During the ARCPAC (Aerosol, Radiation, and Cloud Processes affecting Arctic Climate) airborne field experiment in April 2008 in northern Alaska, about 50 plumes were encountered with the NOAA WP-3 aircraft between the surface and 6.5 km. Onboard measurements and the transport model FLEXPART showed that most of the plumes were emitted by forest fires in southern Siberia-Lake Baikal area and by agricultural burning in Kazakhstan-southern Russia. Unexpectedly, these biomass burning plumes were the dominant aerosol and gas-phase features encountered in this area during April. The influence on the plumes from sources other than burning was small. The chemical characteristics of plumes from the two source regions were different, with higher enhancements relative to CO for most gas and aerosol species from the agricultural fires. In 2008, the fire season started earlier than usual in Siberia, which may have resulted in unusually efficient transport of biomass burning emissions into the Arctic.