Green plants reduce city street pollution up to eight times more than previously believed

Jul 18, 2012
Green plants reduce city street pollution up to 8 times more than previously believed

Trees, bushes and other greenery growing in the concrete-and-glass canyons of cities can reduce levels of two of the most worrisome air pollutants by eight times more than previously believed, a new study has found. A report on the research appears in the ACS journal Environmental Science & Technology.

Thomas Pugh and colleagues explain that concentrations of nitrogen dioxide (NO2) and microscopic particulate matter (PM) — both of which can be harmful to human health — exceed safe levels on the streets of many cities. Past research suggested that trees and other green plants can improve urban air quality by removing those pollutants from the air. However, the improvement seemed to be small, a reduction of less than 5 percent. The new study sought a better understanding of the effects of green plants in the sometimes stagnant air of city streets, which the authors term "urban street canyons."

The study concluded that judicious placement of grass, climbing ivy and other plants in urban canyons can reduce the concentration at street level of NO2 by as much as 40 percent and PM by 60 percent, much more than previously believed. The authors even suggest building plant-covered "green billboards" in these urban canyons to increase the amount of foliage. were also shown to be effective, but only if care is taken to avoid trapping pollutants beneath their crowns.

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More information: Effectiveness of Green Infrastructure for Improvement of Air Quality in Urban Street Canyons, Environ. Sci. Technol., 2012, 46 (14), pp 7692–7699
DOI: 10.1021/es300826w

Abstract
Street-level concentrations of nitrogen dioxide (NO2) and particulate matter (PM) exceed public health standards in many cities, causing increased mortality and morbidity. Concentrations can be reduced by controlling emissions, increasing dispersion, or increasing deposition rates, but little attention has been paid to the latter as a pollution control method. Both NO2 and PM are deposited onto surfaces at rates that vary according to the nature of the surface; deposition rates to vegetation are much higher than those to hard, built surfaces. Previously, city-scale studies have suggested that deposition to vegetation can make a very modest improvement (<5%) to urban air quality. However, few studies take full account of the interplay between urban form and vegetation, specifically the enhanced residence time of air in street canyons. This study shows that increasing deposition by the planting of vegetation in street canyons can reduce street-level concentrations in those canyons by as much as 40% for NO2 and 60% for PM. Substantial street-level air quality improvements can be gained through action at the scale of a single street canyon or across city-sized areas of canyons. Moreover, vegetation will continue to offer benefits in the reduction of pollution even if the traffic source is removed from city centers. Thus, judicious use of vegetation can create an efficient urban pollutant filter, yielding rapid and sustained improvements in street-level air quality in dense urban areas.

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User comments : 10

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xen_uno
3.7 / 5 (3) Jul 18, 2012
... not to mention the benefits for psychological reasons. Can't stand seeing man made garbage everywhere.
The Singularity
not rated yet Jul 18, 2012
here, here...
Howhot
5 / 5 (1) Jul 19, 2012
That's an awesome idea. Very similar to the Green-roofs idea that some cities are pushing.
kochevnik
5 / 5 (3) Jul 19, 2012
City leaders in the USA seem far more interested in killing trees.
antialias_physorg
3 / 5 (1) Jul 19, 2012
Can't stand seeing man made garbage everywhere.

Putting a bush in the way makes having that garbage lie there better?

Climbnig ivy is nice (have smoe on the house my appartment is in), but you definitely get more crawley creatures than you would get without it.
NotParker
2 / 5 (4) Jul 19, 2012
Here we have one story talking about pollution and how bad NO2 is, and anoher story suggests biodiesel is good for city buses, even though it results in "a boost in nitrogen oxides emissions"

http://phys.org/n...ons.html

antialias_physorg
5 / 5 (1) Jul 19, 2012
Here we have one story talking about pollution and how bad NO2 is, and anoher story suggests biodiesel is good for city buses, even though it results in "a boost in nitrogen oxides emissions"

City buses are only a very, very minor contributor to the NO2 produced by city traffic. You have to take these things in perspective and not just look at things naively.

There are man factors which we wish to optimize in real life. NO2 balance is only one of them. CO2 balance is another. At times a big savings in one is worth a small bump in the other.
NotParker
2 / 5 (4) Jul 19, 2012
Here we have one story talking about pollution and how bad NO2 is, and anoher story suggests biodiesel is good for city buses, even though it results in "a boost in nitrogen oxides emissions"

City buses are only a very, very minor contributor to the NO2 produced by city traffic.

"use of ethanol fuels leads to increased levels of toxins called aldehydes and peroxyacyl nitrates (PAN).

Aldehydes are much more reactive in the atmosphere than the alcohols they are made from. They react with other chemicals in urban atmospheres to set off chemical reactions leading to PAN.

Argonne scientists have found that once created, PAN can last for many days in the air if the conditions -- especially temperature -- are right. When it's cold, its lifetime is longer.

PAN is highly toxic to plants and is a powerful eye irritant."
xen_uno
5 / 5 (1) Jul 20, 2012
anti - "Putting a bush in the way makes having that garbage lie there better?"

By garbage I meant endless houses/buildings/pavement/concrete/derelicts more so than refuse which is also bad of course. Green is calming and soothing and good for society. They ought to tear down Detroit and make it a forested wildlife preserve, problem is what to do with all the animals already there.
antialias_physorg
not rated yet Jul 23, 2012
Sure, greenery is good. But we should focus on reducing emissions rather than trying to hide/catch them. 'First consumers' of emissions are children (because they are lower to the ground).

Over here they are already giving out stickers for cars: red, yellow and green depending on how much pollution you create. With a red sticker you aren't allowed to drive in most major cities (yellow will soon follow). But expecially for city traffic it's high time we switch over to some form of locally non-polluting drive (EV or hydrogen fuel cell)