The characteristics of air pollution episodes in autumn over the southern Hebei, China

In this study, the Models-3/Community Multiscale Air Quality Model (CMAQ) coupled with the Mesoscale Modeling System Generation 5 (MM5) was employed to simulate the air pollution episodes over East Asia, northern China Plain (NCP), and southern Hebei (SHB), at a grid resolution of 36, 12, and 4 km, respectively in Oct. 2012. The PM10 concentrations over SHB at 12-km are overpredicted with NMBs of 34.6% to 45.7% and also overestimated with that of 72.1% to 97.5% at 4-km which applied such a fine grid resolution over the SHB for the first time. It indicated that the simulation at 12-km performs better than the 4-km which may be related to the spatial allocation of the emissions, the lack of dust emissions and the limitations of model treatments. Five heavy episodes show the characteristics of sawtooth-shaped cycles over the NCP in fall (i.e. the maximum of PM10 was up to 885.1 mu g m(-3) and PM2.5 was up to 438.4 m(-3) in Handan city) which resulting in the deterioration of visibility and periodically haze days. The concentrations of OC, EC, SO42-, NO3 - and NH4(+) were significantly higher in heavy episodes than non-heavy pollution episodes. In comparison with other cities ([NO3-]/[SO42-] > 1) at 12-km, the monthly-mean mass ratio of [NO3-]/[SO42-] at Taiyuan (0.17-0.73), Shijiazhuang (0.28-2.34) was 0.43, 0.84 respectively, which means the stationary sources emissions were more important than the vehicle emission in the source areas. The influence of the regional transportation for pollutants compared with local emission was also an important factor for heavy pollution episodes. The regional joint framework should be established along with controlling the local emission over the SHB in China to improve the air quality.