Emissions of air pollutants have been dramatically reduced in the Beijing-Tianjin-Hebei (BTH) region of China during 2014-2017. However, impacts of emission reduction on regional air quality are not well quantified. This study evaluates the impacts of emission reduction and inter-annual meteorological conditions on regional air pollution transport in BTH region by employing Community Multiscale Air Quality model embedded with the Integrated Source Apportionment Model (CMAQ-ISAM). Results suggest that the regional transport contributed 32.5%-68.4% of total PM mass concentrations and 52.4%-83.2% of sulfate, nitrate and ammonium in 2017. During 2014-2017, the annual averaged PM concentrations in BTH region decreased by 33%, of which the decrease of local emissions, inter-regional transport and transport from outside the BTH region contributed for 47%, 25%, and 28%, respectively. Emission reductions (91%) mitigate not only the impacts of local sources, but also influence the regional transport with similar magnitude, demonstrating the effectiveness of multiple regional joint controls. The variation of meteorology contributes only 9% to the decrease of PM in BTH, with higher contributions from the change of regional transport compared to local sources since the regional transport is more sensitive to the meteorology variation. The impacts of meteorological variations are considerable, with over 20% on the relative changes of local and regional contributions, and up to 40% on regional transport in spring and winter. Therefore, more strengthened regional joint air pollution control is suggested in winter and spring for this region.
