Evaluation of Vegetation Configuration Models for Managing Particulate Matter along the Urban Street Environment

Urban roadside greenery as a carbon sink: Systematic assessment considering understory shrubs and soil respiration

Roadside greenery is an efficient strategy for maximizing ecosystem services, including carbon sequestration in urban settings. However, the quantification of carbon sequestration is not comprehensive because understory shrubs and soil respiration have not been thoroughly considered. We developed an integrated methodology that combined field measurements and greenhouse incubation to comprehensively assess carbon sequestration in roadside greenery systems. The system was defined as an 8 m long section comprising a single tree (Zelkova serrata), 79 shrubs (Euonymus japonicus), and soil. Annual carbon uptake by a tree was estimated using an allometric equation derived from an official government report. For shrubs, carbon uptake was measured in the field by monitoring CO2 concentration change in the chamber enclosing the leaves and stems. Annual carbon uptake by shrubs was estimated by using the regression equation among carbon uptake, air temperature, and photosynthetically active radiation. We also estimated shrub root respiration by combining net primary production (NPP) from the greenhouse incubation and measured pruning effect in the field. This enabled us to differentiate heterotrophic respiration from the total soil respiration. The overall methodology accurately assessed net ecosystem production (NEP) from the roadside greenery system, which is 0.528 kg C m-2 yr-1. If this figure is extended to all roads in the target city, it can offset daily carbon emitted from the total registered passenger vehicles in the target city. Considering that shrubs sequester an amount equivalent to 29.3 % of the carbon sequestered by tree species, the current greenhouse gas inventory should include shrubs as an important carbon sink. As we also revealed that roadside soil has high carbon vulnerability, proper soil management is needed to enhance NEP. Our systematic approach evaluating the carbon balance within the roadside greenery system can be applied to other cities, contributing to enhance global understanding of urban carbon cycle.

Uneven PM2.5 dispersion pattern across an open-road vegetation barrier: Effects of planting combination and wind condition

The increased traffic-induced emissions contribute to the exacerbation of airborne particulate matter (PM) pollution. The vegetation barrier (VB) provides a means of reducing the traffic-induced pollutants. However, the effects of VB configuration and local environment on PM dispersion and reduction remain unclear, and thereby needs further advancement on VB design and characteristics. This study constructed a 3D numerical model based on field survey in an open-road VB of Shanghai urban area, and then simulated PM2.5 dispersion under various VB configurations and wind conditions. The results consolidated that the presence of the VB reduced PM2.5 concentration by over 15 % across the VB. A greater bush coverage (2/3 and more) reduces over 14 % more PM2.5 pollution across the VB than that for a greater arbor coverage, and reduces 6 % more PM2.5 pollution in the sidewalk canyon. Given a certain bush planting coverage, planting bushes in the windward area is beneficial to the overall PM2.5 reduction by approximately 4-14 %. The wind directions determine the overall pattern of PM2.5 dispersion across the VB plot, decreasing trends for perpendicular winds but fluctuating curves for parallel winds Wind velocities largely contribute to the changing rates of PM2.5 concentration, the increased wind speed from 1 m/s to 7 m/s accumulated 5-11 % more PM2.5 pollution across the VB plot. This study provides practical insights for effective VB designs in order to mitigate the PM pollution and the human's exposure to PM2.5 in urban open-road environments.

Effects of Green Network Management of Urban Street Trees on Airborne Particulate Matter (PM2.5) Concentration

Street trees are crucial for air pollutant reduction in urban areas. Herein, we used computational fluid dynamics (CFD) simulation to identify changes in airborne particulate matter (PM_2.5) concentration based on wind characteristics (direction and velocity) and the green network of street trees. The green network was assessed based on composition of the green area of street trees in the central reserve area and between the motor and pedestrian roads. The PM_2.5 concentration varied according to the presence or absence of major reserve planting and the planting structure of the street trees, but not according to the wind direction or velocity. The concentration was lower when the wind direction was 45° (than when the wind direction was 0°), whereas it showed a more significant decrease as the wind velocity increased. Despite variation at each measurement site, the PM_2.5 reduction was generally higher when the central reserve and street trees had a multi-planting structure. Hence, to ensure an effective reduction in the PM_2.5 concentration on motor roads and reduce its negative impact on pedestrians, both arbors and shrubs should be planted in the central reserve area. The study results will serve as reference for managing the green area network and linear green infrastructure in terms of improving the atmospheric environment.

