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

The largest inorganic, gas-phase reservoir of chlo-rine atoms in the atmosphere is hydrogen chloride (HCl), but challenges in quantitative sampling of this compound cause difficulties for obtaining high-quality, high-frequency mea-surements. In this work, tunable infrared laser direct absorp-tion spectroscopy (TILDAS) was demonstrated to be a su-perior optical method for sensitive, in situ detection of HCl at the 2925.89645 cm(-1) absorption line using a 3 mu m inter -band cascade laser. The instrument has an effective path length of 204 m, 1 Hz precision of 7-8 pptv, and 3 sigma limit of detection ranging from 21 to 24 pptv. For longer averaging times, the highest precision obtained was 0.5 pptv with a 3 sigma 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 ex-pected 5 % tolerance of the reported cylinder concentration (slope = 0.964 +/- 0.008). The use of heated inlet lines and ac-tive chemical passivation greatly improve the instrument re-sponse 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 concentra-tions of gas-phase nitric acid (> 3.0 ppbv) were found to in-crease 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.

Automated summary

In this study, tunable infrared laser direct absorption spectroscopy (TILDAS) was used for sensitive, in situ detection of HCl. The method showed a precision of 7-8 pptv with a detection limit of 21-24 pptv. The accuracy and response time of the method were also demonstrated.