Field measurements showed that there exists a correlation between nitrate and sulfate on mineral dust. In this work, the synergistic mechanism of adsorption and reaction between SO2 and NO2 on gamma-alumina was studied using in situ diffusion reflectance infrared Fourier spectroscopy (in situ DRIFTS) and temperature programmed desorption (TPD). The results revealed that the reaction pathway of NO2 adsorbed on alumina was altered in the presence of SO2. In the absence of SO2, nitrite was found to be an intermediate in the oxidation of NO2 to surface nitrate species. However, in the presence of SO2, the formation of nitrite was inhibited and a new intermediate, dinitrogen tetraoxide (N2O4), was observed. On the other hand, surface tetravalent sulfur species S(IV), including bisulfite and sulfite, were oxidized to sulfate in air condition when NO2 was present. The atmospheric implication of this synergistic effect was also discussed.