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Lifting the fog on the Great Smog
© Wenjie Dong/E+/Getty
The ‘Great Smog’ that killed thousands of people in London in 1952 was triggered by a chemical process currently causing hazy days in China’s megacities.
A major culprit behind London’s fatal fog was sulfate (SO42-) that built up due to increased sulfur dioxide (SO2) from burning coal. How the sulfate formed and why it became so lethal was less clear. A team including researchers from the CAS Key Laboratory of Aerosol Chemistry and Physics in Xi’an, China, studied hazy air in Beijing and Xi’an and found that sulfate forms when nitrogen dioxide (NO2) oxidizes SO2 under specific atmospheric conditions. The reaction occurs either on tiny airborne particles in the presence of ammonia (NH3), or on cloud droplets.
The authors conclude that ammonia from agriculture contributes to China’s pollution problem, yet also prevents sulfates in Beijing and Xi’an from becoming dangerously acidic, whereas in London’s smog, the cloud droplets evaporated each day, leaving behind particles of toxic sulfuric acid.
- PNAS 113, 13630–13635 (2016). doi: 10.1073/pnas.1616540113