Measurement report: Nocturnal subsidence behind the cold front enhances surface particulate matter in the plain regions: observation from the mobile multi-lidar system

Abstract. A multi-lidar system, mounted in vehicle to monitor the profiles of temperature, wind and particle optical properties, was utilized to investigate the winter fine particulate matter (PM_2.5) pollution for a vertical perspective, in four cities in China in winter 2018. We observed the enhancement of surface nocturnal PM_2.5 in two typical plain cities (Changzhou and Wangdu), which was attributed to the subsidence of PM_2.5 transported from upstream polluted areas, with the wind turning north and downdrafts dominating. Combining with the observed surface PM_2.5, the reanalysis meteorological data, and the GEOS-Chem model simulation, we revealed the Transport-Nocturnal PM_2.5 Enhancement by Subsidence (T-NPES) events occurred frequently in the two cities, with percentages of 12.2 % and 18.0 %, respectively during Dec. 2018–Feb. 2019. Furthermore, the GEOS-Chem model simulation further confirmed that the ubiquity of winter T-NPES events in a large scale including North China Plain and Yangtze River Delta. Process analysis revealed that the subsidence was closely correlated with the southeasterly movement of the high-pressure system and the passage of the cold front, resulting in the increase of temperature aloft, a stronger inversion layer, and further PM_2.5 accumulation in the atmospheric boundary layer. Thus, a conceptual model of the T-NPES events was proposed to highlight this surface PM_2.5 enhancement mechanism in these plain regions. However, it was not applicable to the two cities in basin region (Xi’an and Chengdu), due to the obstruction of the weather system movement by the mountains surrounding the basin.