'How air pollutants form and grow in haze decoded'

Alcohol molecules, abundant in the atmosphere, can reduce the formation of hazardous polluting particles that make up haze, according to a study which may lead to more accurate models for air pollution, weather, and climate.

The researchers, including those from the University of Pennsylvania in the US, studied how methanol consumed one of the ingredients of haze -- sulphur trioxide -- and reduced the formation of toxic pollutant particles PM2.5 and PM10 -- which are nearly 10 to 30 times smaller than the width of a single human hair.

According to a study published in The Lancet journal, exposure to particulate matter (PM2.5) caused about 525,000 premature deaths in India in 2015.

 

The results of the current study, published in the journal PNAS, revealed that in dry and polluted conditions with abundant alcohol and sulphur trioxide in the atmosphere, and less availability of water, a chemical reaction drove down the formation of the toxic particles.

"Right now, we're all concerned about PM2.5 and PM10 because these have some real air-quality and health consequences," said study co-author Joseph S. Francisco from the University of Pennsylvania.

The researchers said sulphur trioxide combined with water molecules in the atmosphere to form sulphuric acid -- a major component of acid rain, and one of the most important "seeds" for particles to grow and form networked structures in the atmosphere.

"The question has been, How do you suppress the formation of these kinds of particles? This work actually gives some very important insight, for the first time, into how you can suppress particle growth," Francisco said.

The researchers used powerful computer models and simulations of the molecules and found that the sulphur trioxide in the air could react with alcohols such as methanol when it is abundant in the atmosphere.

The product of this reaction, they said, was methyl hydrogen sulphate (MHS), which they added is sticky enough to seed the particle-formation process.

"Because this reaction converts alcohols to more sticky compounds. Initially we thought it would promote the particle formation process. But it doesn't. That's the most interesting part. Alcohols consume or compete for sulphur trioxide so less of it is available to form sulphuric acid," said study co-author Jie Zhong, a postdoctoral fellow at the University of Pennsylvania.

While the reaction may play a significant role in driving down the rate of PM2.5 and PM10 formation, the study cautioned that MHS formed in the reaction has also been linked to negative health impacts.

"It's a balance. On the one hand this reaction reduces new particle formation, but on the other hand it produces another product that is not very healthy," Zhong said.

According to the researchers, models currently in use to simulate particle formation are not very accurate, and incorporating the newly discovered mechanism may make them better.