Cloud formation slowed by biodiesel emission

Global cloud formation and rainfall could be more dramatically and unexpectedly affected by the increasing use of biodiesel than first thought suggests new research from the Institut Laue-Langevin. Emissions of traditional fuels promote the growth of water droplets in the atmosphere, producing clouds and rain. Although emissions from biodiesels play a similar role, they are very quickly broken down by ozone in the atmosphere, limiting their impact and making cloud formation more difficult. The researchers, who published their results in Physical Chemistry Chemical Physics, believe climate change models may have to be revised as a result. Methyl oleate is being produced in increasing amounts thanks to FAME (Fatty Acid Methyl Ester) biodiesels, and reacts with ozone surprisingly fast. It accumulates naturally on the surface of water, and lowers the surface tension of water droplets. Such surfactants are important in cloud formation as the reduced surface tension allows droplets to grow and produce clouds and rainfall. Without any surfactants, these droplets only grow large enough to form clouds higher in the atmosphere. Researchers performed neutron reflectometry where intense beams of neutrons were skimmed off single layers of methyl oleate molecules floating on water as they were exposed to ozone. Measurements of the intensity of reflected neutrons allowed researchers to determine how the concentration of the molecules changes over time and therefore how susceptible this organic material was to oxidative attack. “Neutrons showed that that the surfactant disappears from the air-water interface surprisingly quickly,” said Dr Christian Pfrang, the lead author of the paper. “The surfactant isn’t stable at the surface in the presence of ozone which means surface tension is increased and droplet growth could be slowed down, making it more difficult to form clouds. Furthermore the products aren’t stable at all at the droplet surface and this would not have been predicted from the results of previous studies.” Researchers attributed the mechanism of reaction of ozone with a carbon-carbon double bond in methyl oleate, breaking its backbone and shearing the molecule in half. This causes methyl oleate to lose its surfactant properties – in the atmosphere this would result in water droplets growing more slowly. The researchers now want to examine the behaviour of different surfactants and their mixtures when exposed to ozone and other oxidants found in the atmosphere. Ozonolysis of methyl oleate monolayers at the air–water interface: oxidation kinetics, reaction products and atmospheric implications