Seasonal cycle of isotope-based source apportionment of elemental carbon in airborne particulate matter and snow at Alert, Canada

Abstract:

Elemental carbon (EC) is a key component of light-absorbing atmospheric aerosol particles. In this study, we quantified the seasonal variation in EC concentrations and sources in airborne particulate matter (PM) and snow at Alert, Canada from March 2014 to June 2015. The EC fraction was isolated with the EnCanTotal-900 (ECT9) protocol and apportioned into contributions from fossil fuel combustion and biomass burning by combining measurements of its stable carbon isotope composition (δ¹³C) and radiocarbon content (∆¹⁴C) with isotope mass balance approaches.

EC concentrations ranged 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). Backward trajectory modeling (HYSPLIT) showed that EC aerosols reached Alert by traveling over the Arctic Ocean from North America (>40°N) during summer and from the Russian Arctic during winter. The isotope analysis revealed that EC was depleted in ¹⁴C relative to current ambient CO₂ year-round, both in PM (Δ¹⁴C=-532±114‰ (ave.±SD, n=20)) and snow (-257±131‰, n=7).

The results show that EC in PM mainly originated from liquid and solid fossil fuels from fall to spring (47-70% fossil), with greater contributions from biomass burning in summer (48-80% modern carbon). EC in snow was mostly from biomass burning (53-88%), indicating that biomass burning-EC is preferentially incorporated into snow because of scavenging processes within the Arctic atmosphere or long-range transport in storm systems. Together, the data provide 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.