Quantifying black carbon from biomass burning by means of levoglucosan – a one-year time series at the Arctic observatory Zeppelin

摘要

Levoglucosan, a highly specific tracer of particulate matter from biomass burning, has been used to study the influence of residential wood burning, agricultural waste burning and Boreal forest fire emissions on the Arctic atmosphere black carbon (BC) concentration. A one-year time series from March 2008 to March 2009 of levoglucosan has been established at the Zeppelin observatory in the European Arctic. Elevated concentrations of levoglucosan in winter (mean: 1.02 ngm~(?3)) compared to summer (mean: 0.13 ngm~(?3)) were observed, resembling the seasonal variation seen for e.g. sulfate and BC. The mean concentration in the winter period was 2–3 orders of magnitude lower than typical values reported for European urban areas in winter, and 1–2 orders of magnitude lower than European rural background concentrations. Episodes of elevated levoglucosan concentration lasting from 1 to 6 days were more frequent in winter than in summer and peak values were higher, exceeding 10 ngm~(?3) at the most. Concentrations of elemental carbon from biomass burning (EC_(bb)) were obtained by combining measured concentrations of levoglucosan and emission ratios of levoglucosan and EC for wildfires/agricultural fires and for residential wood burning. Neglecting chemical degradation by OH provides minimum levoglucosan concentrations, corresponding to a mean EC_(bb) concentration of 3.7±1.2 ngm~(?3) in winter (October–April) and 0.8±0.3 ngm~(?3) in summer (May– September), or 8.8±4.5% of the measured equivalent black carbon (EBC) concentration in winter and 6.1±3.4% in summer. When accounting for chemical degradation of levoglucosan by OH, an upper estimate of 31–45% of EBC could be attributed to EC_(bb)~* (EC_(bb) adjusted for chemical degradation) in winter, whereas no reliable (<100 %) upper estimate could be provided for summer for the degradation rates applied. Hence, fossil fuel sources appear to dominate the European Arctic BC concentrations in winter, whereas the very wide range obtained for summer does not allow us to conclude upon this for the warm season. Calculations using the Lagrangian particle dispersion model FLEXPART show that the seasonal variation of the modeled EC_(bb) (EC_(bb,m)) concentration compared relatively well with observationally derived EC_(bb) from agricultural fires/wildfires during summer, and residential wood burning in winter. The model overestimates by a factor of 2.2 in winter and 4.4 in summer when compared to the observationally derived mean EC_(bb) concentration, which provides the minimum estimate, whereas it underestimates by a factor of 2.3– 3.3 in winter and a factor of 4.5 in summer when compared to EC_(bb)~*, which provides the upper estimate. There are indications of too-low emissions of residential wood burning in northern Russia, a region of great importance with respect to observed concentrations of BC in the European Arctic.