The measurement of peroxy radicals in the marine boundary layer using the PERCA technique
Results from peroxy radical measurements taken at two marine boundary layer sites (Mace Head and Weybourne) in 2002 are discussed. Previous peroxy radical measurements at Mace Head, Cape Grim and Weybourne are compared to the recent data to provide a broader picture of the oxidative capacity of the marine boundary layer. A seasonal comparison between summer and winter peroxy radical measurement at Weybourne reveals a peroxy radical diurnal cycle with a maximum at midday in summer, and a diurnal cycle with maxima during the night in winter. Winter peroxy radical levels were often as high as those during the day in summer. Care must be taken when calculating ozone-production values from peroxy radical data in winter, owing to the non-attainment of an NO-NO2 photostationary state. In winter, this could lead to an exaggeration in the calculation of ozone production. However, when the photostationary state was achieved, winter ozone production levels were often as high as in summer. Peroxy radical measurements at the NAMBLEX campaign at Mace Head provide insights into the photochemistry of the marine boundary layer, and a detailed picture of the effect of NOx and VOCs on peroxy radical and ozone production. Peroxy radicals were found to be linearly dependent on NOx, as well as VOC loading. Night-time peroxy radical formation was driven by a mixture of ozone-alkene and NO3-alkene reactions, with a strong dependence on NO3 when high NOx levels reach the site. Mace Head was seen to have a greater peroxy radical sensitivity to NOx and, subsequently, NOx influences ozone production more than at other marine boundary layer sites, leading to high ozone levels at the site during high NOx episodes.