Occurrence, evolution and degradation of heavy haze events in Beijing traced by iodine-127 and iodine-129 in aerosols

Heavy haze events have become a serious environment and health problem in China and many developing countries, especially in big cities, like Beijing. However, the factors and processes triggered the formation of secondary particles from the gaseous pollutants are still not clear, and the processes driving evolution and degradation of heavy haze events are not well understood. Iodine isotopes (¹²⁷I and ¹²⁹I) as tracers were analyzed in time series aerosol samples collected from Beijing. It was observed that the ¹²⁷I concentrations in aerosols peaked during the heavy haze events. The conversion of gaseous iodine to particular iodine oxides through photochemical reactions provides primary nuclei in nucleation and formation of secondary air particles, which was strengthened as the external iodine input from the fossil fuel burning in the south/southeast industrial cities and consequentially induced heavy haze events. Anthropogenic ¹²⁹I concentrations peaked during clean air conditions and showed high levels in spring and later autumn compared to that in summer. ¹²⁹I originated from the direct air discharges and re-emissions from contaminated seawaters by the European nuclear fuel reprocessing plants was transported to Beijing by the interaction of Westerlies and East Asian winter monsoon. Three types of mechanisms were found in the formation and evolution of heavy haze events in Beijing by the variation of ¹²⁷I and ¹²⁹I, i.e., iodine oxides intermediated secondary air particles, dust storm and mixed mode by both secondary air particles and dust storm induced processes.