Revealing the chemistry of new particle formation and growth

A key criterion to understand how climate has changed since the beginning of the industrial revolution is to reconstruct preindustrial climatic conditions as precisely and reliably as possible. It is not only the increase of greenhouse gases since 1750 that increased the heat load in the Earths atmosphere, atmospheric aerosols are also important for climate change because water droplets form on these particles, resulting in clouds. In this way aerosols cool Earths climate and mask part of global warming. How strongly aerosols hamper global warming is, however, unclear that adds large uncertainties to current climate prediction scenarios owing to the difficulties in constraining the aerosol loading in the preindustrial atmosphere. Aerosols can either be directly emitted to the atmosphere (for example, soot from wood and fossil fuel combustion, and pollen grains) or can be formed in the atmosphere from gaseous precursors. It has been thought that gas-to-particle conversion always requires sulfuric acid and that this mechanism only became of relevance with industrialization. We could show in precisely controlled laboratory experiments that the formation of new particles in the atmosphere and their subsequent growth is possible with substances emitted by trees. This newly found mechanism thus changed our understanding of cloud formation in the preindustrial atmosphere. This pure biogenic formation of new particles is based on complex chemical reactions involving oxidation of organic precursor compounds in the atmosphere. A large fraction of the resulting reaction products and their reaction pathways are unknown. Here we suggest investigating the chemical reactions that are responsible for aerosol formation and growth in precisely controlled laboratory experiments. Besides biogenic precursors, we will also investigate anthropogenic organic substances with similar chemical structures, which may trigger new particle formation and growth in the present-day atmosphere without sulfuric acid. These experiments will be carried out with the newly developed PTR-3-TOF mass spectrometer. This instrument has been developed at the University of Innsbruck to identify and quantify the so far unmeasured fraction of oxidation products responsible for aerosol formation and growth. To disentangle the different reactions pathways, we will consider individual aerosol precursors and oxidants. On one hand these experiments will be performed at the Massachusetts Institute of Technology (MIT) smog chamber to study the chemical reactions that lead to the formation of new particles. On the other hand we will perform experiments at the CERN CLOUD chamber that enables to precisely simulate atmospherically relevant preindustrial and present-day conditions and to observe new particle formation and subsequent growth. With these experiments we hope to better constrain the conditions in the pre-industrial era and thus reduce uncertainties in climate models.