This post is a follow-up to my last two posts about how organic emissions from both forests, and from plankton in the Southern Ocean, are currently masking the climate sensitivity to GHG emissions. The following linked reference cites new evidence that many trees also emit extremely low volatility organic compounds (ELVOC); which are suggested to promote aerosol particle formation and cloud condensation nuclei (CCN) production in the atmosphere, that are also currently masking the true magnitude of Equilibrium Climate Sensitivity, ECS, considering that forests are projected to decline rapidly with continued global warming:
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 (June 9, 2015), "Production of extremely low-volatile organic compounds from biogenic emissions: measured yields and atmospheric implications", PNAS, vol. 112 no. 23 7123-7128, doi: 10.1073/pnas.1423977112 http://www.pnas.org/content/112/23/7123
Significance: "Extremely low volatility organic compounds (ELVOC) are suggested to promote aerosol particle formation and cloud condensation nuclei (CCN) production in the atmosphere. We show that the capability of biogenic VOC (BVOC) to produce ELVOC depends strongly on their chemical structure and relative oxidant levels. BVOC with an endocyclic double bond, representative emissions from, e.g., boreal forests, efficiently produce ELVOC from ozonolysis. Compounds with exocyclic double bonds or acyclic compounds including isoprene, emission representative of the tropics, produce minor quantities of ELVOC, and the role of OH radical oxidation is relatively larger. Implementing these findings into a global modeling framework shows that detailed assessment of ELVOC production pathways is crucial for understanding biogenic secondary organic aerosol and atmospheric CCN formation."
Abstract: "Oxidation products of monoterpenes and isoprene have a major influence on the global secondary organic aerosol (SOA) burden and the production of atmospheric nanoparticles and cloud condensation nuclei (CCN). Here, we investigate the formation of extremely low volatility organic compounds (ELVOC) from O3 and OH radical oxidation of several monoterpenes and isoprene in a series of laboratory experiments. We show that ELVOC from all precursors are formed within the first minute after the initial attack of an oxidant. We demonstrate that under atmospherically relevant concentrations, species with an endocyclic double bond efficiently produce ELVOC from ozonolysis, whereas the yields from OH radical-initiated reactions are smaller. If the double bond is exocyclic or the compound itself is acyclic, ozonolysis produces less ELVOC and the role of the OH radical-initiated ELVOC formation is increased. Isoprene oxidation produces marginal quantities of ELVOC regardless of the oxidant. Implementing our laboratory findings into a global modeling framework shows that biogenic SOA formation in general, and ELVOC in particular, play crucial roles in atmospheric CCN production. Monoterpene oxidation products enhance atmospheric new particle formation and growth in most continental regions, thereby increasing CCN concentrations, especially at high values of cloud supersaturation. Isoprene-derived SOA tends to suppress atmospheric new particle formation, yet it assists the growth of sub-CCN-size primary particles to CCN. Taking into account compound specific monoterpene emissions has a moderate effect on the modeled global CCN budget."
Extract: "According to a new global-scale projection, terrestrial vegetation emits several million tons of extremely low-volatility organic compounds (ELVOCs) per year to the atmosphere, which affect cloud seeds via formation of low-volatility vapors. These oxidation products of compounds such as monoterpenes results in an increase of condensing vapors that can further form cloud condensation nuclei over the continents and have an influence on the formation of clouds.
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.
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."