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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 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 stagnation 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 PM 2.5 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. PM 2.5 measured 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 (SO42- , NO3- , and NH4+) and aerosol liquid water (ALW) compared to that measured under non-transport conditions. The secondary inorganic species and ALW were both increased, particularly in cases with high PM 2.5 levels, and this indicates inorganic species as a major driver of hygroscopicity. We conclude that the urban haze pollution in a 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 urban haze pollution in the downwind region and provides insight into the nonlinearity of domestic and foreign contributions to receptor PM 2.5 concentrations in numerical air quality models. [ABSTRACT FROM AUTHOR]

Air quality in Seoul, one of the highly populated megacities in East Asia, is not only affectedby local emissions but also largely by episodic long-range transport of air pollutants fromChina. Although severe haze episodes in Seoul have been regarded as a result of thecombined effect of local and remote sources in the stagnant condition, the detailedchemical mechanisms and the dynamics with meteorological conditions for suchhaze development are still in vague. In this study, we explored an important roleof aerosol water enhancing nitrate partitioning in the case of the combination oflong-range transport and local stagnant condition by utilizing ∼200 daily PM2.5chemical composition data intermittently measured from October 2012 to June 2014together with the ISORROPIA II thermodynamic model. Using the daily averageresidence time of the HYSPLIT backward trajectories over the local source area (theSeoul Metropolitan Area) and the remote source areas (the North China Plain andthe Yangtze River Delta), the PM2.5 sampling dates were classified into the local,remote, and combined source cases. Our results show that the remote and combinedsource groups are characterized by inorganic-rich wet particles, while the PM2.5samples during the local source cases are organic-rich and aerosol water-short. Asindicated by previous studies in China, the more inorganic fraction in PM2.5, the moreaerosol water fraction, especially for the remote and combined source group. Interms of the meteorological factor, the temperature for the remote and combinedsource groups are generally lower than that for the local source group because thelong-range transport from China occurs in the cold season. The nitric acid–nitratepartitioning analysis based on the thermodynamic model for different groups reveals thatboth the lower temperature and the higher aerosol water content enhanced nitrateuptake during the combined source cases. Our results indicate a synergistic effect oflong-range transport and local high NOx condition on the haze pollution in Seoul. [ABSTRACT FROM AUTHOR]