Properties and evolution of biomass burning organic aerosol from Canadian boreal forest fires

Airborne measurements of biomass burning or- ganic aerosol (BBOA) from boreal forest fires reveal highly contrasting properties for plumes of different ages. These measurements, performed using an Aerodyne Re- search Inc. compact time-of-flight aerosol mass spectrom- eter (C-ToF-AMS) during the BORTAS (quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites) experiment in the summer of 2011, have been used to derive nor- malised excess organic aerosol (OA) mass concentrations (1OA=1CO), with higher average ratios observed closer to source (0.190 0.010) than in the far-field (0.097 0.002). The difference in 1OA=1CO between fresh and aged plumes is influenced by a change in dominant combus- tion conditions throughout the campaign. Measurements at source comprised 3 plume interceptions during a single re- search flight and sampled largely smouldering fires. Twenty- three interceptions were made across four flights in the far- field, with plumes originating from fires occurring earlier in the campaign when fire activity had been more intense, cre- ating an underlying contrast in emissions prior to any trans- formations associated with aging. Changing combustion con- ditions also affect the vertical distribution of biomass burn- ing emissions, as aged plumes from more flaming-dominated fires are injected to higher altitudes of up to 6000 m. Pro- portional contributions of the mass-to-charge ratio (m/z/ 60 and 44 peaks in the AMS mass spectra to the total OA mass (denoted f60 and f44/ are used as tracers for primary and oxidised BBOA, respectively. f44 is lower on average in near-field plumes than those sampled in the far-field, in accordance with longer aging times as plumes are trans- ported a greater distance from source. However, high levels of 1O3=1CO and log(NOx= NOy/ close to source indi- cate that emissions can be subject to very rapid oxidation over short timescales. Conversely, the lofting of plumes into the upper troposphere can lead to the retention of source pro- files after transportation over extensive temporal and spatial scales, withf60 also higher on average in aged plumes. Evo- lution of OA composition with aging is comparable to obser- vations of BB tracers in previous studies, revealing a consis- tent progression fromf60 tof44. The elevated levels of oxy- genation in aged plumes, and their association with lower average 1OA=1CO, are consistent with OA loss through evaporation during aging due to a combination of dilution and chemical processing, while differences in combustion conditions throughout the campaign also have a significant influence on BBOA production and composition.