The atmosphere of large cities undergoes chemical reactions that could be dangerous | Science


In the background, the 'skyline' of Toronto, Canada.  In the foreground the station that captures particles and chemical compounds from the air.
In the background, the ‘skyline’ of Toronto, Canada. In the foreground the station that captures particles and chemical compounds from the air.John Liggio

A century ago, engineers at General Motors and Du Pont created an inert gas they called Freon. It was the first chlorofluorocarbon (CFC) and allowed the democratization of meat processing plants first in the United States and later in the rest of the world. After World War II, its safety and, at that time, the absence of toxicity led to the arrival of air conditioning in buildings and cars and, as a propellant, in all kinds of cans and containers. But, in 1974, the Mexican Mario Molina demonstrated how a chemical that is harmless to living beings could end life: by interacting with solar radiation in the upper parts of the atmosphere, CFCs were opening a hole in the ozone layer. Now a study shows how chemicals in widespread use degenerate into more persistent, bioassimilable, and probably toxic compounds.

For Molina, that discovery earned him the 1995 Nobel Prize in chemistry. To all societies, it helped advance regulations to control the distribution, commercialization and use of each new substance that the chemical industry imagined. Legal frameworks such as the Stockholm Convention on Persistent Organic Pollutants (POPs) require limiting and eliminating those that are proven to be harmful to humans and the environment. They also require lists of new compounds that could be dangerous and the chemical sector to prove that their products are not.

But the analysis of the air of 18 large cities on the planet shows that that effort, although necessary, is almost to put doors to the field: commercially approved compounds present in insulating materials, any furniture with some foam, mobile phones and all kinds of electronic devices are They become dangerous substances upon reaching the atmosphere. The work has focused on a dozen organophosphate flame retardants, a relatively recent class of flame retardant chemicals. This investigation, published in Nature, shows that, subjected to solar radiation, they initiate a series of chemical reactions that converts those primary chemicals into other secondary ones that have not been controlled until now.

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Map with the network of stations that are part of the GAPS Megacities project.
Map with the network of stations that are part of the GAPS Megacities project.GAPS project

John Liggio is a chemist in the air quality research division of Environment and Climate Change Canada, a state agency of the American country. Liggio, along with colleagues from research centers in the United States, China, and Europe, is the lead author of this study from Nature. In the laboratory, they investigated what these ten flame retardants became when subjected to the action of light (photo oxidation). They obtained 186 new compounds. With its structure and composition already known, they reviewed the air samples collected in 18 large cities, finding about thirty secondary substances, that is, degradation products. Liggio assures that none of them “is included until today” in the lists of chemicals to watch. And there can be many more, “each primary compound can form a dozen secondary ones,” he details.

The American Chemical Society has registered about 155 million chemical compounds. Of all of them, only 200,000 are toxic. According to researchers from the Institute of General Organic Chemistry of the CSIC (IQOG-CSIC), the most dangerous are persistent organic pollutants, POPs.

To carry the surname COP, a chemical must meet four criteria: First, that it is persistent in the environment. For example, dichloro diphenyl trichloroethane, an insecticide better known as DDT long banned almost everywhere on the planet, can stay in soils for up to 20 years. Second, that it accumulates and is absorbed by living beings, either by breathing it, by contact or, more commonly, by eating a smaller animal. Humans and other beings located in the highest links of the food chain accumulate the most POPs. Third, that its toxicity is demonstrated. The impact can be systemic or affect essential aspects for living beings, such as the immune or reproductive systems. Finally, that they have long-distance transport capacity. As Begoña Jiménez, IQOG-CSIC researcher and head of the national network for the environmental surveillance of persistent organic pollutants, says, “We have found POPs in Antarctica.”

This surveillance network has stations in nine urban areas and 14 remote areas that take air samples, such as in the Doñana park. The Madrid station is also part of the GAPS Megacities project, which samples the air in large cities and has fed data to Liggio’s study on substances derived from primary flame retardants. These compounds are released from consumer products, volatilizing and passing into the air, where photo oxidation turns them into something else. Liggio’s work detects a dozen new derivative substances in the sky of Madrid. But its concentration, expressed in picograms (trillionth of a gram) per cubic meter of air, is between five and three times less than that detected in New York, London and Beijing.

“The latitude is not relevant. What is relevant is the amount of original chemicals that are produced and used in a city “

John Liggio, atmospheric chemist with the Canadian state agency Environment and Climate Change Canada

“Latitude is not relevant,” says Liggio. “What is relevant is the amount of original chemicals that are produced and used in a city. The cities that have a greater quantity of primary chemical substances also have a greater quantity of by-products ”, he concludes. Of the 18 metropolises analyzed, Madrid is in the third of the least secondary compounds, along with Cairo, New Delhi, Buenos Aires and Santiago.

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Once these chemical pollutants were detected, the study authors investigated in detail the substances created in the atmosphere. They found that, overall, they are 2.5 times more persistent than their parent compounds. Although most accumulate less in aquatic organisms than the materials from which they come, but they are better assimilated by terrestrial organisms. Also in the laboratory, they estimated that its systemic toxicity is greater than that of the primary compounds of origin. And, as Liggio recalls, they have only studied organophosphate flame retardants, the smallest and most recent part of POPs.

The head of the national network for the environmental surveillance of POPs in the CSIC Begoña Jiménez recalls that, although the original compounds are regulated, “the secondary compounds in which they degrade are not, and for which we do not know their persistence or their toxicity”. As Jiménez recalls, “the industry requires toxicity tests for each new compound.” But here it is the interaction with the atmosphere that creates these new by-products. “They would still have to do tests like the ones that have been done in this study,” says Jiménez. Another aspect that the CSIC researcher recalls: “The stations that measure air quality record the concentration of suspended particles, carbon monoxide, ozone, nitrogen dioxide or sulfur dioxide, but none of these chemical compounds.”

“The stations that measure air quality record the concentration of suspended particles, carbon monoxide, ozone, nitrogen dioxide … but none of these chemical compounds”

Begoña Jiménez, head of the national network for the environmental surveillance of persistent organic pollutants (CSIC-CIEMAT)

The Spanish José Luis Jiménez investigates the presence of chemicals in the atmosphere from the University of Colorado in Boulder (United States). You have had the opportunity to read Liggio’s research. As you recall, it is not the first time that the interaction between chemicals and the atmosphere degenerates into dangerous situations. Some, such as the accumulation of ozone in the troposphere, are of natural origin, although it has been spurred by volatiles from engine combustion. Others can be mixed, such as acid rain episodes triggered by atmospheric chemical reactions of oxides of sulfur or nitrogen, which form sulfuric or nitric acid, respectively. And there are the CFCs, mentioned above. “There are thousands and thousands of chemicals in the air. Scientists try to measure and catalog them. But it is complex, and it is normal that from time to time new things are discovered like in this article, ”says Jiménez.

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For the two Jiménezs, chemistry and its products are essential for modern societies and human life. However, studies like the current one show the need for “greater vigilance”, says the IQOG-CSIC scientist and “that we must reduce the use of chemical products as much as possible, because their reactions and evolution can be harmful and difficult to anticipate. ”, Concludes the atmospheric chemist from the University of Colorado.

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George Holan

George Holan is chief editor at Plainsmen Post and has articles published in many notable publications in the last decade.

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