Balloon-borne spectroscopic measurement of stratospheric methane

The absorption due to the ν3 band of methane at a wavelength of 3.3 μm has been observed in solar spectra taken with a balloon-borne monochromator during its ascent from 13 to 28 km and during the float period at peak altitude until sunset. Measurements of the equivalent widths of the R (5) and R (6) lines were used in a single-layer iterative calculation to determine methane abundance as a function of altitude. The Voigt line profile was used throughout. The mixing ratio obtained during the ascent decreases from 0.9 ppmv (parts per million by volume) at 13 km to 0.7 ppmv at 28 km. The variation of equivalent width with solar zenith angle during the float period is interpreted to indicate a rapid decline in mixing ratio above 28 km.