Researchers at the Paul Scherrer Institute PSI have for the first time observed photochemical processes inside organic aerosol particles containing iron. In doing so, they discovered that additional oxygen radicals that can be harmful to human health are formed in these aerosols under everyday conditions. They report on their results in an open-access paper in the journal Nature Communications.

It is well known that airborne particulate matter can pose a danger to human health. The particles, with a maximum diameter of ten micrometers, can penetrate deep into lung tissue and settle there. They contain reactive oxygen species (ROS), also called oxygen radicals, which can damage the cells of the lungs. The more particles there are floating in the air, the higher the risk.

The particles get into the air from natural sources such as forests or volcanoes. Human activities, e.g., in factories and traffic, multiply the amount so that concentrations reach a critical level. The potential of particulate matter to bring oxygen radicals into the lungs, or to generate them there, has already been investigated for various sources. Now the PSI researchers have gained important new insights.

From previous research it is known that some ROS are formed in the human body when particulates dissolve in the surface fluid of the respiratory tract. Particulate matter usually contains chemical components, for example metals such as copper and iron, as well as certain organic compounds. These exchange oxygen atoms with other molecules, and highly reactive compounds are created, such as hydrogen peroxide (H₂O₂), hydroxyl (HO), and hydroperoxyl (HO₂), which cause oxidative stress. They attack the unsaturated fatty acids in the body, which then can no longer serve as building blocks for the cells. Physicians attribute pneumonia, asthma, and various other respiratory diseases to such processes. Even cancer could be triggered, since the ROS can also damage the genetic material DNA.

It has been known for some time that certain reactive oxygen species are already present in particulates in the atmosphere, and that they enter our body as exogenous ROS by way of the air we breathe, without having to form there first. As it now turns out, scientists had not yet looked closely enough.

Previous studies have analyzed the particulate matter with mass spectrometers to see what it consists of. But that does not give you any information about the structure of the individual particles and what is going on inside them.

With the brilliant X-ray light from the Swiss Light Source SLS, we were able not only to view such particles individually with a resolution of less than one micrometre, but even to look into particles while reactions were taking place inside them.

—Peter Aaron Alpert, first author of the new PSI study

To do this, he also used a new type of cell developed at PSI, in which a wide variety of atmospheric environmental conditions can be simulated. It can precisely regulate temperature, humidity, and gas exposure, and has an ultraviolet LED light source that stands in for solar radiation.

Alper worked closely with the head of the Surface Chemistry Research Group at PSI, Markus Ammann. He also received support from researchers working with atmospheric chemists Ulrich Krieger and Thomas Peter at ETH Zurich, where additional experiments were carried out with suspended particles, as well as experts working with Hartmut Hermann from the Leibniz Institute for Tropospheric Research in Leipzig.

The researchers examined particles containing organic components and iron. The iron comes from natural sources such as desert dust and volcanic ash, but it is also contained in emissions from industry and traffic. The organic components likewise come from both natural and anthropogenic sources. In the atmosphere, these components combine to form iron complexes, which then react to radicals when exposed to sunlight. These in turn bind all available oxygen and thus produce the ROS.

Normally, on a humid day, a large proportion of these ROS would diffuse from the particles into the air. In that case it no longer poses additional danger if we inhale the particles, which contain fewer ROS. On a dry day, however, these radicals accumulate inside the particles and consume all available oxygen there within seconds.

This is due to viscosity. Particulate matter can be solid like stone or liquid like water—but depending on the temperature and humidity, it can also be semi-fluid like syrup, dried chewing gum, or Swiss herbal throat drops. This state of the particle, said Alpert, ensures that radicals remain trapped in the particle,. No additional oxygen can get in from the outside.

It is especially alarming that the highest concentrations of ROS and radicals form through the interaction of iron and organic compounds under everyday weather conditions.

Previously, researchers thought that ROS only form in the air—if at all—when the fine dust particles contain comparatively rare compounds such as quinones, said Alpert. Quinones are oxidized phenols that occur, for example, in the pigments of plants and fungi. It has recently become clear that there are many other ROS sources in particulate matter.

As we have now determined, these known radical sources can be significantly reinforced under completely normal everyday conditions.

—Peter Alpert

Around every twentieth particle is organic and contains iron.

The same photochemical reactions likely takes place also in other fine dust particles, said research group leader Markus Ammann.

We even suspect that almost all suspended particles in the air form additional radicals in this way. If this is confirmed in further studies, we urgently need to adapt our models and critical values with regard to air quality. We may have found an additional factor here to help explain why so many people develop respiratory diseases or cancer without any specific cause.

—Peter Alpert

Resources

• Alpert, P.A., Dou, J., Corral Arroyo, P. et al. (2021) “Photolytic radical persistence due to anoxia in viscous aerosol particles.” Nat Commun 12, 1769 doi: 10.1038/s41467-021-21913-x