Multiple perspectives for modeling regional PM2.5 transport across cities in the Beijing-Tianjin-Hebei region during haze episodes

Regional transport always plays a crucial role in the formation and dissipation of haze over the Beijing-Tianjin-Hebei (BTH) region. This study, conducted using pollution and meteorological observations and the Weather Research and Forecasting model (WRF) coupled with the Comprehensive Air Quality Model with Extensions (CAMx), investigated the possible meteorological causes for the occurrence of haze pollution and quantitatively assessed the PM2.5 transport contribution to haze episodes that occurred in Beijing in January and July 2015. The results indicated that modeling system reproduced the spatial-temporal variation in PM2.5 concentrations in the BTH region well. During the study period, haze episodes were primarily attributed to meteorological conditions such as planetary boundary layer height, relative humidity, wind vector, and temperature inversion, in the context of pollution emissions. Analysis of surface PM2.5 transport showed that 62.89% of the surface PM2.5 in Beijing was came from local emissions, with the remaining 23.69% and 13.42%, on average, originating from short- and long-range transport during the study period. The percentage of contribution varied with the evolutionary stage of the haze episodes, showing the joint influence of local emissions and regional transport on haze pollution in Beijing. Additionally, investigation of vertical PM2.5 transport identified the following three major pathways: a northwest-southeast pathway in January (at all layers below 1200 m, though it was stronger above 600 m), a southeast-northwest pathway in July (at all layers below 800 m), and a southwest-northeast pathway during both months (at a height of 200-1200 m). Moreover, the magnitude of daily PM2.5 transport fluxes during the haze episodes was generally stronger than the corresponding monthly average. These results provide a scientific basis for strategic control of both multiple cities and provinces and in-depth knowledge of the mechanisms and sources of haze pollution in the BTH region.