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 1.52×10 -4 m 3 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.