Insights into the rates and processes governing NO2 foliar deposition to native California trees

Nitrogen oxides (NO_x ≡ NO + NO₂) are highly reactive gases that control the oxidative capacity of the atmosphere. In forested regions, where emissions from microbial activity in soils dominate the NO_x source, canopy-level field measurements suggest that absorption of NO₂ through the stomata of plants removes 20 - 70% of soil-emitted NO_x. Atmospheric chemical transport models treat foliar deposition as homogenous among land use category and vegetation type. However, direct leaf flux measurements have suggested a more significant degree of inter-species variability. To understand the leaf-level processes affecting ecosystem scale atmosphere-biosphere NO_x exchange, we have conducted laboratory experiments of branch-level NO₂ deposition fluxes to five 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 0.08 - 0.68 cm s^-1. We also find that the contribution of mesophyllic processing to the overall deposition rate of NO₂ varies significantly by tree species. Additionally, we will discuss the effects of soil nitrogen content on NO₂ deposition to the native California trees Quercus agrifolia and Pseudotsuga menziesii.