As the semester finally begins drawing close to an end, I wish to redirect this blog towards a more positive aspect of air pollution for my final few entries. After learning so much about the various harmful consequences of air pollution, wouldn’t it be nice and perhaps reassuring to also know what kinds of innovative actions have been taken towards redressing it or mitigating its impacts on us?
Kumar et al. (2019) highlights green infrastructure (GI) as an efficient solution to air pollution that will work in tandem with the rapid global urbanisation that will soon render the world devoid of greenery. However, they also remind us that GI are not entirely foolproof; the complex interactions between air pollution and spatially heterogeneous urban landscapes warrant careful planning to ensure that GI does not end up doing more harm than good.
Figure 1. Conceptual diagram showing the linkages of air pollution sources, greening options, optimised benefits and (potential) unintended consequences (Kumar et al., 2019)
So, how has GI been successfully tapped on to redress air pollution? Bermúdez et al. (2023) studied the effects of installing a green barrier (Figure 2) in a UK school playground. In the case of their study, the green barrier entailed 31 different plant species planted along the border of the playground to segregate the playground from street traffic, which is a strong contributor of air pollution. More importantly, the authors noted that 5 species were selected for their properties aiding in air pollution “deposition, deflection, and dilution”, while the low-porosity barrier as a whole served to “screen” and obstruct the transmission of atmospheric pollutants. The green barrier was ultimately a success that reduced NO2 and PM2.5 concentrations in the playground by 13% to 23% and 2% respectively.
Figure 2. Before and after the construction of the green barrier (Bermúdez et al., 2023)
Next, Tomson et al. (2021) explain that hedges are another form of GI efficient at “screening” pollutants from nearby emission sources. Hewitt et al. (2019) crucially note that pollutants mix with clean air as they disperse away from their source. It is through this exact principle that linear structures such as hedges work their magic, extending the effective distance between pollution sources (e.g. vehicles) and receptors (e.g. humans), thereby stimulating more dilution. Although an average of 52% reduction in pollution concentration could be attributed to a 1-metre-high hedge in open settings (Hewitt et al., 2019), “discontinuous” hedges in street canyons have been found to increase pollution concentrations by 3% to 19% (Gromke et al., 2016), defeating the initial purpose they set out to achieve. Therefore, this reinforces the need for a detailed analysis of street canyon characteristics prior to hedge installation to optimise the results produced.
Figure 3. Effect of a hedge on concentrations of pollutants emitted by vehicular traffic (Hewitt et al., 2019)
Finally, Wróblewska and Jeong (2021) share important insight into strategies to maximise and exceed the potential of plants and GI to alleviate air pollution.
The first proposed strategy is to select the species used in GI based on their “particulate matter capture potential”. Given that different vegetation species demonstrate varying “particulate matter capture potential[s]” based on local climatic and environmental conditions, caution must be exercised in implementing GI, especially in regions with large seasonal fluctuations. Moreover, vegetation species are further differentiated by their sensitivity to various aerosols, where efficient lead captures might not be strong particulate matter depositors. Hence, I believe that it would be advisable for a pilot study of the dominant air pollutants to be conducted prior to GI implementation.
Another strategy is to increase biodiversity, not among the plants chosen for GI, but in the general environment. According to the authors, enhanced biodiversity serves dual purposes of boosting plant resilience within stressful urban contexts to sustain their pollution capturing functions, as well as leaving positive climatic effects that reduce the emissions of particulate matter. Finally, planners of GI could consider the use of transgenic plants, which are genetically modified to support the decomposition of pollutants at higher capacities and efficiency levels.
Figure 4. How the pollutant capturing capacity of plants can be further boosted through various methods (Wróblewska and Jeong, 2021)
The range of possible GI is certainly not limited to the green barriers and hedges mentioned in this entry. Check out this video to find out how green walls, green roofs, and even green skyscrapers have joined the war against air pollution. Yet, despite the unequivocal benefits cities stand to gain from GI, one cannot stress enough how important it would be to plan, plan, and plan. Only then can we mobilise GI successfully without it backfiring on us.
Until the next entry, breathe safe and be safe!
References
Bermúdez, M. del R., Chakraborty, R., Cameron, R. W., Inkson, B. J., & Val Martin, M. (2023). A practical green infrastructure intervention to mitigate air pollution in a UK School Playground. Sustainability, 15(2), 1075. https://doi.org/10.3390/su15021075
Gromke, C., Jamarkattel, N., & Ruck, B. (2016). Influence of roadside hedgerows on air quality in Urban Street Canyons. Atmospheric Environment, 139, 75–86. https://doi.org/10.1016/j.atmosenv.2016.05.014
Hewitt, C. N., Ashworth, K., & MacKenzie, A. R. (2019). Using green infrastructure to improve urban air quality (GI4AQ). Ambio, 49(1), 62–73. https://doi.org/10.1007/s13280-019-01164-3
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
Tomson, M., Kumar, P., Barwise, Y., Perez, P., Forehead, H., French, K., Morawska, L., & Watts, J. F. (2021). Green infrastructure for air quality improvement in Street Canyons. Environment International, 146, 106288. https://doi.org/10.1016/j.envint.2020.106288
Wróblewska, K., & Jeong, B. R. (2021). Effectiveness of plants and green infrastructure utilization in ambient particulate matter removal. Environmental Sciences Europe, 33(1). https://doi.org/10.1186/s12302-021-00547-2