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Ambient ammonia concentrations, mainly originating from agricultural activities, have increased in the last few decades in Europe. As a consequence, critical loads on oligotrophic ecosystems such as forests and mires are greatly exceeded. Monitoring of ambient ammonia concentrations is necessary in order to investigate source-receptor relationships. Measuring ambient ammonia concentrations continuously with high time resolution is very expensive and cost-efficient systems are required. Where time resolution is of minor importance, several cost-effective systems, mainly dry denuder and passive samplers, can be applied. In this paper the Zürcher passive sampler, a diffusive sampling system, is presented. It is a Palmes type sampler with an acidic solution as absorbent and is easy to handle. It was tested at 46 sites in Switzerland over one year. The average concentration in ambient air was 2.5 microg m(-3) +/- 0.4 microg m(-3). The average of the blank values were 0.21 microg m(-3). The detection limit (double the standard deviation of the blank values) was 0.36 microg m(-3). Three passive samplers were exposed at each site and each period. The mean standard deviation of these triplicate measurements was 9.5%. Compared with a discontinuous tubular denuder system and a continuous annular denuder system, the deviation was less than 10%. The Zürcher passive sampler is a useful and cost-efficient tool to determine long-term average ammonia concentrations (one- to four-week periods) in ambient air for mean concentrations above 1 microg m(-3).