Measures of ozone concentrations using passive sampling in forests of South Western Europe.

Ambient ozone concentrations were measured with passive samplers in the framework of the EU and UN/ECE Level II forest monitoring programme. Data from France, Italy, Luxembourg, Spain and Switzerland are reported for 2000-2002, covering the period from April to September. The number of plots increased from 67 in 2000 to 83 in 2002. The year 2001 experienced the highest ozone concentrations, reflecting more stable summer meteorological conditions. Average 6-month ozone concentrations above 45 ppb were measured this year in 40.3% of the plots, in contrast with the less than 21% measured in the other 2 years. Gradients of increasing ozone levels were observed from North to South and with altitude. Comments are made on the regional trends and on the time frame of the higher ozone episodes. Also, some recommendations enabling a better comparison between plots are provided.