Prof. Jianzhen YU

Prof. Jianzhen Yu and Prof. Alexis Lau have led their research teams to study the role of nitrogen oxides (NOx) in the rise and fall of airborne sulfates in hazy air pollution. They have developed a conceptual framework to delineate the relationship between sulfate and one set of its controllable precursors, nitrogen oxides.

Among pollutants that are less than 2.5 microns in diameter (PM2.5), airborne sulfate is one of the most common components of hazy air pollution formed atmospherically via the oxidation of sulphur dioxide (SO2). While the link between sulphur dioxide and airborne sulfate formation is common knowledge, the complex oxidants and mechanisms that enable this transformation is not. Managing sulfate pollution has dogged researchers and governments alike as it is not produced directly from pollution sources, unlike nitrogen oxides which are clearly emitted from vehicle exhaust, and the combustion of fossil fuels like coal, diesel and natural gas. This is the first study examining the multiple roles of nitrogen oxides in affecting oxidants that enable this set of chemical reactions.

They revealed three formation mechanism regimes, corresponding to the three distinct roles that nitrogen oxides play in sulfate production depending on the chemical surroundings. Under a low NOx and an extremely high NOx regime common in haze-fog conditions, NOx acts as a dominant oxidant of SO2 and thus promotes formation of sulfate. But in an environment with medium-high level of NOx, nitrogen dioxide (a member of the NOx family) would actually serve as a sink for hydroxyl radicals which fails to initiate reaction with sulphur dioxide and thus inhibits sulfate formation. These findings indicate that in order to reduce sulfate levels in highly polluted haze-fog conditions, co-control of SO2 and NOx emissions is necessary. However, since NOx would inhibit sulfate formation when its emissions are intermediately high, suppressing NOx in such environment would thus bring up sulfate levels in the air. This work offers policymakers new insights into ways to tackle sulfate pollution.

Source: Xue, J., Yu, X., Yuan, Z. B., Griffith, S. M., Lau, A. K. H., Seinfeld, J. H., Yu, J. Z., Efficient control of atmospheric sulfate production based on three formation regimes, Nature Geoscience. 2019, 12, 977-982