OZONE GRADIENTS IN A SPRUCE FOREST STAND IN RELATION TO WIND-SPEED AND TIME OF THE DAY

Ozone concentrations were measured outside and inside a 60-year-old 15 -20 m tall spruce forest at a wind-exposed forest edge in southwest Sw eden, at 3 and 13 m height 15 m outside the forest, and at 3 and 13 m height inside the forest 45 m from the forest edge. Measurements at 3 m were made with three replicate tubes on each site, the replicates be ing separated by 10 m. In addition, horizontal and vertical wind speed s were measured at 8 m height outside and inside the forest. During da ytime, the concentrations inside the forest were generally slightly lo wer. Negative ozone concentration gradients from the open field into t he forest were observed at 3 m height when the wind speed was below ap proximately 1.5 m s(-1). At very low wind speeds, mainly occurring dur ing the night, the ozone concentrations at 3 m height were frequently higher inside the forest than outside the forest. This may be caused b y a very large aerodynamic resistance to ozone deposition, due to very small air movements inside the forest under stable conditions. It is concluded that ozone uptake by the trees is likely to be very small at night, even if stomata are not entirely closed. Results from open-top chamber experiments are also discussed. Copyrigh (C) 1996 Elsevier Sc ience Ltd