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Particle-phase concentrations were measured at 10, 80, and 200 m from the roadside of a national highway near Seoul in January and May 2008. The highway has two lanes each way, with an average hourly traffic volume of 1,070 vehicles. In January 2008, PM10 concentrations decreased from 10 to 80 m but increased at 200 m. Black carbon (BC) decreased only slightly with distance due to the influence of biomass burning and open burning from the surrounding areas. In May 2008, the effect of secondary formation on both PM10 and PM2.5 was significant due to high temperatures compared with January. Because on-road emissions had little effect on secondary formation for a short time, variations in PM10 concentrations became smaller, and PM2.5 concentrations increased with distance. The effects of fugitive dust on PM concentrations were greater in May than in January when the mean temperature was below freezing. In the composition variations, the amounts of primary ions, organic carbon (OC), and BC were larger in January, while those of secondary ions and others were larger in PM10, as well as PM2.5 in May.

Haze pollution is affected by local air pollutants, regional transport of background particles and precursors, atmospheric chemistry related to secondary aerosol formation, and meteorological conditions conducive to the physical, dynamical, and chemical processes. In the large, populated and industrialized areas like the Asian continental outflow region, the combination of regional transport and local stagnant often exacerbates urban haze pollution. However, the detailed chemical processes underlying the enhancement of urban haze induced by the combined effect of local emissions and transported remote pollutants are still unclear. Here, we demonstrate an important role of transported hygroscopic particles in increasing local inorganic aerosols, by studying the chemical composition of PM2.5 collected between October 2012 and June 2014 in Seoul, a South Korean megacity in the Asian continental outflow region, using the ISORROPIA II thermodynamic model. Measured PM2.5 group under the condition of regional transport from the upwind source areas in China was higher in mass concentration and richer in secondary inorganic aerosol (SIA) species and aerosol liquid water (ALW) compared to that under the non-transport condition. The SIA and ALW were both increased, particularly in cases with high PM2.5 levels, and this indicates inorganic species as a major driver of hygroscopicity. We conclude that the urban haze pollution in the continental outflow region like Seoul, particularly during the cold season, can be exacerbated by ALW in the transported particles, which enhances the nitrate partitioning into the particle phase in NOx− and NH3-rich urban areas. This study reveals the synergistic effect of remote and local sources on the urban haze pollution in the downwind region and provides insight into the nonlinearity of domestic and foreign contributions to receptor PM2.5 concentrations in the numerical air quality models. [ABSTRACT FROM AUTHOR]