Particles’ cooling effect on the planet might not be that simple

New particle formation in the atmosphere might not counteract the effect of greenhouse gases quite as we have thought. New research result provides strong evidence that previous assumptions need to be reconsidered.

Until now it has been widely assumed that new particle formation (NPF) – a phenomenon of the atmosphere which has been observed and intensively studied for more than two decades – will cool the planet and so counteract the effect of greenhouse gases.

A new research result counteracts this assumption by stating that the tiniest particles are increasing at the expense of the normal-sized and slightly larger aerosols – and it is only the latter that have a cooling effect on the planet.

This is shown for the first time in an article written by a group of researches such as Pontus Roldin, researcher in nuclear physics at Lund University in Sweden, professor Hanna Vehkamäki and university researcher Michael Boy from the Department of Physics at the University of Helsinki.

Their manuscript The role of highly oxygenated organic molecules in the Boreal aerosol-cloud-climate system was published in Nature Communications in September. The article studied the impact of newly formed aerosols on the cloud-climate system over the Boreal forest.

Unable to reflect light back to space

The atmosphere is full of aerosols with natural and humankind’s origin, like e.g. the combustion of fuel. Some of them are harmful to our health, while others reflect sunlight.

One of the most important natural sources for the precursors forming secondary aerosols is the emission of volatile organic gases from the boreal coniferous forest, the world’s largest coherent land ecosystem on our planet.

Through chemical reactions, the terpenes are transferred to highly oxygenated organic molecules which stick to and grow the tiniest particles or molecule clusters. This way they influence the number of cloud droplets, as each cloud droplet is formed through steam condensing on a sufficiently large aerosol particle.

The newly published article showed that industrialization has changed the picture through emissions of ammonia from agriculture and sulphur dioxide from fossil fuels.

The reactions products from the terpenes are now condensing on many smaller particles, which are formed by sulphuric acid and ammonia, and this way the diameter of the very tiny aerosols is smaller than the wavelength of light and they are unable to reflect light back to space.

“Paradoxically, a larger number of aerosol particles can lead to the cooling effect from the organic molecules released from the forests being reduced or even eliminated”, says Roldin, first author of the article.

In the future, with a warmer climate, the emissions of the terpenes from the boreal forest will increase and so the higher amount of condensable vapours will increase the cooling effect of the organic aerosols. However, the extent of this effect also depends on how humankind’s emissions of sulphur dioxide and ammonia will improve.