RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES1005, doi:10.2205/2007ES000275, 2008
[3] During the last two decades numerous attempts of modelling the nucleation processes in the atmosphere have been made. All of them (with no exception) started from commonly accepted conception that the chemical reactions of trace gases are responsible for the formation of nonvolatile precursors which then give the life to subnano- and nanoparticles in the atmosphere. In their turn, these particles are considered as active participants of the atmospheric chemical cycle leading to the particle formation. Hence, any model of nucleation bursts is included (and includes) coupled chemical and aerosol blocks.
[4] Our main idea is to decouple the aerosol and chemical parts of the particle formation process and to consider here only the aerosol part of the problem. We thus introduce the concentrations of nonvolatile substances responsible for the particle growth and the rate of embryo production as external parameters whose values can be found either from measurements or calculated independently, once the input concentrations of reactants and the pathways leading to the formation of these nonvolatile substances are known. Next, introducing the embryo production rate allows us to avoid rather slippery problem of the mechanisms responsible for embryos formation. Because neither the pathways nor the mechanisms of production of condensable trace gases and the embryos of condense phase are well established so far, our semi-empirical approach is well approved. Moreover, if we risk to start from the first principles, we need to introduce too many empirical (fitting) parameters.
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Figure 1 |
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Figure 2 |
[6] The 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 the reaction graph of all chemical processes responsible for conversion of volatile trace gases to low volatile ones and respective reaction rates are known (+ stoichiometry of the reactions + initial concentrations of all participants and many other unpleasant things). Of course, nothing like this is known and there is no chance to get this information in the near future.
[7] The losses of particles are caused mainly by preexisting submicron and micron particles. There are also other types of losses: deposition of particles on vegetation, soil losses, scavenging by deposits and mists. Here the loss term is introduced as a sink of small particles on preexisting submicron and micron aerosol particles.
[8] Self-coagulation of particles with sizes exceeding 3 nm is entirely ignored in the model. Many authors estimated the characteristic times of the coagulation process and found them to exceed 105s. In what follows we ignore this process. On the contrary, the intermode coagulation (the deposition of newly born particles onto preexisting aerosols) is of great importance and should be taken into account.
[9] Now it is easy to answer the question posed in the title of this Section. Our model is linear because the nucleation mode does not affect the surrounding atmosphere whose chemical state is determined by other numerous external factors. For example, the lifetimes of trace gases and the particles of nucleation mode depend on the concentration and the size distribution of the preexisting aerosol particles.
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Figure 3 |
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Figure 4 |
![]() | (1) |
Here
are linear evolution operators allowing for restoring the full size distribution
functions by the particle source J and the initial conditions n0.
As we will see the first term does not produce
a "burst-like'' picture, although diurnal increases in the detectable particle concentration are well reproduced.
The second term is of special significance. We show that if the source does not work at night time, but
a highly disperse (undetectable) aerosol appears from somewhere, then a running-wave type picture typical for
the nucleation burst arises. We incline to associate the nucleation events to this very mechanism. The overall
situation is displayed in Figure 1. Figure 2 also shows an event picture. Although the daytime increase in the
particle number concentration of the nucleation mode occurs, it is not so clearly expressed as in the case
displayed in Figure 1. The initially existing fine and undetectable aerosol begins to grow in parallel with
the particles from the regular (periodic) source. The linearity of equation (1) is the reason for this running wave
picture. Very likely that this initial aerosol (proto-aerosol in what follows) is closely related to stable clusters
whose existence was theoretically predicted in
[Kulmala et al., 2000].
[11] Another type of nucleation burst is shown in Figure 3. If the source abruptly (at t=tc ) ceases to produce fresh particles then the particles produced before tc begin to grow in the regime of free condensation and their spectrum moves to the right along the size axis as a running wave. There are some reasons to believe that such type of nucleation burst can realize in the atmosphere. The activity of the nucleation process can be suppressed by a slight increase in the concentration of preexisting particles [Lushnikov and Kulmala, 2000].
[12] So the model considered below is linear, i.e., the evolution equation governing the particle size distribution is linear. This is the main and very principle difference of our model from other ones.
Citation: 2008), A model of nucleation bursts, Russ. J. Earth Sci., 10, ES1005, doi:10.2205/2007ES000275.
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