Measurements of NO and NO2 exchange between the atmosphere and Quercus agrifolia

Erin R. Delaria, Megan Vieira, Julie Cremieux, Ronald C. Cohen
2018 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> <span class="inline-formula">NO<sub>2</sub></span> foliar deposition through the stomata of leaves has been identified as a significant sink of <span class="inline-formula">NO<sub><i>x</i></sub></span> within a forest canopy. In this study, we investigated <span class="inline-formula">NO<sub>2</sub></span> and NO exchange between the atmosphere and the leaves of the native California oak tree <i>Quercus agrifolia</i> using a branch enclosure system. <span
more » ... em. <span class="inline-formula">NO<sub>2</sub></span> detection was performed with laser-induced fluorescence (LIF), which excludes biases from other reactive nitrogen compounds and has a low detection limit of 5–50<span class="thinspace"></span>ppt. We performed both light and dark experiments with concentrations between 0.5 and 10<span class="thinspace"></span>ppb <span class="inline-formula">NO<sub>2</sub></span> and NO under constant ambient conditions. Deposition velocities for <span class="inline-formula">NO<sub>2</sub></span> during light and dark experiments were <span class="inline-formula">0.123±0.009</span> and <span class="inline-formula">0.015±0.001</span><span class="thinspace"></span>cm<span class="thinspace"></span>s<span class="inline-formula"><sup>−1</sup></span>, respectively. Much slower deposition was seen for NO, with deposition velocities of <span class="inline-formula">0.012±0.002</span> and <span class="inline-formula">0.005±0.002</span><span class="thinspace"></span>cm<span class="thinspace"></span>s<span class="inline-formula"><sup>−1</sup></span> measured during light and dark experiments, respectively. This corresponded to a summed resistance of the stomata and mesophyll of <span class="inline-formula">6.9±0.9</span><span class="thinspace"></span>s<span class="thinspace"></span>cm<span class="inline-formula"><sup>−1</sup></span> for <span class="inline-formula">NO<sub>2</sub></span> and <span class="inline-formula">140±40</span><span class="thinspace"></span>s<span class="thinspace"></span>cm<span class="inline-formula"><sup>−1</sup></span> for NO. No significant compensation point was detected for <span class="inline-formula">NO<sub>2</sub></span> uptake, but compensation points ranging from 0.74 to 3.8<span class="thinspace"></span>ppb were observed for NO. <span class="inline-formula">NO<sub>2</sub></span> and NO deposition velocities reported here are comparable both with previous leaf-level chamber studies and inferences from canopy-level field measurements. In parallel with these laboratory experiments, we have constructed a detailed 1-D atmospheric model to assess the contribution of leaf-level <span class="inline-formula">NO<sub><i>x</i></sub></span> deposition to the total <span class="inline-formula">NO<sub><i>x</i></sub></span> loss and <span class="inline-formula">NO<sub><i>x</i></sub></span> canopy fluxes. Using the leaf uptake rates measured in the laboratory, these modeling studies suggest that loss of <span class="inline-formula">NO<sub><i>x</i></sub></span> to deposition in a California oak woodland competes with the pathways of <span class="inline-formula">HNO<sub>3</sub></span> and RO<span class="inline-formula">NO<sub>2</sub></span> formation, with deposition making up 3<span class="thinspace"></span>%–22<span class="thinspace"></span>% of the total <span class="inline-formula">NO<sub><i>x</i></sub></span> loss. Additionally, foliar uptake of <span class="inline-formula">NO<sub><i>x</i></sub></span> at these rates could account for <span class="inline-formula">∼15</span><span class="thinspace"></span>%–30<span class="thinspace"></span>% canopy reduction of soil <span class="inline-formula">NO<sub><i>x</i></sub></span> emissions.</p>
doi:10.5194/acp-18-14161-2018 fatcat:zsqbtjnwfbfojn65rczwipcym4