Haze in China has been increasing in frequency of occurrence as well as the area of the affected region. Here, we report on a new mechanism of haze formation, in which coexistence with NOx can reduce the environmental capacity for SO₂, leading to rapid conversion of SO₂ to sulfate because NO₂ and SO₂ have a synergistic effect when they react on the surface of mineral dust. Monitoring data from five severe haze episodes in January of 2013 in the Beijing-Tianjin-Hebei regions agreed very well with the laboratory simulation. The combined air pollution of motor vehicle exhaust and coal-fired flue gases greatly reduced the atmospheric environmental capacity for SO₂, and the formation of sulfate was found to be a main reason for the growth of fine particles, which led to the occurrence of haze. These results indicate that the impact of motor vehicle exhaust on the atmospheric environment might be underestimated.

In this work, graphitic carbon nitride-titanium dioxide (g-C3N4-TiO2) was successfully prepared by a facile calcination route utilizing commercial P25 and melamine as the precursors. The as-prepared g-C3N4/TiO2 photocatalysts were characterized systematically to elucidate their morphological structure and physico-chemical properties. The photocatalytic performance of g-C3N4-TiO2 composites was investigated for the removal of NOx in air. At the optimal g-C3N4 content (∼15wt%, labeled as M400), the conversion of NOx was 27%, which is higher than that of pure P25 (17%) and g-C3N4 (7%) under visible light. The activity of M400 was also enhanced under UV light. However, a mechanically mixed g-C3N4 and TiO2 sample (with the content of g-C3N4 the same as M400, labeled as M0+g-C3N4) did not improve the conversion of NOx. Therefore, the interaction of g-C3N4 and P25 is important for the activity. EPR results indicated that O2− is the main active species for NO oxidation to NO3− under visible and UV light, which is responsible for the difference in activity between M400 and M0+g-C3N4. The present study can improve our understanding of NO removal on the photocatalyst surface and the mechanism for the activity enhancement by the formation of g-C3N4-TiO2