Plants: An efficient green technology for cities

From the American Chemical Society

English: The American Chemical Society Buildin...
English: The American Chemical Society Building, is surrounded by trees in the “urban canyon” of downtown Washington, D.C.  (Photo credit: Wikipedia)

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 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. Trees were also shown to be effective, but only if care is taken to avoid trapping pollutants beneath their crowns.



The authors acknowledge funding from the UK Engineering and Physical Sciences Research Council Sustainable Urban Environment program.

The American Chemical Society is a nonprofit organization chartered by the U.S. Congress. With more than 164,000 members, ACS is the world’s largest scientific society and a global leader in providing access to chemistry-related research through its multiple databases, peer-reviewed journals and scientific conferences. Its main offices are in Washington, D.C., and Columbus, Ohio.

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View from the Solent
July 21, 2012 8:36 am

Only green plants? Do orange, purple, blue, …. ones have the same effect?

gopal panicker
July 21, 2012 8:40 am

absolutely….grow more trees…its a no brainer

July 21, 2012 8:46 am

We see here educated people with no real world experience telling the average gardner or farmer or even just a person with good observational skills what they already know. Our world and our great country is entering the full stupid era as this study is somehow supposed to tell us what ? Ever drive through the desert and observe the vegetation ? The plants by the road are almost always the biggest and most green . This is do to disturbed soil , heat from the roadbed and food from the car exhaust . This has been known for so long that there is no revelation in this statement for anyone who looks. If you plant green stuff around your property and water it , it will grow and if you planted productive plants you will reap a benefit. Aint it amazing.

Restless 1
July 21, 2012 8:51 am

Makes sense. Man has a habit of underestimating nature’s ability to “scrub” itself.
And much more cost efficient than carbon credits or whatever other yoke the greenies would have us under.

July 21, 2012 8:56 am

Just who waould have guessed that the Romans, who mixed gardens and fountains in their cities, were so smart?
I cycle 5 miles to and from work each day in Houston. The streets with overhanging trees are cool and the ones with concrete are hot, anyone have any idea why?

July 21, 2012 9:03 am

Somehow common sense wore through.

July 21, 2012 9:09 am

Joyce Kilmer. 1886–1918
119. Trees
I THINK that I shall never see
A poem lovely as a tree.
A tree whose hungry mouth is prest
Against the sweet earth’s flowing breast;
A tree that looks at God all day, 5
And lifts her leafy arms to pray;
A tree that may in summer wear
A nest of robins in her hair;
Upon whose bosom snow has lain;
Who intimately lives with rain. 10
Poems are made by fools like me,
But only God can make a tree.

July 21, 2012 9:12 am

That is interestingly positive, plus the view is improved. However, the deciduous trees, etc lose their leaves during the winter months. Also, I have seen the roots from many types of that category of tree grow close to the surface uplifting the road surface, busting the sidewalks, and leaves clogging the storm drains. In many cases, the matching funds to improve roads or build new requires some greenery included along the paths. Nice to know there is more benefit than just aesthetics.

R Barker
July 21, 2012 9:14 am

Can I reinvent the wheel and get paid for it?

July 21, 2012 9:25 am

This is obvious for people who have been in cities in other countries.

Mr Lynn
July 21, 2012 9:25 am

I was just delighted to see, for a change, that the word ‘pollutant’ was not referring to carbon dioxide, but instead to REAL pollutants! Is this a sign that rationality is returning to studies of the atmosphere?
/Mr Lynn

