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Quantifying the atmospheric implications of the solid phase and phase transitions of secondary organic aerosols

Final 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. The solid amorphous state of SOA particles may have important implications for a number of atmospheric processes and the ultimate goal of the project was 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 particle phase kinetic limitations on atmospherically relevant processes. To achieve the final goal of the research, measurement method development was needed and the methodology that was be developed in the project QAPPA, enables the quantification of the essential factors affecting the atmospheric processes of the solid phase of SOA particles.
During the project, we have developed new methodological approaches 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 investigate the prevalence of semisolid or solid phase of organic aerosols in different environments, and to quantify the role of semisolid phase on the most central atmospheric processes. Our results shows that at environments dominated by isoprene, and where the humidity and temperature is high, OA is mostly in liquid phase. At northern latitudes, where monoterpenes are the major VOCs and humidity relatively low, the phase state varies from liquid to semisolid. Based on our laboratory results, the particle phase diffusion limitations play a minor role in organic vapour partitioning at temperatures around 293 K and at atmospherically relevant humidity conditions. At lower temperatures, or at dry conditions, the diffusion limitations affect the vapour partitioning, and may also play a role in ice nucleation processes. Our new methodology enabled a quantification of viscosity, vapour pressure and evaporation enthalpy of atmospherically relevant compounds in wide temperature and humidity conditions. This was one of the major goal of the project. Currently we are working to implement the results in large scale models.