Ship-borne observations of OH, HO2, RO2 and OH reactivity in the North Atlantic Ocean

Roughly 90 % of the oxidation of the key greenhouse gas (GHG) methane (CH₄) is driven by reaction with the hydroxyl radical (OH) with ~50 % of this processing occurring over the oceans. Ocean emissions of volatile organic compounds (VOCs) have the potential to modify the oxidation capacity (by acting as a sink for OH) and influence the lifetime of CH₄. The oxidation of ocean-emitted VOCs may also lead to the formation of secondary organic aerosols (SOA) and ozone (another GHG) and further influence the climate.

Combined observations of OH, peroxy radicals and OH reactivity within the remote marine boundary layer are sparse. Ground-based observations can provide insights into factors influencing the oxidation capacity in this type of environment, but do not allow any spatial variability in ocean emissions to be determined.  

Here, ship-borne observations of OH, HO₂, RO₂ and OH reactivity made in the North Atlantic Ocean on board the RSS Discovery in June 2025 are presented. The observed OH, HO₂ and RO₂ will be compared to a preliminary radical budget analysis to assess the major radical sources and sinks. The OH reactivity observed in the North Atlantic as the ship travelled through waters with a range of marine biological activity will be compared to OH reactivity calculated from the coordinated observations of CO, CH₄, O₃, NO_x and VOCs (including oxygenated VOCs measured using PTR-MS and alkanes, alkenes and aromatics measured using canister samples and subsequent GC analysis) to assess the variability in oceanic emissions, the variability in any missing OH reactivity and the impact this missing OH reactivity has on our understanding of the oxidation capacity.