# A brief introduction to COSIMA

The sectional aerosol behaviour code COSIMA (**Co**mputer **Sim**ulation of **A**erosols) treats the structural and optical properties and the dynamical evolution of ramified, apparently irregularly shaped agglomerated or aggregated particles (e.g. freshly emitted soot) employing a formalism based on statistical mechanics and fractal scaling laws. The effect of hydrodynamic interactions and shielding on particle mobility under continuum and transition regime conditions is accounted for within the framework of the Kirkwood-Riseman theory. Rayleigh-Debye-Gans theory is used to deal with light absorption and scattering. While the fractal formalism considers irregularity as the "normal" situation, ensembles of ideal compact spheres are nevertheless included as a limiting case. Therefore, COSIMA can also be operated like a conventional aerosol code.

Alternatively to the fractal formalism the traditional concept of shape factors (dynamical shape factor, coagulation shape factor) has been implemented into COSIMA as well, thus allowing for an unambiguous intercomparison of both approaches under otherwise identical numerical simulation conditions.

Processes included are

- Brownian and gravitational coagulation
- particle removal by sedimentation or diffusion to chamber walls
- aerosol losses due to sampling or leakage
- transport of trace gases to particle surface, surface adsorption, homogeneous and heterogeneous aerosol chemistry (in combination with a chemical rate equation solver)

Simulations are initialised with a given (e.g. measured) particle size distribution or by feeding a well defined particle mode into the control volume. Currently up to 30 subsequent instantaneous or continuous source events can be considered. The number size distribution of the released aerosol has to be specified as a function of the mobility equivalent diameter, either by providing lognormal parameters or by entering a tabulated function of arbitrary shape.

Some characteristic features of COSIMA:

- The volume fractal dimension of the particles is assumed to remain constant in time. This implies preservation of self similarity and neglect of restructuring during coagulation and ageing.
- Particles may grow by coagulation; shrinking due to breakup is currently not implemented.
- The size distribution of the primary particles is assumed to be monodisperse.
- Overlap between adjacent monomers (e.g. as a result of sintering) is accounted for (see Wentzel et al., 2003).
- Homogeneous mixing assumed in entire control volume except for laminar boundary layer adjacent to wall.
- Particle wall losses are assumed to be irreversible.
- The sectional representation of the size distribution is one-dimensional in composition, i.e. the model does not distinguish between internal and external mixing.
- The time integration of the general dynamic equation (GDE) relies on an extensively tested self-governing (explicit) Euler-Cauchy algorithm.

**Selected COSIMA references with regard to dynamics and optics of fractal like aerosols**

Theoretical basis, detailed code description and code validation:

- Naumann, K.-H. (2003). COSIMA - a computer program simualting the dynamics of fractal aerosols.
*J. Aerosol Sci., 34, 1371-1394.*

Characterisation of soot aerosols by combination of TEM image analysis and aerosol dynamics simulations; treatment of structural and dynamical properties of aggregates with overlapping primary particles:

- Wentzel, M., Gorzawski, H., Naumann, K.-H., Saathoff, H., Weinbruch, S. (2003). Transmission electron microscopical and aerosol dynamical characterization of soot aerosols.
*J. Aerosol Sci. 34, 1347-1370.*

Gas-to-surface transport and heterogeneous chemistry of soot aerosols:

- Kamm, S., Möhler, O., Naumann, K.-H., Saathoff, H., Schurath, U. (1999). The Heterogeneous Reaction of Ozone with Soot Aerosol.
*Atmos. Env. 33, 4651-4661.* - Saathoff, H., Naumann, K.-H., Riemer, N., Kamm, S., Möhler, O., Schurath, U., Vogel, H., Vogel, B. (2001). The loss of NO
_{2}, HNO_{3}, NO_{3}/N_{2}O_{5}and HO_{2}/HOONO_{2}on soot aerosol: A chamber and modeling study.*Geophys. Research Lett. 28, 1957-1960.*

Application to soot aerosol optics:

- Wagner, R., Linke, C., Naumann, K.-H., Schnaiter, M., Vragel, M., Gangl, M., Horvath, H. (2009). A review of optical measurements at the aerosol and cloud chamber AIDA.
*J. Quant. Spectrosc. Radiat. Transfer 110, 930- 949.* - Schnaiter, M., Horvath, H., Möhler, O., Naumann, K.-H., Saathoff, H., Schöck, O.W. (2003). UV-VIS-NIR spectral optical properties of soot and soot-containing aerosols.
*J. Aerosol Sci., 34, 1421-1444..*