A review of hydroxyl in the middle atmosphere: Comparison of measured and modeled vertical profiles and ground-based column observations

We present an intercomparison of previous measurements of hydroxyl (OH) and photochemical model results in the middle atmosphere at midlatitudes. The measurements are categorized as functions of altitude and solar zenith angle (SZA). Hydroxyl vertical profiles obtained under similar conditions are integrated over altitude to estimate a total OH column which is compared to model results and to ground-based observations. Of particular interest is the dependence of OH abundances on SZA, where discrepancies have been identified between the observed total OH column and the column abundance calculated by current photochemical models. OH column abundances in models are generally lower (25%) than observed at small solar zenith angles; however, the agreement generally is good for SZA >60°. This suggests that there is a process or processes related to SZA that photochemical models may not be simulating accurately. Currently, there are not enough observations to determine the altitude range where any unknown processes controlling HO_x at high-sun are occurring.