Production of peroxy nitrates in boreal biomass burning plumes over Canada during the BORTAS campaign

Abstract

The observations collected during the BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites (BORTAS) campaign in summer 2011 over Canada are analysed to study the impact of forest fire emissions on the formation of ozone (O₃) and total peroxy nitrates ∑PNs, ∑ROONO₂). The suite of measurements on board the BAe-146 aircraft, deployed in this campaign, allows us to calculate the production of O₃ and of ∑PNs, a long-lived NOx reservoir whose concentration is supposed to be impacted by biomass burning emissions. In fire plumes, profiles of carbon monoxide (CO), which is a well-established tracer of pyrogenic emission, show concentration enhancements that are in strong correspondence with a significant increase of concentrations of ∑PNs, whereas minimal increase of the concentrations of O₃ and NO₂ is observed. The ∑PN and O₃ productions have been calculated using the rate constants of the first- and second-order reactions of volatile organic compound (VOC) oxidation. The ∑PN and O₃ productions have also been quantified by 0-D model simulation based on the Master Chemical Mechanism. Both methods show that in fire plumes the average production of ∑PNs and O₃ are greater than in the background plumes, but the increase of ∑PN production is more pronounced than the O₃ production. The average ∑PN production in fire plumes is from 7 to 12 times greater than in the background, whereas the average O₃ production in fire plumes is from 2 to 5 times greater than in the background. These results suggest that, at least for boreal forest fires and for the measurements recorded during the BORTAS campaign, fire emissions impact both the oxidized NOy and O₃, but (1 ∑PN production is amplified significantly more than O₃ production and (2) in the forest fire plumes the ratio between the O₃ production and the ∑PN production is lower than the ratio evaluated in the background air masses, thus confirming that the role played by the ∑PNs produced during biomass burning is significant in the O₃ budget. The implication of these observations is that fire emissions in some cases, for example boreal forest fires and in the conditions reported here, may influence more long-lived precursors of O₃ than short-lived pollutants, which in turn can be transported and eventually diluted in a wide area.