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 stud ied 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 th e coating layer. The results of this study show that phosphorous acid is a suitable coating layer for a denuder line intended to determine both gaseou s ammonia and particulate ammonium in the atmosphere. It has been found tha t 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 sit es of the denuder glass. The performance of oxalic acid was very good in th e determination of gaseous ammonia, but this coating showed to be unsuitabl e for denuder sampling lines which are intended also for the determination of atmospheric ammonium. The volatilisation of oxalic from the denuder surf ace, in fact, causes a displacement of nitrate from the Teflon filter and a n excess of nitrate ion on the back-up filter. Phosphorous acid-coated denuders were added to the sampling line employed i n the EMEP station of Montelibretti. Reliable and interesting results were obtained, which allowed us to detect the presence of gaseous ammonia adsorb ed on atmospheric particles. (C) 1999 Elsevier Science Ltd. All rights rese rved.