Unexpected: Why planting trees could worsen air quality in urban areas

A study reveals that some tree species, in certain urban contexts, could have the opposite effect and end up worsening air quality.

New York
New York plans to increase its tree coverage from 22 to 30%.

Many cities are planting trees to improve air quality, as a measure of adaptation to climate change. Despite the good intention of these initiatives, the results could go against the proposed objectives, and result in a worsening of air quality.

This is what research from New York City University, Columbia University and Stony Brook University warns.

New York is carrying out an ambitious plan that aims to increase tree coverage from 22% to 30% of the state's surface. For that, they are planting two species: oaks and liquidambars, native trees that are already part of more than half of the parks of the big apple and its surroundings.

The study, which was published this week in Environmental Science & Technology, analyses the impact of urban environmentalisation on air quality in New York, by calculating the emissions of biogenic volatile organic compounds (BVOCs) and their contribution to ozone formation.

The work points out that not all species work the same. Among the volatile organic compounds, trees release isopren, a chemical that reacts with other air-polluting gases and produce ozone, a very harmful substance to breathing.

Although the combination of isoprene and other gases does not represent a problem in rural areas, it can be in areas where the air is very polluted, such as New York.

trees, new york
Isoprene interacts with polluting gases and can produce ozone.

The researchers used a high-resolution MEGAN model (30 m) to estimate the evolution of isoprene emissions in different tree cover scenarios.

According to calculations, in a scenario in which the city increases afforestation to 30%, isoprene emissions could increase between 1.4 and 2.2 times in Manhattan alone.

This could cause ozone levels to rise between 8 and 19 parts per billion, that is, up to 30%. Such an increase could cause the city to exceed, more often, the air quality limits considered healthy in the country.

Researchers point out that not all trees produce the same amount of isoprene. Oaks, for example, generate high levels of this substance. Others, such as ginkgos or lindens, emit much less. That's why it doesn't matter which tree to plant.

The real problem is pollution, not the wrong trees

The researchers defend the initiative and highlight the multiple benefits of planting trees, such as the capture of carbon dioxide, the attenuation of the urban island of heat, and improvements in the physical health and well-being of people.

"We are all in favour of planting more trees. They bring many good things," said the co-author of the Róisín Commane study, atmospheric chemistry from the Lamont-Doherty Land Observatory of the Columbia Climate School. "But if we are not careful, we could worsen the air quality."

The authors point out that trees could never be responsible for air pollution. The real culprit is the high level of nitrogen oxide (NOx), a substance from fossil combustion and industrial activities.

trees
In highly contaminated environments, each species of tree will produce different effects.

If this pollutant did not exist in the environment, there would be no need to worry about isoprene emissions, or the amount or type of tree to reforest. "If we significantly reduced NOx, trees would not be a problem," explains the main author of the study, Dr. Dandan Wei.

We don't want to convey the idea that trees pollute the air. It's the cars, said the leader of the investigation.

In this sense, the reduction of emissions of NOx and other polluting gases is going at such a slow pace that it is estimated that it could take up to 80 years until isoprene emissions from trees do not contribute significantly to the formation of ozone.

As a measure of adaptation to climate change, many cities are implementing urban afforestation plans that aim to improve the air quality and the quality of life of the inhabitants.

This research highlights the importance of having accurate data on soil cover and the composition of tree species to model the emissions of volatile organic compounds, and the need to adopt a comprehensive and holistic view in planning.

The study received financial support from the National Oceanic and Atmospheric Administration (NOAA) and the New York State Energy Research and Development Authority.



Reference of the news:

Wei, A. et al. "High-Resolution Modeling of Summertime Biogenic IsopreneEmissions in New York City". Environmental Science & TechnologyVolume 58, Issue 31August 6, 2024.