Enhanced uptake of methacrolein at the acidic nanoparticle interface: Adsorption, heterogeneous reaction and impact for the secondary organic aerosol formation

Abstract

Considering the moderate acidity of aerosols, the formation of secondary organic aerosols (SOA) through acid-catalyzed heterogeneous reactions has become a recent concern. However, the detailed information on the multiphase chemistry of organic compounds adsorbed onto acidic aerosols remains uncertain. In this work, we investigated the multiphase chemical processes between methacrolein (MACR) and sulfuric acid (SA) and their relationship with SOA formation. Results show that the aqueous nanoparticle interface, especially when it is an acidic nanoparticle interface, is a perfect area to adsorb and accommodate MACR. The occurrence percentage of MACR on the interface is more than 70%. With the increase of SA concentration, the first solvation shell changed from only water to the mixture of SA and water, which facilitates the heterogeneous hydration reaction of MACR. Compared with the neutral nanoparticle interface, the acidic nanoparticle interface exhibits a better ability to uptake and accommodate gaseous carbonyl species. Moreover, SA can catalyze the hydration reaction of MACR inside the aqueous media, and the resulting oligomers contribute to the formation and growth of SOA. The hydration reaction indirectly promotes the continuous adsorption of MACR at the acidic nanoparticle interface. The rate constant shows a positive altitude dependence, and acid-catalyzed reactions have an important impact on environmental chemistry, such as cloud SOA formation, within the range of about 2–6 km. This study reports a complete description of the heterogeneous interactions between unsaturated carbonyl species and acidic nanoparticles by using molecular dynamics and quantum chemistry methods, aiming to provide some insights for the further study on heterogeneous chemistry and its role in the formation of tropospheric SOA.