Expected improved air quality

The Bosque Vertical do Centro Cultural de Belém (CCB) in Lisbon (Vertical Forest at CCB), designed by Francisco Caldeira Cabral, is an innovative vertical garden inaugurated in 2020. Spanning two 17-meter-high walls with 7,524 plant pots, it enhances urban biodiversity by attracting birds and insects (Ref 3). This project offers recreational and aesthetic benefits, making the entrance of the cultural centre into a green space (Ref 4).
The garden helps reduce air pollution by absorbing pollutants and producing oxygen. It uses a drop-by-drop irrigation system made from recycled PVC, ensuring efficient water usage and retention (Ref 1). Furthermore, the garden improves air quality, aims to mitigate urban heat island effects, and conserves water (Ref 2, Ref 5).

Iasi has limited access to green-blue open spaces and fails to meet national and European standards for per capita green space and proximity to larger green areas. Additionally, the city experiences significant fragmentation in its green spaces. To address these issues, the municipality of Iasi launched a project in 2020 to restore a vacant and degraded area, transforming it into a park. Funded by European resources, the project aims to rehabilitate the neglected land by constructing a green park and introducing local vegetation. The primary goal is to provide a leisure and relaxation space for local citizens and visitors while improving the quality of the surrounding air by converting much of the terrain into green space. The vacant land is situated in the historic Ticau neighborhood, which has been affected by municipal neglect, characterized by poor infrastructure and limited connectivity to the city's main areas. Furthermore, the project aims to reduce the heat island effect by creating green oases in a city marked by many “hard surfaces,” such as paved areas, which contribute to this phenomenon expected to intensify in the coming years. Originally scheduled for completion by the end of 2022, the project has been delayed due to a lack of funds and is now expected to be finished by the end of 2024 (Refs 1, 2, 3, 4, 6, 7, 11).

Lima Verde is one of the pillars of the metropolitan mayor's government plan and is supported by SERPAR. It aims to reforest hillsides, hills, and central berms of metropolitan and local roads, as well as public recreational spaces, to create urban lungs that generate oxygen and scenic beauty for the benefit of the health of all residents (Ref 1, 3). Its goal is to plant one million trees in four years (2022 - 2026) in different parts of North, East and South Lima (Ref 3, 5, 6). This project is being carried out to respond to the increasing problem of urban heat islands that are being generated, especially in the most vulnerable districts where there is no shade due to a shortage of trees (Ref 1, 4). It is therefore focused on areas where the environmental quality is low, where the pollution is high and where there is more danger of higher temperatures (Ref 1, 7).
This programme aims to include youth volunteers, district municipalities, educational institutions, grassroots social organisations, neighbourhood leaders, residents and other entities that favour environmental conservation (Ref 1, 2, 3, 5).

The Tbilisi Urban Forest is an ecological restoration project of 5,5 ha spanning two pilot areas initiated in 2020. Located on Mtatsminda ridge around Tbilisi, it is part of a larger reforestation effort within the 700-ha forest territory (Ref. 1,2). The project is funded by the Cartu Foundation and led by the Development and Environment Foundation along with Tbilisi City Hall. Restoration is undertaken to addresses significant challenges such as forest dieback caused by pest infestations, climate change adaptation as well as the creation of recreational infrastructure (Ref. 3,5). Through collaboration among interdisciplinary scientists and urban planners, a novel reforestation strategy has been developed that integrates patches of locally sourced plant communities tailored to diverse soil and slope conditions. This approach incorporates existing trees with new saplings, carefully grafted into existing soil pockets, and focuses on a biodiverse palette of endemic and climate-adapted species (Ref. 1,5). While the project implementation could not be carried out as planned, lacking public participartory due to Covid 19, still 100 jobs were created locally (Ref. 2). Ultimately, the Urban Forest project will enhance ecological connectivity, biodiversity and tourism and also serve the public as an improtant recreational site (Ref. 1,2).

The Monsanto Forest Park Management Plan focuses on enhancing the Monsanto Forest Park as an urban ecosystem, thereby improving ecological health and biodiversity while promoting sustainable resource use and integration with urban areas. Key interventions include managing invasive species, preventing forest fires, and supporting native habitats. The plan also aims to improve recreational and educational facilities, reduce automobile traffic, and enhance public accessibility. It aims to cover conservation efforts, recreational use and urban integration as well as promoting the park as a model for sustainable urban forestry. The implementation is overseen by the Lisbon City Council. (Ref 1,4).
In 2021, this historic urban forest received the Forest Management Recertification under the Forest Stewardship Council®, "the most important global certification in terms of environmental compliance of forestry operations (Ref. 6).

In the city of Cagliari, in the location of Mount Urpinu, 6400 new trees have been planted. The project, initiated in 2022, sought to restore the area's biodiversity and to revitalize the park for the benefits of the citizens (Refs. 1, 3 & 8). In fact, in previous years, the park has been subject to various environmental and social hazards because of its conditions: the area has been subject to wildfires and abandoned wastes, and with time, it became a gathering point for drug addicts and homeless people, thus becoming unsafe for citizens and especially for children (Refs. 1 & 6).
Therefore, the Metropolitan City of Cagliari decided to reclaim the area and plant thousands of trees of various species (holm oaks, oaks, pine, carob and olive trees) over more than 6 hectares of land (Refs. 3 & 8). The project's goal is to restore biodiversity, mitigate temperatures, decrease air pollution, and increase citizens' sense of community (Ref. 1).

Air pollution is one of the most pressing sustainability challenges in the UK, with particulate levels exceeding both WHO guidelines and EU safe limits in many cities (Ref 6). In 2014, Birmingham was identified by the WHO as one of nine UK cities breaching air pollution safety guidelines (Ref 2, 3).

In 2012, a soil-based green wall was installed at Birmingham New Street Train Station as part of a project aimed at improving air quality, as diesel train services contribute significantly to nitrogen oxide (NOx) pollution. However, most of the plants did not survive. In 2017, Network Rail commissioned a private company to reconstruct the wall using a hydroponic system (Ref 3). The new green wall was installed in 2018 and completed in May 2019 (Ref 1, 3).

This hydroponic green wall not only enhances the urban environment with calming green infrastructure, but also helps mitigate air pollution and promotes biodiversity with the addition of 16,600 plants (Ref 1, 2, 3).

The Rio Salado Habitat Restoration Area is a urban green space that transformed a former 5-mile-long city dump into a thriving riparian corridor (2, 3, 7, 9). The project, spearheaded by the City of Phoenix in collaboration with the U.S. Army Corps of Engineers, Maricopa County Flood Control District, and the Arizona Water Protection Fund, has revitalized a degraded ecosystem (9). Through decades of concerted effort, involving government agencies, non-profits such as the Arizona Sustainability Alliance, and community engagement, the area has been restored to a lush habitat supporting diverse wildlife, including over 200 bird species (1, 2, 7). The project aimed to mitigate the pressing issues of urban heat, air pollution, and habitat loss that Phoenix grappled with (1). By creating green spaces, improving air quality, and offering recreational opportunities, the Rio Salado Habitat Restoration Area stands as a testament to the power of collaborative conservation and community resilience (1, 6, 7, 9). The restoration area now boasts hiking and biking trails, a nature center, and various educational programs, making it a community space (3, 7, 9).