Do trees really help clear the air in our cities – or are they trapping pollutants at street level?

Trees: friend or foe? Brooklyn's Prospect Park. Image: Spencer Platt/Getty.

It may sound like a no-brainer to say that trees improve air quality. After all, we know that trees absorb the greenhouse gas carbon dioxide (CO₂), and that their leaves can trap the toxic pollutants nitrogen dioxide (NO₂), ozone, and harmful microscopic particles produced by diesel vehicles, cooking and wood burning.

Yet some recent studies have suggested that trees may in fact worsen urban air quality by trapping pollutants at street level. A closer look at the evidence – and how it was collected – reveals the root of this dispute, and can help us come to a more nuanced understanding of the impacts of trees on our urban environment.


First things first; it is not trees that pollute the air of cities in the developed world. As car manufacturers are all too guiltily aware, it is mainly road vehicles that cause pollution – and their impacts are compounded by the choices we make about how and what we drive.

Many features of the urban landscape influence how air moves around a city. Impervious objects like buildings, and permeable ones like trees, deflect air from the path imposed by weather patterns, such as high and low pressure systems. The urban landscape turns freshening breezes into whorls of air, which can either contain pollution near its source – where it affects vulnerable hearts and lungs – or lift it away from ground level.

Whether the landscape traps or lifts air will depend very sensitively on the exact positioning of roads, buildings, gardens, street trees, intersections, even billboards and other street furniture.

Sticking points

Trees affect the urban environment in several subtle ways. From altering air flows, to collecting pollution deposits, to affecting the chemical make up of the atmosphere, their impacts are both pervasive and difficult to pinpoint.

As air swirls and twists past the urban fabric, microscopic pollutants can deposit on surfaces. Any surface will do, but trees are especially effective at trapping these particles, because of their large, porous surfaces.

We breathe air and – generally speaking – don’t lick leaves

One way we can tell whether trees are helping to reduce air pollution is by estimating the mass of pollutant deposited. Experiments to measure depositing pollutants are usually carried out in the middle of flat fields, where it is easier to interpret the measurements. But, of course, a city is a very different environment, and it’s not clear whether these results hold true in highly variable urban settings.

Experimental studies can certainly show that pollution ends up on leaves. But it is no easy task to convert such measurements into an estimate of how the concentrations – that is, the amount of pollutant per cubic metre of air – change. And it is this concentration change that really counts, since we breathe air and – generally speaking – don’t lick leaves.

Some pollutants, like NO₂, are both emitted by human fuel use and produced when chemical reactions take place in the atmosphere. Other pollutants, notably ozone, are only produced through reactions of nitrogen oxides with fumes from oil-based solvents, petrol and similar chemicals in the air.

The production of toxic ozone can happen purely as a result of our consumption of fossil fuels: particularly when hot, settled summertime conditions provide the light needed to kick-start the chemical reactions, while the stillness prevents dilution of the pollution into the global atmosphere.

That said, trees also release chemicals that react with nitrogen oxides to produce ozone, sometimes in sufficient quantity to make a difference, even in urban areas.

Trees also take up space. Parks and gardens are not usually sites with intense pollutant emissions, so they provide an important volume into which pollution can be diluted. This is evidenced by statistical studies, which show how concentrations of air pollutants vary according to the type of urban neighbourhood: the decrease of pollutant concentrations away from busy roads is modified by how tall the buildings are in the neighbourhood.

Seeing the wood for the trees

When assessing research on the effects of trees on urban air pollution, remember that no single study has yet put all the pieces of the puzzle together. With so many processes to consider, it’s little wonder that experiments based in a range of locations, using varying methods, yield vastly different results.

To produce a definitive study, it would take either many months of measurements before and after the planting of vegetation, or a shorter series of simultaneous measurements in two urban locations identical in all respects, except for the presence of trees or some other form of “green infrastructure” in one location. Both approaches are expensive and difficult to undertake in busy cities, which are constantly subject to all kinds of other changes.


