Unpredictable aerosol particles

Researchers' knowledge of atmospheric fine particles, aerosols, remains incomplete. However, in order to explain global climate change, an increasingly accurate understanding of the life cycle of nanoparticles and their interaction with clouds are necessary.

Global warming is a fact. However, in the report of the Intergovernmental Panel on Climate Change, IPCC, the effect of aerosols on climate change constitutes the biggest uncertainty factor. Unlike greenhouse gases, aerosols cool the climate, but estimates of their cooling effect are highly inaccurate.

"So far, aerosols have reversed some of the global warming and cooled the climate. However, further research is needed to understand exactly how large this cooling effect is," says Professor Ari Laaksonen of the University of Eastern Finland and the Finnish Meteorological Institute.

Aerosols work in the atmosphere in two ways: directly and indirectly. Their direct effect means that they either scatter or absorb solar radiation. Black carbon, i.e. soot, absorbs solar radiation and warms the climate, but the majority of particles scatter solar light, cooling the climate. The other effect is indirect, involving clouds.

"Aerosols activate and become cloud droplets. Clouds, in turn, play a role in the atmospheric radiation balance. Each cloud droplet contains at least one aerosol particle, that is, they need a condensation nucleus," explains Associate Professor Annele Virtanen of the University of Eastern Finland.

In other words, aerosol particles scatter solar radiation and modify cloud properties and cloud abundance. Could their cooling effects come to the rescue of the atmosphere? According to Virtanen, aerosols have slowed down global warming, but they are not a lifeboat worth clinging to in the future.

"There is no reason to believe that their cooling effect would keep pace with the growing amount of carbon dioxide. We don't know how much the cooling effect has increased during the industrial period."

Human-induced aerosol emissions have reduced in Europe and North America since the 1980s, but increased in China and India. Globally, the amount of emissions has reduced only slightly.

Last year, aerosols took scientists by surprise when it turned out that their cooling effect in Asia over the past 20 years was smaller than expected. The transition of human-induced aerosol emissions from Europe and North America to China and India at the turn of the millennium has, rather unexpectedly, warmed the climate.

An important reason behind the low cooling effect in China and India is the fact that in Asia, aerosol emissions were already at such a high level at the beginning of the review period in 1996 that the cooling effect on any additional emissions was relatively low. In turn, the amount of black carbon grew, warming the climate.

With the help of computational methods, researchers are now studying whether the climate could be modified by spraying sea salt particles into the atmosphere or by releasing sulphuric acid from an aeroplane to nucleate with water and form particles, for example. Virtanen and Laaksonen both feel that we shouldn't place too much hope on measures that modify the climate. 

"Climate modification is a last resort. If it were possible to lower the earth's average temperature, it would easily result in areas near the equator cooling down and areas near the poles remaining warmer. Succeeding in bringing average temperatures back to their original level doesn't mean that the distribution of temperatures would be the same as before," Laaksonen says.

Analysing aerosol particles and creating emissions models is more complicated than when working with greenhouse gases. For example, carbon dioxide gets blended into the atmosphere relatively evenly, regardless of the emission source and location. Over a period of a couple of years, carbon dioxide emissions spread in the atmosphere and their effects become global. Aerosols, on the other hand, have a more local effect as they don't spread as far; however, they, too, may have distant repercussions.

"A heavy aerosol emission doesn't just cool the climate locally; it may also cause changes to the weather system further away. When aerosol emissions were reduced in Europe, the largest effects were observed in the middle of Siberia, which was unexpected," Laaksonen says.

"Aerosols remain in the atmosphere for some weeks, whereas the life span of greenhouse gases is hundreds of years," Virtanen says.

It used to be thought that human-induced aerosol particles are emitted directly from the end of a pipe, making it easy to calculate their concentrations and size distributions. Laaksonen's 30-year career has witnessed answers to several major questions relating to aerosol characteristics and clouds.

"I defended my dissertation in 1992 and my PhD thesis focused on the formation of new particles, i.e. nucleation. I wrote that nucleation can also take place in the atmosphere, but that it's very rare. A couple of years later, it was us Finnish researchers who showed that it is in fact very common."

The proportion of aerosols formed in the atmosphere varies. Working together with locals, University of Eastern Finland researchers have measured particle concentrations in the delta of the River Po in Italy, which is the most polluted area in Europe.

"Out of the whole of potential cloud nuclei particles, approximately 25% can be the result of nucleation. That's quite a lot, considering that the area is a source of very significant emissions otherwise," Laaksonen says. 

Virtanen's research group has also found that aerosol particles consisting of organic compounds are often solid and glass-like, not liquid as previously assumed. According to Virtanen and Laaksonen, researchers are very close to discovering what causes the formation of new particles. Finland is one of the global leaders of aerosol research.

"In research addressing aerosol formation, we are, under the lead of the University of Helsinki, number one in the world. The University of Eastern Finland and the Kuopio unit of the Finnish Meteorological Institute participate in that research. We are also at the cutting edge of research when it comes to the interaction between clouds and particles," Laaksonen says.

Outdoor air particles have been a topic of research at the Kuopio Campus ever since the degree programme in environmental hygiene was launched in 1974. In the beginning, the main emphasis was on the health effects of particles, but, since the 1990s, the approach has been increasingly multidisciplinary.

Today, Kuopio is home to extremely diverse research, ranging from the basics of aerosol physics to global climate models and the health effects of particles. In addition to the Aerosol Physics Research Group led by Virtanen, researchers of fine particles, aerosol technology and toxicology from the Department of Environmental Science as well as researchers from the Finnish Meteorological Institute also participate.

Researchers' grasp of aerosols is getting tighter and tighter; however, there are still many questions for physicists, chemists and toxicologists to answer. Physicist Virtanen studies the growth of compounds evaporating from boreal forests into aerosol particles and further to cloud droplets.

"It's not enough to know the method and scale of their formation. We also need to understand in what kinds of conditions aerosols reach a size that has climate-cooling effects."

Text: Virpi Komulainen Photo: Raija Törrönen