July 21, 2012 9:40 am

The role of vegetation in altering air composition has been known for a long time. For example, it explains why the Blue Ridge Mountains are so named.
On a larger scale, knowledge of the role of forests in absorbing or releasing gases is still very limited, especially with regard to quantity. It is not long ago that it was determined the rain forests were a major source of methane.
We only started measuring the input and output of gases in the massive boreal forest a few years ago.
When the promoters of the Kyoto Protocol were trying to convince the US to sign on they asked what accommodation they could make. The request was for carbon credits based on replanting forests. It was rejected because they said they didn’t know how much CO2 was absorbed or released by forests.
The forgotten sphere in climate science that lies between the lithosphere and the atmosphere is the biosphere, Generally less than five meters in depth its impact on the interchange of gases, particulates, moisture is profound. There are so many gaps, not least of which is the most ubiquitous vegetation after trees, namely grasses – a major factor in urban areas. A good resource for what is known about grasslands is available at Sherwood and Craig Idso’s excellent site;
As a measure of the problems with accurate measures of climate of the biosphere, a former technical report from the World Meteorological Organization (WMO) said a weather station in the forest required clearing a 200m area. The billions wasted on the IPCC and actions taken because of their false reports would have been better spent on obtaining more data.

July 21, 2012 9:44 am

1. There are people who want to put plants all over cities.
2. Research is published supporting the position of people who want to put plants all over cities.
3. Be suspicious of #2.
If their results suggested that plants helped LESS than previously believed, do you really think they would have published them? People recognize the logic of ‘follow the money,’ but frequently ignore ‘follow the ideology’ as an equal warning. Air in our cities is already dramatically cleaner than just a few decades ago, when I was a young sprout. And go back to my parent’s day, and there would have been factory smokestacks all over the city, and thousands of houses burning coal for heat in inefficient furnaces. The work of cleaning the air in cities has already been done. Any improvement now will be icing on the cake.

July 21, 2012 9:47 am

Wanted: Gardener/Roof Maintenance Engineer/Electrician/Plumber/Chemical Engineer for maintaining modern building roof gardens, irrigation systems, repairing electrical shorts on hi-voltage industrial A/C units, and roof leaks into upper floors, walls and office furniture/shorted office equipment (copiers, computers, servers etc.). Must be proficient at predicting chemical reactions (pH etc.) from mixtures of fertilizers and other gardening chemicals (pesticides, fungicides etc.) to minimize chemical attacks on roofing materials, conduits, plumbing and typical office furnishings (such as wallboard, paint, wall coverings, carpet etc.) and able to perform remedial operations. Knowledge of “Green” chemistry a plus. Submit your CV/resume to our web-based parser/shredder for eventual consideration.

July 21, 2012 10:03 am

In as much as the Australian and American Chemical Society allowed the name of their organization to be listed on Wiki with thousands of other businesses and organization as supporters of global warming alarmism I think of them mostly as the the AA Chem. Society of IPA swillers.

July 21, 2012 11:06 am

NO2 & volatile organic compounds (VOG) reacting in circumstances with oxygen O2 foster production of ozone (O3). The seemingly incongruous robust trees in cities among all the air pollution is due to the way that O3 upregulates production of secondary metabolites. It is primarily salicylic acid & jasmonic acid pathways which give trees what they need to overcome and thrive under oxidative stress.
Without these secondary metabolites the O3 would reduce respiration/photosynthesis & alter nutrient distribution.Early in a season, but not necessarily all season long, with high O3 around this leads to more nitrogen (N) going into the leaves. While at the same time (with high O3) there is less of a concentration of some plant phenolic glycosides being made in it’s (the tree’s) natural self-defense response a plant does when it registers bugs contact on it’s leaves.
City trees in high O3 compensate when they get low on hydroxycinnamic acid it no longer is deriving in great quantity from phenolics by up-regulatig internal production of protective iso-flavonoids. Plant O3 stress response also causes more glycosides of the quercetin group to be made for better internal cell adaptation.
Of course these are generalizations, because different tree & plant varieties (angiosperm plants have a relatively greater variety of reactions to higher O3 than gynosperm plants) will have their own particular reaction to increased O3. Since city O3 levels are an interplay of day to night changes in temperature/sunlight/VOG the dynamic is not a steady state.
Meanwhile higher CO2 levels set the stage for more C, relative to N, being processed which tends to lessen the Nitrogen in leaves. As far as the main bug defender metabolite jasmonic acid it is depressed by extra CO2, which plays out further along when there is less jasmonic acid around to boost plant’s genetic transcription of proteinase inhibitors (this is what bug would eat & suffer from in it’s bug cycles). On the other hand elevated CO2 makes a plant put out more defensive phenolics and tannins in it’s leaves, which are how plant naturally defends against relentless herbivore insects chewing attacks. To all this CO2 talk it must be added there are plant specific responses & even within the same plant variety but different genes in that plant’s makeup there are no absolutes.
Anthropogenic O3 & anthropogenic O2 apparently aren’t keeping plants from coping.

