Significant reduction in the stratospheric ozone deficit using a three‐dimensional model constrained with UARS data

Abstract

We present a near‐global analysis of the ozone deficit problem by constraining a fully diurnal, three‐dimensional, chemical, radiative, transport model of the middle atmosphere with colocated UARS measurements of ClO, NOx, H2O, and CH4. The domain of the study covers a wide range of altitudes (37.4–49.6 km) and latitudes (62.5°S–27.5°N), for the period of January–February 1992. In this domain, the baseline (no constraints with measurements) model temperatures are mostly warmer than U.K. Meteorological Office (UKMO) observations (by up to 5 K), and the baseline O3 mixing ratios are underestimated by 10 to 25% relative to HALOE measurements. Also, in this domain the model/data discrepancies in concentrations of the ozone‐relevant species are as follows: [H2O] and [NOx] are mostly in good agreement, [CH4] is underestimated by 10–60%, and [ClO] is overestimated by 1.3 to 3 times. We find the following: (1) Constraining the model with UKMO temperatures eliminates about 3–10% of the deficit in the 40–48 km altitude range. (2) Constraining the model with observed NOx or H2O (in addition to temperature) has minimal effect on the ozone deficit in most parts of the domain. (3) When the model temperature and ClO profiles are constrained with observations, the deficit is reduced to about 5–15%, bringing the model ozone predictions in the 40 km region to within the uncertainties of HALOE ozone measurements. (4) A 40% reduction in the rate constant of HO2 + O → OH + O2, in addition to constraining T, NOx, and ClO, eliminates the deficit in portions of the 40 km region and in the upper stratosphere, but it results in 5–10% excess ozone near the equatorial stratopause. (5) When the model methane profile is constrained with HALOE observations and a 6% HCl + O2 channel for the ClO + OH reaction is included in the chemistry, the model ClO abundance agrees well with MLS measurements in most parts of the domain. Further improvement in the ClO abundance can be obtained by decreasing [OH] through reducing the rate constant of the HO2 + O → OH + O2 reaction.