Factors affecting stomatal uptake of ozone by different canopies and a comparison between dose and exposure.

Measured ozone (O(3)) and carbon dioxide (CO(2)) concentrations and fluxes over five different canopies (mixed coniferous-deciduous forest, deciduous forest, corn, soybean and pasture) in the eastern USA were analyzed to investigate the stomatal uptake of O(3). It was found that the ambient O(3) concentration levels had little effect on stomatal conductance. However, the accumulated stomatal uptake of O(3), upon reaching a threshold value on any given day, appears to reduce the rate of further O(3) uptake substantially. This may explain why the maximum O(3) deposition velocity often appeared in the early morning hours over some forest canopies. Substantially reduced CO(2) fluxes over wet canopies compared to dry canopies suggest that stomata were likely partially or totally blocked by water droplets or films when canopies were wet. By using a big-leaf dry deposition model, measured O(3) fluxes were separated into stomatal and non-stomatal portions. It was estimated that stomatal uptake contributed 55-75% of the total daytime O(3) fluxes and 40-60% of the total daytime plus nighttime fluxes, depending on canopy type. This suggests that about half of the total O(3) flux occurred through the non-stomatal pathway. At three locations (deciduous forest, corn and soybean sites), O(3) concentrations of 30-60 ppb and of 60-85 ppb contributed equally to the accumulated stomatal fluxes, while at the other two locations (mixed coniferous-deciduous forest and pasture sites), concentrations of 30-60 ppb contributed twice as much as those from 60 to 85 ppb.