Seasonal cycle and secular trend of the total and tropospheric column abundance of ethane above the Jungfraujoch

Abstract

Total column abundances of ethane (C2H6) above the Jungfraujoch Station, Switzerland, 3.58‐km altitude, have been deduced from infrared solar spectra recorded in 1951 and from 1984 to 1988. The results were derived from the spectroscopic analysis of the ν7 band PQ3, PQ1, and RQ0 subbranches of C2H6 near 2976.8, 2983.4, and 2986.7 cm−1, respectively. The 1984–1988 results obtained from more than 300 observations at 0.005 cm−1 resolution show a definite seasonal variation in the total vertical column abundance of that gas, with a maximum of (1.43±0.03)×1016 molecules/cm2 during March and April and a minimum in the fall. The corresponding C2H6 mixing ratio in the free troposphere was derived to 1.53±0.14 ppb in early April; the ratio of maximum to minimum C2H6 concentration in the troposphere was found to be 1.88±0.12. From the analysis of the same spectral features observed in March and April 1951, we have deduced a total vertical column abundance of (1.09±0.10)×1016 molecules/cm2. Assuming an exponential increase with time and correcting for a small decrease in the stratospheric amount of C2H6 due to the man‐induced increase in stratospheric chlorine, we obtain a secular trend in the tropospheric burden above the Jungfraujoch equal to (0.85±0.3)% yr−1. It is argued that this trend is valid for the entire northern hemisphere.