The results show how a number of natural compounds, which together account for around 70 percent of the biological hydrocarbon emissions, produce low-volatility products and how they can possibly effect the climate via aerosol particles.
Aerosol particles act as cloud droplets and thus reflect solar radiation back to space cooling down the planet. They play a crucial role in cloud formation and therefore also affect rainfall, cloud cover and climate in general. The tiny aerosol particles can originate dust, pollen or sea spray, and are emitted straight into the atmosphere or they can be formed from precursor gases. This gas-to-particle conversion is a complicated process and some parts of its first steps still remain unsolved by scientist.
Laboratory technician Kornelia Pielok at the Laminar flow reactor of TROPOS is shown. Credit:
Photo: Tilo Arnhold/TROPOS
This includes the role of oxidized volatile organic compounds, such as limonene and alpha-pinene, the typical scents of the citrus fruits and coniferous forests, in aerosol formation.
These compounds are first emitted by plants into the atmosphere and then oxidized by common oxidants, ozone or the OH-radicals. Whether these reactions produce condensing vapors that can condense onto the smallest particles or even molecules can have strong impacts on aerosol formation. As long as these processes are poorly understood, it is difficult to give accurate predictions of the future climate by modern climate models.
The discovery of the extremely low-volatility organic compounds (ELVOCs) was published last year in Nature, which observed highly oxygenated organics or ELVOC's that can explain the contribution of organic compounds in aerosol formation. The other discovery was made by the Leipzig-Helsinki team who found the mechanism leading to the rapid formation of these oxidized organic compounds. Auto-oxidation, which can spoil plastics or food, also plays an important role in the atmospheric aerosol formation. In the new paper, the team outlines how different biogenic compounds produce ELVOC's and how relevant these compounds are for the atmospheric processes. For the first time they could estimate the global effects of ELVOC in cloud condensation nuclei production.
In order to investigate the formation of the extremely low-volatility organic compounds the team has studied five abundant biogenic organic compounds with different chemical structures. The compounds all reacted with ozone and OH-radicals producing ELVOC. They found that the ELVOC formation happens very fast and that the chemical structure of the precursor gases determines how effectively they form ELVOC's.
"The structure of biogenic compounds that are emitted into the atmosphere influences how they are oxidized in the air and if they produce vapours that can condensate," says lead author Tuija Jokinen of the University of Helsinki, who carried out these studies in Leipzig at TROPOS. In the experiments, five biogenic organic gases (limonene, α-pinene, β-pinene, myrcene and isoprene) were passed through the laminar flow tube and then examined by a CI-APi-TOF mass spectrometer. "Our results show that the ozonolysis of monoterpenes like α-pinene or limonene leads to a greater efficiency of ELVOC production than a well-known oxidation via OH-radicals. On the other hand, β-pinene, myrcene and isoprene, produce much less ELVOC's, which are an important biogenic sources for the formation of particles in the atmosphere," emphasizes Dr. Torsten Berndt of TROPOS, who was involved in all three publications.
The results from the experiments were incorporated into a global atmospheric model to assess the impact of ELVOC on the particle formation and growth in the atmosphere. The international team used ECHAM5-HAM, an aerosol climate model, which was originally developed at the Max Planck Institute for Meteorology in Hamburg. According to the researchers, the extended model in the study is the first global aerosol model that combines the formation processes of secondary organic aerosol (SOA) with the ELVOC production from experiments.
Citation: Tuija Jokinen, Torsten Berndt, Risto Makkonen, Veli-Matti Kerminen, Heikki Junninen, Pauli Paasonen, Frank Stratmann, Hartmut Herrmann, Alex B. Guenther, Douglas R. Worsnop, Markku Kulmala, Mikael Ehn, and Mikko Sipilä (2015): Production of extremely low-volatile organic compounds from biogenic emissions: measured yields and atmospheric implications. PNAS May 25, 2015.