Acids at the edge: why nitric and formic acid dissociations at air-water interfaces depend on depth and on interface specific area

International audienceWhether the air-water interface weakens or strengthens the acidity of simple organic and inorganic acids compared to the bulk has a critical importance in a broad range of environmental and biochemical processes. However, consensus has not yet been achieved on this key question. Here we use machine learning-based reactive molecular dynamics simulations to study the dissociation of the paradigmatic nitric and formic acids at the air-water interface. We show that the local acidity profile across the interface is determined by changes in acid and conjugate base solvation and that acidity drops abruptly over a transition region of a few molecular layers. At the interface, both acids are weaker than in the bulk due to desolvation. In contrast, below the interface acidities reach a plateau and are all the stronger compared to the bulk as the aqueous phase surface/volume ratio is large, due to the growing impact of the released proton stabilization at the water surface. These results imply that the measured degree of dissociation sensitively depends on the experimental probing length and system size, and 1 suggest a molecular explanation to the contrasting experimental results. The aerosol size dependence of acidity has important consequences for atmospheric chemistry