Laboratory measurements of stomatal NO ₂ deposition to native California trees and the role of forests in the NO _x cycle

Both canopy-level field measurements and laboratory studies suggest that uptake of inline-formulaNO₂ through the leaf stomata of vegetation is a significant sink of atmospheric inline-formulaNO_x. However, the mechanisms of this foliar inline-formulaNO₂ uptake and their impact on inline-formulaNO_x lifetimes remain incompletely understood. To understand the leaf-level processes affecting ecosystem-scale atmosphere–biosphere inline-formulaNO_x exchange, we have conducted laboratory experiments of branch-level inline-formulaNO₂ deposition fluxes to six coniferous and four broadleaf native California trees using a branch enclosure system with direct laser-induced fluorescence (LIF) detection of inline-formulaNO₂. We report inline-formulaNO₂ foliar deposition that demonstrates a large degree of inter-species variability, with maximum observed deposition velocities ranging from 0.15 to 0.51 inline-formulacm 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 inline-formulaNO₂ varies by tree species but has an ultimately inconsequential impact on inline-formulaNO_x budgets and lifetimes. Additionally, we find no evidence of any emission of inline-formulaNO₂ from leaves, suggesting an effective unidirectional exchange of inline-formulaNO_x between the atmosphere and vegetation.