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. Haze pollution is affected by local air pollutants,regional transport of background particles and precursors, atmosphericchemistry related to secondary aerosol formation, and meteorologicalconditions conducive to physical, dynamical, and chemical processes. Inthe large, populated and industrialized areas like the Asian continentaloutflow region, the combination of regional transport and local stagnationoften exacerbates urban haze pollution. However, the detailed chemicalprocesses underlying the enhancement of urban haze induced by the combinedeffect of local emissions and transported remote pollutants are stillunclear. Here, we demonstrate an important role of transported hygroscopicparticles in increasing local inorganic aerosols, by studying the chemicalcomposition of PM2.5 collected between October 2012 and June 2014 inSeoul, a South Korean megacity in the Asian continental outflow region,using the ISORROPIA II thermodynamic model. PM2.5 measured under thecondition of regional transport from the upwind source areas in China washigher in mass concentration and richer in secondary inorganic aerosol (SIA)species (SO42-, NO3-, and NH4+) and aerosolliquid water (ALW) compared to that measured under non-transportconditions. The secondary inorganic species and ALW were both increased,particularly in cases with high PM2.5 levels, and this indicatesinorganic species as a major driver of hygroscopicity. We conclude that theurban haze pollution in a continental outflow region like Seoul,particularly during the cold season, can be exacerbated by ALW in thetransported particles, which enhances the nitrate partitioning into theparticle phase in NOx- and NH3-rich urban areas. This studyreveals the synergistic effect of remote and local sources on urban hazepollution in the downwind region and provides insight into the nonlinearityof domestic and foreign contributions to receptor PM2.5 concentrationsin numerical air quality models.

Synergistic enhancement of urban haze by nitrate uptake into transported hygroscopic particles in the Asian continental outflow