Urban green spaces: A key urban infrastructure for air and acoustic quality

1 Urban green spaces: A key urban infrastructure for buffering air pollution Guillermo Rey-Gozalo Laboratorio de Acústica (Lambda), Departamento de Física Aplicada, Instituto Universitario de Investigación para el Desarrollo Territorial Sostenible (INTERRA), Escuela Politécnica, Universidad de Extremadura (University of Extremadura), Avda. de la Universidad, s/n, 10003 Cáceres, Spain Abstract: Reducing both physical and chemical air pollution is a primary goal for creating sustainable and healthy cities. While advancements have been made in reducing chemical pollutants, noise pollution remains a significant challenge. Urban design and planning, particularly through the development of green spaces, offer a promising avenue for mitigating both issues. Green spaces not only contribute to significant reductions in local and urban air pollution but also create high-quality acoustic environments that benefit human health and well-being. To maximize these benefits, a holistic approach is needed that integrates strategies for addressing both air and noise pollution, considering factors such as human perception and wildlife conservation. Future urban planning should prioritize green infrastructure as a key tool for creating more sustainable and resilient cities. Keywords: urban green spaces, noise pollution, quiet areas, areas of high acoustic quality, soundscape, air pollution, urban planning, urban design. 2 1. Introduction In the European Union (EU), 300,000 premature deaths are caused by fine particulate matter annually [1]. Regarding noise, 12,000 premature deaths are caused by long-term exposure to environmental noise and 22 million people suffer chronic high annoyance [2]. Air pollution levels are not only high in Europe, 6.7 million deaths worldwide in 2019 from exposure to ambient and household air pollution [3]. EU Action Plan “Towards a Zero Pollution for Air, Water and Soil” was adopted by European Commission in 2021 to reduce air, water and soil pollution by 2050 to levels that are not considered harmful to health and natural ecosystems [4]. Zero pollution targets for 2030 include reducing by more than 55% the health impacts (premature deaths) of air pollution and reducing by 30% the share of people chronically disturbed by transport noise. Air pollution reduction was 45% between 2005 and 2022 while noise levels have not changed between 2012 and 2017 in EU member countries (see Figure 1 and Figure 2). This air pollution reduction is also observed in other continents as shown in Figure 1 resulting from the implementation of different actions by different countries: BreatheLife program (Accra, Ghana), Green Bangkok 2030 project (Bangkok, Thailand), Plan Aire 2030 (Bogota, Colombia), etc. Regarding noise, the number of people exposed to harmful noise levels has not decreased significantly between 2007 and 2017 (see Figure 2). However, this trend must be interpreted with caution because of issues of comparability of methodologies [5]. Other countries have also registered a similar trend to European countries in this range of years [6]. [Insert Figure 1; Figure 1: Carbon dioxide emissions per capita from fossil fuels and industry [7].] [Insert Figure 2; Figure 2: Number of people exposed to unhealthy Lden (dBA) levels inside and outside urban areas in Europe [2].] Zero Pollution EU Plan foresees a likely reduction of 66% in air pollution by 2030 (based on a number of proposed measures). However, a likely maximum reduction of 19% in the number of people chronically disturber by noise could be achieved between 2017 and 2030 [8]. Projected growth in transportation (road, rail and air traffic) and population will outweigh the benefits of implementing measures. A conservative scenario for 2030 considering the implementation of current regulations and a small increase in mitigation measures following current trends, the number of people chronically disturbed by transport noise could even increase by 3% [5]. Now, countries need to assess exposure to noise using the same calculation methods (CNOSSOS-EU) in order to improve robustness and comparability between the different EU member states [9]. In addition, the procedures for assessing the effects on human health have been harmonized in Annex III of the Environmental Noise Directive (END) [10]. The report published by the European Commission in 2023 [11] shows a more systematic assessment of noise levels and the adoption of noise management action plans but the number and intensity of actions must be increased. 