Impacts of sea-land and mountain-valley circulations on the air pollution in Beijing-Tianjin-Hebei (BTH): A case study

In the study, observational data analyses and the WRF-CHEM model simulations are used to investigate the role of sea-land and mountain-valley breeze circulations in a severe air pollution event occurred in Beijing-Tianjin-Hebei (BTH) during August 9-10, 2013. Both the wind observations and the model simulations have clearly indicated the evolution of the sea-land and mountain-valley breeze circulations during the event. The WRF-CHEM model generally reproduces the local meteorological circulations and also performs well in simulating temporal variations and spatial distributions of fine particulate matters (PM_2.5) and ozone (O₃) concentrations compared to observations in BTH. The model results have shown that the offshore land breeze transports the pollutants formed in Shandong province to the Bohai Gulf in the morning, causing the formation of high O₃ and PM_2.5 concentrations over the gulf. The onshore sea breeze not only causes the formation of a convergence zone to induce upward movement, mitigating the surface pollution to some degree, also recirculates the pollutants over the gulf to deteriorate the air quality in the coastal area. The upward valley breeze brings the pollutants in the urban area of Beijing to the mountain area in the afternoon, and the downward mountain breeze transports the pollutants back during nighttime. The intensity of the mountain-valley breeze circulation is weak compared to the land-sea breeze circulation in BTH. It is worth noting that the local circulations play an important role when the large-scale meteorological conditions are relatively weak.

Impacts of local circulations on the wintertime air pollution in the Guanzhong Basin, China

Urbanization and industrialization in the recent 30years have caused frequent heavy haze pollution in the Guanzhong basin, China during wintertime. Based on the categorized large-scale synoptic situations, the local circulation and its impact on the air pollution in the basin have been investigated using observational data analyses and model simulations with the WRF-CHEM model. The simulated mountain-valley breeze circulations are well established under most of the large-scale synoptic situations in the south-north direction. The downward mountain-breeze not only causes the convergence zone in the basin but also tends to bring the pollutants back from the mountain areas to the basin and enhance pollutants concentrations in the evening. The intensity of the mountain-valley breeze circulations may be different under different synoptic situations, but in general, aside emissions, every pollution event is the synthetic result of the synoptic situation at the large scale and the local circulation at the small scale in the Guanzhong basin.