Long-term changes of methane and hydrogen in the stratosphere in the period 1978–2003 and their impact on the abundance of stratospheric water vapor



[1] The long-term changes of the stratospheric mixing ratio of CH4 over the period of 1978–2003 are derived from balloon-borne data of H2, CH4 and N2O. The data were obtained by collecting whole air samples and subsequent gas chromatographic analyses. To eliminate the short-term variability attributed to dynamical processes, the N2O mixing ratio is used as a proxy for altitude. A correlation analysis for the individual years is applied and the CH4 mixing ratios are interpolated to four different levels of N2O, corresponding to altitudes of approximately 17, 23, 26 and 30 km at midlatitudes. For the investigated period of 1978 to 2003 we find increases at the four levels of 207 ± 32 ppb, 159 ± 21 ppb, 140 ± 34 ppb and 111 ± 60 ppb, respectively. The CH4 trend has slowed down in recent years and is best fitted by a second-order polynomial. The increase of CH4 can account for only 25–34% of the increase in stratospheric H2O of 1%/yr over the last decades as derived from previous studies. The simultaneously measured time series of stratospheric H2 mixing ratios shows that the contribution of stratospheric H2 to the H2O trend in the period 1988–2003 is minor.