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Tropospheric ozone in the Lower Fraser Valley, British Columbia and the threat of injury to forest plants Krzyzanowski, Judi

Abstract

During the summers of 2001 and 2002 ambient ozone levels were measured as hourly averages in parts per billion (ppb) at four sites of differing elevation (200, 400, 600 and 1200 m) in the Lower Fraser Valley (LFV), British Columbia. A l l sites were located in forest clearings and experienced hourly averages as low as Oppb, and >70ppb. Mean seasonal concentrations show an increase in ambient ozone with elevation due to consistently high nocturnal concentrations and lack of diurnal variation at higher altitudes. Diurnal patterns are in agreement with previous studies showing a peak in concentration in the late afternoon, and a morning increase due to photochemical production and residual layer down mixing. The occurrence of an upper level ridge coinciding with a thermal trough along the coast, cause above average ozone levels to occur, and may cause the National Ambient Air Quality Objective of 82ppb to be exceeded. Cumulative ozone exposures were measured with height in a forest canopy using OGAWA passive samplers mounted to a 10.5m tower. A strong power-law increase in ozone with height was found due to a number of potential processes including deposition and chemical destruction at the surface, uptake by vegetation, and dynamic stability inhibiting down mixing. This relationship shows plants in the LFV may be acting as an effective ozone sink. Plants uptake ozone direcdy through leaf stomates where the pollutant may directly injure foliage. A preliminary survey of native shrubs exhibiting visible ozone injury symptoms suggests that current concentrations of tropospheric ozone in the LFV may be high enough to cause injury to forest species, however more research is required in order to determine the threat of ozone to these economically and culturally important forests.

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