City-level air quality improvement in the Beijing-Tianjin-Hebei region from 2016/17 to 2017/18 heating seasons: Attributions and process analysis.

With the implementation of clean air strategies, PM pollution abatement has been observed in the "2 + 26" cities in the Beijing-Tianjin-Hebei (BTH) region (referred to as the BTH2+26) and their surrounding areas. To identify the drivers for PM concentration decreases in the BTH2+26 cites from the 2016/17 heating season (HS1617) to the 2017/18 heating season (HS1718), we investigated the contributions of meteorological conditions and emission-reduction measures by Community Multi-Scale Air Quality (CMAQ) model simulations. The source apportionments of five sector sources (i.e., agriculture, industry, power plants, traffic and residential), and regional sources (i.e., local, within-BTH: other cities within the BTH2+26 cities, outside-BTH, and boundary conditions (BCON)) to the PM decreases in the BTH2+26 cities were estimated with the Integrated Source Apportionment Method (ISAM). Mean PM concentrations in the BTH2+26 cities substantially decreased from 77.4 to 152.5 μg m in HS1617 to 52.9-101.9 μg m in HS1718, with the numbers of heavy haze (daily PM ≥150 μg m) days decreasing from 17-77 to 5-30 days. The model simulation results indicated that the PM concentration decreases in most of the BTH2+26 cities were attributed to emission reductions (0.4-55.0 μg m, 2.3-81.6% of total), but the favorable meteorological conditions also played important roles (1.9-25.4 μg m, 18.4-97.7%). Residential sources dominated the PM reductions, leading to decreases in average PM concentrations by more than 30 μg m in severely polluted cities (i.e., Shijiazhuang, Baoding, Xingtai, and Beijing). Regional source analyses showed that both local and within-BTH sources were significant contributors to PM concentrations for most cities. Emission controls in local and within-BTH sources in HS1718 decreased the average PM concentrations by 0.1-47.2 μg m and 0.3-22.1 μg m, respectively, relative to those in HS1617. Here we demonstrate that a combination of favorable meteorological conditions and anthropogenic emission reductions contributed to the improvement of air quality from HS1617 to HS1718 in the BTH2+26 cities.