July 21, 2012 11:21 am

Re: Tim Ball
Let’s not forget that the canopy effect is – as I understand it – not properly considered by the GCMs. Sadly enough, land use changes (majority of which – in the GCMs – are the replacement of forest with farmland… to say nothing of the hundreds of thousands of square miles of pavement we have put down in the last 80 years or so) are calculated as having a cooling effect in the IPCC reports. The rational seems to be that forests have a lower albedo… but that neglects the canopy effect.
It’s not rocket science… which is warmer on a summer afternoon: a blacktop road, a cement road, a plowed field, pasture land, or forest?
Plants are good for all sorts of reasons 🙂

July 21, 2012 11:40 am

“Madrid, the capital and largest city in Spain, has more trees and green surface per inhabitant than any other European city. There’s even a jungle growing in Atocha, the main train station. But Madrid is also the third-most populous municipality in the European Union, after London and Berlin, and car congestion has led to large amounts of smog. City officials have taken major steps to combat this issue with a tree-planting campaign. Some 248,000 trees now line the streets, absorbing pollution.” 2008.
Looks like the secret is out!

July 21, 2012 11:47 am

Yes, it’ll be cooler too.
How much of an effect transpiration?

July 21, 2012 12:11 pm

That reminds me of something. Don’t people also exhale water and don’t they spray water on their plants? The main thing I get out of all of this is that much of that which is called AGW is actually ALW and people conflating cities with globes while ignoring termite mounds, beehives, and a whole lot more.

July 21, 2012 12:33 pm

Biodiesel: “NOx increases can range anywhere from less than 1 percent to 15 percent higher than petro-diesel”
One green hand gives and other takes it away …

July 21, 2012 12:44 pm

Plants are not a “green technology” this is a manifestation of researchers egotistical know-it-all mentality into daft terminologies, In aid of funding or for the support of an undisclosed agenda I bet.
Just plant the damn trees and spare us all the idiotic wannabe science.

July 21, 2012 12:58 pm

Grow plants that produce food. So we don’t have to constantly pay more inflated prices for downsized groceries and restaurant menus if we don’t want to.

July 21, 2012 12:59 pm

Also need to bear in mind that trees won’t survive every type of pollution. Back in the ’70s a lead smelter near Blackwell was trying to clean its reputation without cleaning its output, and planted a long avenue of poplars along the side of its facilities. The trees turned gray, bent over and died within a year, leaving a dramatic demonstration of the pollution instead of a covering shield of green.

Kelvin Vaughan
July 21, 2012 1:04 pm

When I was young the streets of London where I lived were full of big trees. Later they cut them all down to save on maintenance costs.

July 21, 2012 1:08 pm

This study has been funded by the No Shit, Sherlock Foundation.

July 21, 2012 1:32 pm

Everything nowadays is more, or less than “previously thought”.
Since they are so thought-less, how could there be a concensus?

Steve in SC
July 21, 2012 1:48 pm

We have here an unprecedented opportunity.
I will be happy to provide any leftist city with all the green plants they want.

July 21, 2012 2:09 pm

I THINK that I shall never see
A Mann lovely as a tree.
A Mann whose hungry mouth is prest
Against the Goverment funding flowing breast;
A Mann that thinks he’s God all day,
And lifts his beafy arms to bray;
A Mann that may in summer wear
A drought or heat wave with which to swear
Upon whose word this totally due,
To CO2, Oh boo, oh Hoo!
Men are mocked by fools like me,
But only Mann can read a tree.
Apologies to Kilmore…

July 21, 2012 2:15 pm

I thought the value of the cambered road was to collect water and direct it to the roadside, where indeed plants can grow well.

