Efficient HO_x Radical Production from Isoprene Nighttime Chemistry

The reactive chemistry of isoprene, the most emitted biogenic hydrocarbon worldwide, has a controlling influence on the composition and cleansing capacity of the atmosphere. Although isoprene emission by plant biosynthesis is negligible at night, heat-stressed vegetation in close proximity to the fire front releases a substantial amount of isoprene, which subsequently reacts with NO₃ radicals, the major oxidant in the dark. In this study, we performed chamber experiments to investigate the efficacy of HO_x recycling through the nighttime chemistry of isoprene. By operating the experiments at the continuous-flow steady-state mode, we created a chemical regime that features sub-ppbv levels of NO, a regime that is highly relevant to the nighttime atmosphere disturbed by fire plumes but was rarely studied in previous chamber experiments. Using measurements of trace levels of cyclohexane at steady state, we derived the total OH radicals produced from isoprene + NO₃ reactions in the range of (4.4–8.0) × 10⁵ molecules cm^–3, accounting for 9.9–17.7% of the total reacted isoprene mass. To explain this high level of observed OH, a simplified mechanism that efficiently generates and converts HO₂ to OH was proposed. By incorporating this mechanism into an observationally constrained box model, we predicted that the mixing ratio of OH radicals can exceed ∼10⁵ molecules cm^–3 when isoprene and NO were present at parts per trillion by volume, a level frequently encountered in environments impacted by biomass burning emissions. Such an efficient production of OH radicals has an important impact on the oxidizing power of the nocturnal atmosphere in regions with intertwined biogenic and anthropogenic activities.