Exploring Oxidation in the Remote Free Troposphere during the Atmospheric Tomography (ATom) Mission

Earth’s atmosphere oxidizes the greenhouse gas methane and other gases, thus determining their lifetimes and oxidation products. Much of this oxidation occurs in the remote, relatively clean free troposphere above the planetary boundary layer, where the oxidation chemistry is thought to be much simpler and better understood than it is in urban regions or forests. The NASA airborne Atmospheric Tomography study (ATom) was designed to produce cross sections of the detailed atmospheric composition in the remote atmosphere over the Pacific and Atlantic Oceans during four seasons. No previous study has covered so much of the troposphere with such a comprehensive measurement suite and in four seasons. As part of an extensive measurement suite, measurements of the atmosphere’s primary oxidant, hydroxyl (OH), and hydroperoxyl (HO₂) are compared to a photochemical box model to test OH and HO₂ chemistry. Generally, observed and modeled median OH and HO₂ agreed to within combined uncertainties at the 2σ confidence level, which is ±40%. However, OH observations increasingly exceeded modeled values at altitudes above 8 km, becoming ~(40-50)% greater for the summer ATom mission, which is at the combined uncertainty. Measurement and model uncertainty and possible undetermined errors complicate tests for unrepresented chemistry or incorrect reaction rate coefficients that would substantially affect the OH and HO₂ abundances.