Abstract
Abstract. In situ airborne measurements were made over eastern Canada in summer 2011 as part of the BORTAS experiment (Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites). In this paper we present observations of greenhouse gases (CO2 and CH4) and other biomass burning tracers (CO, HCN and CH3CN), both climatologically and through case studies, as recorded on board the FAAM BAe-146 research aircraft. Vertical profiles of CO2 were generally characterised by depleted boundary layer concentrations relative to the free troposphere, consistent with terrestrial biospheric uptake. In contrast, CH4 concentrations were found to rise with decreasing altitude due to strong local and regional surface sources. BORTAS observations were found to be broadly comparable with both previous measurements in the region during the regional burning season and with reanalysed composition fields from the EU Monitoring Atmospheric Composition and Change (MACC) project. We use coincident tracer–tracer correlations and a Lagrangian trajectory model to characterise and differentiate air mass history of intercepted plumes. In particular, CO, HCN and CH3CN were used to identify air masses that have been recently influenced by biomass burning. Examining individual cases we were able to quantify emissions from biomass burning. Using both near-field (< 1 day) and far-field (> 1 day) sampling, boreal forest fire plumes were identified throughout the troposphere. Fresh plumes from fires in northwestern Ontario yield emission factors for CH4 and CO2 of 8.5 ± 0.9 g (kg dry matter)−1 and 1512 ± 185 g (kg dry matter)−1, respectively. We have also investigated the efficacy of calculating emission factors from far-field sampling, in which there might be expected to be limited mixing with background and other characteristic air masses, and we provide guidance on best practice and limitations in such analysis. We have found that for measurements within plumes that originated from fires in northwestern Ontario 2–4 days upwind, emission factors can be calculated that range between 1618 ± 216 and 1702 ± 173 g (kg dry matter)−1 for CO2 and 1.8 ± 0.2 and 6.1 ± 1 g (kg dry matter)−1 for CH4.
Highlights
The global burden of greenhouse gases (GHGs) has increased significantly over the last century
For the purpose of providing a useful dataset for wider climatological statistics, we present a broad summary of the BORTAS project using all sampled data, excluding the Atlantic transits to Halifax, to represent composition statistics consistent with boreal biomass burning that may be linked to satellite fire observations
The flights during the BORTAS experiment had the explicit aim of intercepting Biomass burning (BB), and the measurements discussed here do not represent a truly random sample of the atmosphere during this measurement period and will have a slight bias towards air masses that have been influenced by BB
Summary
The global burden of greenhouse gases (GHGs) has increased significantly over the last century This has mostly been attributed to growth and changes in anthropogenic emissions (Forster and Ramaswamy, 2007). There is a large degree of uncertainty and variability in the contributions made by particular regions, both intra- and inter-annually This is due to the unique and episodic nature of individual fire events and the necessary assumptions inherent in the monitoring of such emissions. It has been suggested (van der Werf et al, 2010) that Africa makes the largest contribution to total C emissions (54 %), followed by South America (15 %). Forest fires in Russia alone have been causally linked to the accelerated growth rate of global mean CH4 concentrations that was observed in 1998, 2002 and 2003 (Dlugokencky et al, 2001; Langenfelds et al, 2002; Simpson et al, 2006)
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