Simultaneous 24-hour measurements of gaseous ammonia (NH₃) and particulate ammonium (NH₄⁺) by denuder and filter pack were carried out at five different locations in Denmark. The locations represent areas surrounded by different emission densities of NH₃. In general a good agreement was found for the determination of the sum of NH₃ plus NH₄⁺ by the two methods at all locations. The NH₄⁺ determination by the two methods was in general found to be in good agreement. Even though sampling artifacts are seen for the NH₃ determination by the filter pack, the NH₃ concentrations are in general too low to affect the correlation of particulate NH₄⁺. At a location surrounded by areas with NH₃ emission densities the separate determinations of NH₃ and NH₄⁺ by the filter pack were in general in good agreement with the denuder determination of, respectively, NH₃ and NH₄⁺. At the other locations, the NH₃ determination by filter pack frequently showed underestimation compared to the denuder. The under-estimation was frequently observed at conditions with low NH₃ concentrations, westerly winds and a low ratio of particle NH₄⁺ relative to particulate SO₄^2- and NO₃^-. It probably relates to conditions with unsaturated acidic aerosols absorbing the NH₃. The observations show that the ability of the filter pack method for NH₃ determination is dependent on the climate, pollution climate and surrounding NH₃ emission density. The use of the filter pack method and its restrictions therefore demand an individual investigation of the sampling site characteristics.