Quantifying the magnitude of a missing hydroxyl radical source in a tropical rainforest

Abstract. The lifetime of methane is controlled to a very large extent by the abundance of the OH radical. The tropics are a key region for methane removal, with oxidation in the lower tropical troposphere dominating the global methane removal budget (Bloss et al., 2005). In tropical forested environments where biogenic VOC emissions are high and NO_x concentrations are low, OH concentrations are assumed to be low due to rapid reactions with sink species such as isoprene. New, simultaneous measurements of OH concentrations and OH reactivity, k'_OH, in a Borneo rainforest are reported and show much higher OH than predicted, with mean peak concentrations of ~2.5×10⁶ molecule cm⁻³ (10 min average) observed around solar noon. Whilst j(O¹D) and humidity were high, low O₃ concentrations limited the OH production from O₃ photolysis. Measured OH reactivity was very high, peaking at a diurnal average of 29.1±8.5 s⁻¹, corresponding to an OH lifetime of only 34 ms. To maintain the observed OH concentration given the measured OH reactivity requires a rate of OH production approximately 10 times greater than calculated using all measured OH sources. A test of our current understanding of the chemistry within a tropical rainforest was made using a detailed zero-dimensional model to compare with measurements. The model over-predicted the observed HO₂ concentrations and significantly under-predicted OH concentrations. Inclusion of an additional OH source formed as a recycled product of OH initiated isoprene oxidation improved the modelled OH agreement but only served to worsen the HO₂ model/measurement agreement. To replicate levels of both OH and HO₂, a process that recycles HO₂ to OH is required; equivalent to the OH recycling effect of 0.74 ppbv of NO. This recycling step increases OH concentrations by 88 % at noon and has wide implications, leading to much higher predicted OH over tropical forests, with a concomitant reduction in the CH₄ lifetime and increase in the rate of VOC degradation.