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

Comprehensive measurements were conducted from 15 October to 19 November at Atmospheric Environment Monitoring Super-Station in Heshan of Guangdong province to investigate the characteristics and the specific mechanism of the continuous hazes. According to the definition of haze (visibility < l0 km and relative humidity < 90%), five haze episodes occurred during the observation. Among these hazes, the second haze episode, with the highest concentration of PM2.5 of 187.51 μg/m3, was the most severe. NO3− was always higher than SO42 −, which indicated that motor vehicles played an important role in the hazes, 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 to this result. Relative humidity played a crucial role in the formation of hazes by leading to hygroscopic growth of the diameter of aerosols, and the faster visibility decreased with increasing concentration of PM2.5. To control the occurrence of haze episodes in the Heshan region, the concentration of PM2.5 should be lower than 50 μg/m3.271