Closure of the total hydrogen budget of the northern extratropical lower stratosphere

Methane (CH 4 ), molecular hydrogen (H 2 ), and water vapor (H 2 O) were measured concurrently on board the NASA ER‐2 aircraft during the 1995–1996 Stratospheric Tracers of Atmospheric Transport (STRAT) and 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns. Correlations between these three main hydrogen reservoirs in the northern extratropical lower stratosphere are examined to evaluate H 2 O production from CH 4 and H 2 oxidation. The expected ratio of stratospheric H 2 O production (P H2O )to CH 4 destruction (L CH4 ) = −1.973±0.003 is calculated from an evaluation of CH 4 and H 2 oxidation reactions and the relationship between H 2 and CH 4 mixing ratios measured during STRAT. Correlations between H 2 O and CH 4 were tight and linear only for air masses with mean ages ≥3.8 years, restricting this analysis predominantly to latitudes between 40° and 90°N and potential temperatures between 470 and 540 K. The mean observed ΔH 2 O/CH 4 (−2.15±0.18) is in statistical agreement with the expected P H2O /L CH4 . The annual mean stratospheric entry mixing ratio for H 2 O calculated from this slope is 4.0 ± 0.3 ppm. The quantity H 2 O + 2·CH 4 is quasi‐conserved at 7.4 ± 0.5 ppm in older air masses in the northern extratropical lower stratosphere. Significant departure of H 2 O + 2·CH 4 from the mean value is a sensitive indicator of processes which influence H 2 O without affecting CH 4 , such as dehydration in a polar vortex or near the tropical tropopause. No significant trend is observed in ER‐2 aircraft data for H 2 O + 2·CH 4 in the lower stratosphere from 1993 through 1997.