Green Infrastructure

Green infrastructure (GI) refers to the incorporation of vegetation in urban areas, such as parks, green roofs, and trees. In our rapidly urbanising world, GI is often implemented to mitigate air pollution, since these elements are capable of absorbing and filtering pollutants from the atmosphere [1].

Plants are able to reduce air pollution through two main ways: deposition and dispersion. Deposition, also referred to as air phytoremediation, occurs when plants absorb nutrients from the atmosphere through their stomata. Subsequently, the plants either store these pollutants or neutralise them via metabolic processes [2]. In order to enhance the extent of pollutant deposition on plant surfaces, several factors must be accounted for, such as the leaf area index, proximity to pollution source, and the coarseness of specific pollutants [3].  However, research indicates that dispersion might be a more valuable tool for reducing the direct impact of air pollution on city-dwellers through creating what are known as “vegetation barriers” which restrict the extent and concentration of pollutants that people come in contact with [4]. This is depicted in Figure 1, whereby Hewitt et al (2020) found that deposition dominates for distance d₁ , mixing dominates for d_2, , and that pollutant concentration decreases exponentially between these two.  In the absence of the hedge, one is exposed to a higher concentration of pollutant, c₀, over a shorter distance, d_0.

Unfortunately, green infrastructure comes with its limitations, particularly owing to how it may aggravate the very atmospheric problems it seeks to combat. For instance, trees may contribute to air pollution through releasing biogenic volatile organic compounds (bVOCs). These compounds react with free radicals to form secondary particles which remain suspended in the ambient air for extended periods of time [5]. For example, the outgassed bVOC isoprene tends to react with NO_X in the presence of sunlight to form ground-level ozone which in turn produces photochemical smog in urban areas [6].

However, the interplay between urban vegetation and ozone remains complex (Fig 2), since trees also remain a major sink for ozone and can help regulate its levels in urban areas [7]. Evidently, comprehensive knowledge of species-specific characteristics of plants is essential to understand what type of vegetation best suits different urban areas. Several factors come into play during this assessment, including: sensitivity to environmental stressors, indigeneity, and the balance between species’ ozone deposition capacity and BVOC emission rate [8].

Therefore, while developing green infrastructure, it is vital to account for the impact of urban vegetation on ozone formation to ensure that the project is combatting the negative impacts of air pollution as opposed to exacerbating them. Crucially, reducing emissions at the source remains of utmost priority, and though GI offers many benefits, it should be integrated into a comprehensive approach targeting multiple factors to mitigate air pollution.

References

[1] Omasa, K., Saji, H., Youssefian, S., and Kondo, N. (eds.). (2012). Air Pollution and Plant Biotechnology: Prospects for Phytomonitoring and Phytoremediation, Springer Science & Business Media.

[2] Omasa, K., Saji, H., Youssefian, S., and Kondo, N. (eds.). (2012). Air Pollution and Plant Biotechnology: Prospects for Phytomonitoring and Phytoremediation, Springer Science & Business Media.

[3] Janhäll, S. (2015). Review on urban vegetation and particle air pollution – Deposition and dispersion. Atmospheric Environment, 105, 130–137. https://doi.org/10.1016/j.atmosenv.2015.01.052

[4] Wesołowska, M., & Laska, M. (2019). The use of green walls and the impact on air quality and life standard. E3S Web of Conferences, 116, 00096. https://doi.org/10.1051/e3sconf/201911600096

[5] Kumar, P., Druckman, A., Gallagher, J., Gatersleben, B., Allison, S., Eisenman, T. S., Hoang, U., Hama, S., Tiwari, A., Sharma, A., Abhijith, K. V., Adlakha, D., McNabola, A., Astell-Burt, T., Feng, X., Skeldon, A. C., de Lusignan, S., & Morawska, L. (2019). The nexus between air pollution, green infrastructure and human health. Environment International, 133, 105181. https://doi.org/10.1016/j.envint.2019.105181

[6] Churkina, G., Kuik, F., Bonn, B., Lauer, A., Grote, R., Tomiak, K., & Butler, T. M. (2017). Effect of VOC Emissions from Vegetation on Air Quality in Berlin during a Heatwave. Environmental Science & Technology, 51(11), 6120–6130. https://doi.org/10.1021/acs.est.6b06514

[7] Hardin, P. J., & Jensen, R. R. (2007). The effect of urban leaf area on summertime urban surface kinetic temperatures: A Terre Haute case study. Urban Forestry & Urban Greening, 6(2), 63–72. https://doi.org/10.1016/j.ufug.2007.01.005

[8] Fitzky, A. C., Sandén, H., Karl, T., Fares, S., Calfapietra, C., Grote, R., Saunier, A., & Rewald, B. (2019). The Interplay Between Ozone and Urban Vegetation—BVOC Emissions, Ozone Deposition, and Tree Ecophysiology. Frontiers in Forests and Global Change, 2. https://doi.org/10.3389/ffgc.2019.00050

Hewitt, C. N., Ashworth, K., & MacKenzie, A. R. (2019). Using green infrastructure to improve urban air quality (GI4AQ). Ambio. https://doi.org/10.1007/s13280-019-01164-3