Effects of potential recirculation on air quality in coastal cities in the Yangtze River Delta

Air quality is closely related to the synoptic circulation and local wind field affecting a certain area as they have distinct influence on the path and speed of pollutants. The Yangtze River Delta is located on the eastern coast, and the air returning from coastal areas has a detrimental effect on air quality in the area. This study proposes to analyze if certain circulation types and the occurrence of recirculation are predominantly related to the occurrence of bad air quality in the Yangtze River Delta. Using sea level pressure data from 2006 to 2016, we used T-mode objective classification to classify circulation in the Yangtze River Delta into nine categories. At the same time, using the Allwine and Whiteman (AW) method, we categorized local winds in the region as ventilation, stagnation, and recirculation types, and we found that the local wind tends to be under recirculation conditions when the region was controlled by circulation types 3 (CT3, southeast low pressure), CT4 (northeast low pressure), CT7 (northwest high pressure), and CT8 (north high pressure, south low pressure, with a large pressure gradient). By comparing concentrations of pollutants and different local wind types, we found that recirculation tended to promote high pollution situations. Use of the HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) model to simulate the diffusion of pollutants by recirculation in Shanghai in March 2016 confirmed this conclusion. The outputs of HYSPLIT model can show the track of air mass intuitively, and then reflect the effect of recirculation.