Aspects of the Comparison of Stratospheric Trace Species Measurements with Photochemical Models

Summary

Observation and measurement of ‘reservoir’ species, including H₂ 0₂, Cℓ0N0₂, HN0₄, H0Cℓ, N₂ 0₅ and others, can potentially test some portions of photochemical models of the stratosphere, leading to improvements and increased confidence in model predictions. But abundance measurements of a single reservoir species without accompanying values for other species do not serve, in most cases, to constrain the related model mechanism and parameters within the recognized uncertainties. This follows from the sensitivity of abundance of a species to fluctuations in its long-lived precursors from transport processes. A better test of theory would be provided by correlation studies in which precursor concentrations and photolytic fluxes are measured simultaneously and in the same air volume as the reservoir species. Three recent reported differing observations or upper limits for H₂0₂ give an example of the difficulties. All three reported values could be nearly consistent with the current LLNL one-dimensional stratospheric photochemical model, depending on the values of 0₃, NO_x and H₂0 appropriate to each measurement. The important precursors in each case can be identified by consideration of the appropriate kinetics and observed precursor variability.