Background concentration of atmospheric PM2.5 in the Beijing-Tianjin-Hebei urban agglomeration: Levels, variation trends, and influences of meteorology and emission

The determination of background PM2.5 and the separation of the meteorology-related and emission-related influences on background concentration in urban areas are important to evaluate the effectiveness of local anthropogenic emission control measures. In this study, the baseline separation technique was used to estimate the urban background concentration of PM2.5 in the Beijing-Tianjin-Hebei Urban agglomeration (JJJUA) by comparing its results with the background level monitored at the Shangdianzi regional background site. The Kolmogorov-Zurbenko (KZ) filter and stepwise regression model were further used to isolate the impacts of meteorology and emission on background levels. The results showed that the annual average background levels of PM2.5 in JJJUA were 27.18, 24.77, and 22.20 mu g/m(3) in 2018, 2019, and 2020, respectively. The background concentration showed significant differences in the seasonal and spatial distributions, with the highest levels obtained during winter in the southern inland plains. Boundary layer height, surface net solar radiation, total precipitation, temperature, and wind direction were negatively correlated with the background level, whereas surface pressure was positively correlated with the background level. A significant contribution of anthropogenicemissions (89%, 5.71 mu g/m(3) .yr) on the decrease in trend of long-term background concentration was observed in JJJUA, indicating the strong influence of long-range transport of PM2.5 from surrounding areas. The results emphasize the strong influence of regional air pollution levels on background PM2.5, and coordinated control of regional air pollution is suggested to potentially reduce the regional background level of PM2.5.