Observations of the Hydroxyl (OH) and Hydroperoxyl (HO2) Radicals During the NASA TRACE-P Study in Spring 2000

Oxidation chemistry cleanses the atmosphere of chemical emissions from Earth?s surface, establishes the global ozone balance, and influences climate change. It is dominated by the hydroxyl radical, OH, but involves the hydroperoxyl radical, HO₂, together called HO_x. Measurements of OH and HO₂ were made with our Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) as part of a much larger measurement suit from the NASA DC-8 aircraft during TRACE-P in spring 2000. This mission, which was conducted mainly in the Asian plume, was an excellent test of oxidation chemistry in aged pollution plumes over the remote Pacific Ocean. In general, measured OH and HO₂ were significantly less than expected from models. This difference between measurements and models was a surprise because measured and modeled HO_x generally agreed to within a factor of 1.3 during a previous aircraft study, PEM Tropics B, which occurred over the tropical Pacific in 1998. We consider the implications of these differences between models and measurements and discuss recent activity to ensure that the ATHOS calibration was accurate.