Impact of topographic wind conditions on dust particle size distribution: insights from a regional dust reanalysis dataset

Abstract. The size of windblown dust particles plays a critical role in determining their geochemical and climate impacts. This study investigates the relationship between topographic wind conditions (speed and direction relative to land slope) and the particle size distribution (PSD) of dust emissions on a regional scale. We used the Multiscale Online Nonhydrostatic Atmosphere Chemistry (MONARCH) dust reanalysis dataset, which assimilates satellite data on coarse-mode dust optical depth (DODcoarse). Validation against flight measurements from the 2011 Fennec campaign confirms the effectiveness of the reanalysis in capturing coarse to super coarse dust. A 10-year dust reanalysis underwent selective screening to identify events with fresh emissions and the fraction of coarse dust concentrations was calculated as a surrogate for size distribution. The coarse fractions and associated meteorological and land characteristics obtained from various datasets were incorporated into multiple linear regression and machine learning models. Results indicate that dust particle size increases with wind speed, probably due to a higher fraction of fresh emissions and reduced deposition of coarse dust under stronger winds. A positive correlation between dust size and uphill slope suggests that enhanced vertical transport of dust by topography outweighs the impact of shifting emission microphysics over veering winds. Both positive correlations weaken in the afternoons and summer, probably due to the turbulence during haboob storms, which can suspend coarse dust from aged emissions, competing with the effect of uphill slopes. These findings on dust size dependency on topographic winds could improve representation of dust cycle and its impacts.