July 21, 2012 3:05 pm

“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 canyons”
•Woman blown over at Bay & King, 26 Jan 1978

July 21, 2012 3:08 pm

Max Hugoson says:
July 21, 2012 at 2:09 pm

I THINK that I shall never see
A Mann lovely as a tree.

Apologies to Kilmore…

You should be apologetic, Max (and it’s “Kilmer” not “Kilmore”), but more to the point, you accurately display the vast polar difference between God’s gift to man and Mann’s gift to himself.
Mann’s the “Kilmore”

July 21, 2012 3:30 pm

Are we sure we want trees? Because CO2 levels could drop in our already CO2 starved atmosphere…

July 21, 2012 4:20 pm

And in Australia, part of the opposition’s “direct action plan” involves planting lots of trees.
The AGW bletheren don’t like that idea because it won’t destroy Australian industry like the carbon tax will.
Its amazing how often, when trying to talk to the warmists over here about the carbon tax, the very first thing they do is say “what about Mr Abbott’s direct action plan?” DOH !!!
Even if there wasn’t a CO2 scare scam running, we should always be planting more trees now that we have released some of the long buried carbon and put it back in the atmosphere where it belongs. Lets get the Earth flourishing and fertile again !!

July 21, 2012 4:26 pm

pjssjr says:
“Are we sure we want trees? Because CO2 levels could drop in our already CO2 starved atmosphere…”
That’s why we HAVE to keep releasing lots of CO2, to maintain the balance between the increasing world biomass and CO2.. It is obvious that around 280ppm is the basic subsistence balance level (that has been, apparently, the pretty constant level of many many thousands of years)
We MUST maintain it at a much higher level if plant life is to flourish.

July 21, 2012 4:28 pm

ps: “Towards 700ppm”

July 21, 2012 6:27 pm

Hi DIrkH,
Not all trees & plants put out isoprenes (C5H8); for example Birch trees do not while Poplar trees do. (? Didn’t the N.E. USA cities have a Birch tree crisis a few decades ago ?)
One of the plant’s isoprenes benefits is, among other things, to protect what envelops the chloroplast & conveniently it is near chloroplasts that isoprenes are made.
Counter-intuitively, in city air with lots of ozone those trees making isoprenes actually take up more ozone (than compared to when that same type of isoprene producing tree is in an environment of low ozone , like far or not down wind from a city). The tree isoprenes apparently act like an anti-oxidant; so that despite the extra O3 in their leaves those isoprene making trees still suffer less oxidative damage within their leaves’ “humid” mesophyllic layers.

Tom Harley
July 21, 2012 7:11 pm

My current ‘life’s work’ … Thanks for the post Anthony, makes it all look worthwhile.

July 21, 2012 8:53 pm

Urban planners of the 18th and 19th century included trees and parks as part of the cities which they designed. (vis e.g. Berlin and Paris) For eactly the reason that they understood the improvement in air quality. The understood that from observations.
While trees and other plants won’t filter all types of nasty emissions, they do pretty well on most. And that is a good, effective and affordable way of tackling those emissions. The transpiration from trees also helps to moderate peak temperatures in the urban heat island.

Eugene WR Gallun
July 21, 2012 9:30 pm

by Joyce Kilmer
That is an excruciatingly bad poem. What would be your reaction if the author had become so poetically aroused over a puppy or a bunny rabbit?
I am reminded of the work of Jessica Wood McKeand of Laguna Beach. After her death the Laguna Poets sought to collect and destroy every copy of her chapbook in existence. i possess one of the few remaining extant.
The chapbook is called “Sunshine And Shadows” (1970) and is dedicated to her recently deceased little blonde cocker named Buckles. The majority of the poems are about Buckles but she does toss in a few about her son who had also recently died of cancer. Below are some excepts from her work.
His faithful little tail
Never ceased to wag
I used to think
How tired, it must get!
He knew no guile, as humans do,
No jealousy, nor hate
A kind, and loving spirit —
And Oh! How patiently he’d wait!
Dear,dear little Buckle’s last, and most precious token of devotion, was the night before he died, when, during the night he crawled with supreme difficulty, and super-humane effort, having to drag his dear paralyzed, and helpless back legs, in order to get just as close to my bed, and as near to me, as he possibly could — giving me as always, his last full measure of devotion … be next to me in what I believe he knew,was near the end. His love and devotion, were far greater than any human love. I will always know, he had a touch of the DIVINE!
I know that love never dies.
Love is Eternal.
And i know that your beautiful soul, lives on, little Buckles.
God would not deny you this.
One day, while gazing into your beautiful, soul-searching eyes, a friend said, “Oh Buckles, you must know something we do not know, because you are SO BEAUTIFUL!
Part of the joy of reading McKeand is trying to figure out where her poetic ideas came from. You may recognize the source for this —
There is the beauty of the trees, giving shelter
With their arms outstretched to their God.
Eugene WR Gallun

