Searching for manganese pollution from MMT anti-knock gasoline additives in snow from central Greenland

Abstract

Mn has been measured in snow core samples dated from 1967 to 1989 collected in central Greenland. Observed concentrations range from 16 to 236 pg g⁻¹. A large fraction of Mn in Greenland snow is found to originate from rock and soil dust. The different possible natural sources for `excess' Mn above that contributed from rock and soil dust include volcanoes, natural vegetation fires and continental biogenic emissions. They could explain part of the observed excess. At least part of it is, however, probably related to anthropogenic inputs, especially from North America. Aside from industrial sources, MMT (methyl-cyclopentadienyl manganese tricarbonyl) anti-knock gasoline additives are good candidates, but our data do not allow to clearly identify their influence. Unambiguous identification will require the direct determination of organo-Mn compounds in comprehensive series of Greenland snow samples with sub-seasonal resolution using highly sensitive speciation techniques.

Introduction

From the early 1970s in North America, and later in other parts of the world, there has been a massive decrease in the use of Pb alkyl additives in gasoline, with ever-increasing sales of unleaded gasoline and a pronounced decline in the Pb content of leaded gasoline (see, e.g. Nriagu, 1990, Thomas and Spiro, 1994). It resulted in a well documented decrease in Pb concentrations in central Greenland snow from the 1970s (Boutron et al., 1991, Candelone et al., 1995).

Amongst the anti-knock agents which can be used to replace Pb alkyl additives is methylcyclopentadienyl manganese tricarbonyl (MMT: C₉H₇MnO₃). MMT is an organic derivative of manganese (Mn) (see, e.g. Ter Haar et al., 1975, Cooper, 1984, Abbott, 1987, Lynam et al., 1990) which can be used as an anti-knock agent and to improve octane ratings. It was discovered in the research laboratories of Ethyl Corporation in the 1950s (Ethyl Corporation, 1998). Although various other organo-metal compounds, such as organo-Fe, organo-Ni and organo-Sn species have efficient anti-knock properties, MMT is the only one besides Pb compounds which was produced in commercial quantities (Larbey and Bain, 1988). It was used in North America, especially in Canada, from the mid 1970s (see, e.g. Loranger and Zayed, 1994, Loranger and Zayed, 1995; Chau et al., 1997) but probably never in Europe. At present, it appears that the only countries which use MMT (in unleaded gasoline) are Canada, Argentina and Bolivia (Y. Chau, personal communication, 1997) and probably the US (Ethyl Corporation, 1998).

There has been concern that the use of MMT could be a source of environmental contamination by Mn, especially in the form of small size Mn₃O₄ particles (see, e.g. Forget et al., 1994, Chau et al., 1997). Combined with the lack of toxicological data on the effects of long-term exposure to low Mn concentrations (see for instance US EPA, 1990, US EPA, 1991), it has severely limited the general use of MMT throughout the world.

The aim of this study was to determine if Mn from MMT and other anthropogenic sources have resulted in a detectable enhancement of Mn in Greenland snow above natural levels. For that purpose, we have analysed for Mn various snow samples from central Greenland dated from the late 1960s onward, i.e. a period which includes the year of the beginning of the commercial use of MMT in North America (1976).