Hong-di He

Multifractal nature of particulate matters (PMs) in Hong Kong urban air

In this study, we investigate the persistent variation and the multifractal nature of particulate matter (PM) concentrations from vehicle emissions at a typical traffic intersection of street canyon in Hong Kong. Six size groups of PMs are measured and collected during rush hour sessions on different dates respectively. A recently developed model, namely multifractal detrended fluctuation analysis (MF-DFA), is employed to decompose and analyze the collected database. Through estimating the scaling exponent, it is found that the PM levels from vehicular emissions display long-term correlation characters. By employing MF-DFA method to calculate the generalized Hurst exponent and discuss the multifractal spectrums of all size groups, it is noticed that the fine particulate matters in grain diameter of 0.3-0.499 μm present strong multifractal nature, intensive oscillations of concentration variations, and long-term persistence. For fine particulate matters in the grain diameter ranges from 0.5 μm to 4.99 μm, their similar and weak multifractal natures reflect the self-similarity behaviors among these groups and the gradual decreases of the lasting effects. For large size particulate matters, i.e., grain diameter above 5 μm, certain mono-fractal nature and sharp decay of long-term persistence are obtained, even for intermittent effects. It can therefore be concluded that the fine particulate matter diffuses anomaly and persists for a long time.

Prevision of vehicle headway effect on urban traffic with a new car-following model

In this study, a new car-following model is established aiming to predict the variation of vehicle headways on urban road. The linear stability condition is derived corresponding to the prevision of headway in moving. The modified Korteweg–de Vries (mKdV) equation is deduced through the nonlinear analysis. The kink–antikink soliton solution of the mKdV equation can interpret the urban traffic jams near the critical point under the prevision of vehicle headway. Moreover, it is clear that the prevision of headway effect did improve the stability of urban traffic flow since the traffic jams are alleviated efficiently by taking into account the prevision of headway term in numerical simulations, which are consistent with the theoretical analysis.

Prediction of particulate matters at urban intersection by using multilayer perceptron model based on principal components

The time series of particulate matter at urban intersection consists of complex linear and non-linear patterns and are difficult to predict. Multilayer perceptron (MLP) model has been applied to air quality prediction in urban areas, but it has limited accuracy owing to the co-linearity between the input variables. To overcome it, a novel hybrid model combining MLP model and principal component analysis (PCA) is proposed to improve the prediction accuracy. The PCA was applied before the MLP model was implemented to generate principal components as input variables, rather than using the original data, to reduce the complexity and eliminate data co-linearity. The proposed model is examined with the measured data of particulate matter (PM) concentrations in spring and winter respectively. Experimental results indicated that the hybrid model can be an effective tool to improve the particulate matter prediction accuracy. Additionally, the proposed model is found to perform better for analyzing PM1 levels than PM10. The model is also verified to produce better predictions in winter than that in spring. The outputs of these findings demonstrate the potential of the proposed model to be applied to predict the trends of air pollution in similar meso- to mega- cities.

Multiscale multifractal properties between ground-level ozone and its precursors in rural area in Hong Kong

In rural area, due to the reduction of NOx and CO emitted from vehicle exhausts, the ozone photochemical reaction exhibits relatively weak effect and ozone formation presents different pattern with its precursors in contrast to urban situation. Hence, in this study, we apply detrended cross-correlation analysis to investigate the multifractal properties between ozone and its precursors in a rural area in Hong Kong. The observed databases of ozone, NO2, NOx and CO levels during 2005-2014 are obtained from a rural monitoring station in Hong Kong. Based on the collected database, the cross-correlation analysis is carried out firstly to examine the cross-correlation patterns and the results indicate that close interactive relations exist between them. Then the detrended cross-correlation analysis is performed for further analysis. The multifractal characters occur between ozone and its precursors. The long-term cross-correlations behaviors in winter are verified to be stronger than that in other seasons. Additionally, the method is extended on daily averaged data to explore the multifractal property on various time scales. The long-term cross-correlation behavior of ozone vs NO2 and NOx on daily basis becomes weaker while that of ozone vs CO becomes stronger. The multifractal properties for all pairs in summer are found to be the strongest among the whole year. These findings successfully illustrate that the multifractal analysis is a useful tool for describing the temporal scaling behaviors of ozone trends in different time series in rural areas.

Investigation of exhaust gas dispersion in the near-wake region of a light-duty vehicle

The air pollution induced by urban traffic has addressed much attention in recent years as the rapid urbanization and the fast increase of vehicle number in urban area. In this study, we attempted to investigate the dispersion behavior of exhaust gas from exhaust pipe using computational fluid dynamics approach. The time-averaged CO2 concentration, velocity and temperature profiles along the centerline of the vehicular exhaust plume were simulated in varied situations. The computational results showed good agreement with the experimental data and the numerical model was validated to be an effective method to investigate the pollutant dispersion in the near-wake region of a vehicle. Based on it, the numerical simulations were extended to explore the impacts of the emit concentration, the emit direction and the incoming velocity on the flow dynamics and CO2 dispersion. The outputs indicated that the emit concentration could change the pollution level in the near-wake region of a vehicle and the emit direction may alter the spreading direction of the vehicular exhaust plume. The incoming velocity was found to have dominant influence on the dispersion of pollutant due to induced vortices and turbulence behind the vehicle. These findings are expected to provide important insight into evaluating the design and control strategies for alleviating mobile source emissions.

Nonlinear analysis of a new car-following model accounting for the global average optimal velocity difference

In this paper, a new car-following model is proposed by considering the global average optimal velocity difference effect on the basis of the full velocity difference (FVD) model. We investigate the influence of the global average optimal velocity difference on the stability of traffic flow by making use of linear stability analysis. It indicates that the stable region will be enlarged by taking the global average optimal velocity difference effect into account. Subsequently, the mKdV equation near the critical point and its kink–antikink soliton solution, which can describe the traffic jam transition, is derived from nonlinear analysis. Furthermore, numerical simulations confirm that the effect of the global average optimal velocity difference can efficiently improve the stability of traffic flow, which show that our new consideration should be taken into account to suppress the traffic congestion for car-following theory.

Traffic Jam Formation in Traffic Flow on a Harbor Tunnel

A study on the occurrence and growth of traffic jams on a single-lane in the tunnel by using the optimal-velocity traffic model [1, 2, 3] is reported in this paper. At the low vehicle density, the current increases linearly with density and saturates at certain values of density range. As the vehicle density increases, the traffic jam appears firstly before the uphill section and extends to the downhill section with the increasing density. The relationships of velocity against position in different vehicle density are obtained from simulations, which clarifies clearly where and when the traffic jam occurs. We also derived the critical densities before and after the discontinuous fronts from the theoretical current curves. These values also comply with the results of simulation [4]. The study will be useful to provide suggestions for engineer to control the range of vehicle density in tunnel to avoid the traffic jam.