Biomass Burning as a Source of Mineral Dust and Giant Aerosol to the Free Troposphere

Biomass burning is a dominant emission source of fine mode aerosol to the atmosphere. Fires can also emit supermicron particles that are important for cloud formation. Mineral dust aerosol exert a strong influence on formation of cirrus clouds by acting as efficient ice nuclei. Giant aerosol larger than ~10 microns can dominate condensational growth inside warm clouds and affect precipitation rates. Airborne measurements of aerosol concentration and single particle composition were conducted inside smoke plumes from fires originating in North America, Asia, and Central American. Sub- and supermicron mineral dust concentrations were strongly correlated with biomass burning particle concentrations, both within strong fire plumes and in the background troposphere. Dust concentrations varied with fire intensity and vegetation type. In general, western US wildfires emitted more dust than eastern US and Central American fires. Dust concentrations inside plumes were strongly enhanced relative to background air, demonstrating that biomass burning can provide a significant source of potential ice nuclei to the free troposphere. However, dust particles emitted from fires were coated with more organic carbon material than dust from other sources, which may reduce their ability to act as ice nuclei. Giant aerosol particles up to several 100 microns in size were also observed within some fire plumes and could account for 50% of the particle volume. Particle images show irregular shapes and suggest ash and fibrous plant material. Giant aerosol persisted inside plumes in the free troposphere for 100's of km downwind from fire sources, indicating that particles must have low density to remain aloft. This combination of direct injection above the boundary layer and reduced depositional loss provides a regional source of particles that may act as giant cloud condensation nuclei.