Plant—atmosphere exchange of ammonia

Abstract

The results of recent controlled environment and micrometeorological measurements of NH 3 fluxes are presented to highlight the processes controlling NH 3 plant-atmosphere exchange. The presence of NH +4 in leaf tissues results in the existence of an NH 3 ‘compensation point’ concentration for substomatal tissues (x s ), so that both emission and deposition are possible from stomata. In addition, NH 3 may deposit efficiently on to leaf cuticles, short-circuiting any stomatal emission, so that a ‘canopy compensation point’ (X c ) may be defined that is smaller than X s . Ammonia is generally deposited to nitrogen limited ecosystems, indicating a small X s and small leaf cuticle resistance ( R w ). In contrast, fluxes over croplands are typically bidirectional and may reflect a larger X s as a consequence of greater N supply. The paper discusses the processes defining (humidity, acidic pollutants) and X s (plant phenology, species, N nutrition) and proposes a new resistance approach, which integrates X s and R w into one model. Estimating long term bidirectional NH 3 fluxes is still uncertain, though it is now possible to apply a single model concept to a range of ecosystem types and satisfactorily infer NH 3 fluxes over diurnal time scales.

Atomic-scale analysis of liquid-gallium embrittlement of aluminum grain boundaries

“…125 STGBs were created in Al (<100>, <110> and <111> tilt axes) and equilibrated using MS simulations performed with the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [54].The simulation cell consisted of two single crystal layers meeting at a planar grain boundary with periodic boundary conditions in all three orthogonal directions [45,51,[55][56][57][58][59][60][61]. The thickness of the simulation cell in the direction perpendicular to the boundary plane was ~12 nm (this large value was chosen in order to eliminate any effects associated with the periodic boundary condition).…”

“…The thickness of the simulation cell in the direction perpendicular to the boundary plane was ~12 nm (this large value was chosen in order to eliminate any effects associated with the periodic boundary condition). Several 0 K minimum-energy GB structures were obtained through successive rigid body in-plane translations of the two grains followed by application of an atom-deletion technique and energy minimization using a conjugate gradient method [45,51,54,55,57,58,[60][61][62][63][64]. The embedded atom potential (EAM) developed by Nam and Srolovitz [9] was used to describe the Al-Al interactions in generating the impurity-free GB systems and to describe Al-Ga interaction in the segregation calculations.…”

“…For low-angle boundaries, these units describe discrete dislocations. However, at higher misorientation angles (high-angle GBs), the dislocation cores overlap, dislocations relax to minimize the boundary energy, yet such GBs can be characterized by GB dislocations or structural units [45]. In certain GBs (typically low Σ), (D) coherent twin boundary, respectively.…”

“…In certain GBs (typically low Σ), (D) coherent twin boundary, respectively. The structural units reported here are represented in a convention widely used to represent fcc cubic metals [45,58,59,78]. The Voronoi atomic volume V vor is also computed for the boundaries in Fig.…”

“…Grain boundaries are two-dimensional defects which are characterized by five macroscopic degrees of freedom [40][41][42][43][44][45][46][47][48]. Saylor et al [40] analyzed the GB character distribution (GBCD) in commercially pure Al and suggested that boundaries with low index planes occur with particularly high frequency in polycrystals.…”