. High levels of fine (PM2.5) and ultrafine (PM0.1) particles in the atmosphere can cause adverse effects on the environment and human health. This study aims at determining the mass concentrations of PM and meteorological influencing factors in high pollution event days in Hanoi. Daily samples of PM2.5 and PM0.1 were collected at a mixed site in Hanoi, Vietnam, from the middle of October to December 2020. High pollution events were determined based on PM2.5 concentrations and analyzed for its average concentration and intensity (number of days). The regression analysis and correlation matrix were determined by R software, version 4.04. Seven high pollution events were recorded for 2.5 sampling months. The daily PM2.5 concentrations were in the range of 19 - 147 µg/m3. Those of PM0.1 varied from 2 to 13 µg/m3 with an average of 6 µg/m3. The investigated meteorological factors can explain 70% PM2.5 variation but only 47% of PM0.1 variations.

A composite of TiO2 quantum dots (TiO2QDs) and TiO2 nanoparticles (TiO2NPs) was successfully deposited on glass beads (GBs), with a very low content of TiO2 (<0.3 wt%), by a rapid, simple, inexpensive, and scalable synthesis method. The TiO2QDs-TiO2NPs/GBs catalyst possesses an extremely high photocatalytic performance under simulated solar irradiation, with the complete degradation of the aqueous solutions of methylene blue in 60 minutes of photodegradation. There were no changes in the catalytic activity after six times of recycling. The superior performance of the catalyst should be attributed to the role of the synergistic effect between TiO2NPs and TiO2QDs to prevent the accumulation of TiO2 nanoparticles during the oxidation reaction, to improve the absorption in the visible region, to enhance the immigration of photogenerated electrons and holes in interfaces of two materials in the TiO2QDs-TiO2NPs composite, which leads to a reduced charge recombination rate, and to generate midgap states and suppress the recombination of electron-hole pairs.