3 The main actions proposed for noise reduction focus on the source [12]. Noise control measures in the propagation path and at the receiver or other measures involving landuse planning and design can also achieve high noise reductions [13]. In this context, the effective design of green infrastructure can act as a key tool to address environmental and social challenges [14]. There is no universal definition of urban green space, as it varies depending on the nuance and specific context. Green urban space is defined by European Environment Agency (EEA) as “a plot of vegetated land separating or surrounding areas of intensive residential or industrial use and devoted to recreation or park uses” [15]. US Environmental Protection Agency (EPA) defines green area as “land that is partly or completely covered with grass, trees, shrubs, or other vegetation which includes parks, community gardens, and cemeteries” [16]. The Management and Quality Commission of the Spanish Association of Public Parks and Gardens uses the concept of urban green infrastructure. This is defined as a strategically planned network of high quality natural and semi-natural areas with other environmental elements, designed and managed to provide a wide range of ecosystem services and protect the biodiversity of both rural and urban settlements. Urban green infrastructure includes river space, coastline, forest park, urban park, garden, historical park, square, tree-lined street, urban flower bed, green roof, green façade, urban orchard, sports area, botanical garden, zoological park, cemetery, municipal nurseries, public facilities and educational centers with green areas and undeveloped municipal lots. Over half the world’s population currently resides in cities and nearly 70 per cent of the global population projected to reside in cities by 2050 [17]. Access to green and public spaces is one of the priorities on which policies and practices should focus to make cities more inclusive, safe, resilient and sustainable (Goal 11 of the UN 2030 Agenda). However, three out of four cities worldwide dedicate less than 20% of their area to open public spaces, and only 44% of urban residents have access to an open public space within 400 meters [17, 18]. Urban public space accessibility varies very significantly from less developed countries (less than 30% of citizens) to high-income regions such as Australia, New Zealand, Europe and North America (60 to 70% of citizens). Eighty-four percent of Madrid's population resides within 200 meters of green spaces larger than 0.1 ha, and almost the entire population lives within 750 meters of parks larger than 0.5 ha (see Figure 3). [Insert Figure 3; Figure 3: Urban green spaces in Madrid, Spain [19, 20].] Accessibility to green spaces is a priority given the multiple environmental, social and economic benefits they can deliver [21]. These benefits are the pathways to improved health and well-being [15, 22]. Good health and well-being is Goal 3 of the UN 2030 Agenda. Improved mental health and cognitive function, reduced cardiovascular morbidity, reduced prevalence of type 2 diabetes, better pregnancy outcomes and reduced mortality are some of the evidence of the health benefits of green areas [14, 15]. Other social benefits of green spaces include a greater sense of community and enhanced social cohesion [23, 24]. Although there are a large number of studies that consider social 4 cohesion as a variable of interest in green spaces, few define and measure it. Clarke et al. [24] show that the perception of safety, level of maintenance, accessibility and efforts to include a diversity of users are important for social cohesion in green spaces. Various studies show an increase in housing prices near green areas [25, 26]. While this price rise reflects recognition of their environmental and health benefits, it can also lead to challenges related to equity and accessibility [27, 28]. The health benefits of green spaces also contribute to reduced healthcare costs [29]. Furthermore, green spaces design, installation, and maintenance generate employment opportunities across a broad spectrum of levels of education and professional experience [21]. Environmental benefits of green spaces are particularly important given the current emphasis on reducing urban pollution and the key role these areas could play in achieving this goal. Green areas improve climate resilience and reduce the heat island effect [21]. Taking urgent measures to combat climate change and its impacts (Goal 13 of the UN 2030 Agenda) is a priority for all countries worldwide. The impacts of climate change can be profound, leading to increasingly extreme and unpredictable weather events, as well as rising sea levels. Bowler et al [30] provided evidence of the cooling effect through observational studies in a small number of green spaces. Despite numerous studies linking green spaces and climate change, evidence on their design to improve this relationship is still limited [31]. Green infrastructure can also be designed to absorb and filter stormwater, thereby improving water quality, reducing localized flooding, and increasing groundwater reserves [21, 32]. Access to clean water is one of the fundamental needs for health care and well-being (Goal 6 of the UN 2030 Agenda). In 2022, approximately half of the global population faced severe water scarcity for at least one month during the year [17]. By enhancing water quality, green infrastructure also improves habitats in aquatic environments [21]. Constructed wetlands represent a green infrastructure solution for sustainable urban water management. In addition to contributing to the previously mentioned ecosystem services, constructed wetlands can provide habitats for