The formation of particles in the atmosphere is a well-established phenomenon, involving sulphuric acid. However, repeated attempts to simulate the process in a smog chamber proved unsuccessful: Usually, a higher concentration of sulphuric acid was required than is actually present in the atmosphere. Bart Verheggen from ECN was involved in such aerosol research: “We found that organic compounds can play almost as important a role as sulphuric acid. These findings will allow climate models to be refined.”

Particles in the atmosphere (also known as aerosols or particulate matter, PM) have a major impact on the climate. These particles reflect some of the solar radiation. They also contribute to the formation of clouds, which in turn reflect solar radiation and also prevent thermal radiation from escaping. Verheggen: “Aerosols affect our health as well as the climate. There are gaps in the various theories explaining the formation of aerosols in the atmosphere. The results of our research mean that we have added another piece to the puzzle.”
The particles concerned are 1 to 3 nanometres in size, too small for their development and growth to be observed using conventional measuring instruments. What we know is based on the measurement of particles that have already grown through processes such as condensation and coagulation. So far, this knowledge has been included in climate models to a limited extent. According to Verheggen, it is important to know which factors actually affect the aerosol concentrations in order to improve the models. This has demonstrated that organic substances are a contributing factor.”

Relationship between the concentration of organic vapour (colour scale from purple to red) and sulphuric acid (horizontal) and the formation rate of aerosols in the atmosphere (vertical). The grey dots are field measurements in the forests near Hyytiälä in Finland. The coloured dots are the results of measurements in the Swiss smog chamber (Illustration: ECN, click to enlarge).

Smog chamber
The researchers produced a chart representing the results of their research in the smog chamber. On the chart, the Y axis represents the rate at which particles are formed (“J1.5”). The horizontal X axis represents the concentration of sulphuric acid required to achieve a particular formation rate. For a constant concentration of sulphuric acid (imaginary vertical line), note that there is significant variation in the quantity of aerosol particles formed. It turns out that this variation can be partly explained by the quantity of organic vapour present in the smog chamber: from purple-blue (low concentration) to bright red (high concentration). Verheggen: “We found that TMB (trimethyl benzene) is a direct factor in aerosol formation exerting an almost linear effect. In nature, organic particles, such as terpenes emitted by Finnish forests, may be just as important as sulphuric acid. We have put forward the hypothesis that the formation of aerosol particles in the atmosphere is partly dependent on organic molecules.”

Precursors of organic molecules
This study represents an important step forward in the research into the formation of aerosols in the atmosphere. Digging deeper, we still find many unanswered questions and uncertainties that warrant further investigation. However, Verheggen believes a clear course for follow-up studies has been set. “Now that it is known that organic compounds have a direct effect on the formation of aerosols, it is important to investigate the precursors of these organic molecules. Once they have been identified, it will be possible to determine the extent to which they are the result of natural emissions (for example, terpenes emitted by trees and plants) and the extent to which they are related to human activities (industrial processes, traffic and the generation of power). This information will provide us with hands-on tools to improve climate models.”
Unlike particulate level standards, the number of particles is essential in climate models, rather than their mass. Verheggen: “The more we find out about the size and number of particles in the atmosphere and the amount of time they remain in it, the better and more accurate the input used for climate models. We need to find sufficiently accurate ways of calculating the number of particles from the aerosol mass, which is easy to measure. This is being done already, but there are too many uncertainties. Now we have a chance of developing a sound hypothesis regarding the formation of aerosols and integrating it into climate models.

Greenhouse gases
Global policymaking benefits from reliable climate models. The number of particles in the atmosphere is an important factor in these models, but so are concentrations of greenhouse gases. However, there is a major and characteristic difference between gases and aerosol particles: the length of time they remain in the atmosphere. Verheggen: “Greenhouse gases remain in the atmosphere for long periods of time, aerosols don’t. Depending on their size, they may remain in the atmosphere for a few days or several weeks. Furthermore, their distribution in the atmosphere is particularly uneven. This makes it difficult to determine their effects on the climate system. In the short term (decades), I think that the impact of changes in the concentration of aerosols could be a determining factor. Over a longer period (50 years or more), the concentration of greenhouse gases (mainly CO2) will be the predominant factor affecting the climate.
By and large, greenhouse gases lead to heating of the atmosphere, while aerosols lead to cooling. The fact that the average temperature has increased over the last 150 years is a result of the heating effect (caused by greenhouse gases) exceeding the cooling effect (due to aerosols). Between 1950 and 1980, the concentrations of aerosols increased dramatically, and their cooling effect temporarily limited global warming. After 1980, aerosol concentrations decreased, particularly in Europe, reducing the associated cooling effect and causing an additional rise in temperatures.
For those who doubt this: over the same period, the frequency of mist, fog and haze over Europe decreased. These are atmospheric phenomena that are directly related to the aerosol concentrations.

Energy technology and aerosol research
One would not really expect someone from a research institute for energy technology such as ECN to be involved in research into the formation of aerosol particles, but Bart Verheggen has a simple explanation: “ECN has internationally recognized expertise in the field of aerosols, which goes back to the days when we did research into the spreading of airborne radioactive material into the soil. Today, this knowledge is applied to research into acid rain, particulate matter and also to climate change. Furthermore, the formation of aerosol particles was the subject of my Ph.D. thesis research in Canada. I also worked for three years at the Paul Scherrer Institute in Switzerland, a sister institute of ECN, where I’m working as a member of the Air Quality and Climate Change Group. Within this group, we are also studying aerosols in addition to other activities, such as measuring and modelling of greenhouse gases.”

Contact
Bart Verheggen
ECN Biomass, Coal and Environmental Research / Air Quality & Climate Change
Tel.: +31 (0)22 456 40 33
Email: Bart Verheggen

Info
Evidence for the role of organics in aerosol particle formation under atmosferic conditions. Article in Proceedings of the National Academy of Sciences (PNAS), 2010
Climate change and the impact of aerosol

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