Aging-friendly cities: Investigating the effects of street usage on the psychological satisfaction of older adults in megacities

The psychological satisfaction of older adults is an important evaluation standard for the construction of elder-friendly cities. Meanwhile, as important space carriers carrying the travel activities and social participation of older adults, streets are also of great significance to improve psychological wellbeing. However, few studies pay attention to the street usage of aging population, especially in the context of megacities. Moreover, the previous literature rarely distinguishes the types of streets. Thus, employing a mixed approach, we investigate the relationships between street usage and psychological satisfaction for older adults. Based on the survey in Shanghai, we find that a clear role for different usage indicators in the determination of subjective psychological satisfaction of older adults. More specially, the street usage and psychological satisfaction for older adults are strongly correlated, especially for living streets. Psychological satisfaction for older adults in different types of streets is not always positively related to the positive perception of street usage. The psychological satisfaction of different streets depends on different factors. By focusing on the case of a megacity, our study emphasizes the differences between different types of streets, which will be conducive to the proposal of practical planning policies. In addition, employing mixed research methods not only explains how different street usage affects the psychological welfare of older adults on a macro scale, but also emphasizes the inner world of respondents.

Impact of Different Combinations of Green Infrastructure Elements on Traffic-Related Pollutant Concentrations in Urban Areas

Urban air quality is a major problem for human health and green infrastructure (GI) is one of the potential mitigation measures used. However, the optimum GI design is still unclear. The purpose of this study is to provide some recommendation that could help in the design of the GI (mainly, the selection of locations and characteristics of trees and hedgerows). Aerodynamic and deposition effects of each vegetation element of different GI scenarios are investigated. Computational fluid dynamics (CFD) simulations of a wide set of GI scenarios in an idealized three-dimensional urban environment are performed. In conclusion, it was found that trees in the middle of the avenue (median strip) reduce street ventilation, and traffic-related pollutant concentrations increase, in particular for streets parallel to the wind. Trees in the sidewalks act as a barrier for pollutants emitted outside, specifically for a 45° wind direction. Regarding hedgerows, the most important effect on air quality is deposition and the effects of green walls and green roofs are limited to their proximity to the building surfaces.

Assessment of Air Quality and Meteorological Changes Induced by Future Vegetation in Madrid

Nature-based solutions and green urban infrastructures are becoming common measures in local air quality and climate strategies. However, there is a lack of analytical frameworks to anticipate the effect of such interventions on urban meteorology and air quality at a city scale. We present a modelling methodology that relies on the weather research and forecasting model (WRF) with the building effect parameterization (BEP) and the community multiscale air quality (CMAQ) model and apply it to assess envisaged plans involving vegetation in the Madrid (Spain) region. The study, developed within the VEGGAP Life project, includes the development of two detailed vegetation scenarios making use of Madrid’s municipality tree inventory (current situation) and future vegetation-related interventions. An annual simulation was performed for both scenarios (considering constant anthropogenic emissions) to identify (i) variations in surface temperature and the reasons for such changes, and (ii) implications on air-quality standards according to EU legislation for the main pollutants (PM10, PM2.5, NO2 and O3). Our results suggest that vegetation may have significant effects on urban meteorology due to changes induced in relevant surface properties such as albedo, roughness length or emissivity. We found a net-heating effect of around +0.18 °C when trees are introduced in dry, scarcely vegetated surfaces in the city outskirts. In turn, this enhances the planetary boundary layer height (PBLH), which brings about reductions in ambient concentrations of relevant pollutants such as NO2 (in the range of 0.5–0.8 µg m−3 for the annual mean, and 2–4 µg m−3 for the 19th highest 1 h value). Conversely, planting new trees in consolidated urban areas causes a cooling effect (up to −0.15 °C as an annual mean) that may slightly increase concentration levels due to less-effective vertical mixing and wind-speed reduction caused by increased roughness. This highlights the need to combine nature-based solutions with emission-reduction measures in Madrid.