Ozone photochemistry in boreal biomass burning plumes

We present an analysis of ozone (O-3) photochemistry observed by aircraft measurements of boreal biomass burning plumes over eastern Canada in the summer of 2011. Measurements of O-3 and a number of key chemical species associated with O-3 photochemistry, including non-methane hydrocarbons (NMHCs), nitrogen oxides (NOx) and total nitrogen containing species (NOy), 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 O-3 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 (Sigma AN) and the surrogate species NOz (= NOy - NOx) revealed that the potential for O-3 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 O-3 production in the sampled biomass burning plumes with Delta O-3/Delta CO 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 Delta O-3/Delta CO enhancement ratio was Delta CO in plumes with photochemical ages less than 4 days, and that was a transition to Delta O-3 becoming the main contributing factor in plumes with ages greater than 4 days. In comparing O-3 mixing ratios with components of the NOy 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 Sigma AN and 1.6 +/- 0.9 ppbv ppbv(-1) relative to NOz. In plumes with ages greater than 4 days, OPE increased to 472 +/- 28 ppbv ppbv(-1) relative to Sigma AN and 155 +/- 5 ppbv ppbv(-1) relative to NOz. From the BORTAS measurements we estimated that aged plumes with low aerosol loading were close to being in photostationary steady state and O-3 production in younger plumes was inhibited by high aerosol loading and greater production of Sigma AN relative to O-3. The BORTAS measurements of O-3 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.