MINATROC: Mineral dust aerosol and tropospheric chemistry

• Research unit: EU RFP
• Project number: 99.0431

Key words (English)

Heterogeneous chemistry; atmospheric acidity; HNO3; HCl; interfacial processes; mineral dust aerosols

Alternative project number (English)

EU project number: EVK2-1999-00003

Research programs (English)

EU-programme: 5. Frame Research Programme - 1.4a.2 Global change, climate and biodiversity

Short description (English)

See abstract

Further information (English)

Full name of research-institution/enterprise:
EPF Lausanne
Département de Génie Rural (DRG)
Laboratoire de pollution atmosphérique et sol (LPAS)

Partners and International Organizations (English)

Coordinator: Centre nat. de la recherche scient., Gif s. Yvette

Abstract (English)

The goal of this study is twofold: (1) study of the reaction mechanism and (2) the measurement of the heterogeneous chemical kinetics of the interaction of atmospheric acids (HCl, HNO3) with surrogates (models) of atmospheric dust particles, both in the presence and the absence of other atmospherically relevant gas molecules. All experiments to date have been performed in a Knudsen cell reactor under molecular flow conditions at ambient temperature. The role of adsorbed H2O enabling the solid state reaction has turned out to be of great importance.
The following results have been obtained during this reporting period:
- The interaction of HCl with CaCO3 is controlled by the presence of adsorbed H2O: increasing amounts lead to increasing rates of uptake of HCl according to:
2HCl + CaCO3 => CaCl2 + CO2 + H2O
where both CO2 and H2O stem from the decomposition of the intermediate carbonic acid, H2CO3. However, owing to the hygroscopic nature of CaCl2 no H2O is observed in the gas phase. The structural properties of the solid CaCO3 are important as the rate of uptake of HCl on a polished marble plate is very small. However, the rate increases with increasing amounts of adsorbed H2O.
- Consideration of the mass balance between HNO3 taken up and CO2 produced leads to the conclusion that HNO3 and presumably other atmospheric acids directly interact with basic sites on the solid CaCO3 according to the following equilibrium:
H2O + CaCO3 ó Ca(OH)2 + CO2 (a)
At the start of the HNO3 interaction with CaCO3 according to:
2HNO3 + CaCO3 => Ca(NO3)2 + CO2 + H2O
the emerging CO2 is trapped by the basic sites (Ca(OH)2) thus shifting the above equilibrium (a) to the left, whereas the interaction of CaCO3 with H2O enables the formation of basic sites by shifting equilibrium (a) to the right.
- The (initial and steady-state) uptake coefficient of HNO3 on Kaolinite, Arizone Road Dust and Saharan Dust is large, persisting even after 120s and does not saturate. No reaction products are detected. Apparently, stable nitrates and chlorides (in the case of HCl interaction) are formed on these substrates.
- Rapid and quantitative ozone decomposition takes place on CaCO3 without any signs of saturation according to the following net equation: 2O3 => 3O2.
- SO2 adsorbs onto CaCO3 according to the following equations:
SO2 + H2O => H2SO3 or SO2·H2O (b)
H2SO3 + Ca(OH)2 => CaSO3 + 2H2O (c)
The rate of uptake of SO2 on CaCO3 depends on the quantity of adsorbed H2O and is partially reversible. The indications are that SO2 is first loosely bound as a complex with adsorbed H2O on the substrate (SO2·H2O) or as H2SO3 which subsequently rearranges or reacts according to the rate-limiting reaction (c) above.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 99.0431