Analysis on a rare persistent heavy pollution event in Ningbo

A rare persistent heavy pollution event occurred in Ningbo during 1 to 10 December 2013. Data from conventional weather observations and automatic weather stations in Zhejiang Province, wind profiles from Cixi wind profiler radar and observations at the 370 m tower on the Ningbo Liangmaoshan Island, and the pollutant data collected at Ningbo and Zhoushan were analyzed. The NOAA HYSPLIT4 particle trajectory model and the WRF/CALMET diagnosis model were applied to investigate the particle trajectories in the boundary layer. Results showed that:(1) The synoptic circulation background played a critical role for the outbreak of the pollution event. Accompanied with the pollution enhancement, three weak cold air mass activities affected the levels above 800 m over the pollution region, which provided favorable dynamic conditions for pollutants transport, but they little impact on wind speeds and stability at the lower levels in the boundary layer. The wind speeds were weak and wind directions changed quickly between the cold air intermittent periods, providing very good conditions for pollutants circulation and retention; (2) Persistent weak descending motions, low boundary layer height, and a thermal inversion layer below 200 m were all favorable for the aerosol accumulation and visibility deterioration; (3) Dispersion was poor with the ventilation value less than 1 m²/s at night during the haze lasting period, and the ventilation value rose up significantly during the pollution weakening period. The ventilation value was negatively correlated with aerosol concentration when there existed no exogenous particles.