NOx Measurement Instrumentation
Two-Channel chemiluminescence instrument
The two channel instrument is based on the chemiluminescence detection of NO via reaction with O3 to form excited NO2, which is detected via photon counting. One sample channel is used to measure nitric oxide, NO, and the second measures nitrogen dioxide, NO2, by flowing ambient air through a glass cell illuminated by light-emitting diodes (LEDs) at 395 nm, for the conversion of NO2 to NO via photolysis. The instrument is similar to instruments previously built at NCAR (Ridley and Grahek, 1990; Ridley et al., 2004). The two-channel detector box is at the core of the instrument and includes the reaction vessels, zero volumes, and photomultiplier tubes for simultaneous detection of two sample flows, together with a number of flow controllers (for calibration flows, O3 reagent flows, zero-air flows) and pressure transducers. A second box houses the computer for data acquisition and instrument control, together with power supplies, and a third box houses two ozonizers which produce O3 for reaction with NO to enable detection via production of excited NO2. Additionally, there is a vacuum pump, three gas bottles (zero air, O2 for O3 generation, and NO-in-N2 calibration gas), and inlet components (photolytic converter for NO2, identical dummy cell for NO, two sample flow controllers, valves for calibration gas addition). Dry ice is required for PMT cooling. The precision of 1-s values is estimated to be in the range of 5-10 pptv, dependent on performance characteristics to be determined in flight. Overall uncertainty of 1-sec values is estimated to be 10% or better for large mixing ratios (> 50 pptv).
Laser Induced Fluorescence (LIF) NOx instrument
The LIF instrument we are currently developing is based on the design described in Rollins et al. (2020). It uses single-photon laser-induced fluorescence in the ultraviolet at around 215 nm. The technique is based on early research by Bradshaw and colleagues (1982, 1985), and Bloss et al, (2003). The new technique utilizes a pulsed, fiber coupled distributed feedback (DFB) laser tunable between 1074 and 1076 nm with narrow linewidth. The laser pulses are then amplified and passed through a series of three nonlinear crystals to produce the fifth harmonic near 215 nm wavelength, which is used to pump the NO AΠ (v′′ = 0) ← X2Π (v′′ = 0) vibronic transition. We expect to achieve performances close to the ones described by Rollins et al. (2020), i.e., a limit of detection of 1 pptv for a 1 second measurement. This is almost an order of magnitude better than the chemiluminescence (CL) technique and it's due to a much lower photon count background because of the shorter wavelength detection range compared to the near-IR of the CL technique.
Bloss W. J., et al., “Application of a Compact All Solid-State Laser System to the in Situ Detection of Atmospheric OH, HO2, NO and IO by Laser-Induced Fluorescence,” Journal of Environmental Monitoring 5, no. 1 (January 27, 2003): 21–28, https://doi.org/10.1039/B208714F;
Bradshaw, J. D., M. O. Rodgers, and D. D. Davis, “Single Photon Laser-Induced Fluorescence Detection of NO and SO2 for Atmospheric Conditions of Composition and Pressure,” Applied Optics 21, no. 14 (July 15, 1982): 2493–2500, https://doi.org/10.1364/AO.21.002493;
Bradshaw J. D., et al., “A Two-Photon Laser-Induced Fluorescence Field Instrument for Ground-Based and Airborne Measurements of Atmospheric NO,” Journal of Geophysical Research: Atmospheres 90, no. D7 (1985): 12861–73, https://doi.org/10.1029/JD090iD07p12861;
Ridley, B. A. and F. E. Grahek, “A Small, Low Flow, High Sensitivity Reaction Vessel for NO Chemiluminescence Detectors,” Journal of Atmospheric and Oceanic Technology 7, no. 2 (April 1, 1990): 307–11, https://doi.org/10.1175/1520-0426(1990)007<0307:ASLFHS>2.0.CO;2;
Ridley B., et al., “Florida Thunderstorms: A Faucet of Reactive Nitrogen to the Upper Troposphere,” Journal of Geophysical Research: Atmospheres 109, no. D17 (2004), https://doi.org/10.1029/2004JD004769;
Rollins A. W., et al., “Single-Photon Laser-Induced Fluorescence Detection of Nitric Oxide at Sub-Parts-per-Trillion Mixing Ratios,” Atmospheric Measurement Techniques 13, no. 5 (May 15, 2020): 2425–39, https://doi.org/10.5194/amt-13-2425-2020.
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