Abstract
Abstract. 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 HO2 in a clean (i.e. low NOx), tropical marine environment. It was found that concentrations of OH and HO2 were typically higher in the summer months (June, September), with maximum daytime concentrations of ~9 × 106 and 4 × 108 molecule cm−3, respectively – almost double the values in winter (late February, early March). HO2 was observed to persist at ~107 molecule cm−3 through the night, but there was no strong evidence of nighttime OH, consistent with previous measurements at the site in 2007. HO2 was shown to have excellent correlations (R2 ~ 0.90) with both the photolysis rate of ozone, J(O1D), and the primary production rate of OH, P(OH), from the reaction of O(1D) with water vapour. The analogous relations of OH were not so strong (R2 ~ 0.6), but the coefficients of the linear correlation with J(O1D) 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 HO2 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.
Highlights
The hydroxyl radical, OH, is the dominant daytime oxidant in the troposphere
The shaded areas in the top panels represent the contributions to the air mass from five possible sources, as calculated using the NAME dispersion model (Ryall et al, 2001) using the technique described in detail in Carpenter et al (2010) – Atlantic continental air from over North America, Atlantic marine, polluted marine air from over Europe, African coastal and Saharan dust
This study was the first time that both OH and HO2 have been measured in a tropical location in order to assess the seasonal variability of these species
Summary
The hydroxyl radical, OH, is the dominant daytime oxidant in the troposphere. A major pathway for its formation is via the reactions. [OH] and [HO2] was measured by Whalley et al (2010) at the CVAO as part of the Reactive Halogens in the Marine Boundary Layer Experiment (RHaMBLe) in 2007 – the maximum daytime concentrations of OH and HO2 were 9 × 106 and 6 × 108 molecule cm−3, respectively. Almost all these studies of HOx have been “short-term”; i.e. the observation periods are typically of the order of a few weeks at one particular period of the year.
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