All Parks are not created equal… in terms of air quality benefits

The ability of trees to remove carbon dioxide from the air and capture carbon makes reforestation a way to combat global warming. It turns out, carbon dioxide is not the only pollutant that trees and other vegetation can help reduce. They can also remove ozone and particulate matter (PM) from the air through absorption through their stomata and deposition on foliage and bark. A group of researchers recently showed that park size and location, as well as types of vegetation make a difference in an urban park’s ability to improve air quality. 

Diversity of Parks

In a recent research study, a cross-institutional team of Italian scientists sought to better estimate the benefit of urban parks to air quality. These benefits have been estimated previously, but they did not take the diversity of parks into account. Specifically in this study, the researchers  wanted to examine how different characteristics of parks, like what man-made pollutants parks are exposed to, and where they are located, might impact the parks’ ability to reduce air pollution. Understanding what park-specific factors impact the ability of parks to reduce air pollution will help in future city planning and landscaping efforts. The researchers collected data from two different Italian parks in their study: Park of Castel di Guido, a 3673 hectare (ha) reforested area located northwest of Rome, and Park of Valentino, a 42 ha urban park in downtown Turin. 

Studying these two parks allowed the research team to compare the pollution-removal potential of (1) different types of vegetation, (2) different climates in central v. northern Italy, (3) different size parks, and (4) different proximity to urban areas. They conducted field surveys in each of these locations, collecting a variety of detailed information including tree species, diameter, and height. They then used additional techniques to estimate Leaf Area Index (LAI) and tree crown cover from collected images. They input these details as parameters into a model simulator called AIRTREE.

Tracking pollutants

In both Rome and Turin, climatic and air quality data are continuously monitored by a network of micrometeorological stations installed and maintained by the Regional Agency for Environmental Protection (ARPA). The researchers tracked the removal of three contaminants: carbon dioxide, particulate matter, and ozone. All three of these pollutants come from a variety of sources common in urban environments, such as vehicle emissions. Carbon dioxide (CO₂) is a greenhouse gas and contributes to global warming. The removal of carbon dioxide in this study was actually tracked as “net primary productivity” (NPP), or the amount of CO2 removed via photosynthesis- the amount of CO2 released during respiration.  Particulate matter (PM) represents a collection of fine (on the order of micrometers) liquid and solid particles that contribute to haze and can lead to various health effects including aggravated asthma, as well as decreased lung and heart function. Ozone (O₃) is an air pollutant at ground level and contributes to smog as well as respiratory health issues in humans.

How much pollution was removed?

The researchers estimated that the parks of Castel di Guido and Valentino could collect the PM emission of around 15,000 and 160 vehicles per year, and the CO₂ emission of around 2190 and 13 vehicles per year, respectively. This might not seem like a lot, but there are many urban parks and green spaces that work simultaneously to remove pollutants in these cities.

The large difference between the impact of these two parks is mostly due to their difference in size, but after normalizing collection rates per park area, the research team found that Castel di Guido still removed considerably more carbon and ozone, with a slight advantage in terms of PM. What causes this difference in removal rates? They found that the warmer climate (and resulting higher growth rates) at Castel di Guido led to higher removal rates. Generally, evergreen species removed more carbon dioxide and PM than broadleaf deciduous species due to enhanced opportunity for deposition on foliage. However, ozone removal was approximately equivalent between types of species due to enhanced rates of stomatal uptake in broadleaf trees during spring and summer compensating for their loss of leaves in fall and winter.

The researchers predicted that with realistic city planning, urban parks may only reduce pollutant concentrations by 1-7%… so it probably won’t make a huge difference in air quality. However, there are plenty of other benefits of urban parks, and they can be combined with other methods to improve air quality..

The benefits of urban parks

Based on this research publication, it looks like a large expansion of urban parks might not be the sole solution to urban air quality issues. However, thoughtful selection of trees with improved pollution-reducing capabilities for landscaping projects can increase their effectiveness as air-quality enhancers. Urban parks also provide measurable benefits to communities in terms of happiness, stress relief, and other forms of mental health. Furthermore, there is a large effort underway for companies to reduce their carbon footprint. Here is a list of companies with aggressive planning and goals in that area. 

Source article: S. Fares, A. Conte, A. Alivernini, F. Chianucci, M. Crotti, I. Zappitelli, F. Petrella, and P. Corona. Testing removal of carbon dioxide, ozone, and atmospheric particle by urban parks in Italy. Environ. Sci. Technol. 2020. https://pubs.acs.org/doi/abs/10.1021/acs.est.0c04740

Cover Image source: https://commons.wikimedia.org/wiki/File:Dublin_Stephen%27s_Green-44_edit.jpg