University of Essex

Impacts of air pollution and mitigation by green infrastructure Ian Colbeck Email: colbi@essex.ac.uk

Air pollution and health § 40,000 deaths per year from NO2 and PM2.5 § ~9,000 indoor air § Costs to health services and to business of more than £20 billion every year. § 5.7% deaths in Essex due to long term exposure to PM2.5 (PHE, 2014) h"ps://www.rcplondon.ac.uk/projects/outputs/every-­‐breath-­‐we-­‐take-­‐lifelong-­‐impact-­‐air-­‐pollu=on

Nature environments - human health and wellbeing

Har=g et al. Annu. Rev. Public Health 2014. 35:21.1–21.22

To tree or not to tree?*

*Vos et al., Environ Poll 183 (2013) 113-­‐122

Trees and air quality § Temperature reduction and other microclimatic effects § Removal of air pollutants § Emission of volatile organic compounds and tree maintenance emissions § Energy effects on buildings

Trees and air quality (T) § Affect air temperature, radiation absorption and heat storage, wind speed, relative humidity, turbulence, surface albedo and mixing-layer height § In UK studies report reductions of up to 5oC § Cooling effects differ depending on species and growth conditions. Higher leaf area index cools the soil beneath them more than those with a low leaf area index

Trees and air quality (R) § Trees remove gaseous pollutants either through uptake via leaf stomata or the plant surface § Increasing tree cover from 3.7% to 16.5% - estimated reduction in PM10 of 10% (West Midlands) § increasing tree cover from 3.6% to 8% - estimated reduction in PM10 if 2% (Glasgow) § London – current urban tree canopies estimated reduction in PM10 of 0.7% to 1.4%

Trees and air quality (E) § Adverse effect on air quality via emissions of VOCs § Production of ozone and particles § VOC emissions are temperature dependent so increased tree cover can lower overall VOC emissions and, consequently, ozone levels in urban areas. § Emission rates vary by species e.g. sycamore, poplar and oak have high isoprene emissions § Tree maintenance emissions

Trees and air quality (E) § Reduce building energy use by lowering temperatures and shading buildings during the summer, and blocking winds in winter. § Can increase energy use by shading buildings in winter, and may increase or decrease energy use by blocking summer breezes. § Tree placement near buildings is critical to achieve maximum building energy conservation benefits.

US Forest Service § Trees are saving more than 850 human lives a year and preventing 670,000 incidents of acute respiratory symptoms § Trees can help save $7 billion a year in health costs by reducing respiratory illness. § Most of the pollution removal occurred in rural areas, while most of the health impacts and values were within urban areas.

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Manchester: 2.3% of PM removed in a year (sedum) Larger plants would have a bigger impact but more expensive to install and maintain Vegetables grown on rooftops will receive a reduced pollution load, including heavy metals, than crops grown in the ground near roads Plants on green roofs are likely to remove less air pollution than ground level vegetation due to the effect of building elevation on the vertical gradient of PM2.5

Green walls § Plants growing vertically on building walls could remove nearly 10 times as much NO2 and nearly 12 times as much PM10 from streetcanyon air as horizontally grown rooftop vegetation § Vertical vegetation in street canyons can reduce street level concentrations by as much as 40% for NOx and 60% for PM

Species choice § Rural-evergreen, rural-deciduous, urban-evergreen, and urbandeciduous trees in this order had greater effects on air quality (Hirabayashi and Nowak, 2106) § Evergreen species contribute to pollutant scrubbing year-round § When in leaf, broadleaf species may be more efficient than needleleaf species, due to the higher leaf surface area. § Leaves with complex shapes, large circumference-to-area ratios, waxy cuticles or fine hairs on their surfaces collect particles more efficiently § Leaf surfaces are important, with ridged hairy leaves giving the highest particle deposition § To improve deposition, the vegetation should be hairy and have a large leaf area index, but still be possible to penetrate § Beware of VOC emission

Urban vegeta=on (RHS)

Roadside vs canopy : NO2

Mango tree

Jamun tree

The Essex Natural Capital Asset Check (ENCAC) aims to model the value of natural assets in Essex

Greener but not cleaner?

Generally improve air quality Physical and mental health Customers willing to pay more if businesses located on tree-lined streets Clear benefits but also downsides Abhijith et al. Atmos Environ 162 (2017)

Impact on indoor air § ~1860s § “in what way can plants assist our health? They can purify the air, which is a very important matter”

Additional reading § Natural Capital Investing in a Green Infrastructure for a Future London § Demystifying green infrastructure § Building a Green for Europe § Urban vegetation (RHS) § Urban air quality – The Woodland Trust § Defra - What impact do trees have on air pollutant concentrations?