Molecular diffusion of organics in secondary organic aerosols and impacts on particle chemistry
Molecular diffusion of organics within secondary organic aerosol (SOA), a main class of tropospheric particles, controls predictions of particle mass, size, mixing state, and cloud formation properties, thus SOA’s role for air quality, atmospheric chemistry and climate. Despite that, measurements of diffusion coefficients of organics in SOA at low, tropospheric relevant temperatures (T) are largely missing.
The objectives of MICROSCOPE are to directly measure diffusion coefficients of organic molecules in SOA particles at T < 290 K, improve parametrizations used to estimate diffusion, test predictions of diffusivity in atmospheric models and assess the impacts on particle chemistry, by the combination of development of innovative instrumentation, experimental and modelling work. 
Measuring diffusion coefficients as a function of water activity (aw) and temperature, will be achieved by developing a new flow cell with simultaneous and in-situ T and aw-control for rectangular area fluorescence recovery after photobleaching measurements. The chemical composition of the SOA samples will be determined using high-resolution mass spectrometry, with the goal to improve existing parametrizations used to estimate the diffusion of organics in SOA and derive new ones that directly relate chemical composition to diffusion coefficients. The new T and aw-dependent parametrization will be used along with model output to verify if tropospheric mixing times of organics in SOA particles are < 1 h. Finally, the impact of diffusion of organics on SOA particle reactivity and chemistry will be determined through measuring the degradation rates of peroxides within SOA particles, using aerosol flow tube and X-ray microscopy experiments. 
By combining the expertise of two research groups in North America and Europe, state-of-the-art laboratory facilities, and small and large-scale instrumentation, both the scientific as well as the training goals of this action will be reached.