Calculating how trees improve urban air quality

Particulate matter and why it matters

The air that we breathe is full of particles. It can’t be avoided. City or village, farmland or beach, there will be some trace of dust, soot, pollen, smoke or even liquid particles like sea-spray. What varies enormously is the level of particulate matter (PM). In measuring this, the common standard is PM2.5 which means particles no larger than 2.5µm in diameter. To put this in context, it doesn’t include dust mites which are a full 3µm across and certainly not bits of hair which range from 100 to 150µm in diameter.

There is no safe level of PM2.5 – just the lower the better – although the WHO recommends that concentrations should not exceed an annual average of 10µg/m3 and a daily average of 25µg/m3. Increased levels of PM2.5, whether permanent or temporary, have serious effects on public health, exacerbating lung and heart conditions. Children, the elderly and people with existing respiratory or cardiovascular conditions are, not unexpectedly, particularly affected.

The University of Leicester, which has a history of air quality research, collaborated with aerial survey specialists Bluesky on a project to calculate the benefits of green infrastructure within urban areas in reducing PM2.5. Trees and grassy areas have obvious immediate benefits in making urban environments more pleasant places to live, work and play – but they can also have practical benefits in reducing particulate matter, which they do in two ways: dispersion and deposition. Dispersion is reducing the concentration of particulate matter by breaking it up and spreading it over a wider area. Whereas in deposition particles remove themselves from the air by attaching to surfaces.

Simulating the City of Leicester

Since it’s not really practical to crawl around a city centre measuring numbers of microscopic particles, this research required some sophisticated computer modelling. A 2km by 2km square of Leicester (the city centre and the surrounding area) was modelled in detail, starting with a 3D LIDAR model originally constructed in 2007 with a resolution of 25cm. Trees were added, based on the average urban tree density for the East Midlands, modelled as porous objects using Bluesky’s National Tree Map Crown Polygon software. Areas identified as grass (based on Ordnance Survey mapping) were modelled at a surface roughness of 0.03m while non-grass areas were modelled at a roughness of 0.10m, thereby providing an approximation of things like bollards and benches.

The wind was simulated at two different speeds - 4.6m/s (the UK average) and 1m/s – and from a clock-face of 12 different directions at 30 degree intervals. The research considered different combinations of data, such as just the city centre, just the surrounding areas or just roadsides. After some serious number-crunching, what this research showed was an average 9.0 percent reduction in PM2.5 caused by the aerodynamic dispersive effects of trees. Deposition on trees resulted in a 2.8 percent reduction in PM2.5 but deposition on grass only reduced PM2.5 by 0.6 percent.

Introducing EarthSense

This research – more detailed and over a larger area than any previous similar projects – was the first output from EarthSense Systems, a spin-out company established as a collaboration between BlueSky and the University of Leicester. EarthSense Systems aims to deliver products that enable the world to visualise and solve its air quality issues, providing policy makers, planners and those responsible for delivering results, with access to real world information in order to support decision making.

With a mix of hardware (air quality sensors), software (bespoke modelling), data (derived and complementary) and people, EarthSense is uniquely poised to take a lead in air quality monitoring solutions and services, making a difference to people’s lives and delivering high value information to a range of consumers and decision makers. Among a range of projects, EarthSense is providing a network of Zephyr sensors to Berlin to assist with a major initiative to understand NO2 and O3 concentrations across a diverse environment.

Researcher profile: Professor Paul Monks

profile of paul monksAs Pro-Vice-Chancellor and Head of our College of Science and Engineering, Professor Monks has overall strategic and operational responsibility for the College whilst being a member of the University Leadership team. A Professor in Atmospheric Chemistry and Earth Observation Science, he is a Fellow of the Royal Meteorological Society and the Royal Society of Chemistry, and the European representative on the Environmental Pollution and Atmospheric Chemistry Scientific Steering Committee (EPAC SSC) of the World Meteorological Organisation and iCACGP (international Commission on Atmospheric Chemistry and Global Pollution).

Professor Monks’ research covers the broad areas of air quality, atmospheric composition and climate change. He is the chair of the Department for Environment, Food and Rural Affairs’ (DEFRA) Air Quality Group that provides independent science advice on air quality, and a member of DEFRA’s Science Advisory Council. He also sits on the Satellite Applications Catapult advisory group, the Royal Society Global Environmental Research Committee and the NERC audit committee. 

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ScienceDirect article: Modelling the effectiveness of urban trees and grass on PM2.5 reduction via dispersion and deposition at a city scale

Read the ScienceDirect article

College of Science and Engineering research: Air quality innovations

Find out more about the research

Air Quality News: University of Leicester backs air monitoring firm

Read the Air Quality News article