Insights into ozone deposition patterns from decade-long ozone flux measurements over a mixed temperate forestPresented at the COST International Conference on Ozone, Climate Change and Forests, 14-16 June 2011, Prague, Czech Republic

• Main Authors: Neirynck, J, Gielen, B, Janssens, I. A, Ceulemans, R
• Format: Journal Article
• Language: English
• Published: 30.05.2012
• ISSN: 1464-0325; 1464-0333
• DOI: 10.1039/c2em10937a

Long-term fluxes of ozone (O 3 ) were measured over a mixed temperate forest using the aerodynamic gradient method. The long-term average O 3 flux ( F ) was −366 ng m −2 s −1 for the period 2000-2010, corresponding to an average O 3 concentration of 48 μg m −3 and a deposition velocity v d of 9 mm s −1 . Average nocturnal ozone deposition amounted to −190 ng m −2 s −1 , which was about one third of the daytime flux. Also during the winter period substantial O 3 deposition was measured. In addition, total O 3 fluxes were found to differ significantly among canopy wetness categories. During the day, highest deposition fluxes were generally measured for a dry canopy, whereas a rain-wetted canopy constituted the best sink at night. Flux partitioning calculations revealed that the stomatal flux ( F s ) contributed 20% to the total F but the F s / F fraction was subject to seasonal and diurnal changes. The annual concentration-based index AOT40 (accumulated dose over a threshold of 40 ppb) and the Phytotoxic Ozone Dose (POD 1 or accumulated stomatal flux above a threshold of 1 nmol m −2 s −1 ) were related in a curvilinear way. The O 3 deposition was found to be largely controlled by non-stomatal sinks, whose strength was enhanced by high friction velocities ( u * ), optimizing the mechanical mixing of O 3 into the canopy and the trunk space. The long-term geometrical mean of the non-stomatal resistance ( R ns ) was 136 s m −1 but lower R ns values were encountered during the winter half-year due to higher u * . The R ns was also subject to a marked diurnal variability, with low R ns in the morning hours, when turbulence took off. We speculate that non-stomatal deposition was largely driven by scavenging of ozone by biogenic volatile organic compounds (BVOCs) and especially NO emitted from the crown or the forest floor. Long-term fluxes of ozone (O 3 ) were measured over a mixed temperate forest using the aerodynamic gradient method.