Aerodyne Tunable IR Laser Direct Absorption Spectrometer (TILDAS) model HCl-TILDAS is an infrared laser spectrometer producing measurements of hydrogen chloride mole fractions at 1 Hz. Sample air is pumped through a cell into which the single laser beam is directed. The laser beam passes many times across the cell before exiting onto a photodetector. The laser frequency is tuned across an methane absorption line, and used to find the hydrogen chloride absorption line.