So, we are left with piecing together the evidence as it is presented to us in reports. When doing so, look first at whether the study is, in fact, concerned with how air is dispersed in urban areas, and remember that such dispersion really depends on every part of the cityscape, not just on trees. Look to see if, and how, removal of pollutants by deposition is considered, and then check whether the study considers the effects of dilution or atmospheric chemistry.

Finally, consider the results of any single study in the light of the best available systematic approach to trees in the townscape before drawing conclusions.

Asking whether cities should have trees in it is a bit like asking whether a suit should have a person in it. There is every chance that urban trees could provide a “nature-based solution” to several pressing problems with the urban environment, but perhaps not in the way scientists and policy-makers seem currently to be thinking.

Rather than providing a technical fix that disguises our obsession with the diminishing returns of the internal combustion engine, increasing urban tree numbers could change our entire perspective on cities, facilitating the creation of liveable cities that value nature as an integral part of social, economic and environmental capital.The Conversation

Rob MacKenzie is professor of atmospheric science at the University of Birmingham.

This article was originally published on The Conversation. Read the original article.

 
 
 
 

To build its emerging “megaregions”, the USA should turn to trains

Under construction: high speed rail in California. Image: Getty.

An extract from “Designing the Megaregion: Meeting Urban Challenges at a New Scale”, out now from Island Press.

A regional transportation system does not become balanced until all its parts are operating effectively. Highways, arterial streets, and local streets are essential, and every megaregion has them, although there is often a big backlog of needed repairs, especially for bridges. Airports for long-distance travel are also recognized as essential, and there are major airports in all the evolving megaregions. Both highways and airports are overloaded at peak periods in the megaregions because of gaps in the rest of the transportation system. Predictions for 2040, when the megaregions will be far more developed than they are today, show that there will be much worse traffic congestion and more airport delays.

What is needed to create a better balance? Passenger rail service that is fast enough to be competitive with driving and with some short airplane trips, commuter rail to major employment centers to take some travelers off highways, and improved local transit systems, especially those that make use of exclusive transit rights-of-way, again to reduce the number of cars on highways and arterial roads. Bicycle paths, sidewalks, and pedestrian paths are also important for reducing car trips in neighborhoods and business centers.

Implementing “fast enough” passenger rail

Long-distance Amtrak trains and commuter rail on conventional, unelectrified tracks are powered by diesel locomotives that can attain a maximum permitted speed of 79 miles per hour, which works out to average operating speeds of 30 to 50 miles per hour. At these speeds, trains are not competitive with driving or even short airline flights.

Trains that can attain 110 miles per hour and can operate at average speeds of 70 miles per hour are fast enough to help balance transportation in megaregions. A trip that takes two to three hours by rail can be competitive with a one-hour flight because of the need to allow an hour and a half or more to get to the boarding area through security, plus the time needed to pick up checked baggage. A two-to-three-hour train trip can be competitive with driving when the distance between destinations is more than two hundred miles – particularly for business travelers who want to sit and work on the train. Of course, the trains also have to be frequent enough, and the traveler’s destination needs to be easily reachable from a train station.

An important factor in reaching higher railway speeds is the recent federal law requiring all trains to have a positive train control safety system, where automated devices manage train separation to avoid collisions, as well as to prevent excessive speeds and deal with track repairs and other temporary situations. What are called high-speed trains in the United States, averaging 70 miles per hour, need gate controls at grade crossings, upgraded tracks, and trains with tilt technology – as on the Acela trains – to permit faster speeds around curves. The Virgin Trains in Florida have diesel-electric locomotives with an electrical generator on board that drives the train but is powered by a diesel engine. 

The faster the train needs to operate, the larger, and heavier, these diesel-electric locomotives have to be, setting an effective speed limit on this technology. The faster speeds possible on the portion of Amtrak’s Acela service north of New Haven, Connecticut, came after the entire line was electrified, as engines that get their power from lines along the track can be smaller and much lighter, and thus go faster. Catenary or third-rail electric trains, like Amtrak’s Acela, can attain speeds of 150 miles per hour, but only a few portions of the tracks now permit this, and average operating speeds are much lower.

