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Road traffic is an important source of urban air pollutants. Due to increasingly strict controls of exhaust emissions from road traffic, their contribution to the total emissions has strongly decreased over time in high-income countries. In contrast, non-exhaust emissions from road vehicles are not yet legislated and now make up the major proportion of road traffic emissions in many countries. Brake wear, which occurs due to friction between brake linings and their rotating counterpart, is one of the main non-exhaust sources contributing to particle emissions. Since the focus of brake wear emission has largely been on particulate pollutants, little is currently known about gaseous emissions such as volatile organic compounds from braking and their fate in the atmosphere. This study investigates the oxidative ageing of gaseous brake wear emissions generated with a pin-on-disc tribometer, using an oxidation flow reactor. The results demonstrate, for the first time, that the photooxidation of gaseous brake wear emissions can lead to formation of secondary particulate matter, which could amplify the environmental impact of brake wear emissions. [ABSTRACT FROM AUTHOR]

This study has found that the wintertime (November–March) air quality in Seoul, Korea had been deteriorated in weekends during the period of 2016–2019. Specifically, the concentration of particulate matters (PMs) of aerodynamic diameter less than 2.5 μm (PM2.5) in weekends (Saturday–Sunday) was up to 30% higher than that in the mid-week (Wednesday–Thursday) days (probability value < 0.01). As the weekend PM concentration had been lower than the mid-week values by 10% until 2015, such a sudden change in the intra-weekly air quality is unexpected. This study finds out that the deterioration of air quality in weekends can be attributed primarily to secondary particle formations and external transports from China (Shandong and northeast provinces) and domestic provinces (southern Gyeonggi and Chungcheong in Korea) to Seoul. High concentration of PM2.5 on weekend could be partially explained by the differences in the concentrations of inorganic PM components including nitrate, ammonium, and sulfate between weekends and mid-week days. About 40% of the differences are attributed to the domestic sources located in the southern region to Seoul. However, domestic emission from power generations and industry sector in southern source region on weekends does not well explain the variations of the PM precursors in weekends. Therefore, a clear strategy for improving air quality on the weekend in Seoul requires steady efforts to accurately calculate regional emissions and to reveal missing emissions sources. [ABSTRACT FROM AUTHOR]

Experiments were carried out to investigate the effects of surface charge density on emulsion kinetics and secondary particle formation in emulsifier-free seeded emulsion polymerization. Three monodisperse seed latices with different surface charge densities were prepared from styrene/NaSS comonomers using the two-stage shot-growth process. After purification of the seed latices, they were used in seeded emulsion polymerization of methyl methacrylate. The initial rate of polymerization and the average number of radicals per particle for the high-charged seed latex system were lower than that of the low-charged case. The low rate of polymerization resulted from the low rate of radical adsorption in the beginning of the reaction due to the electrical repulsion between seeds and oligomeric radicals. In this case, because of the secondary particles, particle size distribution became bimodal. The low rate of radical adsorption and the formation of secondary particles reduced the average number of radicals per particle. The rate of polymerization (Rp) increased, but the rate of polymerization per particle (Rp/Np) decreased. [ABSTRACT FROM AUTHOR]