Upper stratospheric and lower mesospheric HOx: Theory and Observation

Observations of OH and HO2 from the MLS instrument on Aura provide new insights into two long-standing scientific issues of middle atmosphere photochemistry: the ozone deficit problem and the HOx dilemma. The ozone deficit problem refers to the tendency of models to under-estimate observed ozone, in a region where the ozone photochemical time constant is short and where ozone is controlled by HOx photochemistry. The HOx dilemma refers to the fact that suggested kinetics changes needed to obtain good agreement with prior space-based observations of OH below ~40 km results in poorer agreement (relative to standard photochemistry) for comparisons in the upper stratosphere and lower mesosphere. We show, contrary to prior results, that the new measurements of HOx are well described by photochemical theory at all altitudes. However, an imbalance between production and loss of ozone persists for vast altitude regions, suggesting the middle atmosphere ozone budget is not "closed".