Recent Advances in Airborne Aerosol Measurements and Implications for Cold Cloud Formation (Invited Presentation)

Laboratory studies have identified a variety aerosol types that heterogeneously nucleate ice at upper tropospheric conditions. High altitude airborne measurements in various cirrus environments implicate mineral dust particles as predominant heterogeneous ice nuclei (IN). However, field evidence also suggests that other particle types can act as heterogeneous IN. We summarize recent airborne aerosol measurements and what they can tell us about cold cloud formation mechanisms.

The NASA ATom (Atmospheric Tomography) 2016-2018 campaign was an airborne global mapping survey that spanned 165 degrees of latitude, two ocean basins, and four seasons, giving an unprecedented view of aerosol properties in the remote troposphere. This novel data set helps fill critical measurement gaps for mineral dust and other potential ice nuclei. We consider mineral dust in detail by examining spatial patterns, seasonal variations, inter-hemispheric gradients, secondary coatings, and the implications for cold cloud formation. Furthermore, we compare measured dust concentrations to a global simulation using a sectional aerosol scheme to investigate dust sources and assess our predictive ability for this important IN. Model accuracy was highly sensitive to aerosol transport and wet removal schemes, and discrepancies with measurements were often greatest at cirrus altitudes.

Other potential IN are considered in light of recent laboratory studies, including biomass burning particles, sea salt, crystalline ammonium sulfate, and glassy organic particles. In particular, ammonium sulfate and organic particles are ubiquitous throughout the upper troposphere. At the highest altitudes where mineral dust concentrations can be too low to influence cirrus, these inefficient yet abundant IN have the potential to dominate heterogeneous nucleation. Recent advances in online detection of primary biological particles and industrial fly ash help constrain their roles in cirrus formation.