Possible alternatives to fast enough trains

True electric high-speed rail can attain maximum operating speeds of 150 to 220 miles per hour, with average operating speeds from 120 to 200 miles per hour. These trains need their own grade-separated track structure, which means new alignments, which are expensive to build. In some places the property-acquisition problem may make a new alignment impossible, unless tunnels are used. True high speeds may be attained by the proposed Texas Central train from Dallas to Houston, and on some portions of the California High-Speed Rail line, should it ever be completed. All of the California line is to be electrified, but some sections will be conventional tracks so that average operating speeds will be lower.


Maglev technology is sometimes mentioned as the ultimate solution to attaining high-speed rail travel. A maglev train travels just above a guideway using magnetic levitation and is propelled by electromagnetic energy. There is an operating maglev train connecting the center of Shanghai to its Pudong International Airport. It can reach a top speed of 267 miles per hour, although its average speed is much lower, as the distance is short and most of the trip is spent getting up to speed or decelerating. The Chinese government has not, so far, used this technology in any other application while building a national system of long-distance, high-speed electric trains. However, there has been a recent announcement of a proposed Chinese maglev train that can attain speeds of 375 miles per hour.

The Hyperloop is a proposed technology that would, in theory, permit passenger trains to travel through large tubes from which all air has been evacuated, and would be even faster than today’s highest-speed trains. Elon Musk has formed a company to develop this virtually frictionless mode of travel, which would have speeds to make it competitive with medium- and even long-distance airplane travel. However, the Hyperloop technology is not yet ready to be applied to real travel situations, and the infrastructure to support it, whether an elevated system or a tunnel, will have all the problems of building conventional high-speed rail on separate guideways, and will also be even more expensive, as a tube has to be constructed as well as the train.

Megaregions need fast enough trains now

Even if new technology someday creates long-distance passenger trains with travel times competitive with airplanes, passenger traffic will still benefit from upgrading rail service to fast-enough trains for many of the trips within a megaregion, now and in the future. States already have the responsibility of financing passenger trains in megaregion rail corridors. Section 209 of the federal Passenger Rail Investment and Improvement Act of 2008 requires states to pay 85 percent of operating costs for all Amtrak routes of less than 750 miles (the legislation exempts the Northeast Corridor) as well as capital maintenance costs of the Amtrak equipment they use, plus support costs for such programs as safety and marketing. 

California’s Caltrans and Capitol Corridor Joint Powers Authority, Connecticut, Indiana, Illinois, Maine’s Northern New England Passenger Rail Authority, Massachusetts, Michigan, Missouri, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, Texas, Vermont, Virginia, Washington, and Wisconsin all have agreements with Amtrak to operate their state corridor services. Amtrak has agreements with the freight railroads that own the tracks, and by law, its operations have priority over freight trains.

At present it appears that upgrading these corridor services to fast-enough trains will also be primarily the responsibility of the states, although they may be able to receive federal grants and loans. The track improvements being financed by the State of Michigan are an example of the way a state can take control over rail service. These tracks will eventually be part of 110-mile-per-hour service between Chicago and Detroit, with commitments from not just Michigan but also Illinois and Indiana. Fast-enough service between Chicago and Detroit could become a major organizer in an evolving megaregion, with stops at key cities along the way, including Kalamazoo, Battle Creek, and Ann Arbor. 

Cooperation among states for faster train service requires formal agreements, in this case, the Midwest Interstate Passenger Rail Compact. The participants are Illinois, Indiana, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, and Wisconsin. There is also an advocacy organization to support the objectives of the compact, the Midwest Interstate Passenger Rail Commission.

States could, in future, reach operating agreements with a private company such as Virgin Trains USA, but the private company would have to negotiate its own agreement with the freight railroads, and also negotiate its own dispatching priorities. Virgin Trains says in its prospectus that it can finance track improvements itself. If the Virgin Trains service in Florida proves to be profitable, it could lead to other private investments in fast-enough trains.

Jonathan Barnett is an emeritus Professor of Practice in City and Regional Planning, and former director of the Urban Design Program, at the University of Pennsylvania. 

This is an extract from “Designing the Megaregion: Meeting Urban Challenges at a New Scale”, published now by Island Press. You can find out more here.