Optimization of the coating layer for the measurement of ammonia by diffusion denuders

The performance of citric acid, oxalic acid and phosphorous acid as denuder coating layers for the determination of atmospheric ammonia have been studied by means of laboratory and field tests. The parameters evaluated during the study include: collection efficiency, selectivity of the coating layer, stability of the reaction product, operative capacity and stability of the coating layer. The results of this study show that phosphorous acid is a suitable coating layer for a denuder line intended to determine both gaseous ammonia and particulate ammonium in the atmosphere. It has been found that the citric acid coating suffers from an insufficient strength of the bond between collected ammonia and the coating layer, which causes a release of the collected ammonia both towards the air flow and towards the active sites of the denuder glass. The performance of oxalic acid was very good in the determination of gaseous ammonia, but this coating showed to be unsuitable for denuder sampling lines which are intended also for the determination of atmospheric ammonium. The volatilisation of oxalic from the denuder surface, in fact, causes a displacement of nitrate from the Teflon filter and an excess of nitrate ion on the back-up filter. Phosphorous acid-coated denuders were added to the sampling line employed in the EMEP station of Montelibretti. Reliable and interesting results were obtained, which allowed us to detect the presence of gaseous ammonia adsorbed on atmospheric particles.