Indirect cloud effects from Alaskan smoke: Evidence for ice formation below water saturation

Particularly over the past two summer seasons, forest fires in the interior of Alaska have been active on a maximum historic level. Regular polarization lidar observations from the Arctic Facility for Atmospheric Remote Sensing (AFARS) in Fairbanks reveal that deep smoke layers can be a common occurrence, ranging from dense and unhealthy to weak but prolonged aerosol concentrations. Many cases show supercooled liquid water cloud formation at the top of the smoke boundaries, and at temperatures of ~-15 degree C or colder, ice crystals often occur at slightly lower heights with and without the altocumulus cloud layers being present. An explanation, which is supported by the cloud model simulations we present, is that the heterogeneous nucleation of haze particles is taking place somewhat below water saturation as the insoluble fraction of solute initiates ice nucleation in swelling haze particles. (A similar mechanism, but reliant on homogeneous nucleation, is regarded as important in cirrus cloud formation at colder temperatures.) Model results using heterogeneous freezing contact parameters of m ~ 0.5 reproduce the observations. Previous studies of the composition of smoke indicate a mixture of organic vapors, soot, and minerals, which may certainly act in this capacity. Clearly, changes in midlevel cloud phase due to smoke effects, perhaps linked to climate change itself, may have impacts on radiative transfer in addition to the gross effects of the smoke aerosol itself.