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The Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO) instrument on the SCISAT satellite is a simple, compact spectrophotometer for the measurement of atmospheric extinction, ozone, nitrogen dioxide, and other trace gases in the stratosphere and upper troposphere as part of the Atmospheric Chemistry Experiment (ACE) mission. We provide an overview of the instrument from requirements to realization, including optical design, prelaunch and on-orbit performance, and a preliminary examination of retrievals of ozone and NO(2).

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 HO x is proportional to O 3 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 HO 2 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 NO y 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.