ATMOCHEM - Secondary organic aerosol production in the lower free troposphere over western Europe

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Recent studies suggest a significant large abundance of secondary organic aerosol (SOA) present in a large part of the low and middle troposphere, both, for present-day as well as over the past. A basic understanding of this important aerosol fraction is therefore crucial to evaluate the net impact of aerosol (organic and inorganic) on past and future climate change. Most of very recent models systematically underestimate the mass of SOA present in the atmosphere, likely due to incorrect empirical parameterizations of its production. To progress on this question, studies dedicated to processes are urgently needed. Given its importance for climate issues, the long term trend of SOA preserved in the Alpine ice core archive needs to be profoundly explored. In this context, the proposal aims at (1) to understand nucleation/condensation processes for which documentation remains scarce, (2) to evaluate the relative importance of these processes with respect to the total mass of SOA present at different altitudes within the troposphere, (3) to quantify the long term trend of SOA from Alpine ice cores thereby inferring its controlling cause.The proposal approach combines intensive atmospheric field campaigns (physical and chemical aerosols properties, likely precursors of SOA) at three mountain sites, along with transport-chemistry simulations and Alpine ice cores investigations. The coupling of the aerosol physical measurements with the simulations from a atmosphere-chemistry/aerosol model to quantify the role of nucleation/condensation throughout the troposphere has never been addressed so far in a combined study between atmospheric observations and modelling. A continuous long term trend of SOA inferred from alpine ice core and backed up by radiocarbon constraints will be documented for the first time. This attempt includes also HULIS suspected to be one of the major contributor to the mass of SOA contributor.