The Picarro SI2108 gas concentration analyzer can provide real-time monitoring of hydrogen chloride (HCl) at a parts-per-trillion (ppt) sensitivity for a variety of applications, including indoor and outdoor air quality, atmospheric science, and hazard assessment.

The Picarro SI2108 operates using Cavity Ring-Down Spectroscopy (CRDS). CRDS utilizes the unique infrared absorption spectrum of gas-phase molecules to quantify isotope ratios and concentrations. Using a small 3-mirrored cavity with a long effective path-length of up to 20 km, measuring the decay rate rather than absolute absorbance, and using a patented wavelength monitor that provides highly accurate measurement and control of the laser’s wavelength, allows the instrument to measure HCl at <15 ppt level precision, with a maximum drift over 24 hours of ±50 ppt.

Here we present examples of Picarro instruments being used to define sources of HCl indoors. HCl levels inside are generally a few hundred pptv. Using Picarro analyzers, HCl was observed from dishwasher use and electric stovetop use. Reproducible increases in HCl concentrations (~100 pptv) were also observed from bleach applications on a house floor at night, with room lights turned on. The possibility of direct emission or chemical formation of HCl via chlorine atoms is indicated by these observations of HCl derived from household activities.