wildlife [33]. Small green spaces provide habitats for birds, mammals, amphibians, reptiles, and insects [34, 35]. Climate change has accelerated the decline of amphibians [36]. Creating or protecting their habitats would also contribute to mitigating climate change. Preserving the life of terrestrial ecosystems is the UN's Goal 15. The expansion of urban green spaces leads to an increase in bird populations in cities [37], with bird diversity serving as an indicator of urban quality of life [38]. Birds, as sound sources, can contribute to enhancing the naturalness and quality of the sound environment in urban green spaces, an environmental physical variable associated with population well-being [14]. Among the environmental benefits provided by urban green spaces, one of the most significant—given the health issues caused by these pollutants—is their ability to reduce air pollution. Improving air quality is also a goal of the United Nations' Sustainable Development (Goal 11). Particulate air pollution is a major contributor to the global 5 disease burden [39]. Air quality is improving worldwide but still far from the levels recommended by the World Health Organization (WHO), especially in developing countries as shown in Figure 4. Therefore, WHO has established incremental steps in air pollution reduction to be used in areas of high pollution [40]. [Insert Figure 4; Figure 4: Average PM2.5 (μg/m3) concentrations in urban areas from 2015 to 2019 compared with Air Quality Guideline level (AQG) and Interim Targets (IT) recommended by the WHO [17, 40].] The air quality management plan of the city of Accra (Ghana) promotes the development of green spaces under the BreatheLife campaign. Universities in the United Kingdom have developed a guide for designing green spaces in London to reduce public exposure to air pollution [41]. Urban green spaces offer a promising approach to address air quality problems. Deposition, dispersion, and modification are the three PM mitigation mechanisms that take place in these green spaces [42]. There is some controversy about the efficiency of green areas in reducing air pollution [43, 44], because their efficiency depends on different factors: type and density of vegetation, design and location of green areas, ventilation, maintenance, local environmental conditions, etc [45]. Therefore, the air pollution reducing effects of green spaces are varied depending on the scale, context and characteristics of the vegetation [42]. There is no single design strategy, but adequate and well-located green infrastructure can efficiently reduce exposure to air pollution [41]. In fact, a poor vegetation layout can even lead to poor air quality [42]. Green space design and characteristics influence the quality of its sound environment just as they do air pollution [14]. This potential benefit of green areas has not been relevant until recently. In fact, most of the green areas have not been designed to have a quality sound environment [46]. Urban spaces close to large infrastructures have become green areas with an exclusive function of attenuating sound levels [47]. This affects their use and the benefits of these green areas. In addition to being potential quiet areas, green areas can also be environments of great sound quality [15]. These sound oases should be considered in the cities of the future to compensate for the impossibility of improving the acoustic situation in other urban environments. Considering the above, an analysis of the potential benefits of green spaces on air quality and the acoustic environment—two major urban stressors—becomes highly relevant. Therefore, both are the main objectives of the present study. 2. Urban green quiet areas The negative effects of noise on health are very significant, similar to other air pollutants, and this has led to a growing concern and interest in reducing this pollutant, especially in urban environments [2]. Exposure to high sound levels or prolonged exposure to noise affects health in various ways, causing hearing loss, stress, sleep disorders, cardiovascular problems, decreased cognitive performance, and an increased risk of diseases, negatively impacting both physical and emotional well-being [48–52]. In 2017, across Europe, 6 39,800 individuals were affected by ischemic heart disease, and 10,600 deaths were attributed to premature mortality caused by traffic noise [53]. Road traffic was the primary source of unhealthy noise levels as shown in Figure 2. Railways and aircraft are significant sources of noise at the local level, despite their lower overall impact on the population. To reduce noise pollution at both global and local scales, various strategies and measures are being implemented in areas such as urban planning, legislation, technology, and public awareness and education. The WHO recommends exposure limits for noise levels to mitigate negative health effects [54]. However, these noise exposure limits may be considered ambitious [55]. Numerous countries have established legislation to regulate noise in specific areas, during certain time periods, or for particular activities. The development and application of