Are Ammonia Sensors Ready for Outdoor Use?

Ammonia (NH3) gas is primarily an agriculture atmospheric pollutant, and measuring near-emission sources is essential for understanding NH3 emission plumes. Sensors in theory are attractive alternative monitoring methods due to their high-time data resolution, size, and lower costs; however, there are metrological and technical challenges. NH3 sensors operating in the subppm range (typical of near-emission source concentrations) are relatively new to the market, and while promising, there are metrological and technical challenges, especially in outdoor environments. Six NH3 sensors were evaluated under field conditions at a poultry house emission simulation site, Whim Bog, Scotland (3.4 g of NH3 min-1). Five electrochemical (TB600B, PS1, ECtox, duplicated AM1 low concentration (LC), and AM2 high concentration (HC)) and one chemiresistive-polymer (MELBA) sensors were tested and compared to a cavity ring-down instrument (Picarro G2103 NH3 analyzer) as a reference method. Only the TB600B (R 2 = 0.59-0.84), AM1 LC (R 2 = 0.7-0.9), and AM2 HC (R 2 = 0.71-0.9) demonstrated a positive correlation to the reference analyzer, being potentially capable of delivering indicative NH3 concentrations, with the caveat that the AM sensors had very low data capture and their performance may improve once the sensors operate for longer. All sensors tested have major technical challenges including accuracy, precision, response time, manufacturer deployment guidelines, sensor lifecycle metrics, software engineering, and data traceability. This study highlights the need for improvement in the NH3 sensor industry and among suppliers, and concludes that outdoor ammonia sensor measurements are not yet ready for routine use.