Insights into ozone deposition patterns from decade-long ozone flux measurements over a mixed temperate forest
Faculty of Sciences. Biology
Journal of environmental monitoring. - Cambridge, 1999 - 2012
, p. 1684-1695
University of Antwerp
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 mg m(-3) and a deposition velocity nu(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 (POD1 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 Rns was also subject to a marked diurnal variability, with low Rns 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.