Pollution Characteristics And Modeling Of Volatile Organic Compounds On Typical Urban Roads In Beijing

The high-speed growth of motor vehicles in the urban areas in China imposed a severe impact on the local air quality. Volatile organic compounds (VOCs) are a group of major pollutants in the atmosphere of the urban traffic roads. The levels of VOCs can be influenced by the traffic fleet profiles, street geometry and meteological conditions, therefore show some dynamics. Further understanding of the characteristics of the VOCs, their emission factors and the disperstion patterns would help much the effective control of the VOCs from road traffic.Non-methane hydrocarbons (NMHCs) as one of the major groups in VOCs as well as the group of benzene, toluene, Ethylbenzene and xylene as BTEX were chosen in this study for investigating the dynamics of NMHCs and BTEX in the three typical roads in Beijing and Guangzhou, namely open roads, intersections and street canyons. The results show that the levels of NMHCs and BTEX in the traffic atmosphere in Beiing in summer were noticeably higher than those in the other three seasons. The levels of NMHCs and BTEX followed the order of street canyons>intersections>open roads. Except for the two rush periods, the levels of NMHCs in summer also increased during the noon time. This can be attributed to an increase in the evaporation of the fuel with high temperatures near noon time. The levels of NMHCs in the summer in Guangzhou were notable higher than those in Beijing. This can be explained by the fact that the evaporation of NMHCs was enhanced with high temperatures in the summer of Guangzhou and the more sun light led to more emission of NMHCs from the plants. The levels of BTEX in Guangzhou was in the order of open roads>street canyons>intersections, differing from the order found in Beijing. This can be due to the selection of the main road as the open road in Guangzhou.Based on the results of the correlation analysis of the pollutants, the levels of NMHCs and BTEX had a close positive correlation, which was also ture among the BTEX components. The NMHCs and BTEX had close correlations with their backgrounds, suggesting that the local backgrounds contributed largely to tested concentrations. The correlation between the levels of the pollutants and the traffic volume was low, which was consistent with the B/T values of<1in most cases. The background values of BTEX in the winter decreased and its contribution dropped. The ratios of B/T were thus seen<1, suggesting that BTEX in that period was of a local origin.A parameter sensitivity-based simplified MOBILE6.2was used to calculate the emission factors of NMHCs and benzene for the traffic roads in Beijing, and then CALINE4and OSPM packages were adopted to model the dispersion of NMHCs and benzene in the road atmosphere. The results showed that, among the input parameters of MOBILE6.2package, calendar year, temperature, registration distribution, vehicle mileage traveled, arerage speed, I/M program and Reid vapor pressure were sensitive for the modeling of the emissions of NMHCs and benzene. Aromatic content, benzene content and E300were found to be sensitive parameters only for the calculation of benzene emission. The calculation results by using the simplified MOBILE6.2based on the parameter sensitivity analysis still had a high accuracy for the modeling of NMHCs and benzene.When using CALINE4for the modeling of the dispersion of a line-source pollution, traffic volume, wind speed and wind direction were the sensitive parameters in determining the dispersion of the air pollutants. The modeling results of NMHCs and benzene from CALINE4agreed well with the tested values. In terms of OSPM model, wind speed and direction was shown as sensitive parameters in the modeling of traffic emission dispersions in the street canyons. The correlation between the modeled NMHCs and benzene agreed well with the measured. When the concentrations of benzene in the control site increased, a positive deviation of the modeling value was expected.