The mechanism of SO2 + O-2 and O + SO3(D-3h) reactions have been investigated at the MP2/6-31 + G(d) and CCSD(T)/cc-pV(Q + d)Z//MP2 levels on the triplet and singlet PESs. Although, no stable collision complexes have been found for the SO2 + O-2((3)Sigma(-)(g)), O(P-3) + SO3(D-3h) and O(D-1) + SO3(D-3h) reactions, (IN)-I-1(O2S-O-2) has been considered on the singlet PES for the SO2 + O-2((1)Delta(g)) reaction. The results show that there are no favorable paths for the OSO ring formation of SO4(C-2v) in the atmospheric reactions of the SO2 + O-2((3)Sigma(-)(g)), SO2 + O-2((1)Delta(g)) and O(P-3) + SO3(D-3h) while, the O(D-1) + SO3(D-3h) reaction can be suitable for the SO4(C-2v) formation on the singlet PES. (C) 2012 Elsevier B.V. All rights reserved.