Modeling study of source contributions and emergency control effects during a severe haze episode over the Beijing-Tianjin-Hebei area

In February 2014, the Beijing-Tianjin-Hebei (BTH) area experienced a weeklong episode of heavy haze pollution. Cities such as Beijing (BJ) and Shijiazhuang (SJZ) issued heavy pollution alerts for the first time and took emergency control measures. This study employed the Nested Air Quality Prediction Modeling System (NAQPMS) to simulate and analyze the three-dimensional structure of the source contributions of PM2.5 in the BTH area during this pollution episode and quantitatively assessed the effects of the emergency control measures. The results showed that during the polluted period (February 19–26), surface PM2.5 mainly originated from local sources (48%–72%). In the entire BTH area, southern Hebei (SHB) represented the largest internal contribution (33%), while the main external contributions came from Shandong (SD) (10%) and Henan (HN) (4%). Vertically, the local contribution was constrained below the near-ground layer, and rapidly decreased with altitude. The regional transport path from SHB and Shanxi (SX) to BJ appeared at 0.5–1.5 and 1.5–2.5 km, with contributions of 32%–42% and 13%–27%, respectively. The non-local source regions for the BTH area were SD below 1 km and mainly SX and HN above 1 km. Compared to the non-polluted period (February 27–28), the contribution from regional transport increased during the polluted period, indicating the key role of regional transport in the pollution formation. The emergency control measures had a relatively large effect on NOx and SO2 concentrations, but a limited effect on PM2.5. The stronger regional transport during the polluted period may have weakened the effects of the local emergency control measures. These results indicated that a coordinated emission control should be implemented not only over the BTH area but also over its surrounding provinces (e.g. SD, HN).