Inorganic ammonium salts and carbonate salts are efficient catalysts for aldol condensation in atmospheric aerosols

Abstract

In natural environments such as atmospheric aerosols, organic compounds coexist with inorganic salts but, until recently, were not thought to interact chemically. We have recently shown that inorganic ammonium ions, NH₄⁺, act as catalysts for acetal formation from glyoxal, a common atmospheric gas. In this work, we report that inorganic ammonium ions, NH₄⁺, and carbonate ions, CO₃²⁻, are also efficient catalysts for the aldol condensation of carbonyl compounds. In the case of NH₄⁺ this was not previously known, and was patented prior to this article. The kinetic results presented in this work show that, for the concentrations of ammonium and carbonate ions present in tropospheric aerosols, the aldol condensation of acetaldehyde and acetone could be as fast as in concentrated sulfuric acid and might compete with their reactions with OH radicals. These catalytic processes could produce significant amounts of polyconjugated, light-absorbing compounds in aerosols, and thus affect their direct forcing on climate. For organic gases with large Henry's law coefficients, these reactions could also result in a significant uptake and in the formation of secondary organic aerosols (SOA). This work reinforces the recent findings that inorganic salts are not inert towards organic compounds in aerosols and shows, in particular, that common ones, such as ammonium and carbonate salts, might even play important roles in their chemical transformations.