Geoscience Reference
In-Depth Information
Because neither the pathways nor the mechanisms of production of condensable
trace gases and the embryos of the condensation phase are well established so far,
our semi-empirical approach is well approved. Moreover, if we risk starting from
the first principles, we need to introduce too many empirical (fitting) parameters.
Aerosol particles throughout the entire size range beginning with the smallest
ones (with sizes of the order 1 nm) and ending with sufficiently large particles
(submicrometer and micrometer size) are shaped by some well-established mecha-
nisms: condensation and coagulation. Little is known, however, about atmospheric
nucleation. This is the reason why this very important process together with
intramode coagulation is introduced here as an external source of the particles of
the smallest sizes. The final productivity of the source is introduced as a fitting
parameter whose value is controlled by two of these processes simultaneously and
thus is always lower than the productivity of the nucleation mechanism alone.
Next, coagulation produces particles distributed over a size interval, rather than the
monodisperse ones of a critical size (as in the case of nucleation alone). Productivity
should be introduced as a function of particle size and time, respectively. In
principle, the size dependence of the source can be found theoretically, but it is
better to refuse this idea and to introduce it as the product of a lognormal function
and the time-dependent total production rate.
Condensational growth depends on the concentrations of condensable vapors,
with the condensational efficiencies being known functions of the particle size.
The concentrations of condensable trace gases are introduced as known functions.
They can also be calculated, once all reactions responsible for conversion of
volatile trace gases to low volatile ones and respective reaction rates are known
(
C
stoichiometry of the reactions
C
initial concentrations of all participants and
many other unpleasant things). Of course, nothing like this is known, and there is
no chance to get all this information in the near future.
The losses of particles are caused mainly by preexisting submicrometer and
micrometer particles (Boy et al.
2003
). There are also other types of losses:
deposition of particles onto leaves of trees, soil losses, scavenging by deposits, and
mists. Here the loss term is introduced as a sink of small particles on preexisting
submicrometer- and micrometer-sized aerosol particles.
Self-coagulation of particles with sizes exceeding 3 nm is entirely ignored in the
model. Many authors (e.g., Kulmala et al.
2004
and references therein) estimated
the characteristic times of the coagulation process and found them to exceed 10
5
s.
In what follows, we ignore this process. In contrast, the intermode coagulation (the
deposition of newly born particles onto preexisting aerosols) is of great importance
and should be taken into account.
The remainder of this chapter focuses on the interactions of a single aerosol
particle with surrounding carrier gas that can contain some condensible vapors
(condensation and evaporation), ions (particle charging processes), and the energy
exchange between the particle and the surrounding gas. The kinetics of all these
processes is very complex, because the particle sizes are comparable to the molec-
ular mean free path. In principle, in this situation one must solve the Boltzmann
