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. 2023 Jul 18;23(14):6498.
doi: 10.3390/s23146498.

Open-Path Laser Absorption Sensor for Mobile Measurements of Atmospheric Ammonia

Soran Shadman  1 , Thomas W Miller  2 , Azer P Yalin  1
Affiliations

Open-Path Laser Absorption Sensor for Mobile Measurements of Atmospheric Ammonia

Soran Shadman et al. Sensors (Basel). .

Abstract

Anthropogenic emissions of ammonia to the atmosphere, particularly those from agricultural sources, can be damaging to the environment and human health and can drive a need for sensor technologies that can be used to detect and quantify the emissions. Mobile sensing approaches that can be deployed on ground-based or aerial vehicles can provide scalable solutions for high throughput measurements but require relatively compact and low-power sensor systems. This contribution presents an ammonia sensor based on wavelength modulation spectroscopy (WMS) integrated with a Herriott multi-pass cell and a quantum cascade laser (QCL) at 10.33 µm oriented to mobile use. An open-path configuration is used to mitigate sticky-gas effects and achieve high time-response. The final sensor package is relatively small (~20 L), lightweight (~3.5 kg), battery-powered (<30 W) and operates autonomously. Details of the WMS setup and analysis method are presented along with laboratory tests showing sensor accuracy (<~2%) and precision (~4 ppb in 1 s). Initial field deployments on both ground vehicles and a fixed-wing unmanned aerial vehicle (UAV) are also presented.

Keywords: ammonia; laser absorption sensor; open-path sensor; quantum cascade laser; wavelength modulation spectroscopy.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
(Left): Open-path WMS based ammonia sensor. Green arrows show the approximate beam path (neglecting the multiple cell passes). (Right): Photograph of front mirror of multi-pass when illuminated with a red beam showing the 28 passes (input/output hole is top left + 27 beam spots).
Figure 2
Figure 2
(Top): Photograph of sensor vehicle roof mount. (Bottom): Pictorial view of components used to power the sensor for vehicle deployments.
Figure 3
Figure 3
(Left): Sensor on flying UAV. (Right): Zoom-in (rear) view of ammonia sensor mounting.
Figure 4
Figure 4
Simulated lock-in signals for 2f and 1f frequency amplitudes. See text for detail.
Figure 4
Figure 4
Simulated lock-in signals for 2f and 1f frequency amplitudes. See text for detail.
Figure 5
Figure 5
Comparison of simulated and experimental spectra for NH3 concentration of 150 ppb.
Figure 6
Figure 6
(Left): Comparison between measured and expected ammonia concentrations with closed-path configuration Red bars show the fix delivered concentrations. (Right): Allan variance analysis shows the sensitivity of ~4 ppb (at 1 s).
Figure 7
Figure 7
(a) Concentration data imported and viewed in Google Earth. (b) Ammonia concentration time series from the same data (~1 h measurement).
Figure 8
Figure 8
(Top) The aerial view of the flight path and the feedlot. (Bottom) The measured ammonia concentration timeseries.
See this image and copyright information in PMC

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