Hydroxyl radical buffered by isoprene oxidation over tropical forests

Abstract

The hydroxyl radical is a key oxidant in the Earth’s atmosphere. The inclusion in an atmospheric chemistry model of a detailed mechanism of isoprene oxidation, involving the buffering of hydroxyl radical concentrations, improves agreement between model simulations of hydroxyl radical levels and observations. The hydroxyl radical is a key oxidant in the Earth’s atmosphere. This short-lived highly reactive molecule plays an important role in the degradation of volatile organic compounds, leading to the production of ozone and the formation and growth of aerosol particles1,2,3. In this way, hydroxyl radicals influence air quality and regional climate. Measurements over tropical forests suggest that hydroxyl radicals are recycled following reaction with the volatile organic compound isoprene4,5, but the chemistry underpinning this observation is uncertain. Here, we propose a detailed chemical mechanism for the oxidation of isoprene by hydroxyl radicals. The photo-oxidation of unsaturated hydroperoxy-aldehydes—a product of isoprene oxidation—is a central part of the mechanism; their photolysis initiates a hydroxyl radical production cascade that is limited by the reaction of hydroperoxy-aldehydes with the hydroxyl radical itself. We incorporate this mechanism into a global atmospheric chemistry model and find that measurements of hydroxyl radical concentrations over a pristine region of the Amazon, and in moderately polluted conditions, are captured well. On the basis of this agreement, we suggest that isoprene oxidation can buffer hydroxyl radical concentrations, by serving as both a sink and source for these radicals.