Though tailpipe emissions will fall in the years ahead as more zero-emission vehicles hit the streets and old vehicles are retired, one major source of highway air pollution shows little sign of abating: brake and tire dust. More can be done about this for example by having better regenerative braking, the smoother traffic flow that comes with autonomy, better tires and brake disks and forced earlier retirement of old vehicles following the admirable recent lead of Barcelona.
Metals from brakes and other automotive systems are emitted into the air as fine particles, lingering over busy roadways. Now, researchers at Georgia Institute of Technology have shown how that tiny metal particles could wreak havoc on respiratory health.
In a study published January 31 in the journal Environmental Science & Technology, the researchers described how vehicle-emitted metals such as copper, iron and manganese interact with acidic sulfate-rich particles already in the air to produce a toxic aerosol.
"There's a chain reaction happening in the air above busy highways," said Rodney Weber, a professor in Georgia Tech's School of Earth & Atmospheric Sciences. "Acidic sulfate in the atmosphere comes into contact with those metals emitted from traffic and changes their solubility, making them more likely to cause oxidative stress when inhaled."
The study, which was sponsored by the National Science Foundation and the U.S. Environmental Protection Agency, showed how the metals are emitted mainly in an insoluble form but slowly become soluble after mixing with sulfate.
"Sulfate has long been associated with adverse health impacts," said Athanasios Nenes, a professor and Georgia Power Scholar in the School of Earth & Atmospheric Sciences and the School of Chemical & Biomolecular Engineering. "The old hypothesis was that the acidic sulfate burns your lung lining, which in turn leads the bad health effects. But there is not enough acid in the air alone to really have that impact."
Sulfate plays a key role in making metals soluble before they are inhaled, maybe explaining sulfate linking with adverse health so samples of ambient particulate matter were taken near a major interstate highway and 420 meters away. They analyzed the chemical content, size distribution and acidity.
Much of the ambient sulfate found was similar in size to the metal particles, implying ambient sulfate and metals were mixed within particles, which over hours or days would allow the acidic sulfate to make the metal soluble.
To quantify aerosol danger, researchers developed oxidative potential analysis simulating toxic response on cellular organisms. Large data sets resulted on ambient aerosol oxidative potential, which when utilized in an earlier epidemiological study, revealed that the chemical assay was statistically associated with hospital admissions in Atlanta for asthma and lung disease. Peak toxicity closely correlated with particles containing most soluble metals, created by metallic particles mixed with highly acidic sulfate.
"That's the smoking gun," Nenes said. "The sulfate essentially dissolves those metals; when you breathe in those particles, the metals could be absorbed directly into the blood stream and cause problems throughout the body. For the first time, a mechanism emerges to explain why small amounts of acidic sulfate can adversely affect health."
It was even shown that roadway pollution could travel through the air and potentially cause problems in surrounding areas as well. The amount of particulate sulfate in the southeastern United States has decreased over 15 years as sulfur dioxide emissions from power plants have fallen, but enough acidic sulfate still gets in the air to keep the pH of particles very low at 0 to 2, transforming insoluble ambient metals to a soluble material.
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Top image source: AA1car.com
