Planting trees is the most popular strategy to help make cities ‘greener,’ both literally and figuratively. However, according to the American Chemical Society, scientists have found a counterintuitive effect of urban vegetation during heat waves: Urban trees can increase air pollution levels and the formation of ground-level ozone.

What Is a Ground-Level Ozone?

Ground-level ozone or “bad” ozone is a colorless and highly irritating gas that forms just above the earth’s surface. It is not emitted directly into the air but is created through chemical reactions of different volatile organic compounds. It is referred to as a “secondary” pollutant because it is produced when two primary pollutants react in sunlight and stagnant air. These two primary pollutants are nitrogen oxides (NOx) and volatile organic compounds (VOCs).

According to a previous research, planting trees in cities have multiple benefits, including storing carbon, controlling storm water and cooling areas off by providing shade.

This drove efforts in cities across U.S. and Europe to encourage the practice. However, according to research,  it’s also known that trees and other plants release volatile organic compounds or VOCs, that can interact with other compounds that worsen air pollution. When it’s hot, plants release higher levels of VOCs.

Galina Churkina of the Institute for Advanced Sustainable Studies in Potsdam, Germany, and her colleagues investigated what effects heat waves and urban vegetation might have on air pollution.

These team of researchers compared air pollutant concentrations in the Berlin-Brandenburg metropolitan area in Germany in the hot summer of 2006, when there was a heat wave, and the summer of 2014, which had average seasonal temperatures. Their test showed that during the summer of 2006, VOCs from urban greenery contributed to about 6 to 20 percent of the ozone formation and that during the heat wave period, the contribution spiked to up to 60 percent.

How to Prevent Formation of The “Bad” Ozone?

Churkina’s team suggested that in addition to tree-planting campaigns,  there should be efforts to improve cities’ environments such as reducing vehicular traffic, which is a major source of nitrogen oxides that can react with VOCs and form ozone.

Another study from the  University of Surrey also said that the harmful impact of air pollution can also be prevented by strategically placing low hedges along roads in a built-up environment of cities instead of taller trees.

This study was published in the Journal of the Atmospheric Environment with the collaborative efforts of partners from UK, Europe, and USA, led by Professor Prashant Kumar under the H2020 project of iSCAPE (Improving Smart Control of Air Pollution in Europe).

The research also stressed that higher trees only have more of an impact in reducing air pollution in areas which are more open and are less densely populated, and do not have taller buildings.

As of now, urban air quality continues to be a primary health concern as most of the world’s population currently lives in urban areas (54% in 2014), and percentage is projected to rise to 66% by 2050; this is coupled with the fact that one of the main global sources of air pollution in cities is traffic emissions.

What This Means for Yemen

Yemen as of now is one of the most polluted countries, according to Numbeo, with a pollution index of 71.26. Aside from air pollution, the country is also suffering from the accessibility of water supply and contamination. Although there are very few vehicle owners in Yemen, and most people commute from one place to another, emissions from vehicles, power plants and industrial saws, contribute to these, especially in cities.

Also, thousands of Yemenis returned to their homes and brought their decade-old cars with them after the 1990 Gulf war. The majority of these vehicles run on leaded gasoline and local diesel which contain high levels of impurities.

This should start by eventually banning the use of old cars that are proven to be harmful to the environment, followed by greening up.

Green infrastructure in cities is the best urban planning solution for improving air quality as well as enhancing the sustainability of cities for growing urban populations.

Planting more street trees, vegetation barriers (including hedges), green (or living) walls, and green roofs are the main solution. They act as porous bodies that influence dispersion of pollution and aid the deposition and removal of airborne pollutants.

Haitham Alaini