Ice-nucleating particle emissions from biomass combustion and the potential importance of soot aerosol

Ice-nucleating particles (INPs) are required for initial ice crystal formation in clouds at temperatures warmer than about -36 degrees C and thus play a crucial role in cloud and precipitation formation. Biomass burning has been found to be a source of INPs in previous studies and is also a major contributor to atmospheric black carbon (BC) concentrations. This study focuses on isolating the BC contribution to the INP population associated with biomass combustion. Emissions of condensation mode INPs from a number of globally relevant biomass fuels were measured at -30 degrees C and above water saturation as fires progressed from ignition to extinguishment in a laboratory setting. Number emissions of INPs were found to be highest during intense flaming combustion (modified combustion efficiency > 0.95). Overall, combustion emissions from 13 of 22 different biomass fuel types produced measurable INP concentrations for at least one replicate experiment. On average, all burns that produced measureable INPs had higher combustion efficiency, which is associated with higher BC emissions, than those that did not produce measureable INPs. Across all burns that produced measureable INPs, concentrations ranged from 0.1 to 10 cm(-3), and the median emission factor was about 2 x 10(7) INPs per kilogram of fuel burned. For a subset of the burns, the contribution of refractory black carbon (rBC) to INP concentrations was determined by removing rBC via laser-induced incandescence. Reductions in INPs of 0-70% were observed, indicating an important contribution of rBC particles to INP concentrations for some burns, especially marsh grasses.