Abstract
Abstract. We describe a newly developed single-photon laser-induced fluorescence sensor for measurements of nitric oxide (NO) in the atmosphere. Rapid tuning of a narrow-band laser on and off of a rotationally resolved NO spectral feature near 215 nm and detection of the red-shifted fluorescence provides for interference-free direct measurements of NO with a detection limit of 1 part per trillion by volume (pptv) for 1 s of integration, or 0.3 pptv for 10 s of integration. Uncertainty in the sensitivity of the instrument is typically ±6–9 %, with no known interferences. Uncertainty in the zero of the detector is shown to be <0.2 pptv. The instrument was deployed on the NASA DC-8 aircraft during the NASA/NOAA FIREX-AQ experiment (Fire Influence on Regional to Global Environments Experiment – Air Quality) during July–September 2019 and provided more than 140 h of NO measurements over 22 flights, demonstrating the ability of this instrument to operate routinely and autonomously. Comparisons with a seasoned chemiluminescence sensor during FIREX-AQ in a variety of chemical environments provides validation and confidence in the accuracy of this technique.
Highlights
Nitric oxide (NO) is central to radical chemistry in Earth’s atmosphere
Since the pulse-pair resolution is greater than the signal lifetime, the system will at most count one fluorescent photon per laser shot and at very high signal levels the observed count rate will start to deviate from a linear response to the rate at which photons strike the photocathode
A new instrument has been described for performing direct measurements of atmospheric NO using single-photon laserinduced fluorescence
Summary
Nitric oxide (NO) is central to radical chemistry in Earth’s atmosphere. In the troposphere the catalytic reaction of NO with the hydroperoxy and organic peroxy radicals. Almost all of the research in the past 2 decades associated with direct detection of atmospheric NO has relied on the chemiluminescence (CL) detection technique (Ridley and Howlett, 1974) In this method, a sample of air is mixed with a high concentration (∼ 1 %) of O3, resulting in the formation of electronically excited nitrogen dioxide, which produces intense luminescence in the near infrared. The single-photon excitation scheme employed by Bradshaw et al used a dye-based laser system near 226 nm to excite the v = 0 manifold of the A2 electronic state and observe the red-shifted fluorescence emission from relaxation near 259 nm (v → v = 0 → 3) This system had a reported detection limit of 28 pptv for 1 min of integration. We describe the instrument and its performance during this initial deployment
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