Open-Path High Sensitivity Atmospheric Ammonia Sensing with a Quantum Cascade Laser Instrument
Abstract
Atmospheric trace-gas sensing with quantum cascade laser (QCL) spectroscopy offers the potential for high sensitivity, fast, selective mid-infrared absorption measurements of atmospheric species such as ammonia (NH3). As the third most abundant nitrogen species and most gaseous base in the atmosphere, ammonia plays important roles in neutralizing acidic species and as a gas-phase precursor to ammoniated fine particulate matter. High precision gas phase measurements are necessary to constrain highly uncertain emission sources and sinks with implications for understanding how chemical components of fine particulate matter affect air quality and climate as well as nitrogen deposition to ecosystems. Conventional ammonia sensors employing chemical ionization, denuder or filter techniques are labor-intensive, not gas-selective and exhibit low time resolution. As an advantageous alternative to conventional measurement techniques, we develop an open-path quantum cascade laser-based ammonia sensor operating at 9.06 μm for ground-based measurements. A continuous wave, thermoelectrically cooled quantum cascade laser is used to perform wavelength modulation absorption spectroscopy (WMS). Room-temperature, unattended operation with minimal surface adsorption effects due to the open-path configuration represent significant improvements over cryogenically cooled, closed path systems. The feasibility of a cylindrical mirror multi-pass optical cell for achieving long path lengths near 50 m in a compact design is also assessed. Meaningful ammonia measurements require fast sub-ppbv detection limits due to ammonia’s large dynamic range and temporal and spatial atmospheric variability. When fully developed, our instrument will achieve high time resolution (up to 10 Hz) measurements with ammonia detection limits in the 100 pptv range. Initial results include ambient laboratory ammonia detection at 58 ppbv relative to a 0.4% ammonia reference cell based on the WMS signal integrated area. We estimate a limit of detection based on our signal to noise ratio of ~400 pptv NH3. Non-cryogenic, unattended operation of this compact sensor offers the potential for applications in particulate matter gas-phase precursor monitoring networks. Future sensor measurements can also be utilized for evaluations of and data assimilation into air quality and aerosol forecast models of particular importance for regions where ammonia plays a critical role in fine particulate matter formation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.A33B0248M
- Keywords:
-
- 0305 ATMOSPHERIC COMPOSITION AND STRUCTURE / Aerosols and particles;
- 0322 ATMOSPHERIC COMPOSITION AND STRUCTURE / Constituent sources and sinks;
- 0345 ATMOSPHERIC COMPOSITION AND STRUCTURE / Pollution: urban and regional;
- 0394 ATMOSPHERIC COMPOSITION AND STRUCTURE / Instruments and techniques