Alkyl and multifunctional nitrates (RONO2) and peroxynitrates (RO2NO2) influence the atmosphere through their role in sequestering or recycling atmospheric NO and NO2 (ie. NOx). This effect modulates the production of tropospheric ozone (O3), which has consequences for human and ecosystem health. The chemical processing of RONO2, RO2NO2, and NOx typically leads to the recycling of NOx, which in most environments leads to higher ozone production. The deposition of these nitrogen oxide compounds, however, leads to the permanent loss of these compounds from the atmosphere. Recent results have indicated that nitrogen oxides can deposit rapidly from the atmosphere, but the rates, mechanisms, and properties that influence deposition are not well understood. Further, an understanding of the impact of nitrogen oxide deposition on the lifetime of nitrogen oxides as well as its role in the NOx cycle is needed.
Laboratory chamber experiments were run using a variety of RONO2 and RO2NO2 compounds and tree species to gain insight into the leaf-level processes driving organic nitrate deposition to vegetation. These experiments revealed that the deposition of RONO2 and RO2NO2 occurred solely through leaf stomatal uptake. The deposition of RO2NO2 was found to scale linearly with stomatal conductance. The deposition of RONO2 did not scale with stomatal conductance and the deposition rates differed for each RONO2 compound tested. The rates of uptake observed for all RONO2 and RO2NO2 studied were too fast to be explained through a dissolution and hydrolysis mechanism within the leaf, suggesting uptake via an alternative mechanism. Scaling the observed RO2NO2 deposition rates to the canopy-level indicated that the deposition of these organic nitrates from the atmosphere could compete with their thermochemical losses. Scaled canopy deposition rates for RONO2 led to the conclusion that deposition was unlikely to be an important atmospheric loss of these compounds.
In parallel with these laboratory experiments, a remote sensing canopy conductance model was developed to estimate the stomatal uptake of atmospheric nitrogen oxides on a regional scale. The model was shown to accurately capture real-time canopy stomatal conductance diffusion rates across the continental United States using satellite retrievals of solar-induced fluorescence. The model was able to successfully reproduce the spatial distribution of nitrogen oxide fluxes that were estimated from leading chemical transport models. A key advantage of the canopy conductance model was its ability to capture real changes in deposition over the growing season driven by environmental factors such as drought. The canopy conductance model was used to estimate the lifetime of PAN and NO2 over the USA and indicated that the lifetime to deposition of these compounds was shortest in heavily forested coastal regions.
