Abstract
Abstract. Forest fires in British Columbia in August 2017 caused a pyrocumulonimbus event that injected a polluted air mass into the lower stratosphere. The Microwave Limb Sounder (MLS) on the Aura satellite first observed the polluted air mass on 14 August 2017 and continued to observe it for 60 d (100 d in water vapour). We estimate the mass of CO injected into the stratosphere to be 2400 Gg. Events in which a fire injects its burning products directly into the stratosphere are rare: this is the third of four such events in the 16 years since the launch of Aura, the second largest of the four events, and the only one in the Northern Hemisphere. The other three events occurred in Australia in December 2006, February 2009 and from December 2019 to January 2020. Unlike the 2006 and 2009 events, but like the 2019–2020 event, the polluted air mass described here had a clearly elevated water vapour content: between 2.5 and 5 times greater than that in the surrounding atmosphere. We describe the evolution of the polluted air mass, showing that it rose to an altitude of about 24 km (31 hPa) and divided into several identifiable parts. In addition to CO and H2O, we observe enhanced amounts of HCN, CH3CN, CH3Cl and CH3OH with mixing ratios in the range to be expected from a variety of measurements in other biomass burning plumes. We use back trajectories and plume-dispersion modelling to demonstrate that the pollutants observed by MLS originated in the British Columbia fires, the likeliest source being at 53.2∘ N, 121.8∘ W at 05:20 UTC on 13 August 2017.
Highlights
Fires are an important natural process in many ecosystems, forests in particular (Bowman et al, 2009)
Outside of the tropics it is rare for Microwave Limb Sounder (MLS) to observe the immediate effect of fires; we describe one such event in this paper
Khaykin et al (2018) suggest that the pollution is from fires near Lake Athabasca; we show clearly that it is from British Columbia and from the vicinity of the Pacific Northwest Event as described by Peterson et al (2018)
Summary
Fires are an important natural process in many ecosystems, forests in particular (Bowman et al, 2009). The intensity and frequency of fires can be sensitive to human intervention (e.g. forestry practices), even when humans have not provided the source of ignition (Williams and Abatzoglou, 2016). The Microwave Limb Sounder (MLS) instrument (Waters et al, 2006) observes several chemical species emitted by forest fires. In tropical regions these tend to be transported to altitudes observable by this instrument by the wide-scale circulation of the atmosphere and by the strong convection along the intertropical convergence zone (Pumphrey et al, 2018). Outside of the tropics it is rare for MLS to observe the immediate effect of fires; we describe one such event in this paper
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