Insights into ozone deposition patterns from decade-long ozone flux measurements over a mixed temperate forest

Abstract

Long-term fluxes of ozone (O₃) were measured over a mixed temperate forest using the aerodynamic gradient method. The long-term average O₃ flux (F) was −366 ng m⁻² s⁻¹ for the period 2000–2010, corresponding to an average O₃ 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 O₃ deposition was measured. In addition, total O₃ 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 O₃ 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 O₃ 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.