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 Compos...

Properties and evolution of biomass burning organic aerosol from Canadian boreal forest fires

“…However, CO concentrations are also diminished and consistently below 100 ppb, relative to an average of 200 ppb for aged plumes, while OA concentrations are comparable to levels in higher altitude, flaming-type plumes (∼ 20 µg m −3 ). These trends, which diverge from the expected characteristics for emissions of this origin, are further compounded by high O 3 / CO (> 0.2), indicative of an elevated level of oxygenation and photochemical activity (Mason et al, 2001;Parrington et al, 2013). Formation of SOA from biogenic precursors has previously been observed in the forests of Ontario (Slowik et al, 2010).…”

“…Research flights primarily originated from Halifax and largely involved surveying areas adjacent to the Gulf of St. Lawrence and the North Atlantic. A predominant source region in northwestern Ontario (approximately 52.5 • N, 93.5 • W) has been identified for the majority of plumes sampled throughout BORTAS, although more disperse fires were also active in northern Alberta and the Northwest Territories Parrington et al, 2013). As the majority of plumes from fires in this region were encountered at a distance of several thousand kilometres downwind, emissions would have undergone substantial processing prior to sampling, with estimated photochemical ages between 1-11 days.…”

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends

“…Most observations have shown that ozone is produced from fire emissions, yet some have reported no ozone enhancement or even ozone depletion in fire plumes depending on plume age, effects of co-emitted aerosols, and mixing with urban pollution (Singh et al, 2010(Singh et al, , 2012Jaffe and Wigder, 2012;Parrington et al, 2013;Baylon et al, 2015). Observations of ozone enhancements in fire plumes indicated by the O 3 /CO enhancement ratio show a wide range of -0.1 to 0.9 nmol mol -1 /nmol mol -1 , and tend to increase with plume age (Jaffe and Wigder, 2012;Wigder et al, 2013).…”

Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer

“…31). Parrington et al (2013) showed that ozone formation increases with plume age. They used NMHC ratios (extensively described for this BORTAS campaign in to estimate photochemical ages of the observed biomass burning plumes between 0 and 10 days.…”