Real-time monitoring of atmospheric ammonia based on modifier-enhanced vacuum ultraviolet photoionization ion mobility spectrometry

Abstract

Ammonia (NH₃) plays an important role in the atmospheric environment such as the formation of PM_2.5, the concentration monitoring of which could hence help in the air quality assessment. In this study, a method for quantitative monitoring of atmospheric NH₃ was developed based on modifier-enhanced selectivity detection using a homemade vacuum ultraviolet photoionization ion mobility spectrometry (VUV-PI-IMS). To enhance the resolution and sensitivity of measuring NH₃, 2-butanone as the gas modifier was introduced into the drift tube with the drift gas. Atmospheric NH₃ can be selectively detected, where the peak-to-peak resolution (R_P-P) of 7.69 was obtained. The product ions were identified to be [C₄H₈O]₂NH₄⁺ by using a homemade time-of-flight mass spectrometer. The calculated limit of detection (LOD) was 0.39 ppbv improving about 10 times. For the most common concentration variation of atmospheric NH₃ in the range of 10–100 ppbv, the linear curve was obtained with R² of 0.997. Lastly, the VUV-PI-IMS was used to monitor the evolution of atmospheric NH₃ near our laboratory and mounted on a car for monitoring the regional distribution of atmospheric NH₃ in Dalian, China. The results also showed that VUV-PI-IMS has a promising application prospect in monitoring the concentration of atmospheric NH₃ and supporting the air quality assessment.