Introducing the Green Infrastructure for Roadside Air Quality (GI4RAQ) Platform: Estimating Site-Specific Changes in the Dispersion of Vehicular Pollution Close to Source

Abstract

The benefits of ‘green infrastructure’ are multi-faceted and well-documented, but estimating those of individual street-scale planting schemes at planning can be challenging. This is crucial to avoid undervaluing proposed schemes in cost–benefit analyses, and ensure they are resilient to ‘value engineering’ between planning and implementation. Here, we introduce prototype software enabling urban practitioners to estimate the site-specific air quality impacts of roadside vegetation barriers: highly localised changes in pollutant concentrations due to changes in the dispersion of vehicular emissions close to source. We summarise the recent shift in understanding regarding the impacts of vegetation on urban air pollution towards changes in pollutant dispersion (cf. deposition) and describe our prototype software, offering rapid estimates thereof. First tests of the underlying model’s performance are promising, reproducing: annual mean NO2 and PM2.5 concentrations in a street canyon (Marylebone Road, London, UK) to within 10% and 25%, respectively; and changes in pollutant concentrations of the right order of magnitude behind roadside barriers in a wind tunnel simulation of a street canyon and a real open-road environment. However, the model underestimates the benefits of a barrier in a simulated street canyon under perpendicular wind conditions. The prototype software is a first step towards informing practitioners of the site-specific impacts of vegetation barriers, which should always be additional to (i.e., no substitute for) essential emission reductions. The code is open-source to engage further researchers in its continued development.

Protecting playgrounds: local-scale reduction of airborne particulate matter concentrations through particulate deposition on roadside ‘tredges’ (green infrastructure)

“…Recent modelling-based papers and expert reviews (e.g. 45 ) state that the magnitude of PM deposition on urban vegetation is small in most street contexts; ‘dwarfed’ 46 by the effects that street vegetation has on dispersion of local PM. Such studies mostly use computational fluid dynamics (CFD) to simulate PM emission, dispersion and deposition, and, typically, indicate small reductions (i.e., a few percent) in PM 10 or PM 2.5 concentrations by deposition onto (mostly existing, often very tall) roadside vegetation (e.g.…”

Efficacy of Green Infrastructure in Reducing Exposure to Local, Traffic-Related Sources of Airborne Particulate Matter (Pm)

“…This approximation is consistent with the magnitude of the drop in PM 10 magnetic loading of the filter samples on either side of the tredge. It is important to note that % removal of PM is an estimate, and only tell us about the pollution that is removed from air passing through the vegetation and nothing about whether the PM-laden air either moves through the vegetation or around it (Pearce et al, 2021). However, our permeability experiment, using CO 2 as a tracer, confirms that the tredge is permeable enough to allow 'slow filtering' and subsequent deposition of locally sourced anthropogenic particles on leaves.…”

Realistic Forests and the Modeling of Forest‐Atmosphere Exchange

“…Windbreaks, also known as shelter belts, exploit the ability of trees and shrubs to adapt to windy environments. Windbreaks have been used for centuries to shelter coastal buildings, to modify the microclimates around crops and grazing land, to protect woodland and wildlife, and, more recently, to modify pollutant transport around urban or industrial areas (Palmer et al., 1997; Pearce et al., 2021). Figure 3 shows a sketch of the streamlines around a windbreak for wind blowing left to right.…”