From fine to giant: Multi-instrument assessment of the dust particle size distribution at an emission source during the J-WADI field campaign

Abstract. Mineral dust particles emitted from dry, uncovered soil can be transported over vast distances, thereby influencing climate and environment. Its impacts are highly size-dependent, yet large particles with diameters d_p > 10 μm remain understudied due to their low number concentrations and instrumental limitations. Accurately characterizing the particle size distribution (PSD) at emission is crucial for understanding dust transport and climate interactions.

Here we characterize the dust PSD at an emission source during the Jordan Wind Erosion and Dust Investigation (J-WADI) campaign, conducted in Wadi Rum, Jordan, in September 2022, focusing on super-coarse (10 < d_p ≤ 62.5 μm) and giant (d_p > 62.5 μm) particles. This study is the first to continuously cover the full range of diameters from d_p = 0.4 to 200 μm at an emission source by using a suite of aerosol spectrometers with overlapping size ranges. This overlap enabled a systematic intercomparison and validation across instruments, improving PSD reliability.

Results show significant PSD variability over the course of the campaign. During periods with friction velocities (u_∗t) above 0.22 m s⁻¹, the approximate onset of local dust emissions, super-coarse and giant particles were observed, with concentrations increasing with u_∗. These large particles accounted for about 90% of the total mass concentration during the campaign. A prominent mass concentration peak was observed near d_p = 60 μm in geometric diameter. While particle concentrations for d_p < 10 μm showed good agreement among most instruments, discrepancies appeared for larger d_p due to reduced instrument sensitivity at the size range boundaries and sampling inefficiencies. Despite these challenges, physical samples collected using a flat-plate sampler largely confirmed the PSDs derived from the aerosol spectrometers. These findings help to advance our understanding of the dust PSD and the abundance of super-coarse and giant particle at emission sources.