accurate, standardized methods for noise assessment are critical for analyzing and comparing the acoustic environment and evaluating the effectiveness of implemented mitigation or conservation measures [9, 11]. With regard to technology, the development of electric vehicles, low-noise tires, quiet pavement surfaces, noise barriers, and building insulation are among the most common measures implemented, resulting in a significant reduction in sound levels [13]. Environmental education, particularly for children, is regarded as a key measure for building healthier and quieter cities in the future [56]. Various international programs have been developed to raise awareness among children and parents about the effects of noise [57, 58]. A recent EEA report highlights that transportation noise significantly impacts children's reading comprehension and behavior difficulties [59]. Urban design and planning are key strategies for reducing urban noise, despite the challenges involved in quantifying their effects in some cases [13]. The segregation of residential urban areas from high acoustic impact zones, such as industrial zones, airports, or major roadways, offers significant advantages in minimizing noise exposure levels [60, 61]. Urban land uses can have a significant correlation with noise levels. In fact, regression models based on land use characteristics are frequently applied to estimate noise levels across various urban areas [62, 63]. This acoustic zoning is also currently used to establish low-noise emission zones. Many of these low-emission zones were initially created to reduce air pollution; however, given the relationship between both types of pollutants [64], the measures implemented also benefit the acoustic environment [65]. Restricting the circulation of internal combustion vehicles and reducing speed limits are among the most common measures applied in these low-emission zones. Another common approach in urban design is the promotion of pedestrian streets and green infrastructure. The wide range of environmental benefits of green areas have been mentioned above, including noise reduction [14, 15]. The END aims not only to reduce noise levels but also to protect and establish quiet areas [66]. Urban green spaces are potential quiet areas [14]. In fact, green areas, forests, conservation areas, water features, natural monuments, moors, and agricultural lands are the most commonly used land indicators for quiet [67]. Thus, quiet areas are closely 7 linked to green spaces, as evidenced in the city of Nantes (see Figure 5). Nantes was awarded the title of European Green Capital in 2013. [Insert Figure 5; Figure 5: Green (a) and quiet areas (b) in Nantes [19, 68].] Most urban green areas have not been designed as quiet areas. Their size, location, the presence of certain features, etc., have resulted in some cases where they provide a healthy acoustic environment. Additionally, through certain projects or urban renovations, measures (such as acoustic barriers, quiet pavements, etc.) have been implemented to improve the acoustic environment of these urban areas [69]. Given the multiple environmental benefits of green spaces [15], many cities have implemented continuous programs to increase their green areas by using non-buildable zones or converting major roads into green spaces [70]. These green spaces have also served as connections to pedestrianized urban streets and areas where traffic has been restricted due to their tourist appeal [71]. As a result, pedestrians are progressively replacing vehicular traffic, especially in urban centers with high levels of tourism, commerce, and entertainment activity [72]. The European Commission annually awards the title of European Green Capital for cities over 100,000 inhabitants and the European Green Leaf for smaller cities as of 20,000 inhabitants, recognizing those that have implemented bold actions and fostered local pride in advancing towards a greener and more sustainable future. These awards are important because urbanization emerges as a dominant force driving the decline of urban green spaces [73]. Different types of green spaces were sampled in the city of Madrid (Spain) for the R&D&I project “Sounds and green spaces for healthy and sustainable cities (SANATUCITY)” [74]. Most of the green spaces located in peripheral neighborhoods (parks from 9 to 15 in Figure 6) exhibited an average daytime equivalent sound level within the range of 45–55 dB, which aligns with the criteria for potential quiet areas [66]. These peripheral urban residential areas are newly developed and are characterized by wide streets and avenues. The Ana Tutor park (park 11 in Figure 6), due to its geometric layout and the presence of natural barriers, has low sound levels despite its proximity to a major urban road. Similarly, Vaguada park (park 12 in Figure 6) is close to a main road, but this urban road is underground. However, Monte las Tablas (park 13 in Figure 6), a leveling near the M30 highway, exhibits sound levels exceeding 55 dB. Parks 3 and 4 share a similar location and origin. These parks act as buffers, mitigating the sound levels from this major roadway in relation