Unusual meteorological conditions were responsible for the abnormally high concentrations of surface ozone observed last year during winter in the US, according to a study published online in Nature Geoscience. Ozone production at the Earth’s surface, which is a significant threat to human health, was previously considered to be a summer phenomenon.

Russell Schnell and colleagues used atmospheric chemistry records and meteorological datasets to find the cause of excessively high levels of surface ozone near a gas field in Wyoming in February last year. They discovered that ozone precursors released from the gas field became trapped at the Earth’s surface due to strong temperature inversions, in which warm air overlies cold surface air and acts as a lid. The build-up of ozone precursors with nowhere to go resulted in high levels of ozone production.

The authors warn that there may be other areas around the world that also produce ozone during winter.