. We use a box model and reference simulations with 1000 particle size bin to test the accuracy of both approaches with 4 to 30 bins, in simulating (1) the mineral dust number and mass PSD evolution due to dry and wet deposition, and (2) the derived visible aerosol optical thickness (AOT). Both schemes show a trend in increasing accuracy when increasing the number of bins. The classical isolog scheme, however, exhibits strong, embarrassing oscillations in accuracy when varying the number of bins. For a given number of particle size bins, the isogradient scheme significantly improves the accuracy of results in terms of mass PSD and greatly suppresses oscillations of errors in both mass and number distributions. We conclude that at least eight isogradient size bins are necessary to secure an 8% accuracy on the total suspended dust mass after two days of transport. In terms of particle number distribution, errors remain small for both schemes but they become somewhat larger ( 8%) in terms of AOT for 12–30 isogradient size bins. This drawback can be compensated by weighting the average aerosol extinction cross section within each size bin by the initial dust particle number distribution. We conclude that at least eight isogradient size bins are necessary.