to the nearby residential areas. The botanical gardens (parks 16 and 17 in Figure 6) can be considered quiet areas [75]. These results are of particular interest due to their frequent use by tourists and residents for the study of plant species. The parks located in the city center, except for Retiro park (park 1 in Figure 6), have average sound levels above 55 dB. Buen Retiro is one of the main green areas in Madrid, covering 118 hectares. Avenues with green infrastructure (parks 2 and 7 in Figure 6) and small parks (parks 5, 6, and 8 in Figure 6) have the highest sound levels. This type of green infrastructure is the most complex to isolate from road traffic noise. However, the functionality of urban roads close to green spaces is an important urban planning factor 8 that can influence the surrounding acoustic environment [71, 76]. A clear example can be seen when comparing the sound levels in parks 14 and 15 with those in parks 2 and 7. [Insert Figure 6; Figure 6: Sound levels measured in green spaces in Madrid, Spain [74].] The size and shape of green spaces significantly influence their sound levels. As shown in Figure 6, large parks with regular shapes, even when situated in central urban areas with high vehicular traffic, contain zones characterized by low noise levels. Regular and linear shapes are the most prevalent forms of urban green spaces. The functionality of urban roads is a key factor to consider in the analysis of sound levels [77, 78] and is closely associated with urban planning [76]. Careful urban design of building facades and street geometry can reduce noise exposure levels by more than 10 dB [79]. On the other hand, locating green areas in landscape depressions results in noise level reductions of 6– 7 dB [80]. The Príncipe and Buen Retiro parks are two of the main green areas in the cities of Cáceres and Madrid, respectively. Despite being surrounded by urban roads with similar traffic flow, Príncipe park, located in a landscape depression, registered lower noise levels than Buen Retiro park [81]. Numerous cities around the world, including Madrid and Nantes (see Figure 5 and 6), have a significant proportion of green areas with noise levels harmful to human health. [82, 83]. A variety of measures can be implemented to mitigate noise levels. Elevated berms or dense vegetation bordering the park are some of the most commonly used natural barriers. Berms are low-cost and provide theoretical reductions of approximately 3 dB [84]. Experimental studies have shown similar attenuation to concrete or metal barriers [85], although influenced by ground type. Artificial barriers such as walls, shops, or cafes, commonly found in some green spaces, also contribute to noise attenuation, especially when located in border areas and near roads with high traffic flow [86]. A belt of trees bordering an urban road can significantly reduce noise levels if specific guidelines are followed [87]. Sound insulation is primarily achieved through the combination of the basal area of the trees and the presence of acoustically soft ground [88]. Uncompacted ground where leaf litter is not removed allows for greater noise reduction [89]. Thus, forest ground can reduce traffic noise by up to 3 dB more than grassland ground, using the noise generated by a light vehicle traveling at 70 km/h as a reference [90]. Zhang et al [91] quantified noise reductions up to 10 dB higher in forest grounds with tree or shrub vegetation compared to grass-covered grounds. A 15-meter deep tree belt with a basal area of 1% produces 3–5 dB reductions for a nearby 4-lane road with a uniform flow of light vehicles at a speed of 70 km/h [92]. Similar noise reductions are reported by Ow and Ghosh [93], who employed trucks and ambulances with sirens as noise sources. The attenuation of noisy events is of interest due to the lack of studies evaluating them and their negative effects on health [94, 95]. Increasing the basal area to 2% results in a noise reduction of 4 to 8 dB [92]. Consequently, the noise attenuation achieved by this green belt is comparable to that of a thin 2-meter concrete barrier. High basal area percentages can hinder light, nutrient, and water penetration 9 required for tree growth. To optimize tree health and noise reduction, rectangular planting arrangements with increased spacing, row omission, and selective thinning are recommended [88]. Shrubs can be used as an alternative to trees for noise reduction. A 2meter-high shrub zone with a 15-meter depth and a total aboveground dry biomass of 4 kg/m² produces an average noise level reduction of 4.7 dB from a light vehicle traveling at 70 km/h [90]. However, tree belts are more efficient in attenuating road traffic noise due to their predominance in low and medium frequencies, while shrubs can attenuate high-frequency noise [96]. Tree density and soil type are not the only factors that significantly influence the sound attenuation capacity of green belts. Other attenuating factors include leaf type, tree crown, and trunk. The presence of leaves in the tree crown increases absorption at high frequencies but also the source-receiver angle [97]. Therefore, seasonal