Seasonal observations of OH and HO[subscript 2] in the remote tropical marine boundary layer

Field measurements of the hydroxyl radical, OH, are crucial for our understanding of tropospheric chemistry. However, observations of this key atmospheric species in the tropical marine boundary layer, where the warm, humid conditions and high solar irradiance lend themselves favourably to production, are sparse. The Seasonal Oxidant Study at the Cape Verde Atmospheric Observatory in 2009 allowed, for the first time, seasonal measurements of both OH and HO[subscript 2] in a clean (i.e. low NO[subscript x]), tropical marine environment. It was found that concentrations of OH and HO[subscript 2] were typically higher in the summer months (June, September), with maximum daytime concentrations of ~9 × 10[superscript 6] and 4 × 10[superscript 8] molecule cm[superscript −3], respectively – almost double the values in winter (late February, early March). HO[subscript 2] was observed to persist at ~10[superscript 7] molecule cm[superscript −3] through the night, but there was no strong evidence of nighttime OH, consistent with previous measurements at the site in 2007. HO[subscript 2] was shown to have excellent correlations (R[superscript 2] ~ 0.90) with both the photolysis rate of ozone, J(O[superscript 1]D), and the primary production rate of OH, P(OH), from the reaction of O([superscript 1]D) with water vapour. The analogous relations of OH were not so strong (R[superscript 2] ~ 0.6), but the coefficients of the linear correlation with J(O[superscript 1]D) in this study were close to those yielded from previous works in this region, suggesting that the chemical regimes have similar impacts on the concentration of OH. Analysis of the variance of OH and HO[subscript 2] across the Seasonal Oxidant Study suggested that ~70% of the total variance could be explained by diurnal behaviour, with ~30% of the total variance being due to changes in air mass

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

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[subscript 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'[subscript OH'], in a Borneo rainforest are reported and show much higher OH than predicted, with mean peak concentrations of ~2.5×10[superscript 6] molecule cm[superscript −3] (10 min average) observed around solar noon. Whilst j(O[superscript 1]D) and humidity were high, low O[subscript 3] concentrations limited the OH production from O[subscript 3] photolysis. Measured OH reactivity was very high, peaking at a diurnal average of 29.1±8.5 s[superscript −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 HO[subscript 2] 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 HO[subscript 2], a process that recycles HO[subscript 2] 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 CH[subscript 4] lifetime and increase in the rate of VOC degradation