Final Activity Report Summary - VOCAT (The role of Versatile Organic Compounds in ATmospheric processes) VOCAT had for objective to develop a cutting-edge research program on the chemistry of organic compounds in the atmosphere. Four directions of research were identified and ground-breaking results have been achieved as the result of this project. 1. Ionic organic chemistry in aerosols This topic is new in atmospheric science and Dr Noziere's group has played a central role in developing it. VOCAT was a turning point, with the discovery of several new classes of catalysts for these reactions: amino acids and inorganic ions. These discoveries led to two patents and change the understanding of aerosol chemistry by showing that common inorganic material, such as ammonium sulfate, is not inert towards organic compounds as previously thought but triggers important reactions. These reactions play important roles in the atmosphere and for climate: they produce light-absorbing compounds increasing the warming radiative forcing of aerosols on climate; they remove organic gases such as glyoxal from the atmosphere, affecting ozone chemistry and producing secondary organic aerosols (SOA); they can produce surface-active compounds increasing the cloud-forming efficiencies of atmospheric aerosols and their cooling effect on climate. These effects are supported by a number of observations and more work is now underway to further characterise this new chemistry. 2. Direct determination of secondary organic material in aerosols SOAs have been investigated for decades but mostly under artificial conditions or by using results of such investigations (oxidation products identified, for instance). In VOCAT, new methods were developed to determine the primary or secondary origin of organic material in aerosols without making any assumption on their formation or origin. The first results show that compounds believed to be secondary products from isoprene (methyl tetrols) are actually partly primary, i.e. emitted directly as particulate matter by biological sources, which has tremendous implications on the estimates of aerosol mass produced globally in the atmosphere. These first results illustrate the critical importance of developing such independent approaches. 3. Role of biological material in the atmosphere Until now, most of the physical and chemical transformations considered in the atmosphere were non-biological (abiotic). A potential role of living organisms, in particular airborne microorganisms, in the formation of cloud droplets has been investigated but has remained unclear. In VOCAT, organic substances produced by microorganisms at the Earth's surface were show to be more efficient in forming cloud droplets than any material studied until now, including inorganic salts, and to be present in aerosols from various origins. These results open new perspectives in the understanding of cloud formation and reveal an unsuspected link between the biosphere, the atmosphere, the hydrological cycle, and climate. Although the corresponding article is still under review, the considerable implications of these results have already received a large attention from the media (one article in the popular science magazine New Scientist and in numerous websites). 4. Development of a new monitoring technique for organic peroxyl radicals Organic peroxyl radicals play a critical role in the atmosphere as intermediates in the formation of ozone. Yet, no techniques exist to monitor them in the atmosphere and they remain today the last group of chemical species never observed. The development of a chemical ionisation mass spectrometric technique for the monitoring of these compounds initiated in the United States was pursued in VOCAT. Although this project is still under development, the CIMS instrument built for this purpose is now operational and a first article is in preparation. In conclusion, important milestones have been achieved in VOCAT and a cutting edge research program has been established and is on its way to make more important contributions to the field of atmospheric chemistry.