Certain Gas Molecules Found to Release Harmful Particles in Atmosphere

At the University of Copenhagen, scientists have identified an unexpected phenomenon in a process through which specific gas molecules create harmful particles.

The effect of this phenomenon may increase in urban areas with decreasing pollution. This understanding can serve to assist politicians to implement improved measures to fight air pollution and help enhance climate models.

NO_x emissions in urban areas are mainly caused by diesel emissions. In spite of the clear public health benefits resulting from reduced NO_x emissions, a reduction in NO_x gases does not necessarily imply that air pollution has been completely eradicated.

Other airborne health hazards also exist, such as ultrafine particles. A study carried out by the University of Copenhagen, Denmark, indicates that as the NO_x levels fall, people could be exposed to more particles than scientists had earlier assumed.

We have found a fundamental shortcoming in the models that assess and predict air pollution. Our discovery allows us to improve these models and provide politicians with a stronger foundation for making greener decisions.

Henrik G. Kjaergaard, Professor, Department of Chemistry, University of Copenhagen

Professor Kjaergaard and his colleague Kristian Holten Møller, in association with other scientists from Caltech, have identified a unique mechanism in the process through which specific molecules produce particles in the atmosphere.

As the volatile organic compounds (VOCs) break down, these specific molecules form radicals in both left- and right-handed form—a phenomenon in chemistry called chirality. The scientists have shown that one of these forms can produce particles that are up to 1000 times faster than the other form.

According to Kristian Holten Møller, postdoc from the Department of Chemistry, “Previously, no one knew that right- and left-handedness made a difference in how many airborne particles were created. This is important because ultimately, the amount of particles directly correlates with the number of air pollution-related deaths.”

The mechanism takes place when a VOC molecule breaks down in the atmosphere by reacting with itself rather than with other kinds of molecules. When this self-reaction takes place, the molecular radicals grow increasingly bigger as they absorb more and more oxygen, ultimately developing into ultrafine particles.

Such a process takes place with extremely different rates based on whether the radicals have a left- or right-handed form. Later, very different amounts of particles are also produced.

Fewer NO_x Gases Result in More Particles

Although VOC molecules are discharged in forested regions as plant and tree odors, they are also discharged as anthropogenic pollution. In urban regions, VOCs evolve from several different origins, like detergents, cosmetics products, paints, cars, and solvents.

Earlier research carried out by Henrik G. Kjærgaard has shown that the recently discovered phenomenon indeed occurs with a specific level of NO_x in the atmosphere.

Urban NO_x gases limits this oxidation and prevent radicals from growing into particles. However, as we reduce NO_x emissions, particles formed via oxidation are likely to become more prominent in cities.

Henrik G. Kjaergaard, Professor, Department of Chemistry, University of Copenhagen

Kjaergaard also pointed out that retaining diesel vehicles in cities does not provide a complete solution: “Diesels not only emit NO_x—they emit particles directly. We are in no way implying that it is a good idea to keep diesel vehicles in urban areas.”

One possible solution—according to the scientists—is to control VOC emissions and substitute the VOCs that are responsible for the majority of particles with others that have a lower impact. The researchers also emphasized that it is a difficult area to control and that more understanding is required about how particles are created by various VOCs.

Path to More Accurate Climate Models

The scientists further indicated that this finding would help develop more precise climate models. Ultrafine particles have an impact on climate by either absorbing or reflecting sunlight. The presence of these particles results in the greatest source of ambiguity in global climate models.

With the enormous differences between right- and left-handed radicals, uncertainties arise in climate models if failing to distinguish between their form—as is the case today. This leads to an over- or underestimation of the number of particles created in the atmosphere.

Kristian Holten Møller, Postdoc, Department of Chemistry, University of Copenhagen