Characteristics and formation mechanism of regional haze episodes in the Pearl River Delta of China.

To investigate the characteristics and the specific mechanism of continuous haze, comprehensive measurements were conducted from 15 October to 19 November in the Atmospheric Environment Monitoring Super-Station in Heshan of Guangdong province. Five haze episodes occurred in October and November 2014 in the Pearl River Delta (PRD) region. The meteorological parameters, gas data, chemical compositions, and optical parameters of the aerosols were obtained. Among these events, the second haze episode, with the highest concentration of PM2.5 of 187.51μg/m3, was the most severe. NO3- was always higher than SO42-, which indicated that motor vehicles played an important role in the haze, even though the oxidation rate from SO2 to SO42- was faster than that of NOX to NO3-. The difference between the hourly averages of Na+ and K+ during the haze episode and clean days was small, implying that straw combustion and sea salt had no significant effect on the occurrence of haze, and the backward trajectories of the air masses also conformed with this result. The air pollutants were difficult to disperse because of the significant decrease in the planetary boundary layer (PBL) height. Relative humidity played a crucial role in the formation of haze by leading to hygroscopic growth of the diameter of aerosols.