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Controls over ozone deposition to a high elevation subalpine forest

Turnipseed, Andrew A., Burns, Sean P., Moore, David J.P., Hu, Jia, Guenther, Alex B., Monson, Russell K.
Agricultural and forest meteorology 2009 v.149 no.9 pp. 1447-1459
montane forests, ozone, air pollution, atmospheric deposition, seasonal variation, wind, photosynthesis, growing season, light intensity, relative humidity, vapor pressure, stomatal conductance, precipitation, leaf conductance, diurnal variation, conifer needles, heat transfer, Colorado
Ecosystem level ozone (O₃) fluxes during four different years were examined at a subalpine forest site in the Colorado Rocky Mountains. The local mountain-valley wind system and the proximity of the Denver Metropolitan area leads to high summertime ozone episodes on many afternoons. The timing between these episodes and the ecosystem processes controlling photosynthesis during the growing season plays a critical role in determining the amount of ozone deposition. Light and vapor pressure deficit (VPD) were the most dominant environmental drivers controlling the deposition of O₃ at this site through their influence on stomatal conductance. 81% of the daytime O₃ uptake was predicted to occur through the stomata. Stomatal uptake decreased at high VPD and temperatures leading to an overall decrease in O₃ flux; however, we did observe a non-stomatal conductance for O₃ that increased slightly with temperature before leveling off at higher values. During the growing season, O₃ deposition fluxes were enhanced after midday precipitation events and continued at elevated levels throughout the following night, implying a role for surface wetness. From nighttime data, evidence for both the presence of water films on the needles and non-closure of the plant stomata were observed. During the winter (non-growing) season, the ozone deposition velocity showed a consistent dependency on the latent heat flux. Although the mechanism is unclear, it is apparent that precipitation events play a role here through their influence on latent heat flux.