Hydration, dehydration, and the total hydrogen budget of the 1999/2000 winter Arctic stratosphere

Stratospheric H₂O, CH₄, and H₂ measured during the 1999/2000 Arctic winter are used to examine the total hydrogen budget and to quantify hydration and dehydration within the Arctic polar vortex. The mean volume mixing ratio of equivalent water, H₂O_eq = H₂O + 2∗CH₄, was found to be 7.38 ± 0.11 ppmv in the vortex and 7.35 ± 0.10 ppmv in the extravortex and vortex edge regions, implying insignificant net loss of H₂O from the 50-90 hPa layer of the Arctic stratosphere. However, isolated occurrences of hydration and dehydration were observed on several flights. In particular, an average dehydration of 0.31 ppmv was encountered for 440 km along the NASA ER-2 aircraft flight path on 27 January 2000 (0.63 ppmv maximum dehydration). Diabatic back-trajectory calculations indicate that this dehydrated air parcel may have encountered temperatures below the ice frost point two weeks earlier. In addition to these isolated events, measurements of equivalent water indicated a decrease with altitude throughout the lower stratospheric vortex and vortex edge. Potential causes of this altitude dependence are quantified.