A case of the vapours: source of 'climate-active' organic aerosol particles pinned down

From Forschungszentrum Juelich , comes what looks to be a pretty important discovery about how plant emitted aerosols like Great Smoky Mountains National Park haze comes about and grows large enough to reflect significant sunlight, something climate models don’t yet fully account for.

Major enigma solved in atmospheric chemistry

Nature: Researchers discover source of climate-active organic aerosol particles

According to their results, these extremely low-volatile organic compounds consist of relatively large molecules which contain an almost equal number of carbon, oxygen, and hydrogen atoms. The scientists present a plausible explanation supported by numerous experimental findings of how these vapours are formed almost immediately when plant emissions (e.g. monoterpenes) are released into the air. The vapours can then condense on small aerosol particles (starting from clusters of only a few nanometres in diameter) suspended in the air, causing them to grow to around 100 nanometres – at which size they can reflect incoming sunlight and act as condensation nuclei for cloud formation in the atmosphere.

The researchers’ findings have bridged a major gap in knowledge in atmospheric and climate research. “Thanks to our much improved understanding of the role that naturally occurring substances in the atmosphere play in the formation of organic aerosol particles, we will in future be able to make more reliable assessments of their impact on cloud formation and sunlight scattering, and thus on climate,” says Dr. Thomas F. Mentel from Jülich’s Institute of Energy and Climate Research – Troposphere (IEK-8).

The findings are based essentially on measurements performed at Forschungszentrum Jülich in a special 1450 litre glass chamber using a combination of several recently developed mass spectrometry methods, with instruments from Jülich, the University of Helsinki (Finland), and the University of Washington (Seattle, USA). Combined, these produced one of the most comprehensive data sets ever acquired, showing how organic emissions from trees can oxidize to form organic aerosols.

Experts consider a good understanding of the relationship between the increase in soil temperature, plant emissions, aerosol formation, and cloud formation to be essential for predicting future climate development correctly. “Our current research findings will help to improve computer models of the atmosphere and reduce existing uncertainties in climate prediction,” says Prof. Andreas Wahner, director at IEK-8.

“What really made these new findings possible were the new mass spectrometry methods, together with the combined efforts and expertise of all the international collaborators involved”, says the article’s lead author Dr. Mikael Ehn, currently university lecturer at the University of Helsinki. In addition to the institutions at Jülich, Helsinki, and Seattle, the Leibniz Institute for Tropospheric Research (Leipzig, Germany), the University of Copenhagen (Denmark), Aerodyne Research Inc. (USA), and Tampere University of Technology (Finland) contributed to the study.

Original publication:

A large source of low-volatility secondary organic aerosols, Mikael Ehn et al; Nature 506, DOI: 1038/nature13032  http://www.nature.com/nature/journal/v506/n7489/full/nature13032.html

Abstract:

Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol^{1, 2}, which is known to affect the Earth’s radiation balance by scattering solar radiation and by acting as cloud condensation nuclei³.

The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non-volatile organic vapours^{4, 5, 6}, but the sources and compositions of such vapours remain unknown.

Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies².

We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere–aerosol–climate feedback mechanisms^{6, 7, 8}, and the air quality and climate effects of biogenic emissions generally.

UPDATE: Figure 4 courtesy of Lance Wallace, who writes in comments:

This does seem to be a blockbuster from Kulmala’s group and others. They used various time-of-flight mass spectrometers to identify what they call ELVOCs (extremely low volatility organic compounds) that are created in the atmosphere and immediately condense on nano-condensation nuclei (nano-CN) particles irreversibly, such that the nanoparticles can grow to eventually become CCN (cloud condensation nuclei) and create clouds. These ELVOCs are large molecules, with example formulae of C10H15O10 or C20H32O12. The main mass spectrometer employed nitrate ions to collide with the particles. Since HNO3 is a pretty common atmospheric constituent, the authors conclude that once the nanoparticles grow to about 1.5 nm (say from a cosmic ray collision or a radon decay) the ELVOCs can continually condense on them to grow them to 50 nm (CCN size) in a matter of hours.

The authors do not mention Kirby or Svensmark, but I wonder if this is the missing mechanism from Svensmark’s 2013 study allowing cosmic-ray-initiated particle growth to proceed to CCN size.