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 cédex, France

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

pH Influence

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 dissolution fractions:

R = k[H+]a + k'[H+]-b

with two different situations :

  1. If k[H+]a << k'[H+]-b; log R = log k' + b pH. This equation corresponds to the part of the curve where the slope is positive. As the H+/OH­ equilibrium is ever very fast, we can write k'[H+]­b = k'.Kw­b.[OH­]b, to emphasize reaction control by OH- (Kw is the equilibrium constant of water ionic dissociation).

  1. If k[H+]a >> k'[H+]-b; log R = log k - a pH. This equation corresponds to the part of the curve where the slope is negative. The reaction is controlled by protons.

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 (28).

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.

Leaching mechanism

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 been subjected.


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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Figures Captions

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 aerosol states.

Figure 1

Figure 2