Elevated ozone concentrations have been measured in rural areas where local anthropogenic sources of ozone precursors are insignificant. The long-range transport of ozone and its precursors from upwind source areas (or possibly ozone from a stratospheric origin) is thought to be responsible for the high concentrations found. Maximal hourly ozone concentrations exceeding 200 µg/m3 and 300 µg/m3 have been observed in rural areas of Norway and Sweden and in rural areas of the USA, respectively (1). Maximal hourly ozone values of 430 and 520 µg/m3 were measured in the Netherlands and the United Kingdom,respectively (6).