Modelling trace metals acting as catalysts in clouds: multiphase chemistry in non polluted area.

CMD project APP 8, Karlsruhe 23-25 September 1998

R. Losno, K. Desboeufs

Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA)

Universités Paris 7 Paris 12, UMR CNRS 7583



Trace metals, and especially d-block metals (Fe, Cu and Mn) are found to be essential to describe the aqueous atmospheric chemistry. The influence of dissolved trace metals in cloud droplets should be compared to other factors. We will present case studies applied to non polluted air masses, including hydrogen peroxide, ozone and small amounts of background crustal aerosols. Two coupled catalytic cycles drive all the reaction scheme: One with iron and other with copper. Gas exchange is fast enough to be considered as equilibrium, but metals exchanges between particulate and dissolved phase are of major importance. All the concentrations are given in mol.L­1 (M) in the figures.

Cloud drop chemical sheme

The chemistry inside the cloud droplet in remote areas can be well described by around 40 reactions [1-5]. But most of them are not important because not fast enough. The initial concentrations are taken following next table:
CompoundInitial concentration Reference
O33.0 E-10 mol.L­1 25 ppb gaseous
H2O210 µmol.L­1 [6]
H+10 µmol.L­1 (pH = 5) [7], [8]
Soluble Fe5.0 E­8 mol.L­1 [10], [11]
Soluble Mn3.0 E­9 mol.L­1 [10], [12]
Soluble Cu1.0 E-9 mol.L­1 [12], [13], [14]

Reduced system

The system can be summarised by the following main equations, giving two catalytic cycles, one with FeIII/FeII and another with CuII/CuI:

H2O2 + hv OH + OH J1=5.70 10-07 s­1

H2O2 + OH HO2 k4=2.70 10+07 M­1.s­1

O2- + FeIII FeII + O2 k19,20=1.50 10+08 M­1.s­1

H2O2 + Cu+ Cu2+ + OH k35=4.00 10+05 M­1.s­1

O2- + Cu2+ Cu+ + O2 k38=5.00 10+09 M­1.s­1

hv + Fe(OH)2+ FeII + OH J22=5.90 10-04 s­1

H2O2+ Fe(OH)+ FeIII + OH k28=6.01 10+01 M­1.s­1

A photostationary state is reached when (Ka;HO2 is the acidity constant of HO2):

Reactivity dynamics

As the system reacts very fast to irradiation changes, this state is reached after less than 1 second (next figure).

But FeII/FeIII variations are slower which allows iron speciation measurements. By opposite, the copper speciation is faster relaxed because reaction constant between copper and radicals are greater than between iron and radicals. Next figure shows long term variations of CuI, FeII and FeIII concentrations in a periodically illuminated system. Right scale is for CuI, which concentration varies around 10­11 mol.L­1. CuII remains constant. "LIGHT ON" is a period where all the J are to their nominal values, and "LIGHT OFF" where all J are switched to zero.

Thus, the chemical system is also more stable against chemical variations. The following example shows the OH radical concentrations versus time if methanol (10­7 mol.L­1) is added to the system after 500 s. The lines taking into account trace metals (Met & No_Oz and Met & Oz) are showing smaller concentration variations than those which do not (No_Met & No_Oz and No_Met & Oz). Moreover, the presence of ozone do not change anything if metal is added (Met & Oz is with ozone, Met & No_Oz is without), but become determinant is trace metals are neglected (No_Met & Oz with and No_Met & No_Oz without ozone).

Very small amounts of dissolved metals, 0.01 µM for Fe and 0.001 µM for Cu, decrease the radical concentration in presence of ozone but increase without ozone. That is the H2O2 importance against ozone in non polluted cloud drops is expanded. In presence of trace metals, the methanol decrease is close to a first order law, showing a straight line if "log[Methanol]" is expressed as function of time. This make computation easier.

But the system becomes very sensitive to trace metals concentrations, as shown by the figure left, plotting methanol concentration variations versus time with various dissolved trace metal amounts, the ratio to iron remaining constant.



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