An assessment of the applicability of ambient NH3 instrumentation under field conditions

Ammonia (NH3) is an important atmospheric base which can contribute to eutrophication, acidification of ecosystems and the formation of secondary aerosol. In order to monitor potential policy driven reductions and provide information on NH3 emissions, transport and deposition, active sampling is required. There are, however, many challenges in measuring NH3 as it is a ‘sticky’ reactive molecule which is readily soluble and rapidly interacts with other trace gases to form secondary inorganic aerosol. When undertaking ambient NH3 measurements, inlet set up, use of filters, reliable calibration standards and potential chemical interferences in the analytical technique thus need particular attention. Due to these complexities of NH3 active measurements, a metrology-centred project on “Metrology for Ammonia in Ambient Air” (MetNH3) funded by the European Metrological Research Programme (EMRP), was started in 2014, with the aim of developing traceable NH3 monitoring methods and NH3 certified reference material, which will be applicable under field conditions. In this project a commercial cavity ring down instrument is being characterised and further developed to be the traceable method of choice, alongside with the development of an open path absorption spectrometer. The following study presents the first results of a field intercomparison of different NH3 instrumentation verified against traceable methods developed and characterised as part of the MetNH3 project, held in South East Scotland. In addition, the applicability of dynamic calibration systems, under field conditions which have developed within the project is assessed. The overall objective of this study will be to establish recommendations for ambient NH3¬ measurements.