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Study Confirms It: Trees Pollute
November 2, 2015   
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A new study confirms speculation that trees contribute to rather than reduce air pollution in many areas. Trees, along with car engines and furnaces, are the main culprits responsible for airborne particulate matter—tiny particles that can get lodged in people’s lungs, leading to asthma and cancer—according to the study, which was recently published in the journal Chemical Reviews.

Trees produce and emit isoprene, an abundant molecule in the air known to protect leaves from oxygen damage and temperature fluctuations. Isoprene is involved in the production of particulate matter.

The microscopic specks that comprise particulate matter are composed of a mixture of inorganic and organic chemicals. Despite growing interest among scientists and improved analytical techniques, the chemical complexity of particulate matter and the complicated reaction pathways leading to its formation are largely unknown. More study is needed to understand the complex impact of particulate matter on human health, quality of life and climate change, says the extensive report, which was written by an international team of researchers in Chemical Reviews, a peer-reviewed scientific journal published by the American Chemical Society.

“Particulate matter, rich in organic compounds, is literally omnipresent,” says Rafał Szmigielski, Ph.D., from the Polish Academy of Sciences’ Institute of Physical Chemistry in Warsaw, one of the authors. “With each breath we take, the chemical components of this cocktail enter our lungs and once there, they penetrate cell membranes and go directly into the bloodstream. Information about what we are really breathing is therefore of absolutely key importance for our lives.”

Particulate matter is an aerosol, an array of multiple solid or liquid particles with diameters no larger than one-tenth the diameter of a human hair. These particles are composites of hundreds or even thousands of chemical compounds: from very simple ones comprising only a few atoms, to polymer chains of several tens of thousands of atoms or even fragments of DNA. According to recent estimates, simple mineral (inorganic) substances constitute far less than half of the components of aerosol particles.

“We now know that particulate matter contains mainly nitrates and sulfates,” Szmigielski says. “We also understand where they come from—usually from a complicated sequence of reactions as a result of which nitrogen and sulfur oxides in the atmosphere are converted into acids; or else they are the result of natural rock erosion processes. However, the composition and origin of the main, organic fraction of the ambient aerosol remained virtually unknown until last year.”

The scale of the challenges associated with the measurement and analysis of particulate matter turned out to be comparable to that of penetrating such exotic environments as the ocean floor or the surfaces of other planets. The researchers had to develop new methods of chemical analysis and to adapt existing ones.

Today, particulate matter is collected at measuring stations where it settles on special filters. “We have been collecting the aerosol particles for research at the institute from the air of the cleanest Polish regions, using the most modern dust collectors equipped with quartz filters for this purpose,” Szmigielski says.

Filters with concentrated aerosol particles, after being isolated from the environment, are transported to the laboratory. Here, the chemical compounds forming the dust are transferred into solutions in which they undergo testing with ultrasensitive analytical techniques: liquid chromatography coupled with mass spectrometry.

“Liquid chromatography enables us to separate mixtures of organic compounds into single components,” Szmigielski says. “And using mass spectrometry, we can determine the molecular weight of each compound studied and postulate its chemical structure and elemental composition,”
The use of liquid chromatography coupled with mass spectrometry has enabled scientists to better understand the interactions taking place in the atmosphere between the gaseous substances contained within it and the particulate matter that is formed. In cities, the role of nuclei mopping up successive chemicals from the environment is predominately played by dust and carcinogenic soot particles arising as a result of the combustion processes of garbage and contained in exhaust gases, especially those emitted by cold diesel engines.

However, recent studies have confirmed a hypothesis formulated half a century ago: It is that, on the scale of the whole planet, trees play an important role in the formation of particulate matter. In the course of evolution, trees have developed a smart immune system: while growing, they produce large quantities of highly volatile substances that surround them with a protective gas cloud. The purpose of these gases—mainly isoprene in the case of deciduous trees and alpha-pinene in the case of coniferous trees—is to deter unwanted insects and trap harmful chemicals before they penetrate plant cells. It turns out that nowadays molecules of these compounds act as precursors around which harmful chemicals from human activities condense.

Epidemiological studies show a clear link between pollution in an area with a high concentration of aerosol particles and the morbidity of the local population. The link is particularly evident in the case of lifestyle diseases, including asthma, allergies, heart disease and various dermatological problems.

“The widely known problems of miners with pneumoconiosis are due to exposure to high concentrations of mineral dust containing fairly simple inorganic compounds. In the case of aerosols in big cities we are dealing with a cocktail of really harmful chemicals that we breathe in throughout our lives,” says Szmigielski. “Our exposure is continuous, which means that even quite small concentrations of dust may prove harmful. This is why appropriate education is extremely important, making the inhabitants of a given area aware of the risks associated with the presence of particulate matter and mobilizing local authorities to monitor concentrations and the chemical composition of the dust, and consequently its sources as well.”

The report on particulate matter published in Chemical Reviews was authored by research teams from Belgium, Denmark, Finland, France, Germany, Norway, Poland, Slovenia, the United States, Britain, Italy and Switzerland.
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