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

Methane (CH4), molecular hydrogen (H-2), and water vapor (H2O) were measure d concurrently on board the NASA ER-2 aircraft during the 1995-1996 Stratos pheric 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 H2O production from CH4 and H- 2 oxidation. The expected ratio of stratospheric H2O production (P-H2O) to CH4 destruction (L-CH4) = -1.973 +/- 0.003 is calculated from an evaluation of CH4 and H-2 oxidation reactions and the relationship between H-2 and CH 4 mixing ratios measured during STRAT. Correlations between H2O and CH4 wer e tight and linear only for air masses with mean ages greater than or equal to 3.8 years, restricting this analysis predominantly to latitudes between 40 degrees and 90 degrees N and potential temperatures between 470 and 540 K. The mean observed Delta H2O/Delta CH4 (-2.15 +/- 0.18) is in statistica l agreement with the expected P-H2O/L-CH4. The annual mean stratospheric en try mixing ratio for H2O calculated from this slope is 4.0 +/- 0.3 ppm. The quantity H2O + 2 . CH4 is quasi-conserved at 7.4 +/- 0.5 ppm in older air masses in the northern extratropical lower stratosphere. Significant depart ure of H2O + 2 . CH4 from the mean value is a sensitive indicator of proces ses which influence H2O without affecting CH4, such as dehydration in a pol ar vortex or near the tropical tropopause. No significant trend is observed in ER-2 aircraft data for H2O + 2 . CH4 in the lower stratosphere from 199 3 through 1997.