Articles | Volume 14, issue 24
Research article
23 Dec 2014
Research article |  | 23 Dec 2014

Origin, variability and age of biomass burning plumes intercepted during BORTAS-B

D. P. Finch, P. I. Palmer, and M. Parrington

Abstract. We use the GEOS-Chem atmospheric chemistry transport model to interpret aircraft measurements of carbon monoxide (CO) in biomass burning outflow taken during the 2011 BORTAS-B campaign over eastern Canada. The model has some skill reproducing the observed variability, with a Spearman's rank correlation rs = 0.65, but has a positive negative bias for observations <100 ppb and a negative bias for observations > 300 ppb. We find that observed CO variations are largely due to fires over Ontario, as expected, with smaller and less variable contributions from fossil fuel combustion from eastern Asia and NE North America. To help interpret observed variations of CO we develop a Eulerian effective physical age of emissions (A) metric, accounting for mixing and chemical decay, which we apply to pyrogenic emissions of CO. We find that during BORTAS-B the age of emissions intercepted over Halifax, Nova Scotia is typically 4–11 days, and on occasion as young as two days. We show that A is typically 1–5 days older than the associated photochemical ages inferred from co-located measurements of different hydrocarbons. We find that the frequency distribution of differences between the age measures (Δτ) in plumes (defined by CH3CN > 150 ppt) peaks at 3 days. This corresponds to a chemical retardation of 50%. We find a strong relationship in biomass burning plumes between A and Δτ (r2 = 0.80), which is not present outwith these plumes (r2 = 0.28). We argue that these observed relationships, together with a robust observed relationship between CO and black carbon aerosol during BORTAS-B (r2 > 0.7), form the basis of indirect evidence that aerosols co-emitted with gases during pyrolysis markedly slowed down the plume photochemistry during BORTAS-B with respect to photochemistry at the same latitude and altitude in clear skies.

Short summary
We use the GEOS-Chem chemical transport model to quantify the CO sources responsible for the observed CO during the BORTAS-B campaign over Canada in 2011. We found the largest source was biomass burning from Ontario, with smaller sources from fossil fuel emissions from Asia and NE US. We develop an age-of-emission metric and show values in BORTAS-B are consistent with a slowing of photochemistry in plumes. Indirect evidence suggests this slowing is due to aerosols within the plumes.
Final-revised paper