How effective is ‘greening’ of urban areas in reducing human exposure to ground-level ozone concentrations, UV exposure and the ‘urban heat island effect’? An updated systematic review

Abstract

Background This review updates a systematic review published in 2010 (http://www.environmentalevidence.org/completed-reviews/how-effective-is-greening-of-urban-areas-in-reducing-human-exposure-to-ground-level-ozone-concentrations-uv-exposure-and-the-urban-heat-island-effect) which addressed the question: How effective is ‘greening’ of urban areas in reducing human exposure to ground-level ozone concentrations, UV exposure and the ‘urban heat island effect’? Methods Searches of multiple databases and journals for relevant published articles and grey literature were conducted. Organisational websites were searched for unpublished articles. Eligibility criteria were applied at title, abstract and full text and included studies were critically appraised. Consistency checks of these processes were undertaken. Pre-defined data items were extracted from included studies. Quantitative synthesis was performed through meta-analysis and narrative synthesis was undertaken. Review findings 308 studies were included in this review. Studies were spread across all continents and climate zones except polar but were mainly concentrated in Europe and temperate regions. Most studies reported on the impact of urban greening on temperature with fewer studies reporting data on ground-level UV radiation, ozone concentrations (or precursors) or public health indicators. The findings of the original review were confirmed; urban green areas tended to be cooler than urban non-green areas. Air temperature under trees was on average 0.8 °C cooler but treed areas could be warmer at night. Cooling effect showed tree species variation. Tree canopy shading was a significant effect modifier associated with attenuation of solar radiation during the day. Urban forests were on average 1.6 °C cooler than comparator areas. Treed areas and parks and gardens were associated with improved human thermal comfort. Park or garden cooling effect was on average 0.8 °C and trees were a significant influence on this during the day. Park or garden cooling effect extended up to 1.25 kms beyond their boundaries. Grassy areas were cooler than non-green comparators, both during daytime and at night, by on average 0.6 °C. Green roofs and walls showed surface temperature cooling effect (2 and 1.8 °C on average respectively) which was influenced by substrate water content, plant density and cover. Ground-level concentrations of nitrogen oxides were on average lower by 1.0 standard deviation units in green areas, with tree species variation in removal of these pollutants and emission of biogenic volatile organic compounds (precursors of ozone). No clear impact of green areas on ground level ozone concentrations was identified. Conclusions Design of urban green areas may need to strike a balance between maximising tree canopy shading for day-time thermal comfort and enabling night-time cooling from open grassy areas. Choice of tree species needs to be guided by evapotranspiration potential, removal of nitrogen oxides and emission of biogenic volatile organic compounds. Choice of plant species and substrate composition for green roofs and walls needs to be tailored to local thermal comfort needs for optimal effect. Future research should, using robust study design, address identified evidence gaps and evaluate optimal design of urban green areas for specific circumstances, such as mitigating day or night-time urban heat island effect, availability of sustainable irrigation or optimal density and distribution of green areas. Future evidence synthesis should focus on optimal design of urban green areas for public health benefit.

Parks Under Stress: Air Temperature Regulation of Urban Green Spaces Under Conditions of Drought and Summer Heat

“…In terms of air temperature differences between larger coherent green spaces (e.g., parks or cemeteries) and their built-up surroundings, our findings correspond to those of previous studies assessing temperature gradients, although, they are at the lower margin of the range of effect sizes. Bowler et al (2010) and Knight et al (2021) reported mean effect sizes of 0.94 and 0.8 °C, respectively. In our study, the largest average temperature difference of approximately 1 °C occurred in the morning between the large Friedenspark and the buffer zone of the Lene-Voigt-Park, i.e., not between the parks and their immediate surroundings.…”

“…In particular, urban green spaces help regulate the microclimate through shading, evapotranspiration and air flow regulation (Lehmann et al, 2014;Rahman et al, 2020;Meili et al, 2021). The potential of urban green spaces to regulate the microclimate through cooling was shown to be close to 1 °C during the daytime in parks and over 1.5 °C in urban forests with dense tree canopies compared to urban surroundings (Bowler et al, 2010;Knight et al, 2021). Dense vegetation cover, however, may reduce wind velocity and heat emission (radiative cooling) at night (Yuan et al, 2017;Knight et al, 2021).…”

“…Studies on the microclimate effect of urban green areas can be broadly categorized into empirical and modeling studies (Bartesaghi Koc et al, 2018;Knight et al, 2021). Empirical data are gathered through both remote sensing approaches and in-situ observations.…”

Analysis and Valuation of the Energy-Efficient Residential Building with Innovative Modular Green Wall Systems

“…The technological and organizational parameters were studied in the experiments with the thermal assessment of green buildings [3][4][5][6]. Most studies focus on the green systems assessment of the vegetation types, physical properties and thermal insulation effects of the green systems [4].…”

Modular Green Roofs for the Sustainability of the Built Environment: The Installation Process

“…Approximately 40.9% of the articles focus on regulating services in water, 30.0% on the thermal environment, and 3.5% on air quality [3]. Green roofs showed a surface temperature cooling effect [4,5]. The effects of urban heat islands [6] and global warming have led to an increased greening of cities as a tool for urban heat island mitigation in three main climate types: hot-humid, temperate, and dry [7][8][9][10][11].…”