Arctic Aerosols, Springtime Forest Fires, and Climate

Layers of dense smoke mixed with fossil fuel combustion products were frequently encountered at altitudes from near the surface to 7 km in the Alaskan Arctic in early spring 2008 during NOAA's Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC) airborne research project. Transport models demonstrate that the smoke originated from agricultural and forest fires in Russia, and was transported over distances >5000 km. These smokes appear to be a frequent occurrence in the Arctic in April-June due to agricultural burning and wildfires near the Kazahkstan border and the Chita/Amur River regions, respectively. Aerosol particles in the Arctic in springtime have been postulated to affect climate through direct radiative forcing and strong feedbacks between deposited absorbing particles and snow physics. However, in 2008 there was little substantive evidence for precipitation scavenging and removal of the smoke particles between the time they were emitted and the time they were observed above the Arctic surface. Given the lack of scavenging, the dense smoke layers aloft may not have contributed substantially to changes in snow albedo. Furthermore, direct radiative forcing from the smoke aerosol is calculated to be small and to cool the surface. Independent of the smoke layers, aerosol concentrations very near the sea-ice surface were reduced, suggesting a deposition process from within this shallow layer to the sea-ice. The relevance of these findings to the 'Arctic haze' phenomenon and to climate forcing in the Arctic will be discussed.