The Influence of Trace Metals Leaching from Mineral
Aerosols on Atmospheric Chemistry
K. Desboeufs and R. Losno
Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), CNRS-UMR 7583,
61, avenue du Général de Gaulle, 94010 Créteil
Aims of the research
Several studies emphasised the potential role of transition metals
in the formation of SO2, ozone and organic pollutants
into the cloud droplets, especially Mn, Fe and Cu (1-10). Under
conditions typically observed in atmospheric waters, transition
metals like Fe, Cu and Mn can undergo a series of chemical and/or
photochemical reactions that result in their rapid cycling between
the oxidation states. The trace metals concentrations have a critical
influence on the kinetic of catalysed reaction (11, 12, 13, 14)
and on the synergistic effect of transition metals (14). Generally,
in models, data measured in rainwater and on collected aerosols
are used to integrate trace metals concentrations. These concentrations
are considered to be identical with cloud waters ones. The rain
physicochemical conditions, however, contrast with those in clouds
(15-18). Moreover, a cloud is a renewed system as opposed to precipitation.
Thus, in view of the Cu(I) lifetime (1 s), it is strongly
probable that concentrations of this metal are different in the
two systems (13). Therefore, in order to obtain a fair estimate
of the contribution from this metal catalysis on the atmospheric
chemistry, it is of particular importance to find a relation between
measured and required data. Since the only source of trace metals
in the aqueous phase is the solubilisation of aerosols, solid
to soluble phase transfer mechanisms have to be studied. Some
preliminary results on the kinetic and mechanism of transfer between
dissolved and particular phases in clouds are presented.
Before aerosol particles are removed by precipitation, they are
subjected to repeated wetting and drying cycles during cloud formation
and evaporation. They sustain a partial weathering which causes
some solubilisation of trace metals. Thus, experiments of particle/water
interaction were performed in an open flow reactor, using aerosol
concentration, pH conditions and timescales representative of
cloud water (19). This experimental system aims to simulate the
water condensation on an aerosol particle likely to be encountered
in clouds, and assess whether metal solubility is affected by
pH. Aerosols used in this experiment are Saharan aerosol like
material (Loess from Capo Verde Island).
Principal scientific results
It is known that pH is a major control of the solubility of metals,
in cloud waters and precipitation (20-26). For some trace metals,
the relationship between pH and solubility is not, however, a
simple one (27). Dissolution experiments have been conducted for
typical pH of cloud and rain waters. To point out the effects
of varying pH on dissolution, the advancement is a more useful
comparison parameter. The advancement is defined as the number
of reaction's mole and corresponds with the state of aerosol weathering.
It appears that Fe dissolution rate at the beginning of the experiment (low ) increases regularly with pH (Figure 1). Later (higher ), the iron dissolution rate has a minimum at pH 4.3, suggesting two dissolution behaviours operate over longer periods of time ; one at pH > 4.3, and a different one at pH < 4.3. The same trend can be observed for Cu with a rate minimum at pH 4.5. Manganese does not present this difference between the beginning and the end of the experiment. These observations suggest that both Cu and Fe are released by dissimilar forms at the beginning and the end of the experiment. The form released at the beginning, probably an amorphous phase, seems to be completely removed after several minutes of dissolution.
Moreover, the dissolution rate law can be expressed to sum both
R = k[H+]a + k'[H+]-b
with two different situations :
Similar compound rate laws describing mineral dissolution have
yet been summarised (28). Typically, the proton-controlled mechanism
dominates dissolution for pH < pHpzc and
hydroxyl-controlled dissolution dominates overall dissolution
for pH > pHpzc. The point of zero charge of mineral
depends on pH, concentrations of all ions, hydration and crystallinity
Influence surface crystallinity
In order to clarify the probable impact of the surface crystallinity on trace metals solubilisation, two experiments were carried out. The one with original aerosol, and a second with weathered aerosol, i.e. these aerosols have already sustained a dissolution experiment, so as to simulate a condensation/evaporation cycle.
The results of these experiments (Figure 2) show two different shapes according to the aerosol. For weathered aerosols, the dissolution rate is first very quick, as opposed to fresh particles, then the rate decrease up to relative low values.
Accordingly, the solubilisation behaviour is different if aerosols
have already sustained an evaporation. A probable amorphous phase
is formed in surface, which is very more soluble than the initial
crystalline phase. Thus, the history of the aerosol is an important
point to consider for the leaching model.
From these results, a weathering mechanism can be proposed for original Saharan aerosol. First, the dissolution of the crystalline phase is activated by H+ and/or OH- : the most soluble elements therefore pass into the aqueous phase. However, a residual layer containing the less soluble elements will form. Thus, for a continued efficient dissolution, the H+ and OH- must diffuse through this layer. Similarly, soluble elements must also diffuse through this layer to the bulk.
Thus, from this mechanism, it is easy to make an inventory of
mineral parameters interfering on the solubilisation process.
The origin of elements is primordial, since amorphous and crystalline
phase do not solubilise in the same way. The history of the aerosol
is also paramount since it determines the weathering state of
particles according to the number of evaporation cycles they have
Two important points have been clarified to model the metal leaching
influence on the atmospheric chemistry. Firstly, the pH influence
on metals solubilisation has been pointed out and a relationship
between pH and metal solubility is given. Secondly, the effect
of the surface crystallinity has been emphasised. Finally, a possible
solubilisation mechanism is proposed in respect to these results.
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Figure 1: Evolution of the dissolution rate as a function of pH
for Iron. A dissimilar rate behaviour versus pH is observed between
the beginning ( = 6.5 10-9 ) and the end ( = 1.42
10-8) of dissolution reaction.
Figure 2: Dissolution rate of Mn in function of time for two different