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The Vietnamese government has issued several directives of lockdown in Hanoi, the capital city, for a month (1–30 April 2020) to prevent the human-to-human transmission of COVID-19. This action has affected air pollution due to a decline in transportation. Therefore, this study investigates spatial and temporal change in the PM2.5 concentrations during the first 4 months of 2020 in Hanoi metropolitan area. Spatial distribution maps of the PM2.5 concentration in Hanoi were provided for the first time. The average PM2.5 concentrations at the 22 air monitoring stations were strongly correlated with the population. April had a significantly lower level of PM2.5 than the other 3 months. In particular, the concentrations of PM2.5 and NO2 decreased by 12 and 54%, respectively, between March and April, especially at areas for commercial activities. In April, a higher level of PM2.5 was recorded between Tuesday–Thursday, which is a reverse trend with that in March. Furthermore, Monday and Friday did not show rush hour peaks for the PM2.5 levels in April. A decrease in the PM2.5 concentrations was partly influenced by the long-range transport from the outside to Hanoi. This study implies that a reduction in traffic volumes and public activities may possibly improve the air quality. [ABSTRACT FROM AUTHOR]

PM2.5 samples were collected from a semi-rural site in the industrial city of Ulsan, South Korea during 2018–2019 to characterize the chemical composition and determine source contributions on polluted and normal days. The daily PM2.5 concentration exceeded the United States National Ambient Air Quality Standards (35 µg/m3) on 11 of the 56 days. The mean concentration of Σ9 water-soluble inorganic ions (WSIIs) was 17.1 and 6.07 µg/m3 on polluted and normal days, respectively, with SO42–, NO3–, and NH4+ the dominant species. The elements were dominated by Mg and Fe, with a mean concentration of Σ11 elements of 1973 and 855 ng/m3 on polluted and normal days, respectively. The mean concentration of Σ21 polycyclic aromatic hydrocarbons (PAHs) was 4.44 and 3.26 ng/m3 on polluted and normal days, respectively, indicating a higher human health risk on polluted days. High PM2.5 concentrations were due to an increase in the concentration of almost all WSIIs, elements, and PAHs on polluted days. Potential sources of PM2.5 were identified using positive matrix factorization (PMF), the conditional bivariate probability function (CBPF), and the potential source contribution function (PSCF). According to PMF results, secondary inorganic aerosols (39.8%), primary sulfate (18.2%), and coal/biomass burning (16.3%) were dominant on polluted days, whereas marine sources (23.1%), secondary inorganic aerosols (19.3%), and primary sulfate (16.2%) were the largest contributors to PM2.5 on normal days. Results from the CBPF and PSCF indicate that polluted days may be influenced by both long-range atmospheric transport (LRAT) and local emission sources, while LRAT became more important on normal days. Based on these findings, other studies are necessary to further investigate and distinguish the PM2.5 source contributions in industrial and urban areas. [ABSTRACT FROM AUTHOR]

This study identifies the emission source areas for the atmospheric polycyclic aromatic hydrocarbons (PAHs) detected in Ulsan, South Korea. To achieve this, in addition to a conditional bivariate probability function (CBPF), two hybrid receptor models – the three-dimensional potential source contribution function (3D-PSCF) model and the 3D concentration weighted function (3D-CWT) model – were used, both of which adopt trajectory segments within the mixing layer. Notably, the fraction-weighted trajectory (FWT), a combination of PAH gas/particle partitioning with a hybrid receptor model, was introduced for the first time in this study to support the identification of emission source areas using other approaches (i.e., 3D-PSCF, 3D-CWT, and CBPF). Consequently, it was found that gaseous PAHs in Ulsan mostly originated from local emission sources (i.e., transportation and industrial emissions) throughout the year, whereas particulate PAHs were likely to originate from emission sources in China (e.g., Shandong, Hebei, and Liaoning) during spring and winter via long-range transport. However, in summer and fall, the influence of local emissions on particulate PAHs appeared to be stronger. The FWT was able to distinguish between local and distant sources more effectively, especially in summer and fall, i.e., the periods when local sources increased their contribution. This study thus increases the understanding of the long-range transport of PAHs in Northeast Asia, and the novel FWT approach exhibits the potential to be employed in the source area identification of various semi-volatile organic chemicals. [ABSTRACT FROM AUTHOR]

