Abstract
Abstract. 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 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. PM2.5 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 PM2.5 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 PM2.5 concentrations in numerical air quality models.
Highlights
Fine particulate matter in urban areas consists of inorganic species (SO24−, NO−3, and NH+4 ) and organic matter (OM) produced by the gasto-particle conversion of anthropogenic and biogenic precursors (Seinfeld and Pandis, 2016)
Average concentrations of PM2.5 and chemical components were highest in the “local stagnation with regional transport” (S-T) group (e.g., PM2.5 dry of 72.2 μg m−3) and lowest in the “local ventilation with no regional transport” (V-nT) group (e.g., PM2.5 dry of 19.9 μg m−3), and the average concentrations for the “local ventilation with regional transport” (V-T) group and the “local stagnation with no regional transport” (S-nT) groups (e.g., PM2.5 dry of 53.3 and 34.4 μg m−3, respectively) were located between those of the S-T and VnT groups (Table 2 and Fig. 3f)
The composite difference between the regional-transport (V-T and S-T) groups and no regional-transport (V-nT and S-nT) groups reveals the regional effect of transported haze particles and precursors from the North China Plain and Yangtze River Delta, while that between the local-stagnation (S-T and S-nT) groups and local-ventilation (V-T and V-nT) groups shows the effect of accumulation or diffusion of both local and transported pollutants in the Seoul metropolitan area
Summary
Fine particulate matter (under 2.5 μm in diameter; PM2.5) in urban areas consists of inorganic species (SO24−, NO−3 , and NH+4 ) and organic matter (OM) produced by the gasto-particle conversion of anthropogenic and biogenic precursors (Seinfeld and Pandis, 2016). We explore the combined effects of local and remote sources, ALW and particle pH, and meteorological factors on the formation and growth of urban haze particles, based on daily measurement of PM2.5 chemical compositions in Seoul, backward trajectory analysis, and the ISORROPIA II thermodynamic model (Fountoukis and Nenes, 2007). Different chemical compositions and characteristics of Seoul haze according to the regional transport from China and the local stagnation in the Seoul metropolitan area is investigated from the perspective of inorganic partitioning and water uptake processes. An effective strategy for PM2.5 reduction in Seoul is further discussed using ambient NOx and NH3 levels and analytic calculation of the HNO3–NO−3 partitioning ratio as a function of ALW, pH, and temperature
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
