MEASUREMENTS OF AMMONIA AND AMMONIUM BY DENUDER AND FILTER PACK

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 high 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 underestimation was frequently observed at conditions with low NH3 concentrations, westerly winds and a low ratio of particle NH4+ relative to particulate SO42- 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.