The combined impact of canopy stability and soil NOx exchange on ozone
removal in a temperate deciduous forest
Abstract
Dry deposition is an important ozone sink that impacts ecosystem carbon
and water cycling. Ozone dry deposition in forests is regulated by
vertical transport, stomatal uptake, and non-stomatal processes
including chemical removal. However, accurate descriptions of these
processes in deposition parameterizations are hindered by sparse
observational constraints on individual sink terms. Here we quantify the
contribution of canopy-atmosphere turbulent exchange and chemical ozone
removal by soil-emitted nitric oxide (NO) to ozone deposition in a
North-Italian broadleaf deciduous forest. We apply a multi-layer canopy
exchange model to interpret campaign observations of nitrogen oxides
(NOx=NO+NO2) and ozone exchange above and inside the
forest canopy. Two state-of-science parameterizations of in-canopy
vertical diffusivity, based on above-canopy wind speed or stability, do
not reproduce the observed exchange suppressed by canopy-top radiative
heating, resulting in overestimated dry deposition velocities of
10-19\% during daytime. Applying observation-derived
vertical diffusivities in our simulations largely resolves this
overestimation. Soil emissions are an important NOx source despite the
observed high background NOx levels. Soil NOx emissions decrease the
gradient between canopy and surface layer NOx mixing ratios, which
suppresses simulated NOx deposition by 80% compared to a sensitivity
simulation without soil emissions. However, a sensitivity analysis shows
that the enhanced chemical ozone sink by reaction with soil-emitted NO
is offset by increased vertical ozone transport from aloft and
suppressed dry deposition. Our results highlight the need for targeted
observations of non-stomatal ozone removal and turbulence-resolving
deposition simulations to improve quantification and model
representation of forest ozone deposition.