Effect of relative humidity on HCl formation from the reaction of H2SO4 and HNO3 with NaCl particles

Short path gas cell Fourier transform infrared spectroscopy was used to analyze the production of hydrochloric acid (HCl) gas from the reaction of sodium chloride (NaCl) particles and sulfuric acid (H2SO4) or nitric acid (HNO3) at a range of relative humidity (2–95 %). HCl was produced from both acids, and reached equilibrium in 10 min for both H2SO4 and HNO3 acids. The equilibrium is thought to be a result of two competing reactions which produce HCl and sequester it back into the NaCl particles. The HCl production from H2SO4 was instantaneous, but its production from HNO3 involved an induction period of a few minutes. The equilibrium HCl conditions observed here have implications in influencing the pH of aerosol particles, and consequently the acidity of precipitation in the atmosphere.