A Chinese-led international study has found a link between strong El Niño events and rises in levels in China of the air pollutant ozone . The study, published last month in the quarterly, peer-reviewed journal Environmental Research Letters , found that the climate phenomenon ENSO, or El Niño-Southern Oscillation, was a crucial factor in ozone variation. Apart from particulate matter, near-surface ozone is the biggest air pollutant in the country and high levels are harmful to human health and crops. “The study suggests that when we make policies on pollution control, we should consider the impact of ENSO,” said Yang Yang, lead author of the study and a professor at Nanjing University of Information Science and Technology. “If it is an El Niño year, it will have a big impact on the ozone concentration.” US embassy in Beijing turns attention to ozone levels The researchers, from mainland China, Hong Kong and the United States, used model simulations, ground measurements and reanalysis data to look at the impact of ENSO on the year-to-year variations of near-surface ozone concentrations in China in the summer from 1990 to 2019. ENSO contains two opposite phases, “El Niño” and “La Niña”, and a neutral phase. The team found that summertime near-surface ozone levels in El Niño years were higher over China than in La Niña years, with increases of up to 20 per cent over southern China. In the last decade, certain summer ozone readings have risen by 5 per cent per year in China, faster than any other part of the world, according to the study. Yang said the study was the first of its kind to look at the impact of ENSO on ozone concentrations in China – previous studies focused on the impact of ENSO on general concentrations in the atmosphere. “ENSO is the most important internal variability on Earth and its changes may impact global temperature, precipitation and atmospheric circulation,” Yang said. “In our study, we want to relate the large-scale atmospheric circulation to our atmospheric environment.” Previous research revealed that meteorological factors, such as atmospheric temperature, winds and humidity, had a significant impact on regional ozone concentration. A study in 2019 showed that in northern China, high ozone concentrations were accompanied by high temperatures, low relative humidity and downward moving air. The new study found that compared to the La Niña years, southwest winds from the ocean weakened during the El Niño years, leading to a build-up of ozone in southern China. “In an ordinary year, when the southwestern winds blow from the ocean to the land, the water vapour will reduce and dilute the ozone concentration,” Yang said. “But during the El Niño years, the winds are weaker, so there is an unusual increase in the ozone concentration.” A separate study by Yang and his colleagues published last month found that near-surface ozone concentrations increased during the developing summer of both the 1997/98 and 2015/16 El Niño in southern China. Those increases were also linked to the weaker prevailing monsoon winds. In contrast, in the summers when the El Niño reversed to the cooling phase La Niña, ozone levels fell over many parts of China.