Using tunable infrared laser direct absorption spectroscopy for ambient hydrogen chloride detection: HCl-TILDAS

• Published in: Atmospheric measurement techniques 2023-03, Vol.16 (5), p.1407-1429
• Main Authors: Halfacre, John W, Stewart, Jordan, Herndon, Scott C, Roscioli, Joseph R, Dyroff, Christoph, Yacovitch, Tara I, Flynn, Michael, Andrews, Stephen J, Brown, Steven S, Veres, Patrick R, Edwards, Pete M
• Format: Article
• Language: English
• Subjects: Absorption; Absorption spectroscopy; Acids; Aircraft; Analytical methods; Cascade lasers; Chloride; Chlorides; Chlorine; Chlorine compounds; Cylinder liners; Cylinders; Detection; Dichloropropane; Flow rates; Flow velocity; Frequency measurement; Gas cylinders; Gases; Hydrochloric acid; Hydrogen; Hydrogen chloride; Infrared lasers; Infrared spectroscopy; Inlet flow; Inlets; Laboratories; Lasers; Measurement techniques; Mixing ratio; Nitric acid; Nitric acids; Relative humidity; Slopes; Spectroscopy; Spectrum analysis; Tunable lasers; VOCs; Volatile organic compounds
• ISSN: 1867-8548; 1867-1381; 1867-8548
• DOI: 10.5194/amt-16-1407-2023

The largest inorganic, gas-phase reservoir of chlorine atoms in the atmosphere is hydrogen chloride (HCl), but challenges in quantitative sampling of this compound cause difficulties for obtaining high-quality, high-frequency measurements. In this work, tunable infrared laser direct absorption spectroscopy (TILDAS) was demonstrated to be a superior optical method for sensitive, in situ detection of HCl at the 2925.89645 cm−1 absorption line using a 3 µm inter-band cascade laser. The instrument has an effective path length of 204 m, 1 Hz precision of 7–8 pptv, and 3σ limit of detection ranging from 21 to 24 pptv. For longer averaging times, the highest precision obtained was 0.5 pptv with a 3σ limit of detection of 1.6 pptv at 2.4 min. HCl-TILDAS was also shown to have high accuracy when compared with a certified gas cylinder, yielding a linear slope within the expected 5 % tolerance of the reported cylinder concentration (slope = 0.964 ± 0.008). The use of heated inlet lines and active chemical passivation greatly improve the instrument response times to changes in HCl mixing ratios, with minimum 90 % response times ranging from 1.2 to 4.4 s depending on inlet flow rate. However, these response times lengthened at relative humidities >50 %, conditions under which HCl concentration standards were found to elicit a significantly lower response (−5.8 %). The addition of high concentrations of gas-phase nitric acid (>3.0 ppbv) were found to increase HCl signal (