Non-monotonic response of dust deposition kinetics to low wind speeds

Abstract. Dry deposition is a major sink for mineral dust, yet its behavior under weak winds remains poorly constrained and is often parameterized monotonically with wind speed. Here we quantify size-resolved dust deposition kinetics in a large closed-circuit recirculating wind tunnel using a two-stage protocol (brief high-wind loading followed by low-wind deposition at 0, 2, 4, and 6 m s⁻¹). Cumulative deposition time series were measured with an array of collection trays, with the initial suspended dust loading recorded by an online concentration monitor. Particle sizes were characterized by laser diffraction and grouped into six diameter-threshold classes (PM₂.₅–PM₆₃). A first-order kinetic model captured the deposition evolution and yielded an asymptotic cumulative deposition (𝑎), rate coefficient (𝑘), characteristic time scale  (𝜏 = 1/𝑘), and initial deposition flux (𝐽₀ = 𝑎𝑘). Across the tested wind speeds, net deposition for fine-to-medium particles (PM₂.₅–PM₄₀) is highest at 2 m s⁻¹ and becomes suppressed at higher speeds, with the strongest evidence for a peaked response in the 30 μm class. Quadratic fits across the four wind-speed levels suggest a potential maximum near 2–3 m s⁻¹. These measurements provide process-based constraints on low-wind dust deposition and highlight a potential intermediate-wind window that can inform and evaluate dry-deposition parameterizations.