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 dp>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 62.5 μm) particles. This study is the first to continuously cover the full range of diameters from dpCombining double low line0.4 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*) above 0.22 ms-1 (or ∼3.3 ms-1 threshold 4 m wind speed), the approximate threshold for local dust emission by saltation, both dust concentrations and the contributions of super-coarse and giant particles typically increased with increasing u∗, especially under neutral to unstable atmospheric stability conditions. These large particles accounted for about 90 % of the total mass concentration during the campaign. A prominent mass concentration peak was observed near dpCombining double low line60 μm in geometric diameter. While particle concentrations for dp<10 μm showed good agreement among most instruments, discrepancies appeared for larger dp 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.