Computational fluid dynamics-based optimal installation strategy of air purification system to minimize NOX exposure inside a public bus stop

At public bus stops, NO_X pollutants discharged by regularly stopping buses quickly accumulate, exposing waiting passengers to high levels of air pollutants, which creates a threat to public health. The environmental protection agency (EPA) presents air quality standards for NO_X, a significant pollutant that causes lung diseases such as asthma when exposed to the human body. To handle this problem, air purification systems are installed inside bus stops in many public places. However, it is challenging to maintain a low concentration of NO_X inside public bus stops due to the persistent inflow of bus exhaust gas. Therefore, it is crucial to design an optimal location for an air purification system to meet air environment standards for respiratory areas. This study proposed a computational fluid dynamics (CFD)-based optimal installation strategy for an air purification system to minimize NO_X exposure inside a public bus stop. The CFD model was developed to numerically analyze NO₂ exposure with the actual design value for a public bus stop in Ulsan, South Korea. The local NO₂ concentration was evaluated in the human breathing zone. The case study was performed according to the locations of the inlet and outlet of the air purification system. A transient CFD simulation was performed to analyze the effect of the air purification system on pollutants generated from the stationary bus by time flow in various cases. NO₂ concentration and exposure reduction effectiveness (ERE) were analyzed and compared for each case in the breathing zone. In the optimal case, the ERE of NO₂ was confirmed to be 35.9 %, and the NO₂ concentration according to the air quality standards of EPA could be maintained at 0.1 ppm or less. The theoretical framework proposed in this study can be generalized to design air purification systems for general external facilities.

Bus stop design and traffic safety: An explorative analysis

One way to prioritize public transport over private vehicle mobility, is to implement curbside rather than layby bus stop designs. There is, however, uncertainty about the consequences of implementing curbside rather than layby stops for traffic collision risks. To begin investigating this issue, we describe an exploratory analysis in which national data describing bus stops, road properties, traffic collisions and built-up areas were merged based on geographical location. Analysis of the resulting data set suggests that the relative rates of traffic collisions resulting in personal injury within 60 m of the bus stop, is higher for curbside than for layby stops in built-up areas (0.32 vs. 0.22 collisions per ten million passing vehicles, respectively). Our analyses suggest that the higher risk of nearby collisions for curbside stops is not necessarily due to bus stop design, but rather because they tend to be located closer to junctions and side roads, where collisions are more likely. Our data are not consistent with hypotheses that curbside stops are associated with greater shares of head-on or rear-end collisions than layby stops, nor that layby stops are associated with greater shares of side-on collisions than curbside stops. The limitations of this exploratory analysis, and of the use of national-level data for studying the effects of bus stop design on collision risk, are related to lack of control of bus stop design features other than curbside vs. layby, statistical power, data registration and compromises made when coupling data based on geographical location. Future work should attempt to build on our approach, and supplement database analyses with analysis of in-depth reports of bus stop collisions, observations of road user conflicts near bus stops, and before-after studies following conversion from layby to curbside stops or from curbside to layby stops.

Cyclists' exposure to air pollution under different traffic management strategies

We characterized the air pollution exposure of cyclists in the city center of Curitiba (Brazil) and then systematically analyzed the influence of several traffic management strategies (bus lanes, bicycle lanes, traffic calming area, traffic lights, and cleaner vehicle technologies) on the exposure. We focused on concentrations of particulates monitored on-board bicycles: PM_2.5, black carbon mass (BC) and particle number concentration (PNC), and also reported on total volatile organic compound concentrations (TVOC). Overall, mean (± standard deviation) exposure was moderate compared to other cities around the world (BC: 6.98 ± 11.53 μg m^--3, PM_2.5: 33.22 ± 25.64 μg m^-3, PNC: 3.93 × 10⁴ ± 4.17 × 10⁴ cm^-3, TVOC: 361 ± 99 ppb). Concentrations were higher in the morning rush hour than in the afternoon traffic peak, and exhibited a large spatial variability. Bus stops and signalized traffic intersections emerged as hotspots when compared to the rest of the journey, increasing all particulate concentrations. Lower exposure was found on streets with low traffic (particularly, small number of heavy-duty vehicles) and within shallow canyon structures. The impact of traffic calming areas on cyclists' exposure is still inconclusive and further experimental and modelling studies are needed. Simple emission calculations based on traffic activity and real-world emission factors suggested that replacing the diesel bus fleet with hybrid electric buses might largely decrease (64%) the exposure to BC in the city center. Urban planners could use this valuable information to project new cycleways, which would lead to healthier active transportation. Synchronizing traffic signals might further reduce exposure at intersections.