Measurements and experimental budget analysis of OH, HO2 and RO2 radicals in a remote tropical marine location

The OH radical is the dominant daytime tropospheric oxidising agent, reacting with almost all Volatile Organic Compounds (VOCs). The majority of global methane is removed in the tropical troposphere by OH. The oxidation of VOCs by OH forms peroxy radicals, HO₂ and RO₂, with formaldehyde (HCHO) often formed as a product. In remote marine environments, ozone (O₃) is destroyed during the day by reaction with OH or HO₂, or photolysis.

Ground-based measurements of OH, HO₂, RO₂, OH reactivity and HCHO, together with a comprehensive suite of supporting measurements, were made at the Cape Verde Atmospheric Observatory (CVAO), situated on the island of São Vicente located in the Tropical North Atlantic Ocean, during February 2023 as part of the NERC-funded PEROXY project. With no nearby emissions and prevailing winds from over the ocean, the clean marine air sampled was representative of the open ocean.

In this work, time series and diurnal variations of the measured species are presented, and production and destruction rates of OH, HO₂ and RO₂ have been calculated. Reactions of the halogen oxides, IO and BrO, are shown to be important for understanding the chemistry of OH, HO₂ and RO₂. The CH₃O₂ reactions with halogen oxides are an important sink for RO₂ and a possible source of HO₂ that likely enhances O₃ destruction in remote marine environments. Heterogeneous losses are also shown to be important for HO₂. The OH budget analysis shows a possible minor missing source of OH while net HO₂ production is observed, indicating either an overestimation of HO₂ sources or an underestimation of HO₂ sinks. Net RO₂ production is observed, with the reactions of CH₃O₂ with halogen oxides and heterogeneous losses of CH₃O₂ to aerosols and the ocean surface needed to close the RO₂ budget, but the magnitude of these loss processes is currently highly uncertain.