The study from Chinese scientists is the first detailed examination of nitrogen oxides and the role this compound plays in the level of airborne sulfates, and the causes of hazy air pollution.
The focus is with the type of dense, hazy fog episodes that occurs during periods of high humidity, low visibility and where there is a very high level of PM2.5 particles (these are airborne particles, both liquid and solid, less than 2.5 microns in diameter). Such events are common to the sprawling cities of Mainland China, as well as other regions of the globe characterized by extensive urbanization.
With the polluting particle, airborne sulfate is the most common reason for hazy air pollution. This is formed atmospherically through the the oxidation of sulfur dioxide. The mechanisms that enable this transformation to take place are not commonly understood, and it is this area that the scientists from the Hong Kong University of Science and Technology have been examining.
This revealed three formation mechanisms at play. The first is is under Under low nitrogen oxide conditions, hydroxyl radicals, are catalyzed to produce sulfur dioxide. The second mechanism is under extremely high nitrogen oxide conditions, nitrogen oxides act as dominant oxidants of sulfur dioxide leading to the formation of sulfate. Thirdly, within an environment of medium-level of nitrogen oxide, the compound acts as a sink for hydroxyl radicals which suppress the oxidation of sulfur dioxide and thus inhibits sulfate formation.
This means to reduce sulfate levels and to reduce the formation of haze-fog conditions, co-control of sulfur dioxide and nitrogen oxide emissions is required. However, as nitrogen oxide inhibits sulfate formation when emissions are intermediately high, suppressing nitrogen oxide in such environment would raise bring up sulfate levels in the air. In other words, careful control of nitrogen oxide levels is key to improved air quality.
The implications of the research are to provide governments with new options for tackling smoggy weather.
The research has been published in the journal Nature Geoscience, where the research paper is headed “Efficient control of atmospheric sulfate production based on three formation regimes.”
