Efficacy of green infrastructure in reducing exposure to local, traffic-related sources of airborne particulate matter (PM)

H A Sheikh; B A Maher; A W Woods; P Y Tung; R J Harrison

doi:10.1016/j.scitotenv.2023.166598

Efficacy of green infrastructure in reducing exposure to local, traffic-related sources of airborne particulate matter (PM)

Sci Total Environ. 2023 Dec 10:903:166598. doi: 10.1016/j.scitotenv.2023.166598. Epub 2023 Aug 25.

Authors

H A Sheikh¹, B A Maher², A W Woods³, P Y Tung⁴, R J Harrison³

Affiliations

¹ Department of Earth Sciences, University of Cambridge, Downing Site, CB2 3EQ, UK. Electronic address: has57@cam.ac.uk.
² Center for Environmental Magnetism and Palaeomagnetism, University of Lancaster, LA1 4YQ, UK.
³ Department of Earth Sciences, University of Cambridge, Downing Site, CB2 3EQ, UK.
⁴ Department of Earth Sciences, University of Cambridge, Downing Site, CB2 3EQ, UK; Department of Materials Science and Metallurgy, University of Cambridge, CB3 0FS, UK.

PMID: 37634712
DOI: 10.1016/j.scitotenv.2023.166598

Abstract

One aim of roadside green infrastructure (GI) is to mitigate exposure to local, traffic-generated pollutants. Here, we determine the efficacy of roadside GI in improving local air quality through the deposition and/or dispersion of airborne particulate matter (PM). PM was collected on both pumped air filters and on the leaves of a recently installed 'tredge' (trees managed as a head-high hedge) at an open road environment next to a primary school in Manchester, U.K. The magnetic properties of PM deposited on leaves and filters (size fractions PM₁₀ and PM_2.5) were deduced from hysteresis loops, first-order reversal curves (FORCs), and low-temperature remanence measurements. These were complemented with electron microscopy to identify changes in magnetic PM concentration downwind of the tredge/GI. We show that the tredge is permeable to airflow using a simple CO₂ tracer experiment; hence, it allows interception and subsequent deposition of PM on its leaves. Magnetic loadings per m³ of air from filters (PM₁₀ saturation magnetisation, M_s, at 5 K) were reduced by 40 % behind the tredge and a further 63 % in the playground; a total reduction of 78 % compared to roadside air. For the PM_2.5 fraction, the reduction in magnetic loading behind the tredge was remarkable (82 %), reflecting efficient diffusional capture of sub-5 nm Fe-oxide particles by the tredge. Some direct mixing of roadside and playground air occurs at the back of the playground, caused by air flow over, and/or through gaps in, the slowly-permeable tredge. The magnetic loading on tredge leaves increased over successive days, capturing ~23 % of local, traffic-derived PM₁₀. Using a heuristic two-dimensional turbulent mixing model, we assess the limited dispersion of PM < 22.5 μm induced by eddies in the tredge wake. This study demonstrates that PM deposition on leaves reduces exposure significantly in this school playground setting; hence, providing a cost-effective mitigation strategy.

Keywords: Air pollution; Green infrastructure; Magnetism; Mitigation; Particulate matter; Vehicular pollution.