Dense, hazy fog episodes characterized by relatively high humidity, low visibility and extremely high PM2.5 have been a headache to many megacities including those in Mainland China. 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 reactant-product link between sulphur dioxide and airborne sulfate formation is common knowledge, the complex oxidants and mechanisms that enable this transformation are not. In particular, the role of nitrogen oxides in sulfate production is unclear. 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 systematically examining the multiple roles of nitrogen oxides in affecting oxidants that enable this set of chemical reactions.

In collaboration with the California Institute of Technology, a research team led by Prof. YU Jianzhen, Professor at HKUST’s Department of Chemistry and Division of Environment and Sustainability, identified three formation mechanism regimes, corresponding to the three distinct roles that nitrogen oxides play in sulfate production depending on the chemical surroundings. Under low NOx conditions, NOx catalyze the cycling of hydroxyl radicals, an effective oxidant of SO2, and thus promote formation of sulfate. Under extremely high NOx common in haze-fog conditions, NOx act as dominant oxidants of SO2 and thus also promote 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 supresses the oxidation of 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.

Read more at Hong Kong University of Science and Technology