Insights into the size-resolved dust emission from field measurements in the Moroccan Sahara
Authors: González-Flórez Cristina , Klose Martina , Alastuey Andrés , Dupont Sylvain , Escribano Jerónimo , Etyemezian Vicken , Gonzalez-Romero Adolfo , Huang Yue , Kandler Konrad , Nikolich George , Panta Agnesh , Querol Xavier , Reche Cristina , Yus-Díez Jesús , Pérez García-Pando Carlos 
Published: December 2022

Abstract. Mineral dust effects upon climate are strongly affected by its particle size distribution (PSD). In particular, the emitted dust PSD partly controls the dust lifetime and its global distribution. Despite the extensive research performed on this topic over the last decades, there are still substantial gaps in our understanding of the emitted PSD along with its potential variability and associated causes. In this study, we provide insights into the saltation and size-resolved dust emission process based on measurements obtained during a comprehensive wind erosion and dust emission field campaign that took place in the Moroccan Sahara in September 2019 in the context of the FRontiers in dust minerAloGical coMposition and its Effects upoN climaTe (FRAGMENT) project. The measurement site located in a remote ephemeral lake, consisting of a smooth hard-crusted paved sediment surface surrounded by small sand dunes, is characterized by strong and frequent saltation and dust emission conditions, and relatively low sandblasting efficiencies. Our study, which thoroughly analyses the number and mass PSDs of both the concentration and diffusive flux (the latter typically assumed to be equivalent to the emitted dust PSD), detects statistically significant dependencies upon friction velocity (u*), wind direction and type of event (regular events vs haboob events). We discuss the potential underlying causes of such variability, including the effect of dry deposition, an enhanced fragmentation of aggregates, and the impact of the haboob gust front. We clearly identify and quantify the major role played by dry deposition in shaping the diffusive flux PSD variations, modulated by the wind direction-dependent fetch length of our measurement location and u*. Our estimates show the importance of dry deposition relative to emission, representing up to ∼40 % for super-coarse particles (> 10 μm) and up to ∼20 % for particles as small as ∼5 μm in diameter. While we attribute the enhancement (reduction) in submicron (supermicron) particles with u* to the effect of dry deposition, an enhanced fragmentation of aggregates with u* could still play a complementary yet arguably smaller role. We additionally find clear differences in the PSDs associated to haboob events in comparison with the regular events, i.e., a higher (lower) proportion of supermicron (submicron) particles for equivalent or higher u* values, and more vigorous dry deposition and variability in the coarse and super-coarse dust mass fractions. We hypothesize that these differences are due to 1) a smaller horizontal (spatial) extent of the haboob events (which is equivalent to the effect of a smaller fetch), 2) the effect of the moving haboob gust front, where u* and dust emission are maximized, along with its changing proximity to the measurement site (which is equivalent to a variable fetch), and/or 3) the increased resistance of soil aggregates to fragmentation associated to the observed increases in relative humidity along the haboob outflow. We finally compare the obtained PSDs with both the PSDs predicted by the original and a recently updated version Brittle Fragmentation Theory (BFT), the latter accounting for super-coarse dust emission. For the comparison with the updated BFT we transform our optical diameters into geometric diameter PSDs, assuming dust particles are tri-axial ellipsoids with an index of refraction consistent with measured optical properties during the campaign. We obtain a substantially lower (higher) proportion of submicron (supermicron) particles in the diffusive flux PSDs in comparison with the original BFT PSDs. Also, our PSDs show a higher proportion of particles above ∼2 μm and a higher mass fraction of super-coarse particles, despite large effect of dry deposition upon this fraction. All in all, our results indicate that dry deposition needs to be adequately considered to estimate the emitted PSD, even in studies limited to the fine and coarse size ranges (< 10 μm), and particularly in measurement locations with long fetches.