Abstract. During the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) campaign from 21 July to 3 August 2016,
field experiments on leaf-level trace gas exchange of nitric oxide (NO), nitrogen dioxide (NO2), and ozone (O3) were conducted for the
first time on the native American tree species Pinus strobus (eastern white pine), Acer rubrum (red
maple), Populus grandidentata (bigtooth aspen), and Quercus rubra (red oak) in a temperate hardwood forest in
Michigan, USA. We measured the leaf-level trace gas exchange rates and
investigated the existence of an NO2 compensation point, hypothesized
based on a comparison of a previously observed average diurnal cycle of
NOx (NO2+NO) concentrations with that simulated using a
multi-layer canopy exchange model. Known amounts of trace gases were
introduced into a tree branch enclosure and a paired blank reference
enclosure. The trace gas concentrations before and after the enclosures were
measured, as well as the enclosed leaf area (single-sided) and gas flow rate to obtain the trace gas fluxes with respect to leaf surface. There was no
detectable NO uptake for all tree types. The foliar NO2 and O3
uptake largely followed a diurnal cycle, correlating with that of the leaf
stomatal conductance. NO2 and O3 fluxes were driven by their
concentration gradient from ambient to leaf internal space. The NO2 loss rate at the leaf surface, equivalently the foliar NO2 deposition velocity toward the leaf surface, ranged from 0 to 3.6 mm s−1 for bigtooth aspen and from 0 to 0.76 mm s−1 for red oak, both of which are
∼90 % of the expected values based on the stomatal
conductance of water. The deposition velocities for red maple and white pine
ranged from 0.3 to 1.6 and from 0.01 to 1.1 mm s−1, respectively, and were lower than predicted from the stomatal conductance, implying a
mesophyll resistance to the uptake. Additionally, for white pine, the
extrapolated velocity at zero stomatal conductance was 0.4±0.08 mm s−1, indicating a non-stomatal uptake pathway. The NO2
compensation point was ≤60 ppt for all four tree species and
indistinguishable from zero at the 95 % confidence level. This agrees with
recent reports for several European and California tree species but
contradicts some earlier experimental results where the compensation points
were found to be on the order of 1 ppb or higher. Given that the sampled
tree types represent 80 %–90 % of the total leaf area at this site, these
results negate the previously hypothesized important role of a leaf-scale
NO2 compensation point. Consequently, to reconcile these findings,
further detailed comparisons between the observed and simulated in- and above-canopy NOx concentrations and the leaf- and canopy-scale
NOx fluxes, using the multi-layer canopy exchange model with
consideration of the leaf-scale NOx deposition velocities as well as
stomatal conductances reported here, are recommended.
Measurement report: Leaf-scale gas exchange of atmospheric reactive trace species (NO<sub>2</sub>, NO, O<sub>3</sub>) at a northern hardwood forest in Michigan