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

We demonstrate a compact, open-path, quantum cascade-laser-based atmospheric ammonia sensor operating at 9.06 mu m 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 (1 sigma) of 0.15 ppbv NH3 at 10 Hz, a mass of similar to 5 kg and consumes similar to 50W 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.