Improvement of online monitoring technology based on the Berthelot reaction and long path absorption photometer for the measurement of ambient NH₃: Field applications in low-concentration environments

Abstract. In the last few decades, various techniques, including spectroscopic, wet chemical and mass spectrometric methods, had been developed and applied for the detection of gaseous ammonia (NH₃). We developed an online NH₃ monitoring system (SAC-LOPAP) based on Berthelot reactions and a long path absorption photometer (LOPAP), which could run statically for a long time and be applied to the continuous online measurement of low concentrations of ambient air by optimizing the reaction conditions, adding a constant temperature module and liquid flow controller. The detection limit reached with this instrument was 40.5 ppt under stable conditions. In addition, the range of NH₃ measurement varied from background contamination (<40.5 ppt) to approximately 100 ppb in the current condition (a stripping liquid flow rate of 0.49 ml min^-1 and a gas sample flow rate of 0.70 L min^-1). An inter-comparison of our system with another established system in a field campaign in Beijing was presented, and the results showed that the two instruments had good correlation, indicating that the SAC-LOPAP involved in this study could be used for the accurate measurement of NH₃.