July 22, 2012 1:59 am

gringojay says:
July 21, 2012 at 11:06 am
Anthropogenic O3 & anthropogenic O2 apparently aren’t keeping plants from coping.

interesting post, gringojay, but I think you meant anthropogenic CO2 in your last line.

July 22, 2012 2:39 am

London is perhaps the greenest city in Europe with many large and small parks and roadside trees. The City of London, without much in the way if greenery, has poor air quality compared to the rest of the city. Despite the PM2.5 from diesels engined vehicles the air is breathable.
I fully endorse Tim Ball’s recommendation of Good site with reports of real CO2 science.

July 22, 2012 9:11 am

Urederra thanks for catching the big “C” I omitted above & giving me the benefit of the doubt I hadn’t just captured that carbon & traded it!
As to why NO2 is cleared so much more than anticipated consider my surmise. High CO2’s inorganic carbon (C) processing done by trees/plants drives carboxylation in the plant cell’s carboxysome’s photosynthetic Rubisco makes 3-­phospho-glycerate which is processed into glycer-aldehyde 3-­phosphate.
Glycer-aldehyde 3-phosphate is used to do several things inside the plant besides just cobbled together into chains of 6 carbon carbohydrate molecules. The portion that is shunted through the plant cell’s Krebs pathway spins off alpha keto-glutarate which forms up into something like the backbone of a carbon skeleton which is a scaffold for tying nitrogen (N) atoms to. This is where the NO2 absorbed unloads it’s N ; and so, the more C uptake the more scaffolds to assimilate N giving room for more NO2 to be cleared from the air.
If/when CO2 too low then inside the leaf there is not the ideal concentration of CO2 to drive carboxylation. So then in order to make the inorganic carbon taken in into the plant’s own organic carbon there’s a default reaction. Oxidation, as opposed to carboxylation, gives that leaf only 2-phospho-glycollate & this is a 2 carbon chain molecule. But this molecule isn’t made downstream into a scaffold for assimilation of N atoms. Then the plant must expend energy stores to perform photo-respiration in order to process those 2 carbon chain molecules.
Anthropogenic NO2 & anthropogenic CO2 apparently have a standing date in city tree leaves.

Gail Combs
July 22, 2012 1:23 pm

Steve in SC says:
July 21, 2012 at 1:48 pm
We have here an unprecedented opportunity.
I will be happy to provide any leftist city with all the green plants they want.
Don’t forget the $%#$@# Johnson grass. I am still fighting the blasted stuff. Just introduce it to all the city parks (snicker)

…Today, Johnsongrass is a weed in tropical and temperate climates throughout the world, and is considered among the world’s 10 worst weeds. It has become naturalized throughout much of the U.S. and is spreading northward into Canada…
Johnsongrass forms an extensive system of stout, creeping, fleshy rhizomes (horizontal underground stems) that can grow over 6 feet long…. Stems are stout, erect, round, hollow, unbranched (sometimes branched) and smooth, growing 3 to 8 feet tall (sometimes up to 10 feet)….
Johnsongrass can produce toxic amounts of cyanide if growing under stressful conditions….

