DEVELOPMENT OF VEGETATION METRICS FOR REDUCING PARTICULATE MATTER POLLUTION AT URBAN ROADSIDES: THE CASE OF CENTRAL DOHA, QATAR

Abstract

The Public Works Authority in Qatar, 'Ashghal,’ beautifies roads with greenery and creates space for exercise in central Doha. However, without strategic planning, this vegetation could increase roadside users’ exposure to traffic pollutants. Studies on the effectiveness of vegetation in reducing exposure to traffic pollutants in hot, arid cities are scarce. This research aims to develop vegetation metrics to reduce trafficborne fine particulate matter (PM2.5) concentrations in roadside user zones to support physical activities in central Doha. The study focuses on roads with an aspect ratio (H/W) of 0.5, as these have potential for vegetation implementation in central Doha. The research objectives are: (i) to inquire about roadside users’ preferences for the benefits and types through an inception study; (ii) to investigate the impact across seven road orientations (0°, 15°, 30°, 45°, 60°, 75°, and 90°) relative to the prevailing wind on PM2.5 concentrations in roadside user zones for five scenarios: Base Case (BC), Centre Hedgerow (CH), Sidewise Hedgerows (SH), Centre Trees (CT), and Sidewise Trees (ST); (iii) to examine the influences of variations in the meteorological conditions of the four meteorologically distinct months (January, June, September, and December) on PM2.5 concentrations in roadside user zones for three typical orientations (0°, 45°, and 90°) for the four vegetation scenarios (CH, SH, CT, and ST); (iv) to develop the vegetation metrics based on the key factors that significantly influence the PM2.5 concentrations in roadside user zones. This research employs a descriptive survey for the inception study and utilizes ENVI-met simulation to develop vegetation metrics. Results show that roadside users value vegetation for air quality enhancement and prefer abundant greenery and shade. Spearman’s rank correlation analyses indicate that both road orientation to the prevailing wind and meteorological conditions differently influence the effectiveness of CH, SH, CT, and ST. Minor Road geometry variations have no effect on the performance order of CH, SH, CT, and ST. The findings reveal that increasing the angle of orientation of roads to the prevailing wind leads to a consistent impact of meteorological conditions on PM2.5 concentrations. This research successfully developed vegetation metrics to reduce PM2.5 concentrations in roadside user zones. It recommends the CH scenario for up to 29.80% concentration reduction in roads oriented at 0°; the SH scenario for up to 7.74%, 11.79%, 5.83%, and 8.82% reductions in roads oriented at 15°, 30°, 75°, and 90°, respectively; and the CT scenario for up to 9.29% reductions in roads oriented at 60° to the prevailing wind. In conclusion, strategic vegetation planning can reduce PM2.5 concentrations in roadside user zones. Vegetation metrics serve as a valuable tool, offering guidance for Ashghal and designers to reduce roadside users’ exposure to traffic-borne PM2.5 and support their physical activities in central Doha.