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.
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