Peroxy radicals (HO₂+?RO₂) were measured at the Weybourne Atmospheric Observatory (52° N, 1° E), Norfolk using a PEroxy Radical Chemical Amplifier (PERCA) during the winter and summer of 2002. The peroxy radical diurnal cycles showed a marked difference between the winter and summer campaigns with maximum concentrations of 12 pptv at midday in the summer and maximum concentrations as high as 30 pptv (10 min averages) in winter at night. The corresponding nighttime peroxy radical concentrations were not as high in summer (3 pptv). The peroxy radical concentration shows a distinct anti-correlation with increasing NO_x during the daylight hours. At night, peroxy radicals increase with increasing NO_x indicative of the role of NO₃ chemistry. The average diurnal cycles for net ozone production, N(O₃) show a large variability in ozone production, P(O₃), and a large ozone loss, L(O₃) in summer relative to winter. For a daylight average, net ozone production in summer than winter (1.51±0.5 ppbv h^?1 and 1.11±0.47 ppbv h^?1 respectively) but summer shows more variability of (meteorological) conditions than winter. The variability in NO concentration has a much larger effect on N(O₃) than the peroxy radical concentrations. Photostationary state (PSS) calculations show an NO₂ lifetime of 5 min in summer and 21 min in the winter, implying that steady-state NO-NO₂ ratios are not always attained during the winter months. The results show an active peroxy radical chemistry at night and the ability of winter to make oxidant. The net effect of this with respect to production of ozone in winter is unclear owing to the breakdown in the photostationary state.