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

Abstract

A multi-lidar system, mounted in a vehicle to monitor the profiles of temperature, wind, and particle optical properties, was utilized to investigate the winter fine particulate matter (PM2.5) pollution from a vertical perspective in four cities in China in winter 2018. We observed the enhancement of surface nocturnal PM2.5 in two typical plains cities (Changzhou and Wangdu), which was attributed to the subsidence of PM2.5 transported from upstream polluted areas, with the wind turning north and downdrafts dominating. Combining the observed surface PM2.5, the reanalysis meteorological data, and the GEOS-Chem model simulation, we revealed the transport nocturnal PM2.5 enhancement by subsidence (T-NPES) events that occurred frequently in the two cities, with percentages of 12.2 % and 18.0 %, respectively, during December 2018–February 2019. Furthermore, the GEOS-Chem model simulation further confirmed the ubiquity of winter T-NPES events on a large scale, including North China Plain and the 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 PM2.5 accumulation in the atmospheric boundary layer. Thus, a conceptual model of the T-NPES events was proposed to highlight this surface PM2.5 enhancement mechanism in these plains regions. However, it did not apply to the two cities in the basin region (Xi'an and Chengdu) due to the obstruction of the weather system movement by the mountains surrounding the basin.