Hedging against traffic emissions? Cut roadside pollution with vegetation barriers

Despite reductions in emissions, air quality remains poor in Europe in many areas. For example, in large parts of Europe in 2017, concentrations of particulate matter (PM) continued to exceed the EU limit values and the World Health Organization’s air quality guidelines, as stated in a report by the European Environment Agency. PM, also known as particle pollution, refers to a complex mixture of extremely small particles and liquid droplets that get into the air. Once inhaled, these particles can affect the heart and lungs and cause serious health problems. In recent years, the use of green infrastructure like trees, hedges and individual shrubs along busy urban roadsides has gained prominence in reducing air pollution exposure.

Partially supported by the EU-funded iSCAPE project, a team of researchers conducted a 5-month experiment measuring traffic pollutants behind and in front of a hedge that shielded a children’s park in Guildford, the United Kingdom. “This site was chosen since it constitutes a long unobstructed hedge next to a busy road, and thus constitutes an ideal location for evaluating the effect of the hedge itself,” the researchers say in an article published in the journal ‘Sustainable Cities and Society’.

Factors affecting pollution levels

The researchers used a sensor system to examine whether or not there was a reduction in pollution during the vegetation cycle of a beech hedge. The results showed that the weather, public holidays and the stage of the hedge’s life cycle had an impact on the drop in pollution concentration levels. “The absolute PM concentrations in front and behind the hedge are dominated by meteorological, biological and societal variables as seen by e.g. half term and periods of rain.” The study adds: “For instance, the drop observed in the middle of February is caused by the half-term break (a public holiday in the UK), and the extended period of low concentrations at the beginning of March is caused by rain.”

The experts from project partner University of Surrey’s Global Centre for Clean Air Research (GCARE) who conducted the experiment reported reductions of over 50 % of PM after the hedge’s green-up stage in late April. “Experts believe that this could be because the density of the hedge or the stickiness of the leaves had a sizable impact on particle pollutants passing through it,” as noted in a news release by the University of Surrey. “However, the results also revealed smaller reductions for gaseous pollutants including carbon monoxide and nitrogen dioxide, and that wind direction had little impact on the concentration levels.”

According to the same news release, Prof. Prashant Kumar, founding director of GCARE and corresponding author of the study, said: “This study has not only produced unique evidence and support for our advocacy to install hedges and other forms of green infrastructure (where appropriate) along busy roadsides to protect schools, playgrounds and pedestrians/cyclists from air pollution exposure; it has also provided a clear indication that evergreen species should be favoured for barriers against air pollution to exploit their year-round performance.”

After 36 months, the iSCAPE (Improving the Smart Control of Air Pollution in Europe) project that supported the study concluded in summer 2019.

For more information, please see:

iSCAPE project website