Emerging of surface ozone pollution beyond summer season over the North China Plain

Surface ozone pollution is of great concern in current air quality management in China. The North China Plain (NCP), which is home to 300 million people, has the highest ozone level and increasing trend.  While earlier studies of ozone pollution focused on summer season, here we show that ozone pollution out of summer season over the North China Plain is also very emerging.  Firstly, ozone has been very low during winter haze (particulate) pollution episodes. However, the abrupt decrease of NOx emissions following the COVID-19 lockdown in January 2020 reveals a switch to fast ozone production during winter haze episodes with MDA8 ozone of 60 to 70 ppb. This remarkable switch to an ozone-producing regime in January–February following the lockdown illustrates a more general tendency since 2013 of increasing winter–spring ozone in the North China Plain and increasing association of high ozone with winter haze events, as pollution control efforts have targeted NOx emissions while VOC emissions have limited regulations. Secondly, we find that ozone episodes comparable with those in summer season can also occur in early-autumn over NCP, resulting in the emerging of two-peak ozone pollution (June and September) for some years. The statistical analysis show that this interannual variation is driven by the anomalies in the large-scale sea surface temperature (SST) patterns. These results highlight the urgency of ozone pollution controls out of summer season in China.