Biomass burning in Siberia as a source of BrO to the Arctic free troposphere

During the ARCPAC (Aerosol, Radiation, and Cloud Processes affecting Arctic Climate) campaign in April 2008, several flights of the WP-3D research aircraft over the Arctic Ocean and Alaska sampled relatively rich bromine monoxide (BrO > 5 pptv) air masses in the free troposphere. In most cases air masses were identified as either stratospheric intrusions or as being recently in contact with sea-ice and thereby pointing to an established BrO source. However, in at least two instances there was a strong indication that the BrO originated, directly or as a secondary product from forest fires in Siberia. In all cases the origin of the air masses studied (polluted or stratospheric influenced) was confirmed by trajectories calculations and carbon monoxide (CO), ozone (O3), water vapor (H2O), bromine monochloride (BrCl), and acetonitrile (CH3CN) measurements on-board the aircraft. In spite of this finding, the mechanisms of BrO release in the free troposphere, possibly involving heterogeneous reactions, remain unclear.