Ice nucleation behavior of biomass combustion particles at cirrus temperatures

Measurements of ice formation by biomass combustion particles generated in controlled laboratory burns were made using a continuous flow diffusion chamber operating in the temperature range between -45 and -60°C. These measurements are the first of their kind to investigate the role that such particles may play in ice formation in upper tropospheric clouds. Measurements of aerosol water uptake were used to examine the role of hygroscopicity in low-temperature ice nucleation by biomass combustion particles. Ice formation by the smoke particles at below -45°C and below water saturation followed the temperature and relative humidity dependencies predicted for water activity-based homogeneous freezing within measurement uncertainties. As predicted, relative humidity dependence of homogeneous freezing fractions on hygroscopicity was weak in comparison with the temperature dependence for biomass particles of typical atmospheric sizes. The action of heterogeneous ice nucleation in or on these small smoke particles in the haze particle regime could not be distinguished within measurement uncertainties. Impedance of homogeneous freezing below -50°C was inferred for freezing higher number fractions of predominantly organic smoke particles separately reported to contain an abundance of large organic molecules, consistent with the recently predicted onset conditions for and impact of viscous or glassy aerosol phase states. Finally, we note that precision and accuracy requirements of instrumentation for resolving modest impacts (a few degrees less supercooling or a few percent lower relative humidity) of heterogeneous freezing, as compared with homogeneous freezing in the upper tropospheric temperature regime present difficulties for validating aerosol impacts on cirrus.