Open-path, quantum cascade-laser-based sensor for high-resolution atmospheric ammonia measurements

Abstract

We demonstrate a compact, open-path, quantum cascade-laser-based atmospheric ammonia sensor operating at 9.06 μm for high-sensitivity, high temporal resolution, ground-based measurements. Atmospheric ammonia (NH₃ ) is a gas-phase precursor to fine particulate matter, with implications for air quality and climate change. Currently, NH₃ sensing challenges have led to a lack of widespread in situ measurements. Our open-path sensor configuration minimizes sampling artifacts associated with NH₃ surface adsorption onto inlet tubing and reduced pressure sampling cells, as well as condensed-phase partitioning ambiguities. Multi-harmonic wavelength modulation spectroscopy allows for selective and sensitive detection of atmospheric pressure-broadened absorption features. An in-line ethylene reference cell provides real-time calibration (±20% accuracy) and normalization for instrument drift under rapidly changing field conditions. The sensor has a sensitivity and noise-equivalent limit (1σ) of 0.15 ppbv NH₃ at 10 Hz, a mass of ~ 5 kg and consumes ~ 50 W of electrical power. The total uncertainty in NH₃ measurements is 0.20 ppbv NH₃ ± 10%, based on a spectroscopic calibration method. Field performance of this open-path NH₃ sensor is demonstrated, with 10 Hz time resolution and a large dynamic response for in situ NH₃ measurements. This sensor provides the capabilities for improved in situ gas-phase NH₃ sensing relevant for emission source characterization and flux measurements.

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