The OH radical is the dominant oxidising agent in the troposphere and oxidises Volatile Organic Compounds (VOCs) forming peroxy radicals, HO2 and RO2, with formaldehyde (HCHO) often formed as an intermediate. In polluted regions these peroxy radicals can react with NO, reforming OH and producing ozone (O3), while in remote marine regions O3 is destroyed during the day by reaction with OH or HO2.
In this work the first partially speciated measurements of RO2 and OH reactivity at the Cabo Verde Atmospheric Observatory (CVAO) are reported alongside measurements of OH, HO2 and a comprehensive suite of supporting measurements. The radicals were measured using the University of Leeds Fluorescence Assay by Gas Expansion (FAGE) and ROxLIF instruments. This comprehensive set of ROx measurements allows experimental budget analysis to be performed with the results highlighting potential gaps in our understanding of oxidation chemistry in the remote marine boundary layer. Depending on the literature rate coefficient used, the reaction of CH3O2 with IO contributed up to 12 % of the peak HO2 production if HO2 is a product, while the heterogeneous uptake of HO2 onto aerosols was the dominant HO2 sink, contributing 28 % to HO2 destruction. The reaction of CH3O2 and IO and heterogenous uptake were shown to be important loss processes for RO2 contributing a combined 62 % of the RO2 destruction rate. Net HO2, and RO2 production were observed alongside net OH destruction, highlighting that even in a seemingly simple chemical environment there are gaps in our understanding of oxidation chemistry in the remote marine boundary layer.
Measurements of OH, HO2, OH reactivity and HCHO were also measured at the Manchester Air Quality Supersite during a field campaign that was defined by a warmer period at the end of the campaign when elevated OH concentrations of up to 8 × 10^6 molecule cm−3 were measured. Experimental budget analysis revealed a missing source of OH during the day and it was determined that the dominant source of HCHO was secondary production. By comparison of the net HCHO destruction rate and the measured rate of change of HCHO, missing HCHO production was observed throughout the campaign but increased in the warmer period when temperatures were elevated and correlated strongly with a proxy of photochemical production suggesting that unmeasured biogenic emissions may have been the missing source of HCHO.
Finally, the results of the ROxCOMP intercomparison at the SAPHIR chamber at Forschungszentrum Julich (FZJ) are discussed with good agreement found between the Leeds and FZJ RO2 measurements within the instrumental uncertainties. Possible reasons for discrepancies are discussed including inhomogeneous mixing of the chamber and measurement interferences. Nevertheless, excellent agreement was found between the measurements in an ambient experiment providing confidence in the ability of both instruments to measure ambient ROx radicals.