Luther Wu
July 22, 2012 4:26 pm

polistra says:
July 21, 2012 at 12:59 pm
Also need to bear in mind that trees won’t survive every type of pollution. Back in the ’70s a lead smelter near Blackwell was trying to clean its reputation without cleaning its output, and planted a long avenue of poplars along the side of its facilities. The trees turned gray, bent over and died within a year, leaving a dramatic demonstration of the pollution instead of a covering shield of green.
Zinc smelter, as I recall- Blackwell, OK, USA

July 22, 2012 5:16 pm

“The study concluded that judicious placement of grass, climbing ivy and other plants in urban canyons can reduce the concentration at street level…”
So it’s not just about trees. Here in Ontario, Canada it is becoming increasingly difficult to maintain a green lawn, mostly because the eco-extremists have banned the use of what they call “cosmetic” herbicides and pesticides. As a result lawns are being destroyed by weeds and grubs, and some people are even switching to some kind of ornamental rocks and gravel (although I’m sure it has a more appealing name) instead.

July 22, 2012 5:55 pm

Ouch Gail Combs,
Johnsongrass is also a leafhopper Graminella Nigrifrons host. This bug is carrying maize fine streak nucleo-rhabdo-virus (MFSV) leading to stunted/chlorotic corn plants, in addition to being an insect vector of both corn stunt spiroplasma & maize bushy stunt phytoplasma pathogens. That leafhopper must ingest MFSV from a contaminated plant in order to spread it to maize/corn.
One of the under reported effects of elevating CO2 on plant leaves is their relatively reduced nitrogen content leads to proportionately more leaf munching by bugs in attempt to compensate for lower nitrogen amount per bite. So, outside of a greenhouse where plants are exposed to lots of insects there is more herbivore bug feeding being done on the CO2 boosted plant. If the Johnsongrass host leaves are getting even more chewed on by leafhoppers they then go over & chew even more on nearby food crop leaves
Look how the interplay becomes more variable; although I am no longer discussing plant pathogens or their resolution. Jasmonic acid is plants first (but not only ) line of defense against munching insects. WIthin most genetic types of plants the physical contact with a herbivore bug causes that plant to up-regulate it’s output of jasmonic acid. But experiments with higher than contemporary CO2 show some genetic variations of some plants are not able to boost protective jasmonic acid in response to chewing insects and some actually put out less jasmonic acid when bug chews it.
The impracticality of eradicating Johnsongrass points to agriculture’s best hope lying in genetic manipulation to introduce more jasmonic acid genes into food crops. This would carry over in a general sense to provide additional plant resistance against chewing bug attacks anticipated to be more active as CO2 rises. Humans have been breeding plants for a long time so probably some of that resulted in improved jasmonic acid genetics as evidenced in specific seed strains local farmers say they plant because “local” bugs leave it alone enough to harvest a worthwhile yield. Pardon the digression I’ve indulged in.

July 23, 2012 5:13 am

This will be great for the environment. More needs to be done to combat the problems of climate change. Affirmative action is what is required.

Gail Combs
July 23, 2012 5:27 pm

gringojay says:
July 22, 2012 at 5:55 pm
Ouch Gail Combs,
Johnsongrass is also a leafhopper Graminella Nigrifrons host. ….
I have found the easiest way to kill Johnsongrass is to confine my goats in the patch and let them eat it and stomp it to death. Unlike horses goats do not pass viable seed in their feces. Round-up only seems to kill the stems and the stuff comes back a few months later or the next year either from seeds or the rhizomes.
One of the problems with studies like this is the politicians go off half-cocked and do stupid things like intentionally planting Kudzu or Johnsongrass or Kentucky 31 Fescue (another grass with problems – abortion in pregnant herbevoires)
Johnsongrass because of its extensive rhizome system is considered a valuable species for erosion control and use to be planted in road side gullies. see FAO

Brian H
July 23, 2012 7:20 pm

R Barker says:
July 21, 2012 at 9:14 am
Can I reinvent the wheel and get paid for it?

Been done. It was reinvented many times before the current model came into use. Problem was, the first ones were square or triangular, and the corners kept wearing off! It took many iterations and prototypes, each with more sides, till the infinite corners model was derived. Proving that dividing (wear) by infinity really does give zero.

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