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

e-mail: losno@lisa.univ-paris12.fr

Introduction

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.L^­1 (M) in the figures.

Cloud drop chemical sheme

• Trace metals are coming from the dissolution of particulate material included in the cloud droplets.
• The cloud drop system is open: it receive light and exchange gas (mainly water vapour). Exchange will be assumed here under equilibrium conditions for ozone.
• The homogeneous water phase also exchange metals with the included particulate matter.

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:

Compound	Initial concentration	Reference
O3	3.0 E-10 mol.L­1	25 ppb gaseous
H2O2	10 µmol.L­1	[6]
H+	10 µmol.L­1 (pH = 5)	[7], [8]
Soluble Fe	5.0 E­8 mol.L­1	[10], [11]
Soluble Mn	3.0 E­9 mol.L­1	[10], [12]
Soluble Cu	1.0 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 Fe^III/Fe^II and another with CuII/CuI:

H₂O₂ + hv OH + OH J₁=5.70 10^-07 s^­1

H₂O₂ + OH HO₂ k₄=2.70 10⁺⁰⁷ M^­1.s^­1

O₂^- + Fe^III Fe^II + O₂ k_19,20=1.50 10⁺⁰⁸ M^­1.s^­1

H₂O₂ + Cu⁺ Cu²⁺ + OH k₃₅=4.00 10⁺⁰⁵ M^­1.s^­1

O₂^- + Cu²⁺ Cu⁺ + O₂ k₃₈=5.00 10⁺⁰⁹ M^­1.s^­1

hv + Fe(OH)²⁺ Fe^II + OH J₂₂=5.90 10^-04 s^­1

H₂O₂+ Fe(OH)⁺ Fe^III + OH k₂₈=6.01 10⁺⁰¹ M^­1.s^­1

A photostationary state is reached when (K_a;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 Fe^II/Fe^III 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 Cu^I, Fe^II and Fe^III concentrations in a periodically illuminated system. Right scale is for Cu^I, which concentration varies around 10^­11 mol.L^­1. Cu^II 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 (10^­7 mol.L^­1) 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.

Conclusion:

• Adding trace metals completely changes the reaction scheme.
• Adding trace metals makes the system very less sensitive to other reactive species.
• Adding trace metals lets use 1^st order reaction calculations.
• Adding trace metals lets hope to use photostationary equilibrium instead of kinetics.
• The trace metal amounts injected in the model are here an average value measured in rainwater. Particulate-dissolved dynamic modelisation will be of major importance to apply aqueous photochemistry at a global scale.

References

[1] Graedel T.E., Mandich M.L. and Weschler C.J. (1986), "Kinetic Model Studies of Atmospheric Droplet Chemistry: 2. Homogeneous Transition Metal Chemistry in Raindrops", J. Geophys. Res., 91 D4, 5205-5221.

[2] Jacob D.J., Gottlieb E.W. and Prather M.J. (1989), "Chemistry of a Polluted Cloudy Boundary Layer", J. Geophys. Res., 94 D10, 12975-13002.

[3] Lelieveld J. and Crutzen P.J. (1991), "The role of Clouds in Tropospheric Photochemistry", J. Atmos. Chem., 12, 229-267.

[4] Walcek C. J., Yuan H.H., and Stockwell W.R. (1997) "The influence of aqueous-phase chemical reactions on ozone formation in polluted and nonpolluted clouds", Atmos. Environ., 31, 1221-1237.

[5] Weschler C.J., Mandich M.L. and Graedel T.E. (1986), "Speciation, Photosensitivity, and Reactions of Transition Metal Ions in Atmospheric Droplets", J. Geophys. Res., 91 D4, 5189-5204.

[6]Willey J.D., Kiebert R.J. and Lancaster R.D. (1996), "Coastal Rainwater Hydrogen Peroxide: Concentration and deposition", J. Atmos. Chem., 25, 149-165.

[7] Losno R., (1989), "Chimie d'éléments minéraux en trace dans les pluis méditerranéenes", PhD Thesis, Université Paris 7, 215 pp.

[8] Losno R., Bergametti G., Carlier P. and Mouvier G. (1991), "Major ions in marine rainwater with attention to sources of alkaline and acidic species", Atmos. Environ., 25A, 763-770.

[9] Losno R., Colin J.L., Spokes L., Jickells T., Schulz M., Reberts A., Leermakers M., Meulemann C. and Bayens W. (1998), "Non-rain deposition significantly modifies rain samples at a coastal site", Atmos. Environ., 32, 3445-3455.

[10] Hoffmann P., Dedik A.N., Deutsch F., Sinner T., Weber S., Eichler R., Sterkel S., Sastri C.S. and Ortner H.M. (1997), "Solubility of single chemical compounds from an atmospheric aerosol in pure water, Atmos. Environ., 17, 2777-2785.

[11] Jickells T.D., Knap A.H. and Church T.M. (1984), "Trace Metals in Bermuda Rainwater", J. Geophys. Res., 89 D1, 1423-1428.

[12] Lim B., Losno R.,. Jickells T.D and Colin J.L. (1994), "The solubilities of aluminium, lead, copper and zinc in rain samples in the marine environment over the North Atlantic Ocean and the Mediteranean Sea." , Global Biogeochemical Cycles, 8, 349-362.

[13] Ross H.B. (1987), "Trace metals in precipitation in Sweden", Water Air and Soil Pollution, 36, 349-363.

[14] Spokes L.J., Jickells T.D. and Lim B. (1994), "Solubilisation of aerosol trace metals by cloud processing: a laboratory study", Geochim. Cosmochim. Acta, 58, 3281-3287.