A possible atmospheric source of HNO3: the ammonolysis reaction of t-N2O4 in the presence of water monomer, water dimer, and sulfuric acid

Abstract

Although the ammonolysis of t-N₂O₄ is one of the potential sources of HNO₃ formation, the available studies have only focused on its naked reaction. Herein, the effect of important neutral and acidic trace gases, water monomer, water dimer, and sulfuric acid, on the formation of HNO₃ from the ammonolysis of t-N₂O₄ was studied by the quantum chemical method of CCSD(T)/aug-cc-pVTZ//B3LYP-D3/6-311++G(3df,2pd) and the Master equation/Rice–Ramsperger–Kassel–Marcus (ME/RRKM) rate calculations. The quantum chemical results revealed that the ammonolysis of t-N₂O₄ with (H₂O)₂ and H₂SO₄ are barrierless or nearly barrierless reactions, potentially lowering the energy barrier to 3.4–4.1 kcal mol⁻¹. The calculated effective rate constant illustrates that (H₂O)₂ (100% RH) dominates over H₂O and H₂SO₄ within the range of 280–320 K (0 km), with an effective rate constant that is 1–3 orders of larger magnitude, whereas H₂SO₄ (10⁸ mol cm⁻³) is the most favorable catalyst within the troposphere between 5 and 30 km. However, the contributions of H₂O, (H₂O)₂, and H₂SO₄ are not apparent in the gas-phase ammonolysis of t-N₂O₄ within the range of 213–320 K and 0–30 km because their effective rate constants were at least 4 orders of magnitude lower than the corresponding rate constant of the ammonolysis of t-N₂O₄. In general, the current findings shed fresh light on neutral (H₂O and (H₂O)₂) and acidic (H₂SO₄) catalysts that not only affect energy barriers but also have an impact on the ammonolysis of t-N₂O₄ in neutral and acidic conditions.