## 1 Introduction  
Ammonia (NH\({}_{3}\)) is one of the major pollutants emitted from livestock production. It was estimated that agriculture contributed 93.6% to the total NH\({}_{3}\) emissions in European countries [1]. The gas is not only harmful to human health, but also closely related to acidification and eutrophication of soil and water. Because of the substantial environmental impact, member states of the European Union are required to limit the emission level according to the National Emissions Ceilings Directive (2016/2284/EU). Moreover, excessive NH\({}_{3}\) emissions are prohibited in nature reserves under the Habitats Directive (92/43/EEC). In Belgium, starting up or continuing high-emission livestock production near Natura 2000 sites is no longer permitted. On the other hand, various systems have been proposed to reduce NH\({}_{3}\) emissions from livestock farms [2]. Several relevant aspects of these measures were investigated in early studies, such as the theoretical effectiveness on emission reductions [3; 4; 5],energy use efficiency [6; 7], and economic impact to the farms [8]. However, information on the NH\({}_{3}\) reduction efficiency of these systems in practice is still limited due to insufficient or unreliable monitoring. Field assessment methods are developed in order to evaluate the environmental performance of livestock farms and operational stability of the abatement techniques. One of the guidelines is the Verification of Environmental Technologies for Agricultural Production (VERA) test protocol for livestock housing and management systems [9]. In this protocol, a wet-chemistry method in accordance with ISO/DIS 21877 [10], NEN 2826 [11], and VDI 3496 Blatt 1 [12] is regarded as the reference method for quantifying airborne NH\({}_{3}\). Although wet-chemistry is relatively low-cost, the requirement for laboratory analysis after sample collections is practically inconvenient. This type of measurement cannot provide instantaneous concentrations either and it is thereby unsuitable for monitoring emission processes at high time resolutions. In addition, the method is sensitive to human errors, because the sampling and laboratory analysis involve complex manual procedures. As an alternative, the VERA test protocol allows for the use of automated measuring systems such as gas analysers, provided that sufficient equivalence with the reference methods is demonstrated according to the EN 14793 standard.  
In the past decades, an increasing number of gas analysers applying advanced sensing technologies have become commercially available. These systems are designed to perform continuous concentration measurement of one or more gases of interest including NH\({}_{3}\). Still, measuring NH\({}_{3}\) in livestock barns is practically challenging due to the \"sticky\" nature of the gas and the high load of dust particles in livestock houses. Moreover, as the accuracies of these analysers are often matrix dependent, laboratory evaluation using certified gas cylinders do not always reflect their field performance. In this study, five commercially available gas analysers are compared and validated in a poultry barn and a pig barn following the procedure of EN 14793 [13], as prescribed by the VERA test protocol.