Investigation on Characteristics of High PM2.5 Pollution Occurred during October 2015 in Gwangju

A severe haze event occurred in October 2015 in Gwangju, Korea. In this study, the driving chemical species and the formation mechanisms of PM2.5 pollution were investigated to better understand the haze event. Hourly concentrations of PM2.5 organic and elemental carbon, water-soluble ions, and elemental constituents were measured at the air quality intensive monitoring station in Gwangju. The haze event occurred was attributed to a significant contribution (72.3%) of secondary inorganic species concentration to the PM2.5, along with the contribution of organic aerosols that were strongly attributed to traffic emissions over the study site. MODIS images, weather charts, and air mass backward trajectories supported the significant impact of long-range transportation (LTP) of aerosol particles from northeastern China on haze formation over Gwangju in October 2015. The driving factor for the haze formation was stagnant atmospheric flows around the Korean peninsula, and high relative humidity (RH) promoted the haze formation at the site. Under the high RH conditions, SO42- and NO3- were mainly produced through the heterogenous aqueous-phase reactions of SO2 and NO2, respectively. Moreover, hourly O-3 concentration during the study period was highly elevated, with hourly peaks ranging from 79 to 95 ppb, suggesting that photochemical reaction was a possible formation process of secondary aerosols. Over the PM2.5 pollution, behavior and formation of secondary ionic species varied with the difference in the impact of LTP. Prior to October 19 when the influence of LTP was low, increasing rate in NO3- was greater than that in NO2, but both SO2 and SO42- had similar increasing rates. While, after October 20 when the impact of haze by LTP was significant, SO42- and NO3- concentrations increased significantly more than their gaseous precursors, but with greater increasing rate of NO3-. These results suggest the enhanced secondary transformation of SO2 and NO2 during the haze event. Overall, the result from the study suggests that control of anthropogenic combustion sources including vehicle emissions is needed to reduce the high levels of nitrogen oxide and NO3- and the high PM2.5 pollution occurred over fall season in Gwangju.