Effect of H₂O and NH₃ on the synergistic oxidation reaction of SO₂ and NO₂ is investigated by theoretical calculation using the molecule system SO₂-2NO₂-nH₂O (n = 0, 1, 2, 3) and SO₂-2NO₂-nH₂O-mNH₃ (n = 0, 1, 2; m = 1, 2). Calculated results show that SO₂ is oxidized to SO₃ by N₂O₄ intermediate. The additional H₂O in the systems can reduce the energy barrier of oxidation step. The increasing number of H₂O molecules in the systems enhances the effect and promotes the production of HONO. When the proportion of H₂O to NH₃ is 1:1, with NH₃ included in the system, the energy barrier is lower than two pure H₂O molecules in the oxidation step. The present study indicates that the H₂O and NH₃ have thermodynamic effects on promoting the oxidation reaction of SO₂ and NO₂, and NH₃ has a more significant role in stabilizing product complexes. In these hydrolysis reactions, nethermost barrier energy (0.29 kcal/mol) can be found in the system SO₂-2NO₂-H₂O. It is obvious that the production of HONO is energetically favorable. A new reaction mechanism about SO₂ oxidation in the atmosphere is proposed, which can provide guidance for the further study of aerosol surface reactions.

Sulfamethoxazole (SMX) is a typical antibiotic which may bring potential risk to human and aquatic ecosystems. In this paper, the OH-initiated degradation mechanisms of sulfamethoxazole are studied at the MPWB1K/6-311+G(3df,2p)// MPWB1K/6-31+G(d,p) level. The thermodynamic parameters and reaction pathways are analyzed in detail. The addition, abstraction and bond-broken reactions are the three main reaction mechanisms, and bond-broken reactions may lead to the direct degradation of SMX. The rate constants are calculated using the transition state theory and canonical variational transition with small-curvature tunneling correction at the temperature range of 200-500 K. The Arrhenius equations of rate constants are fitted. The rate constants of addition and abstraction reactions at 298.15 K are 3.03×10^-23 cm³molecule^-1s^-1 and 6.86×10^-24 cm³molecule^-1s^-1, respectively. The rate constants of bond-broken reactions are much lower than those of the addition and abstraction reactions, but their significance can not be ignored.