Understanding the effects of roadside hedges on the horizontal and vertical distributions of air pollutants in street canyons

Prashant Kumar; Juan C Zavala-Reyes; Mamatha Tomson; Gopinath Kalaiarasan

doi:10.1016/j.envint.2021.106883

Understanding the effects of roadside hedges on the horizontal and vertical distributions of air pollutants in street canyons

Environ Int. 2022 Jan:158:106883. doi: 10.1016/j.envint.2021.106883. Epub 2021 Sep 25.

Authors

Prashant Kumar¹, Juan C Zavala-Reyes², Mamatha Tomson², Gopinath Kalaiarasan²

Affiliations

¹ Global Centre for Clean Air Research (GCARE), Department of Civil and Environmental Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford GU2 7XH, Surrey, United Kingdom. Electronic address: p.kumar@surrey.ac.uk.
² Global Centre for Clean Air Research (GCARE), Department of Civil and Environmental Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford GU2 7XH, Surrey, United Kingdom.

PMID: 34583097
DOI: 10.1016/j.envint.2021.106883

Abstract

Built-up environments limit air pollution dispersion in street canyons and lead to complex trade-offs between green infrastructure (GI) usage and its potential to reduce near-road exposure. This study evaluated the effects of an evergreen hedge on the distribution of particulate matter (PM₁, PM_2.5, PM₁₀), black carbon (BC) and particle number concentrations (PNCs) in a street canyon in West London. Instrumentation was deployed around the hedge at 13 fixed locations to assess the impact of the hedge on vertical and horizontal concentration distributions. Changes in concentrations behind the hedge were measured with reference to the corresponding sampling point in front of the hedge for all sets of measurements. Results showed a significant reduction in vertical concentrations between 1 and 1.7 m height, with maximum reductions of -16% (PM₁ and PM₁₀) and -17% (PM_2.5) at ∼1 m height. Horizontal concentrations revealed two zones between the building façade and the hedge, with opposite trends: (i) close to hedge (within 0.2 m), where a reduction of PM₁ and PM_2.5 was observed, possibly due to dilution, deposition and the barrier effect; and (ii) 0.2-3 m from the hedge, showing an increase of 13-37% (PM₁) and 7-21% (PM_2.5), possibly due to the blockage effect of the building, restricting dispersion. BC showed a significant reduction at breathing height (1.5 m) of between -7 and -50%, followed by -15% for PNCs in the 0.02-1 µm size range. The ELPI + analyser showed a peak of ∼30 nm. The presence of the hedge led to a ∼39 ± 32% decrease in total PNCs (0.006-10 µm), suggesting a greater removal in different modes, such as a 83 ± 12% reduction in nucleation mode (0.006-0.030 µm), 74 ± 15% in ultrafine (≤0.1 µm), and 34 ± 30% in accumulation mode (0.03-0.3 µm). These findings indicate graded filtering of particles by GI in a near-road street canyon environment. This insight will guide the improved design of GI barriers and the validation of microscale dispersion models.

Keywords: Green infrastructure; Low-cost sensors; Particle size distributions; Passive solutions; Roadside hedges; Street canyons.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Air Pollutants* / analysis
Air Pollution* / analysis
Air Pollution* / prevention & control
Environmental Monitoring
Particle Size
Particulate Matter / analysis
Vehicle Emissions / analysis

Substances

Air Pollutants
Particulate Matter
Vehicle Emissions