Measurements of ammonia and ammonium by denuder and filter pack

Abstract

Simultaneous 24-hour measurements of gaseous ammonia (NH 3) and particulate ammonium (NH 4 +) by denuder and filter pack were carried out at five different locations in Denmark. The locations represent areas surrounded by different emission densities of NH 3. In general a good agreement was found for the determination of the sum of NH 3 plus NH 4 + by the two methods at all locations. The NH 4 + determination by the two methods was in general found to be in good agreement. Even though sampling artifacts are seen for the NH 3 determination by the filter pack, the NH 3 concentrations are in general too low to affect the correlation of particulate NH 4 +. At a location surrounded by areas with NH 3 emission densities the separate determinations of NH 3 and NH 4 + by the filter pack were in general in good agreement with the denuder determination of, respectively, NH 3 and NH 4 +. At the other locations, the NH 3 determination by filter pack frequently showed underestimation compared to the denuder. The under-estimation was frequently observed at conditions with low NH 3 concentrations, westerly winds and a low ratio of particle NH 4 + relative to particulate SO 4 2− and NO 3 −. It probably relates to conditions with unsaturated acidic aerosols absorbing the NH 3. The observations show that the ability of the filter pack method for NH 3 determination is dependent on the climate, pollution climate and surrounding NH 3 emission density. The use of the filter pack method and its restrictions therefore demand an individual investigation of the sampling site characteristics.