variations can significantly affect both leaf attenuation in trees and the absorptive properties of the ground [89, 98]. Higher ground moisture content during winter implies an increase in surface impedance [89]. While Jamaludin et al. [79] found that larger, thicker leaves exhibit higher sound absorption capabilities, reaching 5 dB, Sağlam [99] demonstrated that leaf roughness and apparent density are more influential factors in sound attenuation than leaf thickness. Leaf vibrations can serve as an additional mechanism for attenuating ambient sound. Yang et al. [97] reported leaf attenuation at both high and very low frequency ranges, which they attributed primarily to leaf vibration [100]. While leaf size, mass, and thickness are well-known factors influencing leaf vibration, petiole dimensions also play a significant role [100]. Notably, coriaceous leaves exhibit higher vibration amplitudes compared to succulent leaves Beyond the mere presence of leaves, an increase in the total crown projection area and the mean height of the crown centroid was found to enhance noise level attenuation, as reported by Zhao et al. [101]. These two crown characteristics proved to be more influential than NDVI in estimating noise levels [101]. Conversely, Van Renterghem et al. [90] observed negative effects of tree crown dispersion, with reductions between 0.4 and 0.8 dB for a light vehicle traveling at 70 km/h, given the low source and receiver heights. The acoustic properties of tree bark are influenced by factors such as roughness and the presence of mosses. Coniferous trunks, in particular, exhibit superior sound absorption compared to other species [102]. 3. Acoustic environment quality in urban green spaces A milestone in Environmental Acoustics was the END's requirement to protect quiet areas [103], as this not only mandated the management of these zones but also took into account public opinion [104]. However, the definition of a quiet area can be misleading, particularly given the strong interest in reducing noise levels [4]. In fact, quiet is rarely desired for these areas and often induces fear [104, 105]. For this reason, the term “areas of high acoustic quality”, as suggested by Brown [104], would have been more appropriate. The term “quiet areas” was subsequently defined in the “Good practice guide 16 influenced by reflection, diffraction, dispersion, and absorption that occur on different urban surfaces [161, 162]. Buildings often behave as hard surfaces and shield sound propagation. Noise reduction is greater than that experienced by air pollution [64]. The arrangement of buildings disperses air pollutants, favoring the development of a canyonlike vortex that increases the concentration of air pollutants [163]. With respect to meteorology, atmospheric pollutants are more affected by wind and temperature at small scales than urban noise [64]. In fact, most noise maps consider standard meteorological conditions [164]. As seen in previous chapters, green belts are efficient barriers for both noise and air pollutants. Density, thickness, height, and plant type follow similar criteria for reducing both pollutants, although in the case of air pollution, it is necessary to select species with low VOC emissions. Conifers are species that have been analyzed for both types of pollutants and have shown significant reductions [45, 92, 165]. An advantage of noise reduction in green areas compared to air pollutants is the presence of sound sources and natural elements that improve its perception and mask unwanted sounds, thus improving its quality. 6. Conclusions Green spaces are a key urban infrastructure for achieving healthy and sustainable cities due to their multiple environmental, social, and economic benefits. Proper design and planning of these green areas can contribute to the reduction of noise and air pollution, two major urban environmental stressors. There is some controversy about the efficiency of green areas in reducing air pollution at the urban or regional scale, but this is because the degree of reduction at this scale is not as significant as at the local scale, and in addition, green infrastructure must be complemented by other urban actions to achieve significant improvements. Regarding to noise, the problem is even greater because, unlike air pollutants, the reduction actions carried out in different countries have not achieved significant reductions. Green spaces are not only an urban infrastructure that can attenuate noise but, much more importantly, can provide high-quality sound environments. For this, human perception and the protection of wildlife are key factors to consider when managing high-quality acoustic areas. Green spaces should not be designed independently to reduce noise or air pollution, but rather we should continue to advance towards integrated decisions that benefit the reduction of these two pollutants, as well as other environmental or urban problems. Today we have the tools to carry out multidisciplinary actions that involve a comprehensive improvement of urban environments, necessary to achieve sustainable development. 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