Robert W. Vaughan Lecture in Chemical Engineering

   The effect of human activities on climate is a grand challenge facing society today.  Humans influence climate in many ways.  Emissions of greenhouse gases (GHGs) tend to warm climate by reducing the amount of infrared radiation that is emitted to space.  Increased levels of suspended atmospheric particles ("aerosols") exert a net cooling effect by directly scattering and absorbing incoming solar radiation.  Aerosols also affect clouds by acting as the seed for droplet (or ice crystal) formation.  "Seeding" of clouds by anthropogenic pollution is thought to cool climate by modulating cloud reflectivity and development.  Aerosol variations have also been proposed to affect the development of storm systems, precipitation and the hydrological cycle overall.  Quantitatively constraining aerosol impacts on clouds and climate, however, is very uncertain and significantly affects predictions of climate sensitivity to GHG levels.  The large uncertainty originates largely from the complex and multi-scale coupling of aerosols and clouds.  Added to this complexity is the large variability and range of aerosol types, each of which is characterized with its own ability to nucleate droplets and ice crystals.

   This talk will present key advancements on the description of aerosol-cloud interactions in climate model frameworks through the combination of observations, theory and modeling.  We will first focus on advancements in the physical representation of droplet and ice formation in models, and demonstrate how instrument development efforts helped solve long-standing issues regarding parametric uncertainty for droplet formation from atmospheric aerosol.  We will then focus on the importance of aerosol-cloud interactions in storm development, specifically on the role of aerosol in the rapid intensification of tropical cyclones. We will conclude by presenting work on airborne atmospheric microbes, an understudied class of particles with potentially important and unique impacts on cloud formation.  Results will be presented using a combination of molecular tools and functional experiments on airborne microbes collected in situ from airborne platforms.