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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]

This study quantified carbon storage and uptake for urban parks in Seoul, the capital of the Republic of Korea. A total of 38 study parks were selected using a systematic random sampling method and all the trees in the parks were field-inventoried. Carbon storage and uptake by the park trees were estimated applying a quantitative model for urban open-grown trees of each species. Mean carbon storage per unit of park area, basal area, and crown cover by the trees was 38.5 ± 3.0 t/ha, 27.3 ± 0.8 kg/100 cm2, and 7.4 ± 0.4 kg/m2, respectively. Annual carbon uptake per unit area and cover by the trees averaged 3.5 ± 0.2 t/ha/yr, 2.5 ± 0.1 kg/100 cm2/yr, and 0.7 ± 0.0 kg/m2/yr, respectively. The major determinants of the levels of carbon storage and uptake were species, density, sizes, and layering structures of the planted trees. The trees across all urban parks in Seoul were estimated to store 222.3 kt of carbon and to annually sequester 20.2 kt of carbon. The trees in these parks played an important role in annually offsetting carbon emissions from gasoline consumption by approximately 2.3% of the total population of the city. The economic value of the annual carbon uptake, which was $7.1million/yr, equaled 15.1% of the annual maintenance budget of the parks in the city. However, the role of study parks as a source of carbon uptake was limited due to the distribution of large grass and impervious areas, the single-layered structures, and the dominance of small trees. Planning strategies were explored to enhance carbon reduction effects of the parks. They included the expansion of tree planting spaces through the minimization of unnecessary grass and paving areas, the active tree planting in the potential planting spaces, the multi-layered planting grouped with larger trees, and the planting of tree species having satisfactory growth rates. This study puts an emphasis on finding out the present carbon offset levels of urban parks on which information is limited and suggesting a future direction of park planning based on a detailed actual survey. [ABSTRACT FROM AUTHOR]

Together with emissions of air pollutants and precursors, meteorological conditions play important roles in local air quality through accumulation or ventilation, regional transport, and atmospheric chemistry. In this study, we extensively investigated multi-timescale meteorological effects on the urban air pollution using the long-term measurements data of PM10, SO2, NO2, CO, and O3 and meteorological variables over the period of 1999-2016 in Seoul, South Korea. The long-term air quality data were decomposed into trend-free short-term components and long-term trends by the Kolmogorov-Zurbenko filter, and the effects of meteorology and emissions were quantitatively isolated using a multiple linear regression with meteorological variables. In terms of short-term variability, intercorrelations among the pollutants and meteorological variables and composite analysis of synoptic meteorological fields exhibited that the warm and stagnant conditions in the migratory high-pressure system are related to the high PM10 and primary pollutant, while the strong irradiance and low NO2 by high winds at the rear of a cyclone are related to the high O3. In terms of long-term trends, decrease in PM10 (-1.75µgm-3yr-1) and increase in O3 (+0.88ppbyr-1) in Seoul were largely contributed by the meteorology-related trends (-0.94µgm-3yr-1 for PM10 and +0.47ppbyr-1 for O3), which were attributable to the subregional-scale wind speed increase. Comparisons with estimated local emissions and socioeconomic indices like gross domestic product (GDP) growth and fuel consumptions indicate probable influences of the 2008 global economic recession as well as the enforced regulations from the mid-2000s on the emission-related trends of PM10 and other primary pollutants. Change rates of local emissions and the transport term of long-term components calculated by the tracer continuity equation revealed a decrease in contributions of local emissions to the primary pollutants including PM10 and an increase in contributions of local secondary productions to O3. The present results not only reveal an important role of synoptic meteorological conditions on the episodic air pollution events but also give insights into the practical effects of environmental policies and regulations on the long-term air pollution trends. As a complementary approach to the chemical transport modeling, this study will provide a scientific background for developing and improving effective air quality management strategy in Seoul and its metropolitan area. [ABSTRACT FROM AUTHOR]

In the absence of direct measurements of fine particles (particulate matter with aerodynamic diameters ≤ ∼2.5 μm; PM 2.5), visibility can be a useful alternative indicator of particulate air quality. This study estimated the current status and long-term trends of PM 2.5 in the North Korean capital, Pyongyang, using an empirical visibility–relative humidity (RH)–PM 2.5 relationship obtained from the South Korean capital, Seoul. The visibility–RH–PM 2.5 relationship was derived from PM 2.5 and meteorological variables for the period of 2003–2018 in Seoul, excluding days affected by hydrometeor conditions; it was applied to long-term meteorological data for the period of 1983–2018 in both Seoul and Pyongyang. In recent years (2016–2018), the average estimated PM 2.5 concentration in Seoul was ∼23 μg m−3, which was close to the measured concentration of ∼25 μg m−3. The average estimated PM 2.5 concentration in Pyongyang for the same period was ∼35 μg m−3. The estimated PM 2.5 concentration in Seoul was generally high before 2000 (∼34 μg m−3); it decreased beginning in the 2000s (−5.2 μg m−3 per 10 years) because of the emission control policy and meteorological phenomena. The estimated PM 2.5 concentration in Pyongyang significantly decreased until 1998 (−5.9 μg m−3 per 10 years), then increased from 1999 to 2018 (+8.5 μg m−3 per 10 years). The growth in gross national income, total primary energy supply, and coal-fired thermal electricity generation implied that the long-term changes in the estimated PM 2.5 concentration in Pyongyang were closely related to the economic contraction and recovery of North Korea. This study provides estimates of North Korean air quality, along with information that can be used to validate chemical transport models. [Display omitted] • Long-term PM 2.5 in Pyongyang, North Korea, was estimated based on visibility and RH. • Visibility–RH–PM 2.5 relationship derived from Seoul was applied to Pyongyang. • Recent PM 2.5 concentration in Pyongyang was estimated as ∼35 μg m−3 for 2016–2018. • Long-term trend in estimated PM 2.5 was related to economic contraction and recovery. [ABSTRACT FROM AUTHOR]

