Enhancements in lower stratospheric CH3CN observed by the Upper Atmosphere Research Satellite Microwave Limb Sounder following boreal forest fires

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Abstract
On 25 August 1992, the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite observed a significant enhancement in the abundance of lower stratospheric methyl cyanide (CH3CN) at 100–68 hPa (∼16–19 km altitude) in a small region off the east coast of Florida. The enhancement was seen to decay and move generally westward over ∼6 days. Concentrations up to ∼1500 parts per trillion by volume (pptv) were observed, compared to a typical stratospheric background level of 30 pptv. Aerosol index data from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) instrument, together with trajectory calculations, provide strong evidence that the enhancement arose from the stratospheric injection, associated with strong thunderstorms, of tropospheric air with high CH3CN concentrations, originating in regions of extensive forest fire activity in Idaho (not observed by MLS in this period because of orbital geometry). After being lofted into the lower stratosphere, this air was advected into the regions observed by MLS and subsequently dispersed. These MLS enhanced CH3CN observations add to the growing body of evidence for the occurrence of such episodic injections of tropospheric air into the stratosphere.