Ice nucleation behavior of particles generated in open burning of biomass

Biomass burning represents a large global source of carbonaceous particulate matter, and interactions between smoke aerosols and clouds may have regional and longer-range impacts on radiative balance and precipitation. During the second Fire Lab At Missoula Experiment (FLAME-II), we conducted measurements of the ice nucleation ability of biomass burning particles generated in the laboratory from open burning of a variety of North American fuels. Our measurements were made with dual Colorado State University continuous-flow diffusion chambers, covering two temperature regimes, chosen to address potential impacts on both supercooled midlevel clouds (-30°C) and colder upper tropospheric clouds (-45 to -60°C). Approximately 1 in 100 to 1 in 10,000 particles from 9 of the 20 tested fuels acted as condensation/immersion freezing ice nuclei at -30°C, while emissions from the others were below our detection limit. Below water saturation and at temperatures below -45°C, ice formation by smoke particles followed predictions from water activity-based homogeneous freezing parameterizations, while the action of some smoke particles as heterogeneous ice nuclei in this regime could not be unambiguously distinguished within measurement uncertainties. Our data show that biomass burning emissions from some fuels are capable of significantly enhancing the number concentrations of particles active in ice formation in clouds over a wide range of atmospheric temperatures.