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The rates and processes affecting the deposition of NOx to vegetation at leaf-level and canopy-level scales: impacts on NOx lifetimes and budgets in the troposphere
- Delaria, Erin Rose
- Advisor(s): Cohen, Ronald C
Abstract
Both canopy-level field measurements and laboratory studies suggest that absorption of NO2 through the leaf stomata of vegetation is a significant sink of atmospheric NOx (NOx≡NO2+ NO), removing a large fraction of the global soil-emitted NOx. Understanding the mechanisms of NOx foliar loss is important for constraining surface ozone, constraining NOx mixing ratios, and assessing nitrogen inputs to ecosystems. However, the mechanisms of this foliar NO2 uptake and their impact on NOx lifetimes remains incompletely understood.To understand the leaf-level processes affecting ecosystem scale atmosphere-biosphere NOx exchange, I 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, which excludes biases from other reactive nitrogen compounds and has a low detection limit of 5—50 ppt. I report NO2 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. I 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, I report no evidence of any emission of NO2 from leaves, suggesting an effective uni-directional exchange of NOx between the atmosphere and vegetation.In parallel with these laboratory experiments, I have constructed a detailed 1-D atmospheric model to assess the contribution of leaf-level NOx deposition to the total NOx loss and canopy flux. My model is able to closely replicate canopy fluxes and above-canopy NOx daytime mixing ratios observed during two field campaigns, one in a western Sierra Nevada pine forest (BEARPEX-2009) and the other in a northern Michigan mixed hardwood forest (UMBS-2012). I present a conceptual argument for the importance of NO2 dry deposition and demonstrate that NO2 deposition can provide a mechanistic explanation for the canopy reduction of NOx. Using the leaf uptake rates measured in the laboratory, these modeling studies suggest that loss of NOx to deposition in forests competes with the pathways of HNO3 and alkyl nitrate (RONO2) formation, with deposition making up to∼40% of the total NOx loss. Additionally, foliar uptake of NOx at these rates could account for as much as∼60%canopy reduction of soil NOx emissions, reconciling inferences of canopy NOx reduction with leaf-level deposition processes. Finally, I show NO2 foliar deposition has a significant impact on ozone and nitrogen budgets under both high- and low-NOx conditions.
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