Abstract
Abstract. Dry deposition is an important sink of tropospheric ozone that affects surface concentrations and impacts crop yields, the land carbon sink, and the terrestrial water cycle. Dry deposition pathways include plant uptake via stomata and non-stomatal removal by soils, leaf surfaces, and chemical reactions. Observational studies indicate that ozone deposition exhibits substantial temporal variability that is not reproduced by atmospheric chemistry models due to a simplified representation of vegetation uptake processes in these models. In this study, we explore the importance of stomatal and non-stomatal uptake processes in driving ozone dry deposition variability on diurnal to seasonal timescales. Specifically, we compare two land surface ozone uptake parameterizations – a commonly applied big leaf parameterization (W89; Wesely, 1989) and a multi-layer model (MLC-CHEM) constrained with observations – to multi-year ozone flux observations at two European measurement sites (Ispra, Italy, and Hyytiälä, Finland). We find that W89 cannot reproduce the diurnal cycle in ozone deposition due to a misrepresentation of stomatal and non-stomatal sinks at our two study sites, while MLC-CHEM accurately reproduces the different sink pathways. Evaluation of non-stomatal uptake further corroborates the previously found important roles of wet leaf uptake in the morning under humid conditions and soil uptake during warm conditions. The misrepresentation of stomatal versus non-stomatal uptake in W89 results in an overestimation of growing season cumulative ozone uptake (CUO), a metric for assessments of vegetation ozone damage, by 18 % (Ispra) and 28 % (Hyytiälä), while MLC-CHEM reproduces CUO within 7 % of the observation-inferred values. Our results indicate the need to accurately describe the partitioning of the ozone atmosphere–biosphere flux over the in-canopy stomatal and non-stomatal loss pathways to provide more confidence in atmospheric chemistry model simulations of surface ozone mixing ratios and deposition fluxes for large-scale vegetation ozone impact assessments.
Highlights
Ozone (O3) in the atmospheric surface layer is an air pollutant that is toxic to humans and plants
The contribution of these ozone removal processes to the total non-stomatal term is uncertain (Fowler et al, 2009) and displays temporal variability on diurnal to interannual timescales that is incompletely understood (Clifton et al, 2020a). Given that these non-stomatal removal processes act in parallel to the stomatal removal of ozone, the characterization and quantification of non-stomatal sinks is important for quantification of total and stomatal ozone uptake
To evaluate this bias further, we performed MLCCHEM simulations with a deactivated sink to wet leaves, motivated by the considerable uncertainty in this ozone removal pathway (Clifton et al, 2020a). This simulation resulted in the strongest decrease in Vd(O3) in the relatively humid months of April (Fig. S2), ranging from 0.15 cm s−1 in April 2013 to 0.05 cm s−1 in April 2015. This modification results in an improved representation of seasonality in Vd(O3), suggesting seasonal variation in the ozone sink to wet leaves that might not be properly captured by the RHdependent parameterization of wet leaf uptake (Eq 4)
Summary
Ozone (O3) in the atmospheric surface layer is an air pollutant that is toxic to humans and plants. Ozone removal occurs via a range of non-stomatal removal mechanisms, such as uptake by the leaf exterior and soils, and in-canopy chemical removal involving nitrogen oxides (NOx) or plant-emitted reactive carbon species. The contribution of these ozone removal processes to the total non-stomatal term is uncertain (Fowler et al, 2009) and displays temporal variability on diurnal to interannual timescales that is incompletely understood (Clifton et al, 2020a). Given that these non-stomatal removal processes act in parallel to the stomatal removal of ozone, the characterization and quantification of non-stomatal sinks is important for quantification of total and stomatal ozone uptake
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