Boundary-layer features and regional transport process of an extreme haze pollution event in Nanjing, China

Extreme haze pollution event is a major environmental concern in East China. In this study, we aim at elucidating the relationships among boundary-layer features, regional transportation and low-visibility event of haze pollution in Nanjing in December 2013. The results showed that this haze event was associated with weak cold front activity under relatively stable weather conditions, including weak surface wind speed. Local pollutant accumulation was the main cause during the whole haze process, the surface wind speed was under 2 m/s, and the irradiance 24-hour change (IR24) had negative values. The higher the pollutants' concentrations, the greater the change of IR24. Most of the pollution was circulating and trapped within 130-km range around the observing site. Based on the cluster analysis, the circling trajectory presented the highest PM2.5, and the long-distance pollutant transformation from North China also made the pollutants' concentration higher. Multilayer inversions, lower mixed layer height and thick lower wind speed zone promoted extreme haze maintenance. Both inversion and relative humidity (RH) had obvious diurnal variation. During the serious pollution period (visibility <2000 m), the wind speed below 500 m was under 2 m/s all the time. Surface inversion (SI), low-hanging inversion and multi-layer inversion all occurred. The lower the inversion bottom (IB), the lower the visibility. Many multi-layer inversions had intensity greater than 1°C/100 m, and corresponded to the increase of pollutants concentrations on the ground. Although the low-hanging inversion intensity (LHII) was only 0.32°C/100 m and the low-hanging inversion bottom (LHIB) rose to 420.0 m, the multi-layer and large thickness (124.9 m) of inversions sustained surface pollutants concentrations at high levels. The downward momentum transport by the weak cold front increased surface wind speeds destroyed the inversions and ended the haze event.