Aircraft-Based Observation of Mineral Dust Particles Over the Western North Pacific in Summer Using a Complex Amplitude Sensor

Abstract

Mineral dust accounts for a significant fraction of the atmospheric aerosol burden, impacting climate through its interactions with radiation and clouds. However, limited observations of dust particles in the free troposphere hinder our understanding of its spatial distribution and climatic impacts. In this study, aerosols were collected on filters using aircraft over the western North Pacific at 0.2–8 km altitudes in summer 2022. Individual water-insoluble particles dispersed in water were analyzed using a complex amplitude sensor (CAS), which enabled classifying particle type, sizing, and counting from the particle's complex amplitude data. During the eight observation flights, the number and mass concentrations of dust were 1.2 ± 1.0 cm⁻³ and 0.6 ± 0.4 μg m⁻³ (mean ± 1σ), respectively, in the 0.3–2.5 μm diameter range, which were 2–3 orders of magnitude lower than previously reported values near the major dust sources in East Asia. A comparison with the total aerosol data from an optical particle counter onboard aircraft suggested that dust dominated aerosols larger than 0.8 μm, consistent with electron microscopy analyses. The observed dust size distributions implied more efficient removal of larger dust during transport. Since our samples were collected at −18°C to 21°C, the free tropospheric dust could contribute to ice formation in clouds. Our observation using the CAS revealed microphysical properties of dust over the western North Pacific when no distinct Asian dust events were observed near the ground, aiding in the validation of models that assess the climatic impacts of dust.