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QAPPA Report Summary

Project ID: 335478
Funded under: FP7-IDEAS-ERC
Country: Finland

Mid-Term Report Summary - QAPPA (Quantifying the atmospheric implications of the solid phase and phase transitions of secondary organic aerosols)

In our earlier studies we have shown that the atmospheric Secondary Organic Aerosol (SOA) particles formed in boreal forest can be amorphous solid in their physical state at least several hours after their formation. Our result has already re-directed the SOA related research. In the several follow-up studies, it has been shown that SOA particles generated in the laboratory chamber from different pre-cursors and in various conditions are solid in their physical phase. The solid amorphous state of SOA particles may have important implications for a number of atmospheric processes and the ultimate goal of the project is to quantify the atmospheric implications of the phase state of SOA particles. If the viscosity and diffusion coefficients of diffusing molecule in the particle bulk are known, it is possible to estimate the effect of particle phase kinetic limitations on atmospherically relevant processes. To achieve the final goal of the research, measurement method development is needed and the methodology that will be developed in the proposed study, aims ambitiously to quantify the essential factors affecting the atmospheric processes of the solid phase of SOA particles.

During the first half of the project we have developed methodology suitable to study the particle phase transitions and atmospherically relevant processes affected by the physical phase of the particles. Using the new methodology we were able to show that the water up-take of biogenic SOA particles is not restricted by the diffusion limitations at temperatures around 293K when atmospheric timescales are considered. Instead, the water uptake is limited by the solubility of the SOA material. We were also able to show that the correct description of water uptake at atmospheric humidities improves the radiative forcing estimates in models considerably. By performing measurements at sub-zero temperatures we have been able to demonstrates that solid SOA can act as a seed for ice nucleation while liquid SOA didn’t activate IN. These results have been already adapted in some of the regional and global modelling applications.

By the newly developed methodology we were able to define the viscosity value for biogenic SOA particles over the wide humidity range. Based on our results we were able to show that the diffusion limitations limits the evaporation at dry conditions but at atmospherically relevant humidities (RH>40%) and at temperatures around 20C the diffusion limitations play only minor role. Also, we showed that the viscosity of the particles increases during the evaporation as the higher volatility compounds evaporated from particles and the viscosity reaches values ~10^9 Pas at the end of the evaporation.

By bringing the developed methodology to field campaigns we were able to show that organic aerosols are mostly liquid in Southern-East US and most probably the diffusion limitations don’t play significant role there when atmospheric processes are considered. This information is important e.g. for regional modelling community. In addition, we were able to show by laboratory and field measurements that the atmospheric aging (i.e. increasing oxidation state of aerosol particles) decreases the liquefying humidity of SOA particles.

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