Levels of a pollutant linked to diarrhoea and global cooling in Beijing’s notorious smog can approach those produced by volcanic eruptions, according to a newly published international study. Researchers from Germany, the United States and China recorded extremely high concentrations of sulphate on the Tsinghua University campus in January 2013 during a joint study of air pollution in the Chinese capital. Sulphate is a salt of sulphuric acid that, in nature, is usually formed in the atmosphere after a volcanic eruption. Beijing’s ‘smog refugees’ flee the capital for cleaner air down south The sulphate concentration on the roof of one Tsinghua building hit 300 micrograms per cubic metre of air, comparable to the fallout from the eruption of the Eyjafjallajokull volcano in Iceland in 2010, which produced an average, near-surface concentration of 400 micrograms over Scandinavia. That might explain why some people experience diarrhoea, a typical effect of sulphate poisoning, on smoggy days, alongside other symptoms linked to air pollution. Watch: Outdoor children's classes hit hard by smog in Beijing But the high levels of sulphate in the smog that plagues Beijing each winter puzzled the researchers. In nature, sulphate is formed when massive amounts of sulphur are thrust high into the atmosphere by a volcanic eruption and transformed by sunlight in a process known as photochemistry. “No theory could explain why it happened during a cold, dim winter in Beijing with little photochemistry going on,” said Dr Su Hang, a researcher at the Max Planck Institute for Chemistry in Mainz, Germany, and one of the scientists involved in the study. In a paper published in the journal Sciences Advances on Wednesday, Su and his colleagues pinpointed a culprit: nitrogen oxides – a family of pollutants mainly created by industrial and vehicular emissions. Severe smog in north China grounds flights, closes highways and suspends production Nitrogen oxides, including nitric oxide and nitrogen dioxide, could bind with water vapour and create many floating liquid droplets that would not freeze in sub-zero temperatures. The airborne droplets then served as a chemical reactor, absorbing sulphur dioxide from the atmosphere and turning it into sulphate. The more sulphate produced, the bigger the droplet and the faster the chemical reaction. “It was like a chain reaction,” Su said. “Once started, it would not stop.” That made the smog in Beijing different from the photochemical smog, driven by sunlight, that troubled Los Angeles in the 1970s. The researchers said the amount of man-made air pollutants in China’s lower atmosphere had reached a level unprecedented in human history and that was triggering chemical reactions previously thought impossible. In Beijing, Su said, smog could develop rapidly at night, and residents sometimes woke up to find the air outside “as thick as soup”. Sulphate is also believed to play an important role in planet cooling, with scientists linking the massive spread of sulphate in the atmosphere after volcanic eruptions to numerous episodes of global cooling throughout history due to the chemical’s ability to reflect sunlight back into outer space almost as effectively as a mirror. Smog blankets Beijing as year’s first red alert comes into effect Whether the smog in China could help slow global warming required further investigation, the researchers said. The researchers urged the authorities to treat nitrogen oxides as a major enemy in the battle against smog – with a focus on cutting industrial and vehicular emissions – because the chemistry at work in haze not only produced sulphate but also lots of nitrate, which could cause oxygen levels in the blood to drop, causing dizziness, headaches or even death. “Reducing the nitrogen oxides can shoot several birds with one stone,” Su said.