10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Molecular Understanding of the Suppression of Organic Nucleation by Isoprene
Martin Heinritzi, Lubna Dada, Mario Simon, Dominik Stolzenburg, Lukas Fischer, Andrea C. Wagner, ANDREAS KÜRTEN, Jasper Kirkby, Joachim Curtius, Goethe University Frankfurt
Abstract Number: 497 Working Group: Aerosol Chemistry
Abstract The formation of new particles in Earth’s atmosphere out of gaseous precursors contributes around 50% to global aerosol loadings and has thus large climatic impacts. It was recently shown that the highly oxygenated products of monoterpene (C10H16) oxidation are able to contribute to nucleation and also nucleate on their own, without the help of other precursors, at atmospherically relevant concentrations (Kirkby et al., 2016). However, atmospheric observations show that even if there are sufficient monoterpenes available, new particle formation is suppressed when isoprene (C5H8) is also present at low ppb levels. These observations were made e.g. in the Amazon basin and in the Southeast of the United States, both of which experience high isoprene levels. It was proposed that OH depletion by isoprene inhibits nucleation (Kiendler-Scharr et al., 2009). However, field measurements have reported undiminished OH levels in the presence of isoprene, and it was shown that isoprene oxidation is capable of recycling OH. So it remains an open question as to why isoprene seems to inhibit biogenic nucleation under otherwise favorable conditions for the formation of new particles. Here, we use data from the Cosmics Leaving OUtdoor Droplets (CLOUD) experiment at CERN, Geneva, to investigate this mechanism. The CLOUD experiment is a very clean and controlled aerosol reactor, equipped with a suite of state-of-the-art mass spectrometers and particle counting instruments that enables us to study nucleation under precisely controlled conditions. We measured the oxidation products of the monoterpene α-pinene in the presence and absence of isoprene, using a nitrate based Chemical-Ionization Atmospheric Pressure Interface Time-Of-Flight mass spectrometer (CI-API-TOF).
We find that nucleation of 1.7 nm particles in the α-pinene/ozone system is reduced when isoprene is present. We also find a reduction of the growth rate of particles in the small size range in the presence of isoprene. We are able to explain these observations at the molecular level due to changes in the composition of Highly Oxygenated Molecules (HOMs) that drive nucleation and growth.
We discuss our findings with respect to the results from Kiendler-Scharr et al., 2009. Our results can help explain the absence of nucleation in isoprene rich environments, such as the Amazon and the Southeast of the United States and may help to increase the accuracy of global aerosol models in the future.