EC analysis

The EC and organic carbon (OC) concentrations were determined by a thermal-optical transmission (TOT) analyzer (instrument #227, Sunset Laboratory, Tigard, OR) using the National Institute for Occupational Safety and Health 5040 protocol (34). This method is known to potentially overcorrect charring fraction relative to other methods and to underestimate the EC content (35–39). During charring, parts of the OC could also end up in the EC fraction in the form of pyrogenic carbon, which would then influence the isotopic composition of the EC fraction. This effect was evaluated by a sensitivity analysis in a previous study, which concluded that the radiocarbon-derived fraction of biomass burning could be overestimated by a maximum of 7%, in extreme cases (25). The detection limit for EC was based on the OC concentration of the field blanks, which had ~100 ng C cm⁻² OC (this roughly translates to 2 ng C m⁻³ for Zeppelin samples, which had the lowest EC concentrations measured). EC could not be detected in any of the blanks.

Annual mean concentrations were calculated asEC¯=∑i=1nEC(i)⋅V(i)∑i=1nV(i)(1)where EC is the EC concentration unit (ng C m⁻³), V is the volume collected for the respective aerosol filter sample, and i is the sample index.