Ozone ( O3 ) concentrations , monitored with passive samplers in remote forest areas of south-west Europe , were high in the southern-most areas and at high altitudes . Critical levels of 5,000 and 10,000 parts per billion times hours ( ppb h ) for accumulated O3 concentrations over the threshold of 40 ppb ( AOT40 ) were exceeded at 95 % and 69 % of 100 sites , respectively , averaged over the period 2000 - 2002 .