Passive Ozone Monitoring for Forest Health Assessment

Critical levels of tropospheric ozone, established for the protection of crops and other plants, are now reported as being exceeded over large forested areas, giving rise to the need for an extensive monitoring program to confirm ambient levels within the forest and to detect related forest health effects. The requirement for an inexpensive monitor that can be used in remote locations prompted the development of the Can Oxy Plate^TM passive ozone monitor and a monitoring protocol by the air pollution research group of the Canadian Forest Service, Forest Health Network. The monitors underwent initial trials in 1996 and operational trials during 1997 that involved two 2-3 week mid summer exposures in the canopy at selected forest health monitoring plots across Canada, and at adjacent forest openings. In both trials monitors were also co-located with the nearest instrumental ozone monitor. This allowed for the production of a field calibration for quality assurance assessment under field conditions. Results from 1996 indicate highly significant correlations with accumulated ambient ozone concentrations from the instrumental monitors at the co-located sites (r=0.88, p=0.0002). However, no such relationship was found between these sites and the forest plots which were up to 200 kilometres away. This may indicate spatial heterogeneity in ozone exposure between the continuous air quality monitoring sites and the forest plots. This information, together with our knowledge that strong gradients of ozone exposure are found within the canopy, underlines the importance of in situ monitoring of ozone exposure of forest health plots at risk to ozone effects.