Seasonal Cycle of Isotope-Based Source Apportionment of Elemental Carbon in Airborne Particulate Matter and Snow at Alert, Canada

Elemental carbon (EC) is a major light-absorbing component of atmospheric aerosol particles. Here, we report the seasonal variation in EC concentrations and sources in airborne particulate matter (PM) and snow at Alert, Canada, from March 2014 to June 2015. We isolated the EC fraction with the EnCan-Total-900 (ECT9) protocol and quantified its stable carbon isotope composition (delta C-13) and radiocarbon content ( increment C-14) to apportion EC into contributions from fossil fuel combustion and biomass burning (wildfires and biofuel combustion). Ten-day backward trajectories show EC aerosols reaching Alert by traveling over the Arctic Ocean from the Russian Arctic during winter and from North America (>40 degrees N) during summer. EC concentrations range from 1.8-135.3 ng C m(-3) air (1.9-41.2% of total carbon [TC], n = 48), with lowest values in summer (1.8-44.5 ng C m(-3) air, n = 9). EC in PM (Delta C-14 = -532 +/- 114 parts per thousand [ave. +/- SD, n = 20]) and snow (-257 +/- 131 parts per thousand, n = 7) was depleted in C-14 relative to current ambient CO2 year-round. EC in PM mainly originated from liquid and solid fossil fuels from fall to spring (47-70% fossil), but had greater contributions from biomass burning in summer (48-80% modern carbon). EC in snow was mostly from biomass burning (53-88%). Our data show that biomass burning EC is preferentially incorporated into snow because of scavenging processes within the Arctic atmosphere or long-range transport in storm systems. This work provides a comprehensive view of EC particles captured in the High Arctic through wet and dry deposition and demonstrates that surface stations monitoring EC in PM might underestimate biomass burning and transport.