Nitrated and oxygenated polycyclic aromatic hydrocarbons (NPAHs and OPAHs) are receiving attention because of their high toxicity compared with parent PAHs. However, the experimental data of their physicochemical properties has been limited. This study proposed the gas chromatographic retention time (GC-RT) technique as an effective alternative one to determine octanol-air partition coefficients (K OA) and sub-cooled liquid vapor pressures (P L) for 11 NPAHs, 10 OPAHs, and 19 parent PAHs. The slopes and intercepts of the linear regressions between temperature versus K OA and P L were provided and can be used to estimate K OA and P L for the 40 targeted compounds at any temperature. The internal energies of phase transfer (Δ U OA) and enthalpies of vaporization (Δ H L) for all targeted compounds were also calculated using the GC-RT technique. High-molecular-weight compounds may release or absorb higher heat energy to transform between different phases. NPAHs and OPAHs had a non-ideal solution behavior with activity in octanol (γ oct) in the range of 19–53 and 18–1,078, respectively, which is larger than the unity threshold. A comparison among four groups of PAH derivatives showed that a functional group (nitro-, oxygen-, chloro-, and bromo-) in PAH derivatives increased γ oct for corresponding parent PAHs by tens (mono-group) to hundreds of times (di-group). This study suggests that the GC-RT method is applicable for indirectly measuring the physicochemical properties of various groups of organic compounds. [Display omitted] • The GC-RT technique was applied to determine K OA and P L for 11 NPAHs and 10 OPAHs. • Applicability of the GC-RT method was verified with various alternative techniques. • High-molecular-weight or di-functional-group compounds had higher K OA and lower P L. • High activity in octanol (γ oct) shows that N/OPAHs had a non-ideal solution behavior. • Attachment of diverse functional groups increased γ oct for corresponding parent PAHs. [ABSTRACT FROM AUTHOR]

Evaluation for the controlling policy's effectiveness to mitigate criteria air pollutants (CAPs) in South Korea during December 1, 2019–March 31, 2020 is difficult because of its coincidence with the COVID-19 social distancing. In this study, we differentiated the influence of three major driving factors (intensive controlling policy by the government, meteorological conditions, and social distancing) to the CAP variation in Ulsan, the largest industrial city in South Korea. In 2013–2019, the concentrations of PM 2.5 (2015–2019), PM 10 , SO 2 , and NO 2 decreased by 6.7, 1.6, 4.2, and 3.3%/year, respectively, whereas the O 3 concentration slightly increased by 0.7%/year. Trend analysis was used to predict the CAP concentrations before (January 1–February 21) and during (February 22–March 31) the social distancing in 2020. The difference between the measured and predicted concentrations was designated as the contribution of the three factors. The controlling policy was the most important driver of the CAP reductions. In particular, its contributions were 94.1% (January 1–February 21) and 87.4% (February 22–March 31) to the PM 2.5 decrease. The change in meteorological conditions considerably affected the CAP reductions, with the highest contributions of 35.2% (January 1–February 21) and 39.2% (February 22–March 31) to the O 3 decrease. On February 22–March 31, the effects of social distancing were 1.6, 0.6, 1.3, and 1.4% to the reduction of SO 2 , NO 2 , PM 10 , and PM 2.5 , respectively. Overall, a decrease in the CAP concentrations was apparent during January–March 2020 in Ulsan primarily due to the intensive controlling policies, not by the COVID-19 social distancing. [Display omitted] • Concentrations of air pollutants, except for O 3 , decreased in Ulsan during 2013–2019. • A controlling policy played a vital role in reducing air pollutants in Jan–Mar 2020. • The contribution of controlling policy to the PM 2.5 and PM 10 decrease was 81–94%. • Meteorological conditions contributed 19–39% of the air pollutant reduction. • The pollutant decrease was affected by the social distancing on Feb 22–Mar 31, 2020. [ABSTRACT FROM AUTHOR]

