Abstract. Both canopy-level field measurements and laboratory studies suggest that uptake of NO2 through the leaf stomata of vegetation is a significant sink of atmospheric NOx. However, the mechanisms of this foliar NO2 uptake and their impact on NOx lifetimes remain incompletely understood. To understand the leaf-level processes affecting ecosystem-scale atmosphere–biosphere NOx exchange, we have conducted laboratory experiments of branch-level NO2 deposition fluxes to six coniferous and four broadleaf native California trees using a branch enclosure system with direct laser-induced fluorescence (LIF) detection of NO2. We report NO2 foliar deposition that demonstrates a large degree of inter-species variability, with maximum observed deposition velocities ranging from 0.15 to 0.51 cm s−1 during the daytime, as well as significant stomatal opening during the night. We also find that the contribution of mesophyllic processing to the overall deposition rate of NO2 varies by tree species but has an ultimately inconsequential impact on NOx budgets and lifetimes. Additionally, we find no evidence of any emission of NO2 from leaves, suggesting an effective unidirectional exchange of NOx between the atmosphere and vegetation.

Acyl peroxynitrates are formed in the atmosphere through the oxidation of NO x and are treated as temporary NO x sinks because they typically decompose to rerelease NO x on the time scale of a few hours. Canopy and leaf level measurements of acyl peroxynitrate deposition to vegetation, however, have revealed that this removal process is rapid and may compete with chemical decomposition. In an effort to learn more about the dry deposition of acyl peroxynitrates, we designed experiments to measure the deposition of peroxyacetyl nitrate and peroxypropionic nitrate to ten California-native tree species. The deposition of these two organic nitrate compounds was driven by leaf stomatal uptake. No surface deposition of either nitrate was observed. Maximum deposition velocities ranged from 0.09−0.3 cm s −1 and correlated strongly with maximum leaf stomatal conductance. The stomatal uptake of peroxyacetyl nitrate and peroxypropionic nitrate scaled with factors of 0.73 ± 0.03 and 0.95 ± 0.07, respectively, of the stomatal limit independent of water and nitrogen status of the trees. These measurements suggest that the uptake of acyl peroxynitrates by leaf stomata can be a dominant loss process in areas of high tree cover and moderate temperature.

<p><strong>Abstract.</strong> Both canopy-level field measurements and laboratory studies suggest that absorption of NO<sub>2</sub> through the leaf stomata of vegetation is a significant sink of atmospheric NO<sub><i>x</i></sub>. However, the mechanisms of this foliar NO<sub>2</sub> uptake and their impact on NO<sub><i>x</i></sub> lifetimes remains incompletely understood. To understand the leaf-level processes affecting ecosystem scale atmosphere-biosphere NO<sub><i>x</i></sub> exchange, we have conducted laboratory experiments of branch-level NO<sub>2</sub> deposition fluxes to six coniferous and four broadleaf native California trees using a branch enclosure system with direct Laser Induced Fluorescence (LIF) detection of NO<sub>2</sub>. We report NO<sub>2</sub> foliar deposition that demonstrates a large degree of inter-species variability, with maximum observed deposition velocities ranging from 0.15–0.51 cm/s during the daytime, as well as significant stomatal opening during the night. We also find that the contribution of mesophyllic processing to the overall deposition rate of NO<sub>2</sub> varies by tree species, but has an ultimately inconsequential impact on NO<sub><i>x</i></sub> budgets and lifetimes. Additionally, we find no evidence of any emission of NO<sub>2</sub> from leaves, suggesting an effective uni-directional exchange of NO<sub><i>x</i></sub> between the atmosphere and vegetation.</p>

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