The reaction mechanism of SO3 with the multifunctional compound ethanolamine and its atmospheric implications

Abstract

SO₃ is an important reactive species in sulfur cycle and sulfuric acid formation processes and its reactions with some functional group substances, such as H₂O, NH₃, CH₃OH, and organic and inorganic acids, have been extensively studied. However, its loss mechanism with multifunctional species is still lacking in detail. Herein, the reaction mechanism between SO₃ and monoethanolamide (MEA) was investigated in the gas phase and on water droplets. The quantum chemical calculations indicate that the gas-phase reactions of SO₃ with the OH and NH₂ moieties of MEA hardly occur as their reaction energy barriers are up to 21.9–29.4 kcal mol⁻¹. When a single water molecule is added into the SO₃ + MEA reaction, it not only decreases the reaction barrier by at least 15.0 kcal mol⁻¹ and thus enhances the rate obviously, but can also lead to the main product changing from HOCH₂CH₂NHSO₃H to NH₂CH₂CH₂OSO₃H. The Born Oppenheimer molecular dynamics simulations on a water droplet show that the routes of the NH₂CH₂CH₂OSO₃⁻⋯H₃O⁺ ion pair, HSO₄⁻ and HOCH₂CH₂NH₃⁺ ions and zwitterionic formations of HOCH₂CH₂NH₂⁺–SO₃⁻ and SO₃⁻–OCH₂CH₂NH₃⁺ occur through a loop-structure route or chain reaction process, and can be finished within several picoseconds. Interestingly, the nucleation simulations show that the products of HOCH₂CH₂NHSO₃H and NH₂CH₂CH₂OSO₃H have a potential ability to participate in the formation of new particles as they can form larger clusters with H₂SO₄, NH₃ and H₂O molecules within 20 ns. Thus, the present study will not only give new insight into the reaction between SO₃ and multifunctional compounds, but also provide a new potential formation mechanism for particles resulting from the loss of SO₃.