Understanding the geographic source contributions by particulate polycyclic aromatic hydrocarbons (PAHs) is important for the Korean peninsula due to its downwind location from source areas. Regional influence of particulate PAHs was previously identified using diagnostic ratios applied to mobile source dominated roadside sampling data (Kim et al., 2012b). However, no study has yet been conducted to quantify the regional source contributions. We applied a multivariate receptor modeling tool to identify and quantify the regional source contributions to particulate PAHs in Seoul. Sampling of roadside particulate PAHs was conducted in Seoul, Korea for four years between May 2005 and April 2009, and data analysis was performed with a new multivariate receptor model, Solver for Mixture Problem (SMP). The SMP model identified two sources, local mobile source and transported regional source, and quantified their source contributions. Analysis of the particulate PAHs data reveals three types of episodic periods: a high regional source contribution period with one case, a high mobile source contribution period with three cases, and a normal contribution period with eight cases. Four-year average particulate PAHs source contributions from the two sources are 4.6 ng m −3 and 10.7 ng m −3 for regional and mobile sources, respectively and equivalent to 30% and 70% of the total estimated contribution from each of these sources. [ABSTRACT FROM AUTHOR]

The large emission of volatile organic compounds (VOCs) is an important environmental issue in Seoul, the capital of South Korea. However, the spatial distribution of VOC emissions has not yet been clearly revealed. In this study, passive air samplers (PASs) were seasonally deployed to measure 50 VOCs at 26 urban, 4 road, and 6 background sites in Seoul. Among the target compounds, toluene (summer: 8.72 μg/m3; winter: 5.47 μg/m3), ethyl acetate (summer: 2.36 μg/m3; winter: 3.51 μg/m3), and ethylbenzene (summer: 1.72 μg/m3; winter: 0.88 μg/m3) exhibited higher mean concentrations than other compounds. Printing offices and industrial facilities seem to have strong influences on the VOC levels in the center and southwest of Seoul, respectively. Diagnostic ratios indicated that the main sources of VOCs were related to local volatilization in summer and vehicular exhaust and transport from other areas in winter. Positive matrix factorization (PMF) suggested four major sources of VOCs: residential/commercial solvents for printing and painting (Factor 1); transport from other areas (Factor 2); industrial solvents for painting, manufacturing, and cleaning (Factor 3); and gasoline/diesel vehicular exhausts (Factor 4). Factors 1 and 3 were dominant in summer, and Factors 2 and 4 were dominant in winter. In conclusion, the seasonal meteorological conditions (e.g., temperature and wind patterns) were found to play an important role in the spatial distribution of VOCs in Seoul, and solvent use was a predominant source. Image 1 • Passive air samplers were deployed at 36 sites in Seoul, Korea in summer and winter. • Toluene, ethyl acetate, and ethylbenzene were dominant compounds in both seasons. • The high concentrations of VOCs were observed in the center and southwest of Seoul. • The main source of VOCs in summer was the local volatilization of solvents. • Vehicular exhaust and transport from other areas were dominant in winter. [ABSTRACT FROM AUTHOR]

Seoul, the capital of South Korea, is one of the largest megacities in northeast Asia. Previous studies have monitored atmospheric polycyclic aromatic hydrocarbons (PAHs) in Seoul, but few have produced highly spatially resolved data. In this study, a total of 144 polyurethane foam-passive air samplers (PUF-PASs) were deployed at 36 sites (4 road, 26 urban, and 6 background sites) in Seoul during the summer of 2016 and the winter of 2017. The concentrations of Σ 13 PAHs fell within a range of 2.6–49.9 ng/m3 in summer and 15.5–77.8 ng/m3 in winter. Low-molecular-weight PAHs with 3–4 rings were dominant (accounting for about 97% of all PAHs) at all sampling sites in both seasons compared to high-molecular-weight PAHs with 5–6 rings. Phenanthrene was the most abundant individual compound, accounting for 35–54% and 42–52% of the total PAHs in summer and winter, respectively. The spatial distribution of PAHs was determined by traffic, domestic heating, topography, transport, and meteorological conditions. The concentration of PAHs at the road sites was higher than at the urban and background sites, indicating that vehicular exhaust was a major source in both seasons. In addition, diagnostic ratios and principal component analysis revealed the influence of various sources, such as gasoline and diesel emissions and coal and biomass combustion. Backward air trajectory analysis highlighted the influence of long-range transport from northern China and North Korea during the winter, whereas the influence of local emissions and satellite cities appeared to dominate during the summer. In conclusion, passive air sampling was successfully employed to identify major sources of PAHs in an Asian megacity, indicating that it could produce data with a high spatial resolution for use in human risk assessment and air monitoring networks. • Passive air sampling was conducted at 36 sites in Seoul, Korea in summer and winter. • The mean concentration of PAHs in winter was higher than that in summer. • Road sites exhibited higher levels of PAHs than urban and background sites. • The influence of long-range transport was evident during the winter. • The influence of local emissions and satellite cities was dominant during the summer. [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]