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 driving forces behind changing climate and air quality.Atmospheric aerosol numbers 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. Correct representation of the aerosol growth/shrinkage by condensation/evaporation of atmospheric vapours is thus a prerequisite for capturing the evolution and impacts of aerosols.I propose to start a research group that will address the major current unknowns in atmospheric ultrafine particle growth and evaporation. First, we will develop a unified theoretical framework to describe the mass accommodation processes at aerosol surfaces, aiming to resolve the current ambiguity with respect to the uptake of atmospheric vapours by aerosols. Second, we will study the condensational properties of selected organic compounds and their mixtures. Organic compounds are known to contribute significantly to atmospheric aerosol growth, but the properties that govern their condensation, such as saturation vapour pressures and activities, are largely unknown. Third, we aim to resolve the gas and particulate phase processes that govern the growth of realistic atmospheric aerosol. Fourth, we will parameterize ultrafine aerosol growth, implement the parameterizations to chemical transport models, and quantify the impact of these condensation and evaporation processes on global and regional aerosol budgets.