Ammonia monitoring in Switzerland with passive samplers: Patterns, determinants and comparison with modelled concentrations

Gaseous ammonia (NH3) is an important form of N deposition to ecosystems, but it is not being routinely monitored in Switzerland. Therefore, a study was conducted to estimate annual means and seasonal patterns of NH3 concentrations for different site types in Switzerland, and to compare annual measured and modelled NH3 concentrations. NH3 concentrations were measured using the `Zurcher' passive sampler, a Palmes type sampler with an acidic solution as absorbent. Twenty-four sampling sites were run for one year, and 17 for two years.The samplers were changed fortnightly or monthly. Spatial emissionpatterns were mapped by combining information on (1) the locationof emission sources, (2) national statistics on NH3-emittingactivities and (3) activity-specific emission factors. The spatial resolution was one hectare. The mean annual NH3 concentration in the ambient air of the 41 sites was 2.5±0.3 μg m-3 (mean±standard error). It ranged from 0.4 to 7.5 μg m-3. The site type and the season were the most importantfactors explaining the variation in the seasonal mean concentration. NH3 concentrations were highest in intensively used agricultural areas and in cities, and lowest in Alpine sites remote from emission sources. At 39 out of 41 sites, the NH3 concentrations were higher in summer (3.1±0.3 μg m-3) than in winter (2.0±0.3 μg m-3). Modelled NH3 concentrations did not systematically deviate from measuredconcentrations (r2 = 0.69). With the combined monitoringand modelling approach, it is now possible to obtain areasonable and consolidated picture of the overallNH3 situation in Switzerland.