Hydroxyl Radicals and Oxidation Capacity in the Tropical Troposphere: Measurements from CAFE Field Campaigns using HORUS

In the tropics, intense solar radiation drives photochemistry and strong convection, transporting air from the boundary layer to the upper troposphere. Conditions in the tropics are characterized by high humidity and UV intensity enhancing hydroxyl (OH) radical production. In addition, OH radicals and ozone (O₃) are formed through reactions of HO_x (OH + HO₂) with nitrogen oxides (NO_x), the latter being produced by lightning in abundant convective systems. The convection also transports volatile organic compounds (VOCs), notably from emissions by the tropical rainforest. The VOCs are oxidized by radicals and O₃, resulting in secondary species contributing to new particle formation. To understand and characterize the atmospheric chemistry in these conditions, the Chemistry of the Atmosphere Field Experiment (CAFE) Brazil was conducted from December 2022 to January 2023 with the High Altitude and Long Range Research Aircraft (HALO) in the Amazon region.

In this study, we present preliminary results measured with the Hydroxyl Radical measurement Unit based on fluorescence Spectroscopy (HORUS), focusing on vertical HO_x profiles measured during different times of the day over both the continent and the ocean, including the outflow of both electrified and non-electrified convective systems. In contrast to the conditions over the continents where lightning-generated NO_x aids in the efficient recycling of radicals, over the ocean, the limited availability of NO hinders recycling and results in radical termination. The conditions over the continent are compared to those measured over the Atlantic Ocean during the CAFE Africa expedition in summer 2018 based in the Cape-Verde islands. This unique dataset provides valuable insights into the atmospheric chemistry and oxidation capacity in these tropical regions.