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

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.