Stable metal isotopes trace atmospheric particulates

Sources of particulate pollution can be identified using stable metal isotopes. Plasma source multi-collector mass spectrometry (MC-ICP-MS) uses stable isotopes to detect concentrations of metals, enabling scientists to study the environmental chemistry of pollutant metals such as copper (Cu) and zinc (Zn). The technique can be used to identify and understand transport processes in the atmosphere.

The ISOTRACE (Stable isotopes of metals as pollutant tracers in the atmospheric environment) project used stable isotope ratios to trace pollutant metal sources affecting urban air quality. The aim was to better understand the mechanisms behind isotropic fractionation in source materials and isotope variation in atmospheric particulates from major European cities.

Researchers studied the Zn isotope compositions of feed materials and combustion by-products in three different coal-fired power stations and developed a model that accounted for the observed isotope fractionations. Results demonstrated that the elemental speciation in coal, evaporation and condensation were the processes governing the isotopic variation within coal-fired power stations.

The model developed by ISOTRACE partners can be used to estimate the Zn isotope signatures in atmospheric particulates leaving the power stations based on the analysis of the feed coal only. This enables isotopes to be used as a quantitative tool for the source allocation of Zn from coal combustion in the atmosphere.

Researchers also collected particulate matter from London and Barcelona over different time periods and from different heights. Results showed for the first time that the dominant zinc sources varied between cities and according to the time of year. Therefore, each city requires its own unique source assessment and abatement strategies that take into account the specific time period.

Comparison with the isotope signature of different sources and source tracing techniques indicated that non-exhaust emission from traffic, like tyres are important. However, emissions from high temperature processes, such as the burning of oil, waste or coal are equally important at various times of year.

ISOTRACE also demonstrated that different analytical approaches could yield accurate and precise isotope data, while highlighting the need for an internationally certified isotope standard for aerosols.