Extensive measurement campaigns by the NASA ER-2 research aircraft have obtained a nearly pole-to-pole database of the species that control HOchi (OH + HO2) chemistry. The wide dynamic range of these in situ measurements provides an opportunity to demonstrate empirically the mechanisms that control the HOchi system. Measurements in the lower stratosphere show a remarkably tight correlation of OH concentration with the solar zenith angle (SZA). This correlation is nearly invariant over latitudes ranging from 70 degrees S to 90 degrees N and all seasons. An analysis of the production and loss of HOchi in terms of the rate determining steps of reaction sequences developed by Johnston and Podolske and Johnston and Kinnison is used to clarify the behavior of the system and to directly test our understanding of the system with observations. Calculations using in situ measurements show that the production rate of HOchi is proportional to O-3 and ultraviolet radiation flux. The loss rate is proportional to the concentration and the partitioning of NO, (reactive nitrogen) and the concentration of HO2. In the absence of heterogeneous reactions, the partitioning of NOgamma is controlled by O-3 and HOchi and the concentration of HO2 is controlled by NOgamma and O-3, so that the removal rate of OH is buffered against changes in the correlation of O-3 and NOgamma. The heterogeneous conversion of NO2 to HNO3 is not an important net source of HOchi because production and removal sequences are nearly balanced. Changes in NOgamma partitioning resulting from heterogeneous chemistry have a large effect on the loss rates of HOchi, but little or no impact on the measured abundance of OH. The enhanced loss rates at high NO2/HNO3 are offset in the data set examined here by enhanced production rates resulting from increased photolysis rates resulting from the decreased O-3 column above the ER-2.