[Characteristics and Reactivity of Ambient VOCs in Urban Hangzhou, China]

The ambient concentration of 122 volatile organic compound (VOC) species were continuously measured in urban Hangzhou, China from May 2018 to April 2019. The average mixing ratio of VOCs was (59.4±23.6)×10^-9 and the oxygenated VOCs (OVOC) were the largest component. There was no clear "weekend effect" in urban Hangzhou, while the concentration of VOCs had a sharp decrease during long holidays. The concentration of VOCs had a positive correlation with air quality index (AQI) and reached the highest level when the primary pollutant was PM_2.5. The assessment results of atmospheric chemical reactivity with·OH radical loss rate (L^·OH) and ozone formation potential (OFP) showed the average value of L^·OH was 7.5 s^-1 and that of OFP was 152.1×10^-9, among which carbonyl compounds, aromatics, and alkenes were the most abundant components. The overall chemical reactivity level of VOCs in Hangzhou was equivalent to 2-methylpentane. The average value of toluene/benzene (T/B) was 1.95, which implied the ambient VOCs in Hangzhou were influenced by vehicle exhaust. Secondary formation (17.6%), combustion (11.8%), industrial processing (12.3%), solvent use (18.1%), biogenic source (4.5%), and vehicle exhaust (35.7%) were identified as six major sources of VOCs in Hangzhou through the positive matrix factorization (PMF) model.

[Characteristics and Reactivity of VOCs in Hangzhou During a Typical Photochemical Pollution Episode]

An intensive observation of ambient volatile organic compounds (VOCs) was carried out in Hangzhou, a key city in the Yangtze River Delta, during a typical photochemical pollution episode from September 14-23, 2018. The analysis results of 80 effective samples showed that the average concentration of 122 compounds of VOCs was (59.5±19.8)×10^-9 during the observation period, and oxygenated VOCs (OVOCs) were the most abundant component. The assessment results of atmospheric reaction activity with ozone formation potential (OFP) showed that the average value of OFP was 145×10^-9 during the observation period, of which alkenes and carbonyl compounds were the most abundant components. The chemical reactivity of VOCs in Hangzhou was equivalent to acrylonitrile. Based on the positive matrix factorization (PMF) model, five major sources of VOCs in Hangzhou were identified, including secondary formation (25.2%), combustion and industrial processing (27.2%), solvent use (17.3%), biogenic sources (9.2%), and vehicular exhaust (21.2%). The results can provide guidance for further understanding of VOC characteristics and the basis for scientific prevention and control measures in Hangzhou.

[Space maintenance for the premature loss of the second primary molar]

Space reduction after the premature loss of the second primary molars is one of the important factors affecting the occurrence of malocclusion, often increasing the demand for orthodontic treatment. It has great significance to select and apply appropriate appliances to maintainthe space soon after the premature loss of the second primary molars. The space should be maintained until the adjacent teeth and the successors erupt successfully. This review summarizes the selection and clinical application of the space maintainer for the premature loss of the second primary molars in different periods, to improve dentists' awareness of the importance of space maintainers and to provide advises for clinical choices.

[Characterization of Volatile Organic Compounds from Cooking Emissions]

Volatile organic compounds (VOCs) do great harm to human health, and also have some impact on air quality. Cooking is one of the important sources of VOCs, so the study of cooking emissions is of great significance. By simulating the heating of oil and cooking, the characteristics and chemical composition of VOCs emissions for different types of oil fumes were analyzed by gas chromatography-mass spectrometry (GC-MS), using different oils, seasonings, and dishes as variables. The results show that the emission factors of the oils range from 0.81 to 2.53 g·kg^-1, and the emissions are dominated by halogenated hydrocarbons and alkanes. The emission factors of the seasonings range from 25.06 to 40.18 g·kg^-1, and the seasonings mainly emit alkanes. The quantity of emissions from chili fried meat is much higher than that of tomato scrambled eggs, and the chili fried meat mainly emits halogenated hydrocarbons, while tomato scrambled eggs mainly emit aromatic hydrocarbons and alkanes.