Leaf uptake of nitrogen dioxide (NO₂) in a tropical wet forest: implications for tropospheric chemistry

Abstract

Tropical forest soils are known to emit large amounts of reactive nitrogen oxide compounds, often referred to collectively as NO_y (NO_y = NO + NO₂ + HNO₃ + organic nitrates). Plants are known to assimilate and emit NO_y and it is therefore likely that plant canopies affect the atmospheric concentration of reactive nitrogen compounds by assimilating or emitting some fraction of the soil-emitted NO_y. It is crucial to understand the magnitude of the canopy effects and the primary environmental and physiological controls over NO_y exchange in order to accurately quantify regional NO_y inventories and parameterize models of tropospheric photochemistry. In this study we focused on nitrogen dioxide (NO₂), which is the component of NO_y that most directly catalyzes the chemistry of O₃ dynamics, one of the most abundant oxidative species in the troposphere, and which has been reported as the NO_y species that is most readily exchanged between plants and the atmosphere. Leaf chamber measurements of NO₂ flux were measured in 25 tree species growing in a wet tropical forest in the Republic of Panama. NO₂ was emitted to the atmosphere at ambient NO₂ concentrations below 0.53–1.60 ppbv (the NO₂ compensation point) depending on species, with the highest rate of emission being 50 pmol m^–2 s^–1 at <0.1 ppbv. NO₂ was assimilated by leaves at ambient NO₂ concentrations above the compensation point, with the maximum observed uptake rate being 1,550 pmol m^–2 s^–1 at 5 ppbv. No seasonal variation in leaf NO₂ flux was observed in this study and leaf emission and uptake appeared to be primarily controlled by leaf nitrogen and stomatal conductance, respectively. When scaled to the entire canopy, soil NO emission rates to the atmosphere were estimated to be maximally altered ±19% by the overlying canopy.