University of Helsinki Multicomponent Aerosol model (UHMA) is a sectional box model developed for studies of tropospheric new particle formation and growth and was originally constructed by Korhonen and co-workers (2004).

It has all basic aerosol processes, including nucleation, condensation (Bilde et al., 2015), coagulation and deposition. The model has been under constant development to include latest knowledge in secondary aerosol formation (Vuollokoski et al., 2010a and b, Hermansson et al., 2014).

In the model, particles are assumed to be spherical and to consist initially of sulphuric acid, water and organic compounds. The inputs include an initial size distribution, particle composition, ambient temperature, relative humidity and precursor gas concentrations. The condensation of an "unlimited" number of organic compounds is simulated by first estimating the pure liquid saturation vapour pressures of all oxidation products using the group contribution method described by Nannoolal et al. (2008) and the group contribution method SIMPOL (Pankow and Asher, 2008). By applying both of these two methods, we can evaluate how sensitive the modelled SOA formation is to the estimated saturation vapour pressures, which can vary orders of magnitudes depending on which method is used. The corresponding equilibrium vapour pressures for each particle size bin are derived with Raoult’s law, corrected for non-ideal mixing using activity coefficients calculated by the AIOMFAC thermodynamic model (Zuend et al., 2011), and the Kelvin effect. The densities of the condensing pure organic compounds are estimated based on their molecular mass and atomic volumes, taking into consideration the changes in volume due to intramolecular bonding (Girolami, 1994). The molecular diffusion coefficients of the vapours are based on a method described in Jacobson (2005). An exact estimate for the surface tension of the organic compounds is one of the most challenging topics and in previous studies we assumed a constant value of 0.05Nm−1 following Riipinen et al. (2010), which is a too simple approach and will be further investigated.

References (Publications from our group are highlighted in bold)

  • Bilde M., Barsanti, K., Booth, M., Cappa, C. D., Donahue, N. M., Emanuelsson, E. U., McFiggans, G., Krieger, U. K., Marcolli, C., Topping, D., Ziemann, P., Barley, M., Clegg, S., Dennis-Smither, B., Hallquist, M., Hallquist, Å. M., Khlystov, A., Kulmala, M., Mogensen, D., Percival, C. J., Pope, F., Reid, J. P., Ribeiro da Silva, M. A. V., Rosenoern, T., Salo, K., Soonsin, V. P., Yli-Juuti, T., Prisle, N. L., Pagels, J., Rarey, J., Zardini, A. A., and Riipinen, I.: Saturation vapor pressured and transition enthalpies of low-volatility organic molecules of atmospheric relevance: from dicarboxylic acids to complex mixtures, Chem. Rev., 2015.
  • Girolami, G. S.: A Simple “Back of the Envelope” Method for Estimating the Densities and Molecular Volumes of Liquids and Solids, J. Chem. Educ., 71, 962–964, 1994.
  • Hermansson, E., Roldin, P., Rusanen, A., Mogensen, D., Kivekäs, N., Boy, M. and Swietlicki, E.: Biogenic SOA formation through gas-phase oxidation and gas-to-particle partitioning comparison between process models of varying complexity, Atmos. Chem. Phys., 14, 11853-11869, 11001-11040, 2014.
  • Jacobson, M. Z.: Fundamentals of Atmospheric Modelling (2nd edition), Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, ISBN: 0 521 54865 9, 2005
  • Korhonen, H., Lehtinen, K. E. J., and Kulmala, M.: Multicomponent aerosol dynamics model UHMA: model development and validation, Atmos. Chem. Phys., 4, 757–771, 2004.
  • Nannoolal, Y., Rarey, J., and Ramjugernath, D.: Estimation of pure component properties. Part 3. Estimation of the vapor pressure of non-electrolyte organic compounds via group contributions and group interactions, Fluid Phase Equilibr., 269, 117–133, 2008.
  • Pankow, J. F. and Asher, W. E.: SIMPOL.1: a simple group contribution method for predicting vapour pressures and enthalpies of vaporization of multifunctional organic compounds, Atmos. Chem. Phys., 8, 2773–2796, 2008.
  • Vuollekoski, H., Boy, M., Kerminen, V.-M., Lehtinen, K.E.J. and Kulmala, M.: MECCO: A method to estimate concentrations of condensing organics – description and evaluation of a Markov chain Monte Carlo application, Journal of Aerosol Science 41, 1080-1089 , 2010a.
  • Vuollekoski, H., Nieminen, T., Paasonen, P., Sihto, S.-L., Boy, M., Manninen, H., Lehtinen, K., Kerminen, V.-M. and Kulmala, M.: Atmospheric nucleation and initial steps of particle growth: numerical comparison of different theories and hypotheses, Atmos. Research 98, 229-236, 2010b.
  • Zuend, A., Marcolli C., Booth , A. M., Lienhard, D. M., Soonsin, V., Krieger, U. K., Topping, D. O., McFiggans G., Peter, T., and Seinfeld, J. H.: New and extended parameterization of the thermodynamic model AIOMFAC: calculation of activity coefficients for organic-inorganic mixtures containing carboxyl, hydroxyl, carbonyl, ether, ester, alkenyl, alkyl, and aromatic functional groups, Atmos. Chem. Phys., 11, 9155–9206, 2011.