Final Report Summary - ATMOGAIN (Atmospheric Gas-Aerosol Interface: From Fundamental Theory to Global Effects) Atmospheric aerosol particles are a major player in the earth system: they impact the climate by scattering and absorbing solar radiation, as well as regulating the properties of clouds. On regional scales aerosol particles are among the main pollutants deteriorating air quality. Capturing the impact of aerosols is one of the main challenges in understanding the changing climate. According to the Intergovernmental Panel on Climate Change (IPCC), aerosols have been the most important atmospheric cooling component in the industrial period. At the same time IPCC recognizes the predictions of aerosol impacts as the largest individual source of uncertainty in climate models. To pin down the effects of aerosols on climate and air quality, the processes governing their concentrations need to be understood and represented accurately in large-scale models. Atmospheric aerosol number budgets are governed by the ultrafine (< 100 nm in diameter) particles. Most of these particles have been formed from atmospheric vapours, and their fate and impacts are governed by the mass transport processes between the gas and particulate phases. These transport processes are currently poorly understood, particularly for the smallest nanoparticles. Understanding of the growth and evaporation of ultrafine particles is thus a prerequisite for capturing the evolution and impacts of aerosols.The ATMOGAIN research team has addressed the major current unknowns in the behaviour and impacts of atmospheric ultrafine particles, using interdisciplinary approaches ranging from the nano- to global scales. First, we have developed molecular-level theories to describe the processes happening at the gas-particle interfaces. Second, we have studied the chemical properties of selected organic compounds and their mixtures, which are known to contribute significantly to atmospheric aerosol growth, but their properties have been largely unknown. Third, we have developed modeling tools for describing the growth of atmospheric ultrafine aerosol. Fourth, we have parameterised the complex processes leading to ultrafine aerosol growth into highly simplified forms applicable in the computationally heavy atmospheric models used for e.g. climate predictions or air quality policy analysis.Our main findings within ATMOGAIN can be summarized as: 1) the transport of water and common atmospheric organic molecules through the gas-aerosol interface can be described by coupling molecular-level description of the surface with macroscopic transport theories; 2) the ultrafine particles originating from forests are a lot more stable than previously thought. The organic material condensing on these particles can be formed through a variety of different processes including gas-phase oxidation reactions, interactions with inorganic aerosol components, and other particle phase chemistry; 3) including the growth of ultrafine particles by organic compounds in regional and global scale atmospheric models results in a non-linear response of the simulated ultrafine aerosol loadings. This highlights the need for accurate process-level description of gas-aerosol interactions in studies aiming at simulating the concentration of atmospheric aerosol particles in the past or future where experimental data is not available.
