Fiber coupled, QCL-based open-path sensor for atmospheric NH3 measurements

Abstract:

Ammonia (NH₃), as the dominant base in the atmosphere, plays important roles in aerosol nucleation, particle pH, and PM2.5 composition. The dominant sources include agricultural activities (80%) and biomass burning (10%), and their localized sources coupled to ammonia's short lifetime result in strong spatiotemporal gradients. Accurate determination of NH₃ emission sources are thought to a effective way to track nitrogen cycle transformations and NH₃ chemical processing. Unfortunately, NH₃ can readily adsorb onto any surface in contact with the sample air flow, such as filters, inner walls of inlet tubing, and analyzer parts. The ultimate accuracy of sampling based instruments is therefore particularly limited by this unknown and changing loss mechanism, often the interference from the high instrument backgrounds. In order to overcome these disadvantages, particularly for airborne-based applications, we have modified our existing open-path NH₃ sensor at 9.06 microns to include a 9.06 µm hollow core fiber coupled to a custom quantum cascade laser (HCF-QCL) housing. The hollow core fiber allows the laser to be placed elsewhere on an aircraft (e.g. inside the fuselage) or elsewhere on an sUAS and thereby minimizes the laser thermal control issues in the free airstream and instrument bulkiness. The custom laser housing allows us to use the small amount (~ mW) of light emanating from the back facet of the laser to focus into a short reference cell and thereby serve as a continuous, calibration-maintaining check with a reference signal for the sensor. The schematic of fiber coupled, QCL-based open-path NH₃ sensor was shown in the figure (up) as below (WMS: wavelength modulation absorption spectroscopy; M: mirror; L: lens; PM: parabolic mirror; 2f: second harmonic; Ref.: reference; UAS: unmanned aerial systems). The open-path HCF-QCL based ammonia (NH₃) instrument is currently built with a 60 m optical path with a Herriott cell for precision of 120 pptv at 10 Hz, 43 pptv at 1 Hz and 20 pptv for one minute integrated measurements. Its performance evaluation results on the ground was also given in the following figure (bottom). This instrument detects with high accuracy (±20%) based upon a combination of direct absorption spectroscopic experiments and permeation tube calibrations in the laboratory.