Ozone photochemistry in boreal biomass burning plumes observed during the BORTAS aircraft measurement campaign

We present an analysis of ozone photochemistry observed in boreal biomass burning plumes over eastern Canada in the summer of 2011. Measurements of ozone and a number of key chemical species associated with ozone 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. We show that ozone mixing ratios measured in biomass burning plumes were indistinguishable from non-plume measurements, but comparison against measurements of carbon monoxide (CO), total alkyl nitrates (ΣAN) and a surrogate species for photo-oxidation of NOx (NOz = NOy - NOx) revealed that the potential for ozone production increased with plume age. We calculated photochemical ages, from NMHC ratios, for the plumes sampled during BORTAS to range from 0 to 15 days. Ozone production, calculated from ΔO3/ΔCO enhancement ratios, increased 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. In comparing ozone mixing ratios with components of the NOy budget we show that plumes with ages between 2 and 4 days were characterised by high aerosol loading, relative humidity greater than 40%, and low ozone production efficiencies of 8 ppbv ppbv-1 relative to ΣAN and 2 ppbv ppbv-1 relative to NOz. In plumes with ages greater than 4 days, ozone production efficiency was increased to 473 ppbv ppbv-1 relative to ΣAN and 155 ppbv ppbv-1 relative to NOz. We present the observed plume ozone photochemistry in relation to other geophysical parameters measured from the aircraft and previous measurement campaigns that sampled boreal biomass burning plumes over the same region.