Chemical Composition and Light Absorption of PM 2.5 Observed at Two Sites near a Busy Road during Summer and Winter

: To examine the difference in the major chemical composition of fine particulate matter (PM 2.5 ) between two roadway sites, 24 h integrated PM 2.5 samples were simultaneously collected both 15 m (Buk ‐ Ku District Office (BKO) site) and 150 m (Chonnam National University campus (CNU) site) away from busy roads during the summer and winter periods; these samples were taken to determine the concentrations of organic and elemental carbon (OC and EC), water ‐ soluble organic carbon (WSOC), and water ‐ soluble inorganic species. In addition, the real ‐ time aerosol light absorption coefficients (Abs) were measured using a dual ‐ spot seven ‐ wavelength aethalometer at the CNU site to evaluate the influence of traffic and biomass burning (BB) emissions on the concentrations of organic aerosol particles. The hourly NO 2 concentration was also observed at an air pollution monitoring network that is about 2 km away from the CNU site. During summer, 24 h PM 2.5 concentrations (PM 2.5 episode) which exceeded the Korean PM 2.5 standard (35 μ g/m 3 ) were slight differences in the concentrations of secondary inorganic species (NO 3 − , SO 42 − , and NH 4+ ) between the sites during summer and winter. 81% and 19% of the total OC, respectively, and 4.0 (1.3–9.1) and 1.0 (0.0–3.9) μ gC/m 3 at the CNU site, accounting for 82% and 18% of the total OC, respectively. The results revealed that the primary combustion emissions (e.g., traffic in summer vs. traffic + BB emissions in winter) were major contributors to the OC near roadways during both the summer and winter. the MODIS images and the transport pathways of the air masses that reached the sites. The wintertime PM 2.5 episodes, however, were associated with increases in the OM and NO 3 − concentrations, accounting for on average 26~28% (BKO site) and 22~23% (CNU site) of the PM 2.5 concentration. A good correlation between the gaseous NO 2 and NO 3 − , along with weather conditions such as a low wind speed and high relative humidity, suggest that the wintertime particulate NO 3 − was caused by local production through the heterogeneous oxidation of NO 2 on moist particles at the sites rather than the regional transport of aerosol particles. One interesting feature that was observed was the existence of two distinct slopes in the plots of OC vs. EC concentration in winter; one is associated with a significant contribution of traffic emissions, providing a lower OC/EC ratio (~3.0), while the other is strongly linked to BB and traffic emissions, resulting in a higher OC/EC ratio (~6.5). This was supported by the strong correlations between the OC and EC, the enhanced AAE values, and an increased contribution of light ‐ absorbing BrC particles to the total aerosol absorption. The wintertime WSOC was strongly associated with primary traffic and biomass burning emissions and SOA formation. The higher WSOC/OC values (0.49–0.54) during the winter also support the influence of BB emissions on the WSOC concentration. This was evidenced by the strong relationships of Abs BrC370 nm with the WSOC and K + and a substantial increase in the values of AAE 370–520nm (1.4–2.0) during winter, which is an indicator of light ‐ absorbing brown carbon particles that are linked to BB emissions.