The effects of large dust particles coated with salts on droplet formation near cloud base in convective updrafts

An airborne study of cloud microphysics provided an opportunity to collect particle samples on transmission electron microscopy (TEM) grids in ambient and updraft conditions of natural convection systems. Particles were collected simultaneously on lacey-carbon (LC) and calcium-coated carbon (Ca-C) grids, providing observation of particle morphology and chemistry, as well as a unique record of the particle's physical state on impact. In total, 22 particle categories were identified using TEM, including single, coated, aggregate, and droplet types. The fine-fraction comprised up to 90% mixed-cation sulfates (MCS), whereas coarse particles comprised up to 80% mineral-containing aggregates. Insoluble particles (dry), partially soluble particles (wet), as well as fully soluble particles (droplets) were recorded in Ca-C samples. Dry particles were typically single silicate grains. Wet particles were mineral-containing aggregates with chloride, nitrate, or sulfate components. The two main droplet types were aqueous NaCl and MCS. Higher numbers of droplets were present in updraft conditions (80% relative humidity, RH) compared with ambient conditions (60% RH). Almost all ambient particles had activated at cloud base (100% RH). Greatest changes in hygroscopicity were observed in NaCl-containing aggregates (>0.3 μm diameter) creating more large and giantCCN. Their abundance resulted in high numbers of cloud condensation nuclei (CCN) and cloud droplets, as well as larger droplet sizes in updraft particle populations (giant CCN). A clear compositional-dependence was observed to hygroscopic growth, resulting in different activation behavior for coarse and fine modes. Soluble salts both from local pollution and natural sources clearly affected aerosol-cloud interactions by enhancing the spectrum of particles that formed CCN and by forming giant CCN.