Scientists have long known that a group of common atmospheric particles called secondary organic aerosols can affect human health and the climate. Now an international team of chemists has uncovered an important clue to how these particles add to air pollution, help deflect solar radiation and act as seeds for cloud formation.
Secondary organic atmospheric particles, or SOAs, form from precursor volatile organic chemicals emitted by both natural and human-made sources, including trees and fossil fuel combustion. As primary volatile organic chemicals are oxidized in the atmosphere, they form SOAs.
Different types of SOAs can then mix together into single particles and when they do, the new particles’ physical and chemical properties affect the way they behave in the atmosphere. The number of phases—or states—those particles exhibit is particularly important.
Most models of SOAs’ impact on the climate and human health assume that when SOAs mix into a single particle, that particle has just one phase, says Fabian Mahrt, a post-doctoral fellow at the Paul Scherrer Institute and University of British Columbia department of chemistry.
But Mahrt and his team found that’s not always true, which means models predicting the particles’ climate and health impacts may not be as accurate as they could be. The team’s findings were published in November in the journal Atmospheric Chemistry and Physics.
The researchers found that six out of 15 mixtures of two SOAs commonly found in the atmosphere combined to create two-phase particles. They also showed that the number of phases depends on the difference in the average oxygen-to-carbon ratio between SOAs. When this difference is 0.47 or higher, the particles have two phases – a high polarity and a medium polarity phase.
“It’s a simple metric and can be applied to a vast data set (of SOAs) that people have collected over the past 20 years,” says Mahrt.
The researchers injected the mixed SOA particles with a dye that emits different coloured light depending on the particles’ polarity. Then they used a fluorescence microscope to peer inside the mixed SOA particles.
“People have indirectly determined that SOAs can combine and form two phases, but nobody had directly observed them and showed that the oxygen-to-carbon ratio can predict the number of phases,” says aerosol scientist and senior author Allan Bertram, professor in the UBC department of chemistry.
Perhaps one of the reasons nobody had ever directly observed the mixed SOA particles is that producing enough to study them is a long process. The particles must be made in the lab and “you might only get a milligram for a couple days work,” says Bertram.
University of Toronto atmospheric chemist Arthur Chan praised the finding for its potential for making air quality models more accurate.
“We have always assumed a single phase where all the reactions happen, but if the particle is separated in multiple phases, then then rates of chemical reactions and physical transformations can be drastically different,” says Chan. “Uptake of water, for example, can be vastly different between an organic-rich phase and an aqueous phase, and can affect the ability of particles to form clouds.”