Partitioning ozone fluxes between canopy and forest floor by measurements and a multi-layer model

Abstract

Ozone uptake by plant leaves is essential to studies investigating atmospheric air pollution and plant injury. A major challenge to these investigations is the up-scaling of leaf-level stomatal processes to the ecosystem level, the accounting for forest floor ozone removal mechanics, and resolving the numerous pathways responsible for non-stomatal ozone removal within the canopy sublayer. To progress on the first two and offer constraints on the third, the O3 fluxes above and within a boreal Scots pine forest in Southern Finland are explored using a combination of two-level eddy covariance fluxes and detailed within-canopy concentration profiles. The interpretation of the results is aided by a multi-layer canopy model (MLM), which couples radiation attenuation and turbulent transport within the canopy volume with leaf-level gas exchange, photosynthesis, and stomatal conductance. Validation of the MLM against measured turbulent CO2 and H2O fluxes within and above the canopy, as well as their concomitant mean concentration profiles suggest that the stomatal pathway is reasonably quantified via the proposed MLM approach. The results show that the stomatal pathway alone can explain some 80% of the daytime dry-canopy ecosystem uptake of O3. The non-stomatal O3 uptake is largest during nighttime and early morning hours when between one third and half of it seems to originate from below the overstory canopy. During daytime, almost all the non-stomatal uptake occurs in the sub-canopy region. Sub-canopy and/or understory processes contribute between 25–30% (nighttime) and 35–45% (daytime) ecosystem O3 uptake. In sub-canopy, the non-stomatal component exceeds the stomatal by a factor of 2–4 during daytime. Finally, the MLM was used to study some of the potential non-stomatal pathways, including cuticular and soil uptake, forest floor uptake and disequilibrium between photochemical O3 production and a first-order kinetic chemical destruction mechanism. The results indicate that the likely location of the non-stomatal sink is in the lower trunk-space close the forest floor but the soil surface uptake is insignificant. According to the results, a bulk gas-phase disequilibrium between O3 production (assumed to vary linearly with light at a given level inside the canopy) and destruction (assumed to vary exponentially with mean air temperature) is a plausible explanation for non-stomatal O3 removal inside the canopy.