The Behavior of Global Atmospheric Hydroxyl (OH) and Hydroperoxyl (HO2)

Hydroxyl (OH) and hydroperoxyl (HO₂) are two primary oxidants drive changes in atmospheric composition. These oxidants are being measured using the Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) during the four phases of the Atom mission. For the relatively clean atmosphere encountered during the first two ATom phases, expectations are that measured and modeled OH and HO₂ should be in relatively good agreement. Here we present measurements of OH and HO₂ and compare them to the calculations of a photochemical box model constrained to the other aircraft measurements. Measured and modeled OH and HO₂ generally agree to within uncertainties over a number of important controlling variables, such as altitude, location, nitrogen oxides, and photolysis. However, there are important exceptions, particularly as a function on altitude. Some of these measured-to-modeled differences may be due to heterogeneous uptake of HO₂, which has been proposed as an important mechanism for establishing global oxidant abundances. Finally, these measurements will be put into the context of OH and HO₂ measurements from previous NASA aircraft campaigns.