Airborne Measurements of Mineral Dust Abundance, Mixing State, and Ice Nucleating Properties

Airborne mineral dust aerosol can interact with the global climate system by altering atmospheric radiative balance through direct absorption and scattering and also by acting as heterogeneous ice nuclei for cirrus cloud formation. In a recent airborne study, mineral dust was found to dominate cirrus cloud formation across several northern hemisphere regions. Here we present a summary of single-particle measurements in the free troposphere from more than 10 years of airborne field campaigns, highlighting the geographic and vertical distributions of mineral dust, source attribution, and dust's role in cirrus formation. Clear sky aerosol measurements from 7 aircraft campaigns show that mineral dust is ubiquitous in the background upper troposphere at levels from ~1 to 1000's per liter and typically accounts for 5-40% of the particulate mass. Principal sources of upper tropospheric mineral dust include strong biomass burning events and deep convection, although some evidence suggests that dust particles are preferentially scavenged in convective systems. During transport mineral dust particles accumulate secondary sulfate, nitrate, and organic material that can reduce their efficiency as heterogeneous IN. Most upper tropospheric dust particles contain secondary material, and coating type and thickness depend on co-emissions and the vertical transport mechanism.