AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
The Sources and Evolution of Ice Nucleation Particles Emitted by Biomass Burning
LYDIA JAHL, Leif Jahn, Michael Polen, Thomas Brubaker, Bailey Bowers, Sara Graves, Ryan Sullivan, Carnegie Mellon University
Abstract Number: 667 Working Group: Biomass Combustion: Emissions, Chemistry, Air Quality, Climate, and Human Health
Abstract Surprisingly, the combustion of some biomass fuels has been found to produce ice nucleating particles (INPs) that induce immersion freezing at temperatures as warm as –20 °C. Biomass burning thus represents a potentially large but unconstrained source of INPs to the atmosphere that can significantly change cloud microphysics, precipitation, and climate. The source of these ice nucleating particles (INPs) is still under investigation; soot particles do not appear to account for all INPs emitted by biomass burning. We report measurements of efficient INPs in the bottom ash and aerosol produced from biomass burning. The ash from tall grass fuels that combust intensely were found to contain a larger fraction of crystalline material through X-ray diffraction analysis, which likely explains the higher ice activity of the ash. The biomass burning aerosol from these fuels also contained efficient INPs. Biomass burning can therefore introduce new ice-active mineral phases produced by combustion into the atmosphere, in addition to lofting pre-existing mineral and soil particles that may also act as INPs. Fuels that contain higher mass fractions of elements such as calcium and silicon tend to produce more crystalline content upon combustion, providing a connection between the original fuel and the new mineral phases formed in the aerosol and ash, and their ice nucleation activity.
Biomass burning aerosol undergoes extensive physical and chemical transformations during atmospheric transport, and we examined how this evolution might alter the ice nucleation properties of the aerosol. The chemical aging of combustion aerosol from a variety of biomass fuels including sawgrass, birch, cutgrass, and black needlerush was simulated using a smog chamber reactor. Aging such as through dark ozonolysis or UV photolysis of HONO, was often found to enhance the ice activity of the aerosol. We hypothesize that aging makes the mineral-based ice nucleants more accessible to water and thus ice crystal formation. The chemical composition of the aerosol prior to and following aging was analyzed using two single-particle mass spectrometers (SP-AMS and LAAPTOF), and the particles were collected on filters for SEM/EDX analysis and to determine their ice nucleating abilities. Our novel microfluidic droplet freezing assay allowed us to reliably determine the immersion freezing temperature of the aerosol down to –32 °C, critically enabling measurements at lower temperatures where biomass burning aerosol typically induces freezing. We will present our findings on the changes in the aerosol’s composition following chemical aging, and how this correlates with the ice nucleating abilities of fresh and aged biomass burning emissions.