Organic carbon released from plants affects the air

The team, led by scientists at the California Institute of Technology (Caltech), has discovered an unknown element in the process by which plant-generated gases and other plant species become aerosols - polar elements small in the atmosphere.

Their study of the formation and impact of these chemicals, called epoxides, is published in the journal Science.

Paul Wennberg, R. Stanton Avery emeritus professor of atmospheric chemistry and environmental science, and center director Ronald and Maxine Linde on Global Environmental Science at Caltech, with John Seinfeld, professor For Louis E. Nohl on chemistry, he studied the role of biogenic waste - organic carbon compounds released by plants and plants - in atmospheric chemical reactions that lead to aerosols formation.

Wennberg stressed: 'If you mix city waste and plant-released waste, they will interact and change the chemistry of the atmosphere'.

While most attention is focused on the impact of emissions from automobiles and industrial production, our understanding of what happens to biogenic waste is very limited, especially in areas where there are very few man-made wastes. Wennberg explains: 'What we care about is what happens to the chemicals produced by plants when they are released into the atmosphere'.

In these studies, the team focused on a chemical called isoprene, released by deciduous trees. Wennberg said: 'Oak is the plant that produces the most waste. And the amount of isoprene they emit is one of the reasons why the Smoky Mountains look smoky. '

Isoprene is not a secondary factor in atmospheric chemistry, Wennberg said: 'The amount of isoprene released into the atmosphere is much greater than all other gases - petroleum, industrial chemicals - are discharged. from human activities, except for methane and carbon dioxide. And isoprene only comes from plants. Plants create hundreds of millions of tons of this chemical . for reasons that we have not fully understood. '

Researchers at Caltech Fabien Paulot, the lead author of the paper, said: 'Most of the isoprene is released in areas with very little human origin. Its chemical properties are still not well understood. '

When released into the atmosphere, isoprene is "oxidized" by oxidizing substances such as OH, Wennberg explains. This chemical nature is the focus of research. Specifically, the study was conducted to understand how isoprene oxidation can lead to the formation of atmospheric particulate matter, secondary organic aerosols. Seinfeld said: 'A small amount of isoprene becomes a secondary organic aerosol, but because the amount of isoprene is so large, this fraction is also very important.'

Until now, the chemical process from isoprene to aerosol is still known. Wennberg, Seinfeld, and colleagues discovered that aerosols are very likely to form from chemicals called epoxides.

Wennberg said: 'These epoxides are natural glue'. And similar to the epoxy you buy in tool shops and home appliances - an acid is needed to make glue - epoxides found in the atmosphere also need an acidic effect to become "sticky."

Caltech scientists used spectroscopy on NASA's DC8 plane to measure epoxides above Canada's Boreal Forest in the summer of a foreign year.Epoxides are formed from isoprene, a chemical released by many plants into the atmosphere.(Photo: Yohei Shinozuka)

Wennberg explains: 'Gases of these epoxides meet acidic particles, they make glue. Epoxides precipitate in the atmosphere and stick to particles, causing them to thicken so that visibility in the atmosphere is reduced. Because the acidic properties of aerosols are usually higher when the source-derived activities are present, the efficiency of the conversion of epoxides to aerosols will be higher in polluted environments. This shows another complex interaction between the emissions from the biosphere and from humans. "

Seinfeld said: 'The atmospheric particles have been shown to have an impact on human health, because they are small enough to penetrate into the lungs of people. At the same time, aerosols affect the climate of the Earth by dispersing and absorbing solar radiation and through acting as a nucleus for clouds to form. So knowing the source of the element in the atmosphere is an important thing '.

The research team can achieve this scientific step thanks to the development of a kind of ionization mass spectrometer (CIMIS), led by co-author and graduate student at Caltech John Crounse. 'This new CIMS method opens up new possibilities for the study of new sets of compounds that scientists could not measure before, mainly because they decay when analyzed with transmission techniques. system '.

In general, molecules detected and quantified by spectrometers must be converted to charged ion. They are then directed into an electromagnetic field, where ions are classified by mass. The problem with traditional ionization techniques is that 'fragile' molecules, such as those produced in isoprene oxidation, often break into pieces during oxidation, causing recognition. They are very difficult or impossible. Wennberg commented: 'This new method was developed to allow scientists to perform atmospheric measurements from aircraft. It can ionize the gas, including peroxide compounds that are very fragile, while preserving the original molecular size or mass' information.

This makes it easier to identify individual gases in a complex compound - especially when you look for a compound that you don't intend to search for.

Wennberg and colleagues also used oxygen isotopes - oxygen atoms with different amounts of neutrons in the nucleus, thus having different masses - to give an understanding of the chemical mechanism that makes epoxides. Epoxides have not been discovered to this day because they have the same mass as another chemical thought to form in isoprene, peroxide oxidation. Paulot commented: 'Oxygen isotopes divide peroxides and epoxides and show that when epoxides form, OH is recovered in the atmosphere. Since OH is an atmospheric eraser, cleaning up many chemicals in the atmosphere, regenerating OH has important implications for the oxidative ability of the atmosphere '.

Identifying an important photosynthetic reaction in the formation of epoxides helps explain the effect of the release of plant organic compounds on air in urban and rural conditions. Trees are not entirely 'assassins' like Ronald Reagan was ridiculed for calling them so, the level of isoprene release should be' part of the criteria we use when buying and planting trees in areas Wennberg stressed. In fact, he points out that the Southern Ocean Air Quality Administration has done this with a list of 'approved' plants that do not emit large amounts of organic carbon compounds into the atmosphere. .

In addition to Wennberg, Paulot, Crounse, and Seinfeld, other authors of Science articles include Henrik Kjaergaard of Otago University, New Zealand; Priest scholar Dr. Andreas Kürten, currently at Goethe University in Germany, and Caltech postdoctoral scholar Jason St. Clair.

The study described in the Science article is sponsored by Caltech administrator William Davidow, Department of Science, US Department of Energy, US Environmental Protection Agency, New Zealand Royal Society, and NASA. .

Refer:
1. Paul Wennberg et al.Unexpected epoxide formation in the gas-phase photooxidation of isoprene.Science, August 7, 2009