GRADIENTS OF OZONE AT A FOREST SITE AND OVER A FIELD CROP - CONSEQUENCES FOR THE AOT40 CONCEPT OF CRITICAL-LEVEL

Ozone concentrations were measured at a wind-exposed edge of a 60 year -old 15-20 m tall Norway spruce forest in south-west Sweden and simult aneously over a barley field 5 ion away for 27 days. At the forest sit e, measurements were performed at 3 and 13 m height 15 m in front of t he forest edge, at 3 m height 15 m into the forest, and at 3 and 13 m height 45 m into the forest. Measurements at 3 m were made with three replicate tubes separated by 10 m. Differences between replicates were small. At 13 m height, the concentration (24-hr-average) 45 m into th e forest was 95% of that in front of the forest edge. The average conc entration at 3 m height did not vary strongly with the distance into t he forest, but was 86% of that at 13 m in front of the forest edge. Fo r AOT40 (Accumulated Exposure Over Threshold 40 ppb ozone), the differ ences between different positions were larger. At the 13 m level the A OT40 (day and night) was 88% of that in front of the forest 45 m into the forest. The AOT40 at 3 m was 71% of that at 13 m outside the fores t. At the crop site, the ozone concentration at 1.1 m (0.1 m above the canopy), was 78% of that at 9 m (06.00-22.00). The AOT40 at 1.1 m abo ve the ground, however, was only 50% of that at 9 m, indicating that s erious errors can arise if ozone monitoring data are used uncorrected in dose-response relationships based on measurements performed at plan t height. The ozone concentration for the whole period differed very l ittle between 9 m height at the crop site and 13 m height at the fores t site outside the forest during daytime conditions (06.00-22.00). Nig ht-time (22.00-06.00) values were only 21% at the crop site of those a t the forest site due to the stronger night inversion development in t he agricultural environment compared to the wind exposed forest edge. The results suggest that variations in topography and vegetation are i mportant to consider when combining ozone monitoring data with dose-re sponse functions.