HDO in the mesosphere: Observation and modeling of novel isotopic variability

First measurements of [HDO]/[H 2 O] ratios in the lower mesosphere (50–70 km) are described. HDO and H 2 O abundances were obtained over the period 1992–2002, by ground‐based observation of microwave (λ ∼ 1.4 mm) spectral emission lines. Measured HDO depletions are highly variable, with δ D values ranging from −58% ± 3% to −3% ± 9% (58% to 3% less deuterium than standard mean ocean water). Mesospheric δ D values decrease with increasing H 2 O abundance. This is contrary to the stratospheric increase of δ D with increasing H 2 O abundance, indicating that the physical mechanisms controlling mesospheric HDO depletion are fundamentally different from the mechanism (conversion of methane to water) known to control stratospheric HDO depletion. A photochemical model is used to show the observed mesospheric HDO depletion behavior is consistent with slower photolysis of HDO than of H 2 O. The difference in photolysis rates follows from a difference in cross sections of the two isotopomers at wavelengths >175 nm. Water photolysis in this spectral region is important only in the lower mesosphere. Therefore this mechanism for isotopic separation does not conflict with results of accepted stratospheric studies.