High loadings of fine particulate matter (PM2.5) during haze are mostly produced from the chemical reactions of reactive gas precursors, including sulfur dioxide (SO2), nitrogen oxides (NOx), ammonia (NH3), and volatile organic compounds. In an ideal world, air pollution would be cured by wiping clean any one of these four PM2.5 precursors. However, in the real world, we have to go step by step, considering the technological conditions and the economic costs in the emission control strategies.
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Source: Phys.org