Undergraduate Research and Creativity

URECA

2007-2008

Homogeneous Ice Nucleation from Aqueous Particles Containing
Surrogates of Biomass Burning Aerosols

Miguel D. Lopez, Bing Bing Wang, Paul L. Siegel and Daniel A. Knopf
School of Marine and Atmospheric Sciences

Ice particles in the atmosphere affect the global radiation budget by scattering and absorbing radiation, the water budget in the upper troposphere/lower stratosphere, and precipitation processes. Ice particles form from pre-existing aerosol particles at temperatures below the freezing point. Although its importance is highly recognized, aerosol-ice cloud interactions remain one of the largest unknowns to predict future changes in the global radioactive budget and thus in climate. Aerosol particles from biomass burning plumes originating e.g. from forest fires, can reach altitudes at which temperatures are low enough to promote the freezing of ice. It is expected that the wildfire frequency is increasing due to an increase in global temperature and consequently generating more biomass burning particles. Here, we study the effect of typical biomass burning compounds, such as levoglucosan, on ice nucleation. The formation of ice is observed using optical microscopy and is digitally recorded for subsequent analysis. First, we measure homogeneous ice nucleation from aqueous (NH₄)₂SO₄ droplets which also serves experimental validation purposes. Then, we measure the freezing of ice from aqueous levoglucosan-(NH₄)₂SO₄ particles and pure levoglucosan-H₂O particles. About 50 droplets are deposited on a hydrophobically coated glass plate using a piezo-electric driven droplet dispenser or nebulizer. During sample preparation the droplets are exposed to a controlled humidified flow of helium that, in equilibrium, controls the composition of the particles. In this experiment, the droplets were exposed to relative humidities of 99%, 95%, 90%, 85% and 81%. Then the particles are sealed against ambient air and are exposed to temperatures as low as 190 K using a cryo cooling stage. Previous to the experiments the cryo cooling stage was calibrated measuring the melting points of water, octane, pentane and dodecane. From the digitally recorded ice nucleation experiments the freezing temperature, droplet diameter, and the time the droplets remained liquid are obtained to derive homogeneous ice nucleation rates. The ice nucleation rate is then applied to estimate if biomass burning plumes can be a significant source of ice particles in the atmosphere.