Comprehensive characterization of PM2.5 using chemical, optical, and spectroscopic methods during pollution episodes at an urban site in Gwangju, Korea

In this study, PM2.5 samples collected between October 29 and December 23, 2018 at a roadway site in Gwangju, Korea, were comprehensively characterized by chemical, optical, and spectroscopic methods. Aerosol light absorption was observed with a time resolution of 1 min using a dual-spot aethalometer. An Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy analysis was also applied to characterize organic and inorganic components in the PM2.5 samples. Three PM2.5 pollution episodes which exceeded the 24-hr Korean PM2.5 standard (35 μg/m3) were classified; Episode I was associated with local air stagnation and long-range transportation of aerosol particles from northeast China and upwind regions; Episode II was affected by Asian dust (AD) particles and local pollution under low relative humidity (RH) conditions; and Episode III was strongly associated with accumulation of local pollution due to very high RH and low wind speed, along with regional transport of aerosol particles from eastern China. The contribution of organic aerosols to PM2.5 was higher during episodes I and II than during episode III, while the contribution of NO3− particles was greater during episode III, possibly due to very high RH and low wind speed conditions. Unexpectedly, SO42− was not an important component of PM2.5 during the three episodes. The light absorption coefficient (bBrC,370) of brown carbon (BrC) at 370 nm was estimated to range from 9 Mm−1 during episode II to 14 Mm−1 during episode III, but the contribution of bBrC,370 to total aerosol absorption was the lowest during episode III. Multiple linear regression analysis indicated that the contributions of the traffic emissions, biomass burning (BB) emissions, and secondary formation processes to bBrC,370 absorption were, on average, 37, 39, and 14% of the total estimated bBrC,370, respectively, suggesting that primary traffic emissions are an important contributor to BrC absorption at the roadway site. ATR-FTIR analysis showed that aliphatic carbon group (C–H), carbonyl group (CO), ammonium, sulfate, and nitrate were identified in all filter samples collected during the three pollution episodes; the corresponding bands had the highest intensities in the filter sample from episode III. However, the presence of mineral dusts related to hydroxyl group (-OH), gypsum, Al–Al–OH group, CO32−, and SO32− were clearly detected at the absorbance bands at 3400–3700 and 650–900 cm−1 in the AD sample only (epis