Experimental study on HCl uptake by MgCl₂ and sea salt under humid conditions

We studied HCl uptake by MgCl₂ and sea salt over a relative humidity (RH) range from 0% to 34% at 278–313 K using a differential bead-filled flow tube coupled to a high-pressure chemical ionization mass spectrometer. The results showed that dry MgCl₂ and sea salt are essentially inert to gaseous HCl with a probability of less than 10⁻⁶. However, under humid conditions, HCl was found to be efficiently taken up by wet inorganic surfaces. The HCl uptake coefficient for MgCl₂ and sea salt increased squarely with RH, reaching a value of 0.00123 and 0.00171, respectively, at 29% RH and 298 K. Such wetting behavior is even enhanced at elevated temperatures, with the coefficient reaching 0.00208 and 0.00239, respectively, at 313 K. Based on the study of the dependence of γ_HCl on the initial HCl concentration, we estimate γ_HCl as 0.012 at 24% RH at a typical HCl concentration in the troposphere. In addition, the observation of the remarkable enhancement in the OH uptake by the HCl-treated salts agrees with the results of our previous investigation, which suggested that water absorption on salts enhances γ_OH by lowering the surface pH. The proposed mechanism of HCl uptake by sea salt aerosol has implications for ozone production in the marine boundary layer.