Analysis of a long-lasting haze episode in Nanjing, China

A long-lasting haze episode occurred in Nanjing and its surrounding areas from October 15 to 31, 2009. Aerosol physical, chemical and optical properties during this pollution event were investigated. During the long-lasting haze, the Yangtze River Delta (YRD) region was under the control of a high-pressure system and surface pressure gradients were extremely small. The transport and diffusion of air pollutants were suppressed by very low surface wind speed, stably stratified atmosphere and lower mixing level depth (MLD). Back trajectory analysis showed that local emission and regional transport were important in this pollution process. The average diurnal variation of Aitken mode and coarse mode particles had bimodal distribution, which was mainly influenced by diurnal variation of atmospheric boundary layer (ABL) and anthropogenic emissions. Accumulation mode particles were mostly influenced by diurnal variation of ABL; while new particle formation process was important for nucleation mode particles. The peak value of aerosol particle number concentrations shifted to larger particle sizes, perhaps due to the fact that the haze was favorable to accumulation mode aerosols through collision and coagulation of Aitken mode particles. The high ratio of NO3−/SO42 − indicated that traffic source became more and more important in the YRD region. The average aerosol scattering coefficient was 696.7 ± 445.4 M m− 1, mainly due to particle counts within diameter intervals from 0.6 to 1.4 μm that increased remarkably during the pollution process. Higher accumulation mode particle counts and higher relative humidity were the main reasons for atmospheric visibility impairment during the haze.

► Haze episode lasted for more than 15 days.
► Aerosol physical, chemical and optical properties during pollution episode were discussed.
► Traffic source is becoming more and more important in the YRD region.
► The relationships among the aerosol number concentrations, scattering coefficient and visibility were investigated.