Pugh, T. A. M., A. R. MacKenzie, et al. (2012). "The effectiveness of green infrastructure for improvement of air quality in urban street canyons." Environmental Science & Technology. DOI: 10.1021/es300826w

Green Infrastructure and urban air

quality

Thomas A.M. Pugh, A. Robert MacKenzie,

J. Duncan Whyatt, C. Nicholas Hewitt

1) Introduction

2) Current practice

3) New estimates

4) Conclusions & caveats

IAQM, London, 28 June 2012

Introduction: Emission controls

Policy has focused on emissions control (at tail-pipe or through traffic control). Since ~2004, UK urban air NO2 concentrations have failed to show decreases expected from estimated emissions – especially at roadside. PM concentrations also stubbornly high.

Defra (2010) Air pollution in the UK 2010. (Defra, London).

1990 1990 2010 2010

Concentration = f(Emissions, Mixing, Deposition, Chemistry, Advection)

40

20

50

30

Dry deposition to green roofs and parks

Surface Typical NO2 Vd

(cm s-1) Typical PM10 Vd

(cm s-1)

Concrete/Brick 0.015 - 0.05 0.02 (vertical)

Vegetation 0.3 0.64

C = f( E,M,D …)

e.g., green roof, but same thinking can be applied to parks, gardens, etc.

Land use classification. E.g., West Midlands 900 km2, 27 land use attributes, 8 land use classes

The “constant boundary-layer concentration” method to determine pollutant deposition

Farms and villages

Light suburban

Very light suburban

Dense suburban

Dense urban/transport

Urban

Light urban/open water

Woodlands

Owen et al. (2006) Landscape and Urban Planning, 78, 311-319

F = Vd.C Calculated flux

Literature + land-use

Observed air concentrations

The “constant boundary-layer concentration” method to determine pollutant deposition

E.g.: McDonald, A.G. et al. Atmos. Environ. 41, 8455–8467 (2007)

Currie, & Bass, Estimates of air pollution mitigation with green plants and green roofs using the UFORE model. Urban Ecosyst. 11, 409-422 (2008).

Tallis ,et al., Landscape and Urban Planning 103, 129– 138 (2011)

Method makes two assumptions: • constant pollutant air concentration - implies that depositional loss is too small to

significantly impact the concentration. Otherwise, will over-estimate the deposition.

• a well-mixed urban boundary layer. Will under-estimate deposition.

Effect of urban built form

dC i

dt= −

V d, i

z⋅C iC = f( E,M,D)

How do mixing and deposition interact?

Dry deposition in canyon

Surface Typical NO2 Vd

(cm s-1) Typical PM10 Vd

(cm s-1)

Concrete/Brick 0.015 - 0.05 0.02 (vertical)

Vegetation 0.3 0.64

…CiTTy-Street model parameterises air circulation in street canyons…

cartoon model

Reality check on model

Comparison of modeled (dashed blue lines) NO2 and O3 with measurements (solid black lines) for Göttinger Strasse, Hanover, Germany on 21/06/94.

Model evaluation: temporal concentration variations at street level and above

Green walls can be much more effective than previous calculations predicted…(central London example)

…and can have effective cleaning of air in a single canyon (localism). Bottom-up policy becomes possible. Political traction.

Recent estimates: 50,000 die prematurely every year in the UK, 1.34M worldwide, due to the chronic & acute air pollution effects.

Previously under-estimated effect

In-c

anyo

n c

on

cen

trat

ion

red

uct

ion

(%

)

Deep green canyons can even produce better air quality than shallow canyons under some conditions…and the action of all green canyons is to buffer against low-windspeed pollution events

Deep canyon

shallow canyon

The influence of in-canyon deposition on vegetation increases at low wind speeds

When the pollution source is in the street canyon, trees can cause fumigation. Without traffic, trees can produce filtered avenues, in which air is cleaner than on regional scale.

“Green Infrastructure”…Trees vs green walls

Can’t calculate residence time as function of green infrastructure (yet) – so explore model parameter space

Filtered avenues

Fumigated avenues

Flat grey city

Flat green city

Single street; central London emissions

Example for NO2(single street central London): too polluted for trees to make a positive contribution

Flat grey city

Flat green city

Fumigation

NB: in-canyon greening always beneficial if no street emissions. local activism to increase greening should also try to decrease traffic emissions.

Conclusions:

Green roofs (and other extra-canyon vegetation) act across the whole

boundary layer, with small consequences for street-level air quality.

Deposition can be very important for air quality in combination with

surface forms which trap air at ground-level.

Canyon wall greening is very effective at improving street-level air

quality at low wind speeds as it acts on a small volume of air.

Street trees can be beneficial, especially for PM, but must be used

with caution when in-canyon pollutant sources are large.

1

2

3

4 These benefits can be gained at the single-street level.

5

Further Work/Questions:

What are the effects of wall greening on ventilation?

Better constraints on real Vd to leaves and other surfaces

in a street canyon.

(e.g. is aerodynamic resistance increased?)

What are residence times with non-perpendicular winds?

What is the effect of realistic junctions?

Which plant species viable in urban settings?

Field testing of results!

EPSRC grant Urban Futures – Designing Resilient Cities (BRE Press, 2012): http://www.brebookshop.com/details.jsp?id=326921

Ash

Common alder

Field maple

Larch

Norway maple

Scots pine

Silver birch

Apple

Cherry laurel

Common elm

Common lime

Elder

Grey alder

Hawthorn

Hazel

Holly

Italian alder

Lawson cypress

Leyland cypress

Lilac

Mountain ash

Sycamore

Wild cherry

Crack willow

English oak

Goat willow

Poplar

Red oak

Sessile oak

White willow

Best Worst

Greatest capacity to improve air quality Small capacity to

improve air quality

Potential to worsen air quality

www.es.lancs.ac.uk/people/cnh/UrbanTreesBrochure.pdf

Urban Tree Air Quality Score: UTAQS Groups

Previously we used a well-mixed box to calculate 1-2% air pollutant reduction due to urban vegetation. This is still enough to make a significant contribution to cost-benefit analysis – similar analyses used by city mayors in NY and London to embark on tree planting.

Well mixed air

(no rain)

Emissions Deposition

Chemistry Daytime: rapid mixing to 1000 m

Pugh, T. A. M. etal.: A Lagrangian model of air-mass photochemistry and mixing using a trajectory ensemble: the Cambridge Tropospheric Trajectory model of Chemistry And Transport (CiTTyCAT) version 4.2, Geosci. Model Dev., 5, 193-221, doi:10.5194/gmd-5-193-2012, 2012.

Model evaluation: street/roof concentration anomalies

Anomalies are calculated by subtracting background measurements from in-canyon measurements and normalizing against traffic emissions and the mean concentration anomaly.

Göttinger Strasse, Hannover

Jagtvej, Copenhagen

Schildhornstrasse, Berlin