Laboratory measurements of stomatal NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; deposition to native California trees and the role of forests in the NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; cycle

Erin R Delaria,Amy X Liu,Ronald C Cohen,Bryan K Place

doi:10.5194/acp-20-14023-2020

Laboratory measurements of stomatal NO&lt;sub&gt;2&lt;/sub&gt; deposition to native California trees and the role of forests in the NO&lt;sub&gt;x&lt;/sub&gt; cycle

Erin R Delaria, Amy X Liu + Show 2 more

Open Access

https://doi.org/10.5194/acp-20-14023-2020

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Abstract

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.

Highlights

Nitrogen oxides (NOx ≡ NO + NO2) are a form of reactive nitrogen that play a major role in the chemistry of the atmosphere
In this study we present results from laboratory measurements of NO2 fluxes of 10 native California tree species – six conifers and four broadleaf trees – using a branch enclosure system and laser-induced fluorescence (LIF) detection of NO2
P. menziesii was found to have a compensation point, estimated to be 20 ppt, but this concentration is below the limit of quantification for our instrument, so we consider this measurement to be consistent with a compensation point of zero

Summary

Introduction

Nitrogen oxides (NOx ≡ NO + NO2) are a form of reactive nitrogen that play a major role in the chemistry of the atmosphere. There is evidence that NO2 may be directly scavenged by antioxidants, most notably ascorbate (Ramge et al, 1993; Teklemmariam and Sparks, 2006) These processes may be impacted by the leaf pH, which is known to change under conditions of limited water availability (Bahrun et al, 2002). The role of stomatal conductance (gs) in controlling the deposition of NO2 is well-documented, the impact of mesophyllic processes remains poorly resolved. These mesophyllic mechanisms are complex and include any process taking place between the intercellular air space and the ultimate nitrogen assimilation site.

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