On OH uptake by sea salt under humid conditions

We have investigated OH uptake by a MgCl2‐CaCl2 mixture at 0‐29% RH, using a differential bead‐filled flow tube coupled to a high‐pressure chemical ionization mass spectrometer. The results show that the effect of RH on OH uptake by the mixture reproduces quantitatively that of sea salt. This observation is supported by results of our thermodynamic modeling which predicts that the deliquescence relative humidity of MgCl2·6H2O(cr) decreases in the presence of CaCl2·6H2O(cr), leading to higher surface acidification, and therefore, to greater enhancement in OH uptake under humid conditions. This conclusion is further strengthened by our earlier experimental observations which indicate that the enhancement of the OH uptake coefficient under humid conditions for sea salt is a factor of two larger than that for binary MgCl2‐H2O solutions in equilibrium with MgCl2·6H2O(cr). Based on these results we conclude that surface composition determines, to a large extent, the heterogeneous reactivity of inorganic salt mixtures.