Ozone photochemistry in boreal biomass burning plumes

Abstract. We present an analysis of ozone (O₃) photochemistry observed by aircraft measurements of boreal biomass burning plumes over eastern Canada in the summer of 2011. Measurements of O₃ and a number of key chemical species associated with O₃ photochemistry, including non-methane hydrocarbons (NMHCs), nitrogen oxides (NO_x) and total nitrogen containing species (NO_y), were made from the UK FAAM BAe-146 research aircraft as part of the "quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites" (BORTAS) experiment between 12 July and 3 August 2011. The location and timing of the aircraft measurements put BORTAS into a unique position to sample biomass burning plumes from the same source region in Northwestern Ontario with a range of ages. We found that O₃ mixing ratios measured in biomass burning plumes were indistinguishable from non-plume measurements, but evaluating them in relationship to measurements of carbon monoxide (CO), total alkyl nitrates (ΣAN) and the surrogate species NO_z (= NO_y-NO_x) revealed that the potential for O₃ production increased with plume age. We used NMHC ratios to estimate photochemical ages of the observed biomass burning plumes between 0 and 10 days. The BORTAS measurements provided a wide dynamic range of O₃ production in the sampled biomass burning plumes with ΔO₃/ΔCO enhancement ratios increasing from 0.020 ± 0.008 ppbv ppbv⁻¹ in plumes with photochemical ages less than 2 days to 0.55 ± 0.29 ppbv ppbv⁻¹ in plumes with photochemical ages greater than 5 days. We found that the main contributing factor to the variability in the ΔO₃/ΔCO enhancement ratio was ΔCO in plumes with photochemical ages less than 4 days, and that was a transition to ΔO₃ becoming the main contributing factor in plumes with ages greater than 4 days. In comparing O₃ mixing ratios with components of the NO_y budget, we observed that plumes with ages between 2 and 4 days were characterised by high aerosol loading, relative humidity greater than 40%, and low ozone production efficiency (OPE) of 7.7 ± 3.5 ppbv ppbv⁻¹ relative to ΣAN and 1.6 ± 0.9 ppbv ppbv⁻¹ relative to NO_z. In plumes with ages greater than 4 days, OPE increased to 472 ± 28 ppbv ppbv⁻¹ relative to ΣAN and 155 ± 5 ppbv ppbv⁻¹ relative to NO_z. From the BORTAS measurements we estimated that aged plumes with low aerosol loading were close to being in photostationary steady state and O₃ production in younger plumes was inhibited by high aerosol loading and greater production of ΣAN relative to O₃. The BORTAS measurements of O₃ photochemistry in boreal biomass burning plumes were found to be consistent with previous summertime aircraft measurements made over the same region during the Arctic Research of the Composition of the Troposphere (ARCTAS-B) in 2008 and Atmospheric Boundary Layer Experiment (ABLE 3B) in 1990.