H₂O₂ and CH₃OOH (MHP) in the Remote Atmosphere: 2. Physical and Chemical Controls

Abstract

Hydrogen peroxide (H₂O₂) and methyl hydroperoxide (MHP, CH₃OOH) serve as HOₓ (OH and HO₂ radicals) reservoirs and therefore as useful tracers of HOₓ chemistry. Both hydroperoxides were measured during the 2016–2018 Atmospheric Tomography Mission as part of a global survey of the remote troposphere over the Pacific and Atlantic Ocean basins conducted using the NASA DC-8 aircraft. To assess the relative contributions of chemical and physical processes to the global hydroperoxide budget and their impact on atmospheric oxidation potential, we compare the observations with two models, a diurnal steady-state photochemical box model and the global chemical transport model Goddard Earth Observing System (GEOS)-Chem. We find that the models systematically under-predict H₂O₂ by 5%–20% and over-predict MHP by 40%–50% relative to measurements. In the marine boundary layer, over-predictions of H₂O₂ in a photochemical box model are used to estimate H₂O₂ boundary-layer mean deposition velocities of 1.0–1.32 cm s⁻¹, depending on season; this process contributes to up to 5%–10% of HOₓ loss in this region. In the upper troposphere and lower stratosphere, MHP is under-predicted and H₂O₂ is over-predicted by a factor of 2–3 on average. The differences between the observations and predictions are associated with recent convection: MHP is under-estimated and H₂O₂ is over-estimated in air parcels that have experienced recent convective influence.