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Highlights • A portable laser sensor for trace NH3 is developed based on FPGA. • WMS-2 f /1 f is employed to reduce the random fluctuations. • The adsorption of NH3 is studied and minimized in measurements. • The sensor is deployed for in-situ measurement of NH3 slip in SCR. Abstract A portable laser sensor for trace ammonia (NH 3) is developed based on near-infrared laser absorption spectroscopy. Tunable diode laser absorption spectroscopy in conjunction with wavelength modulation spectroscopy is implemented by a system-on-chip based on field-programmable gate array. A variable temperature multipass cell with a 15 m optical path length is adopted to enhance the absorption path length and avoid the NH 3 adsorption. The physical dimension of the sensor is minimized into 43 × 18 × 16 cm3 on the benefit of home-made electronic system and compact optical design. A unique absorption line of NH 3 around 1.51 µm covered by a distributed feedback diode laser is selected to reduce the influence from carbon dioxide, vapor and other trace gases. Calibration-free modulation spectroscopy is employed to avoid the random fluctuations due to laser noise and non-absorption transmission losses. The performance of sensor including adsorption of NH 3 , stability and flow of gas are examined and discussed in detail. For removing the adsorption of NH 3 , the temperature-dependent curve is obtained by the direct absorption spectroscopy, and it shows that the adsorption is significantly suppressed at (or higher than) 415 K. According to Allan deviation analysis, the minimum detection limit is 0.16 ppm at 184 s, and the rise time and the fall time of system are about 4.5 s and 3.7 s, respectively. The feasibility of the sensor is validated and deployed to monitor the NH 3 slip in selective catalytic reduction (SCR) of coke oven flue gas. The work provides a potentially valuable control sensor for SCR. [ABSTRACT FROM AUTHOR]