THEORY AND PRACTICE OF AEROSOL SCIENCE

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Aerosols affect the climate directly and indirectly. A considerable part of these aerosols is produced by nucleation. Nucleation in the sulfuric acid-water-system in ...

CFD AND NUCLEATION IN THE WATER-SULFURIC ACID-SYSTEM E. HERRMANN1, D. BRUS2, A.-P. HYVÄRINEN2, and M. KULMALA1 1

Department of Physics, University of Helsinki, P.O.Box 64, 00014, University of Helsinki, Finland 2

Finnish Meteorological Institute, P.O.Box 503, 00101, Helsinki, Finland

Keywords: CFD, FLUENT, NUCLEATION. INTRODUCTION Aerosols affect the climate directly and indirectly. A considerable part of these aerosols is produced by nucleation. Nucleation in the sulfuric acid-water-system in particular has recently gained revived interest (Brus et al., 2010; Sipilä et al., 2010) following the geo-engineering suggestions of Crutzen (2006) and Wingenter et al. (2007). However, experiments allow only limited insight into the nucleation process. We simulate nucleation experiments (Brus et al., 2010) with the computational fluid dynamics code Fluent in order to better understand the experimental setup and the nucleation process. METHODS

Figure 1. Schematic picture of the experimental setup. In the experiment (Fig. 1), a furnace provides sulfuric acid vapor. The air flow containing this vapor is then mixed with humid air and and the resulting mixture (warm air with water and sulfuric acid vapor) is subsequently cooled down in the nucleation tube where nucleation is expected to occur. After the nucleation tube, particles are counted with a CPC. In the simulation, we only consider the nucleation tube while everything else in included as boundary conditions. The tube is modelled in a axisymmetric 2D simulation with a resolution of 50 (radial) times 1000 (axial) grid cells.

Simulations were made with the computational fluid dynamics (CFD) software Fluent and the fine particle model (FPM). Fluent models flow based on the Euler equations for mass and momentum conservation. A typical simulation calculates the following quantities for each cell of the simulation domain: pressure, density, velocity, temperature and - in case of multiple species - the mass fraction of each species. Additional quantities can be studied by adding user-defined functions (UDF). The FPM is a complex UDF that adds a particle dynamics model to Fluent. The FPM simulates formation, transformation, transport and deposition of multicomponent particles in gases and liquids. The applicable size range stretches from molecule size up to micrometer particles. The FPM simulates the dynamics of a particle population, which means that the particle size distribution is represented by a continuous function. The FPM solves for the spatial and temporal evolution of a multimodal, multiphase, multispecies particle size distribution. To study nucleation in the sulfuric acid-water-system, we considered classical binary nulceation (in a parameterization by Vehkamäki et al., 2002), kinetic nucleation (McMurry, 1980), and cluster activation theory (Kulmala et al., 2006). RESULTS

Figure 2. Nucleation rate (A) and particle number concentration (B) profiles in the nucleation tube modeled with Fluent-FPM. The residence time in this simulation was 30s, the relative humidity 30%. Nucleation was simulated for relative humidities 10%, 30%, and 50% and for sulfuric acid concentration between 109 to 3·1010cm-3. Figure 2 shows a typical profile. Varying with flow rate and sulfuric acid vapor concentrations, the nucleation maximum in our simulations has been found to be located between 54cm and 65cm (RH = 10%) and 65cm and 76cm (RH = 30% and 50%) into the nucleation tube. With growing H2SO4 vapor concentrations, the nucleation rate maximum moves closer to the tube inlet. Figure 2A illustrates that nucleation, if only at a very small rate, starts earlier in the tube, almost at the inlet, close to the wall. The wall cools the incoming gas-vapor-mixture, thus making nucleation possible. The picture indicates that, in our setup, temperature is the decisive parameter that controls the onset of nucleation. The particle number concentration in figure 5B shows how nucleation rate and particle number concentration relate to one another. The number concentration reaches its peak (between 1.0 and 1.25m) after nucleation (figure 5A) has ceased for the most part.

Concerning the nucleation rate, various theoretical approaches were compared to experimental results. Figure 3 shows the main results. We see that, at 10% relative humidity (Fig. 3a), the kinetic nucleation theory best reproduces the nucleation rate slope found in the experiment. At RH = 50% (Fig. 3b), however, the classical binary nucleation rate yields the best agreement. The results show that determining the role of water is crucial in fully understanding nucleation in the water-sulfuric acid-system.

Figure 3. Comparison of experimental and theoretical nucleation rates at 10% (a) and 50% (b) relative humidity.

ACKNOWLEDGEMENTS This research was supported by the Academy of Finland Center of Excellence program (project number 1118615). All simulations were performed on the servers of the CSC - IT Center for Science where Dr. Thomas Zwinger provided valuable technical assistance. REFERENCES Brus, D.; Hyvärinen, A.-P.; Viisanen, Y.; Kulmala, M.; Lihavainen, H. (2010). Homogeneous nucleation of sulfuric acid and water mixture: experimental setup and first results. Atmos. Chem. Phys. 10, 2631. Crutzen, P. J. (2006). Albedo enhancement by stratospheric sulfur injections: A contribution to resolve a policy dilemma? Climatic Change 77, 211. Kulmala, M.; Lehtinen, K. E. J.; Laaksonen, A. (2006). Cluster activation theory as an explanation of the linear dependence between formation rate of 3nm particles and sulphuric acid concentration. Atmos. Chem. Phys. 6, 787. McMurry, P. H. (1980). Photochemical aerosol formation from SO2: A theoretical analysis of smog chamber data. J. Colloid Interface Sci. 78, 513-527. Sipilä, M.; Berndt, T.; Petäjä, T.; Brus, D.; Vanhanen, J.; Stratmann, F.; Patokoski, J.; Mauldin, R. L.; Hyvärinen, A.-P.; Lihavainen H.; Kulmala, M. (2010). The Role of Sulfuric Acid in Atmospheric Nucleation. Science 327, 1243. Vehkamäki, H.; Kulmala, M.; Napari, I.; Lehtinen, K. E. J.; Timmreck, C.; Noppel, M.; Laaksonen, A. (2002). An improved parameterization for sulfuric acid-water nucleation rates for tropospheric and stratospheric conditions, J. Geophys. Res. 107, (D22), 4622. Wingenter, O. W.; Elliot, S. M.; Blake, D. R. (2007). New Directions: Enhancing the natural sulfur cycle to slow global warming. Atmos. Environ. 41, 7373.