Cloud effects from boreal forest fire smoke: evidence for ice nucleation from polarization lidar data and cloud model simulations

Polarization lidar observations from the interior of Alaska have revealed unusual supercooled altocumulus cloud conditions in the presence of boreal forest fire smoke from local and regional fires. At temperatures of about - 15 degrees C, the lidar data show ice nucleation prior to liquid cloud formation ( i. e. below water saturation), as well as the occasional glaciation of the liquid layer. Thus the smoke aerosol appears to act as ice nuclei that become activated in updrafts before the liquid cloud forms, as the concentrated aqueous organic solutions are diluted sufficiently to allow them to freeze heterogeneously. This haze particle freezing process is similar to the production of cirrus ice crystals homogeneously at much colder temperatures. To test this hypothesis, cloud microphysical model simulations constrained by the measurements were performed. They indicate that this heterogeneous ice nucleation scenario can be supported by the cloud model. Although ice formation in this manner may generally act in the atmosphere, the boreal smoke particles produce an unusually dramatic effect in the lidar data. We conclude that smoke-induced ice nucleation occurs at moderate supercooled temperatures either through the effects of raised soil/dust particles embedded in the smoke droplets, coated soot aerosol or through the nucleation via certain organic solutions.