What trees do to the air around them
Air in cities carries a mix of pollutants: tiny solid and liquid particles from traffic, industry and burning, and gases such as ozone, nitrogen dioxide and sulfur dioxide. Trees can remove some of each. They do this passively, without any special process, simply by being large, leafy surfaces in the path of moving air.
The effect is real, but it is easy to overstate. A fair summary is that urban tree cover helps reduce pollution in a modest way, most noticeably close to the trees, and that it works best as a supplement to cutting emissions rather than a substitute.
Trapping particles
Fine particles, often described by their size, such as PM2.5 and PM10, are among the pollutants most harmful to human health. When air moves past a tree, some particles land on the leaves, bark and twigs and stick. Rough, hairy or waxy leaves tend to catch more than smooth ones, and conifers with needles that stay on all year can capture particles through winter, when deciduous trees are bare. Their differences are described under conifer forests and deciduous forests.
Trapped particles are not permanently gone. Rain can wash them to the ground, wind can blow them off again, and when leaves fall the particles can return to the soil or drains. Trees therefore act more as a temporary holding place than as a final disposal site. The main benefit comes when particles are held long enough to avoid being breathed in at street level.
Absorbing gases
Gases enter the leaf through small pores called stomata, the same openings used for taking in carbon dioxide during tree photosynthesis. Ozone, nitrogen dioxide and some other gases can enter along with it and are then broken down or absorbed by the leaf’s tissues. Uptake is greatest when the tree is healthy, has enough water, and the stomata are open. During drought or in extreme heat the pores close to save water, and the pollutant uptake falls just when ozone is often at its worst.
Trees also release natural compounds, and in some conditions these can contribute to ozone formation when mixed with traffic fumes. This effect is usually smaller than the benefits, but it is one reason that species choice matters.
Where trees help most
Location counts for more than the number of trees. Studies of the topic generally find that:
- Near sources such as busy roads, trees can reduce exposure for people standing downwind, especially with dense, continuous planting like a hedge or a belt of mixed vegetation.
- At the neighborhood scale, a large park or high canopy cover can lower average concentrations slightly.
- Around schools, playgrounds and bus stops, even small improvements may be worth having because of who is nearby.
The amount of improvement reported varies widely between studies and cities, since it depends on climate, pollution levels, tree type and design. Anyone quoting a precise percentage for a whole city should be read with caution.
When trees can make things worse
Planting in the wrong way can trap pollution rather than clear it. In a narrow street with tall buildings on both sides, a dense canopy overhead can reduce the airflow that would normally disperse exhaust, so that fumes build up at breathing height. Planners generally prefer more open spacing or lower, hedge-like vegetation in such places.
Some species release a lot of pollen or volatile compounds that irritate people with allergies or asthma. Sap, fruit and leaf litter can also create maintenance headaches. Choosing low-allergen species that suit local conditions is part of good design.
The smoke problem
Trees remove pollution but they cannot cope with extreme episodes. During heavy smog or a smoky season, concentrations are far above what a canopy can absorb. Wildfire smoke, for example, can blanket a city with fine particles, and a few street trees will make little difference. Trees in a smoke-filled city also face stress of their own.
Keeping the trees healthy
A tree that is struggling does not clean the air well. Trees in cities suffer from compacted soil, limited root space, road salt, drought, and attack by forest insect pests. Pollutants such as ozone can themselves damage leaves. Large, healthy, long-lived trees give much more benefit over time than small, short-lived ones, which is why protecting existing mature trees is often a better use of effort than replacing them with saplings.
Diversity helps too. A city dominated by one species is vulnerable if a pest or disease arrives, and aggressive plants can spread into nearby wild areas, as discussed under invasive species. Selecting a mix of well-adapted species spreads this risk.
Putting it in perspective
The most effective way to improve city air is to produce less pollution in the first place, through cleaner vehicles, better public transport, and controls on industry and heating. Trees are a helpful addition. They also provide shade, habitat and a more pleasant environment, so the case for them rarely rests on air quality alone.
Reasonable expectations are therefore these: trees capture some particles and gases, work best close to sources and when well maintained, can backfire if poorly placed, and are no substitute for reducing emissions. Where they are placed, protected and maintained is a design question that belongs to wider urban forest planning.