Plant-atmosphere exchange of ammonia

The results of recent controlled environment and micrometeorological measure- ments of NH3 fluxes are presented to highlight the processes controlling NH3 plant-atmosphere exchange. The presence of NH+ in leaf tissues results in the existence of an NH3 'compensation point' concentration for substomatal tissues (Xs), so that both emission and deposition are possible from stomata. In addition, NH3 may deposit efficiently on to leaf cuticles, short-circuiting any stomatal emis- sion, so that a 'canopy compensation point' (Xc) may be defined that is smaller than Xs. Ammonia is generally deposited to nitrogen limited ecosystems, indi- cating a small Xs and small leaf cuticle resistance (Rw). In contrast, fluxes over croplands are typically bidirectional and may reflect a larger Xs as a consequence of greater N supply. The paper discusses the processes defining Rw (humidity, acidic pollutants) and Xs (plant phenology, species, N nutrition) and proposes a new resistance approach, which integrates Xs and Rw into one model. Estimating long term bidirectional NH3 fluxes is still uncertain, though it is now possible to apply a single model concept to a range of ecosystem types and satisfactorily infer NH3 fluxes over diurnal time scales

Nonintrusive, Fast and Sensitive Ammonia Detection by Laser Photothermal Deflection

Contains fulltext : 29227pub.pdf (publisher's version ) (Open Access

Measurement of ambient ammonia with diffusion tube samplers

Results of a study to evaluate the suitability of passive samplers for the measurement of monthly averaged ammonia concentrations are presented. Five different samplers were tested. Four were diffusion tube samplers which differed in length and in the way they were modified to minimize disturbing effects of wind turbulence. Also tested was a badge-type passive sampler. Measurements were carried out at five sites and in a small wind tunnel. The concentrations ranged from 1 to 35 μg m-3. Precision of the samplers was derived from triplicate measurements and the accuracy by comparison with results of a reference method. Two types of tubes proved to be suitable for the purpose. One is a 71.2 mm long tube provided with a stainless-steel grid in the inlet and mounted in a cylindrical container against wind influences. The other is a 35.6 mm long tube with a permeable membrane in the inlet, mounted in the shield as well as exposed just in the open air. The shorter tube without shield is preferred because of its higher effective uptake rate and the simple way it can be operated. The uptake rate of the tube is 1.52 x 10-4 m3 h-1; the precision of the method is 5%. Using this tube, the detection limit is 0.3 μg m-3 for a monthly mean ammonia concentration