14 Nov 2022
14 Nov 2022
Status: this preprint is currently under review for the journal AMT.

A novel spectroscopic approach and sampling method for ambient hydrogen chloride detection: HCl-TILDAS

John W. Halfacre1, Jordan Stewart1, Scott C. Herndon2, Joseph R. Roscioli2, Christoph Dyroff2, Tara I. Yacovitch2, Michael Flynn3, Stephen J. Andrews1, Steven S. Brown4,5, Patrick R. Veres4, and Pete M. Edwards1 John W. Halfacre et al.
  • 1Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK
  • 2Aerodyne Research, Inc., Billerica, MA, 01821, USA
  • 3Department of Earth and Environmental Science, Centre for Atmospheric Science, School of Natural Sciences, The University of Manchester, Manchester M13 9PL, UK
  • 4Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, 80305, USA
  • 5Department of Chemistry, University of Colorado, Boulder, CO 80309, USA

Abstract. The largest inorganic, gas phase reservoir of chlorine atoms in the atmosphere is hydrogen chloride (HCl), but the 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 interband 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 21–24 pptv. For longer averaging times, the highest precision obtained was 0.5 pptv and 3𝜎 limit of detection of 1.6 pptv at 2.4 minutes. 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 1.2–4.4 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 (> 4.0 ppbv) were found to increase HCl signal (< 10 %), likely due to acid displacement with HCl or particulate chloride adsorbed to inlet surfaces. The equilibrium model ISORROPIA suggested a potential of particulate chloride partitioning into HCl gas within the heated inlet system if allowed to thermally equilibrate, but field results did not demonstrate a clear relationship between particulate chloride and HCl signal obtained with a denuder installed on the inlet.

John W. Halfacre et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2022-309', Anonymous Referee #1, 09 Dec 2022
    • AC1: 'Reply on RC1', John Halfacre, 24 Jan 2023
  • RC2: 'Comment on amt-2022-309', Anonymous Referee #2, 16 Dec 2022
    • AC2: 'Reply on RC2', John Halfacre, 24 Jan 2023
  • RC3: 'Comment on amt-2022-309', Anonymous Referee #3, 20 Dec 2022
    • AC3: 'Reply on RC3', John Halfacre, 24 Jan 2023

John W. Halfacre et al.

John W. Halfacre et al.


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Short summary
This study details a new sampling method for the optical detection of hydrogen chloride (HCl). HCl is an important atmospheric reservoir for chlorine atoms, which can affect nitrogen oxide cycling and the lifetimes of volatile organic compounds and ozone. However, HCl has a high affinity for interacting with surfaces, thereby preventing fast, quantitative measurements. The sampling technique in this study minimizes these surface interactions and provides a high-quality measurement of HCl.