Measurement of the diurnal variation of the OH radical concentration and analysis of the data by modelling

Daily variations of the hydroxyl radical concentration have been measured d uring a campaign at the Weybourne Atmospheric Observatory (WAO) in June 199 5. These measurements are compared with box model calculations, based on a slightly modified, second generation Regional Acid Deposition Model (RADM2) . Results from eight days of the comparison are presented. A detailed analy sis and discussion of the different source and sink terms is given for two days: Julian Day (JD) 170 (19 June, and 178 (27 June). In both cases excell ent agreement between the measurements and the calculation is obtained, ind icating that the model describes the OH chemistry sufficiently well. Furthe rmore, the analysis of these days demonstrate that JD 170 is dominated by t he NOx catalysed OH production, whereas JD 178 is influenced by OH formatio n via ozone photolysis.