A model-based analysis of foliar NOx deposition

Erin R. Delaria, Ronald C. Cohen
2019 Atmospheric Chemistry and Physics Discussions  
<p><strong>Abstract.</strong> Foliar deposition of NO<sub>2</sub> removes a large fraction of the global soil-emitted NO<sub><i>x</i></sub>. Understanding the mechanisms of NO<sub><i>x</i></sub> foliar loss is important for constraining surface ozone, NO<sub><i>x</i></sub> mixing ratios, and assessing the impacts of nitrogen inputs to ecosystems. We have constructed a 1D multi-box model with representations of chemistry and vertical transport to evaluate the impact of leaf-level processes on
more » ... vel processes on canopy-scale concentrations, lifetimes, and canopy fluxes of NO<sub><i>x</i></sub>. Our model is able to closely replicate canopy fluxes and above-canopy NO<sub><i>x</i></sub> daytime mixing ratios observed during two field campaigns, one in a western Sierra Nevada pine forest (BEARPEX-2009) and the other a northern Michigan mixed hardwood forest (UMBS-2012). Our model demonstrates that NO<sub>2</sub> deposition can provide a mechanistic explanation for canopy reduction factors (CRFs). We show that foliar deposition can explain observations suggesting as much as ~&amp;thinsp;60&amp;thinsp;% of soil-emitted NO<sub><i>x</i></sub> is removed within forest canopies. Stomatal conductances greater than 0.1&amp;thinsp;cm&amp;thinsp;s<sup>&amp;minus;1</sup> result in modelled canopy reduction factors in the range of those used in global models, reconciling inferences of canopy NO<sub><i>x</i></sub> reduction with leaf-level deposition processes. We also show that incorporating parameterizations for vapor pressure deficit and soil water potential has a substantial impact on predicted NO<sub>2</sub> deposition, with the percent of soil NO<sub><i>x</i></sub> removed within one canopy increasing by ~&amp;thinsp;15&amp;thinsp;% in wet conditions compared to dry conditions. NO<sub>2</sub> foliar deposition was found to have a significant impact on ozone and nitrogen budgets under both high and low NO<sub><i>x</i></sub> conditions.</p>
doi:10.5194/acp-2019-538 fatcat:nyb4tny7iba2heabrmob4yltx4