Gas-to-particle conversion of mercury, arsenic and selenium through reactions with traffic-related compounds? Indications from lead isotopes

Abstract

Relationships between metal (Cu, Zn, Pb, Cd, Hg, As, Se) concentrations and lead isotope compositions of Geneva aerosols for the period October 1996–September 1997 are investigated. Lead in Geneva is contributed by petrol, waste incineration and, only in winter, coal. Lead and copper during summer and copper and zinc during winter correlate positively with the lead isotope signature of the incinerator suggesting derivation of relevant amounts of these metals from such source. On the contrary, three volatile metals (Hg, As, Se), which can be present in the atmosphere as gaseous compounds at significant levels, display an anomalous strong correlation with the isotope signature of traffic-lead in summer. These metals are unlikely to be contributed by automotive combustion and their most probable summer source is also represented by waste incineration. We suggest that the correlation of Hg, As and Se with automotive lead could unveil conversion of these volatile metals from the gaseous to the particulate phase in concomitance with increasing concentration of traffic-related compounds of which lead isotopes are a valid tracer. Hg, As and Se do not correlate directly with traffic-related gases (e.g. CO, NO x , O 3, THC) while they do correlate only with automotive lead. This is probably due to an atmospheric residence time more similar to traffic-generated lead aerosols than to automotive gases. It has been suggested elsewhere that gaseous mercury can be transformed to the particulate phase through atmospheric reactions with traffic-related oxidants like ozone. Our data might indicate that similar processes are responsible for the gas-to-particle conversion not only of gaseous Hg, but also of As and Se in the atmosphere of Geneva.