Sources, Sinks, and the Distribution of OH in the Lower Stratosphere

Abstract

Extensive measurement campaigns by the NASA ER-2 research aircraft have obtained a nearly pole-to-pole database of the species that control HO_x (OH + HO_2) chemistry. The wide dynamic range of these in situ measurements provides an opportunity to demonstrate empirically the mechanisms that control the HO_x 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° S to 90° N and all seasons. An analysis of the production and loss of HO_x 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 HOx is proportional to O3 and ultraviolet radiation flux. The loss rate is proportional to the concentration and the partitioning of NO_y (reactive nitrogen) and the concentration of HO_2. In the absence of heterogeneous reactions, the partitioning of NO_y is controlled by O_3 and HO_x and the concentration of HO2 is controlled by NO_y and O_3, so that the removal rate of OH is buffered against changes in the correlation of O_3 and NO_y. The heterogeneous conversion of NO_2 to HNO_3 is not an important net source of HO_x because production and removal sequences are nearly balanced. Changes in NOy partitioning resulting from heterogeneous chemistry have a large effect on the loss rates of HO_x, but little or no impact on the measured abundance of OH. The enhanced loss rates at high NO_2/HNO_3 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.