Mineral dust and NOx promote the conversion of SO2 to sulfate in heavy pollution days

TLDR

Haze in China has become more frequent and widespread. The study reports a new haze formation mechanism in which NOx reduces the atmospheric capacity for SO₂, accelerating its conversion to sulfate on mineral dust surfaces. This mechanism involves synergistic reactions of NO₂ and SO₂ on mineral dust, with motor‑vehicle exhaust and coal‑fired flue gases further lowering SO₂ capacity and promoting sulfate formation. Observations from five severe haze episodes and laboratory simulations confirm that reduced SO₂ capacity and sulfate production drive fine‑particle growth, suggesting that motor‑vehicle exhaust’s impact may be underestimated.

Abstract

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.