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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]

Abstract 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.

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]

The causes of haze formation in Beijing, China were analyzed based on a comprehensive measurement, including PBL (planetary boundary layer), aerosol composition and concentrations, and several important meteorological parameters such as visibility, RH (relative humidity), and wind speed/direction. The measurement was conducted in an urban location from Nov. 16, 2012 to Jan. 15, 2013. During the period, the visibility varied from > 20 km to less than a kilometer, with a minimum visibility of 667 m, causing 16 haze occurrences. During the haze occurrences, the wind speeds were less than 1 m/s, and the concentrations of PM 2.5 (particle matter with radius less than 2.5 μm) were often exceeded 200 μg/m 3 . The correlation between PM 2.5 concentration and visibility under different RH values shows that visibility was exponentially decreased with the increase of PM 2.5 concentrations when RH was less than 80%. However, when RH was higher than 80%, the relationship was no longer to follow the exponentially decreasing trend, and the visibility maintained in very low values, even with low PM 2.5 concentrations. Under this condition, the hygroscopic growth of particles played important roles, and a large amount of water vapor acted as particle matter (PM) for the reduction of visibility. The variations of meteorological parameters (RH, PBL heights, and WS (wind speed)), chemical species in gas-phase (CO, O 3 , SO 2 , and NOx), and gas-phase to particle-phase conversions under different visibility ranges were analyzed. The results show that from high visibility (> 20 km) to low visibility (< 2 km), the averaged PBL decreased from 1.24 km to 0.53 km; wind speeds reduced from 1 m/s to 0.5 m/s; and CO increased from 0.5 ppmv to 4.0 ppmv, suggesting that weaker transport/diffusion caused the haze occurrences. This study also found that the formation of SPM (secondary particle matter) was accelerated in the haze events. The conversions between SO 2 and SO 4 as well as NOx to NO 3 − increased, especially under high humidity conditions. When the averaged RH was 70%, the conversions between SO 2 and SO 4 accounted for about 20% concentration of PM 2.5 , indicating that formation of secondary particle matter had important contribution for the haze occurrences in Beijing. [ABSTRACT FROM AUTHOR]

Abstract: Seasonal average observational data, photochemical modeling analyses, and detailed observational data for a summer air pollution episode were used to assess the spatial and temporal distribution of ammonia-rich and ammonia poor conditions in southeast Texas. Seasonal average observational data and photochemical modeling analyses both suggest ammonia rich conditions throughout southeast Texas, but not enough ammonia present to create elevated concentrations of ammonium nitrate, except during morning hours. Analyses of ambient data available for the summer air pollution episode (which was also the modeled episode) indicate that ammonia-poor conditions may be more pervasive in southeast Texas during periods of intense photochemistry. Overall, the results of all three methods are consistent in suggesting that average ammonia availability in southeast Texas is comparable to the ammonia required to neutralize sulfate. Temporal and spatial variability in sulfate formation creates morning excesses of ammonia (leading to some morning nitrate formation) and ammonia poor conditions in SO2 plumes and during periods of high photochemical activity. [Copyright &y& Elsevier]

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]