In the present study, 11 size classes of particulate matter were collected from a semi-rural site in the industrial city of Ulsan, South Korea in 2019 to investigate the size distribution and day–night variation of water-soluble inorganic ions (WSIIs). Approximately 70% of the detected WSIIs were found in fine particles, with Na+, Ca2+, Cl−, and NO 3 − dominant (~70%) in coarse particles and SO 4 2−, NO 3 −, and NH 4 + (SNA) dominant (~70%) in fine particles. Monthly variation in total WSIIs was observed, with the highest average concentration found in April for both coarse (2.71 μg/m3) and fine particles (5.75 μg/m3). Given that this month is characterized by prevailing southeasterly winds, these high levels may represent the adverse effects of industrial activity. Coefficients of divergence for individual WSIIs exhibited large variability, which is indicative of significant day–night differences in meteorology, chemistry, and source contributions. The size distributions of Na+, K+, F−, Cl−, NO 3 −, and SO 4 2− were bimodal, while Mg2+, Ca2+, and NH 4 + were unimodal. Peaks were generally found at 1.8–5.6 μm and 0.18–0.56 μm for coarse and fine particles, respectively. By plotting the linear relationship between the mole charge ratios of SNA, we found that coarse NO 3 − particles were mainly the result of heterogeneous reactions between HNO 3 and sea salts or crustal species, while the homogeneous reaction of HNO 3 and NH 3 played a crucial role in the formation of fine NO 3 − particles. In conclusion, though day–night variation in WSIIs was apparent, especially in SNA, the formation pathways for both coarse and fine particles were similar for day and night. Based on the verification of the formation mechanisms for WSIIs at this semi-rural site, the influence of industrial activity on the secondary formation of PM in urban and industrial areas can be investigated further. Unlabelled Image • The influence of industrial activitiy on the secondary formation of PM was investigated. • Day–night variation in the concentration and size distribution of WSIIs was apparent. • The formation of secondary inorganic aerosols was similar between day and night. • Reaction of HNO 3 and sea salts or crustal species was dominant in coarse mode. • NH 3 played a crucial role in fine NO 3 − formation by homogeneous reaction with HNO 3. [ABSTRACT FROM AUTHOR]

Atmospheric halogenated polycyclic aromatic hydrocarbons (Halo-PAHs) and parent PAHs were monitored in Ulsan, South Korea for one year (January‒December 2015) to investigate their seasonal patterns, gas/particle partitioning behavior, and the impact of meteorological conditions. The mean concentrations of 24 chlorinated PAHs, 11 brominated PAHs, and 13 parent PAHs in the gaseous and particulate phases were 8.64 and 9.64 pg/m3, 11.6 and 1.62 pg/m3, and 2.17 and 2.40 ng/m3, respectively. Winter had the highest ClPAH and PAH levels, with significant contributions from poly-chlorine groups and high-molecular-weight compounds. However, BrPAHs showed reverse patterns with the highest concentration in summer and the dominant gaseous fraction throughout the year. This finding could be explained by the strong local sources of BrPAHs, related to automobile and petrochemical industries. In contrast, the effects of the temperature inversion layer and atmospheric transport from the outside of Ulsan were more apparent for ClPAHs and PAHs, particularly in winter and spring. Regarding gas/particle partitioning, Halo-PAHs exhibited different seasonal behaviors from those of parent PAHs. The sorption pathway of Halo-PAHs seemed to shift from absorption as the sole dominant mechanism in winter and spring to both adsorption and absorption in summer and fall, while both partitioning mechanisms contributed equally for parent PAHs during the entire year. This study implies that Halo-PAHs and parent PAHs might not share the same atmospheric behavior, possibly due to different characteristics in atmospheric reactions with other chemicals and particle-size distribution. However, there have been limited studies about the formation of Halo-PAHs and their physicochemical properties; hence, further in-depth investigations are of vital importance. Image 1 • Winter had the highest ClPAH and PAH levels with significant contributions of HMWs. • Gaseous BrPAHs reached the maximum concentration in summer due to local sources. • The influence of LRAT on ClPAHs and PAHs was more apparent than on BrPAHs. • TILs were more frequent in winter, leading to higher levels of ClPAHs and PAHs. • The gas/particle partitioning mechanisms for Halo-PAHs were seasonally different. [ABSTRACT FROM AUTHOR]