Multicomponent aerosol dynamics model UHMA: model development and validation H. Korhonen, K. E. J. Lehtinen, and M. Kulmala Department of Physical Sciences, University of Helsinki, Helsinki, Finland
Abstract. A size-segregated aerosol dynamics model UHMA (University of Helsinki Multicomponent Aerosol
model) was developed for studies of multicomponent tropospheric aerosol particles.
The model includes major aerosol microphysical processes in the atmosphere with
a focus on new particle formation and growth;
thus it incorporates particle coagulation and multicomponent condensation,
applying a revised treatment of condensation flux onto free molecular regime particles
and the activation of nanosized clusters by organic vapours (Nano-Köhler theory), as well as
recent parameterizations for binary H2SO4-H2O
and ternary H2SO4-NH3-H2O homogeneous nucleation and dry deposition.
The representation of particle size distribution can be chosen from three sectional
the hybrid method, the moving center method, and the retracking method in which
moving sections are retracked to a fixed grid after a certain time interval.
All these methods can treat particle emissions and atmospheric transport consistently, and are therefore
suitable for use in large scale atmospheric models.
In a test simulation against an accurate high resolution
solution, all the methods showed reasonable treatment of new particle formation
with 20 size sections
although the hybrid and the retracking methods suffered from artificial widening
of the distribution.
The moving center approach, on the other hand, showed extra dents in the
particle size distribution and failed to predict the onset of detectable
In a separate test simulation of an observed nucleation event,
the model captured the key qualitative behaviour of the system well.
Furthermore, its prediction
of the organic volume fraction in newly formed particles,
suggesting values as high as 0.5 for 3–4 nm particles and approximately 0.8
for 10 nm particles, agrees with recent indirect composition measurements.
Citation: Korhonen, H., Lehtinen, K. E. J., and Kulmala, M.: Multicomponent aerosol dynamics model UHMA: model development and validation, Atmos. Chem. Phys., 4, 757-771, doi:10.5194/acp-4-757-2004, 2004.