Use of B isotopes as a tracer of anthropogenic emissions in the atmosphere of Paris, France

In order to test the potential of B isotopes as a tracer of contamination of the atmosphere, the B isotopic composition of rainwater samples monitored over a year in the centre of Paris, France were determined. Boron concentrations range from 19 nmol/L to 500 nmol/L and δ11B range from 0‰ to +38‰. Mean annual values are 148 nmol/L and +25‰, respectively. The results suggest that variability in B isotopic compositions is mainly caused by mixing of two main sources, although isotopic fractionation during the evaporation–condensation processes may also be important. One source is a marine component, which exhibits a heavy B isotopic composition. The decrease of δ11B in rainwater with increasing NO3/B and SO4/B molar ratios suggests that a second source may be anthropogenic emissions. To constrain this end-member, B was determined in urban particulates, which were enriched in the light isotope and the lowest values were consistent with a B-rich fossil fuel composition. These results confirm the great sensitivity of B to anthropogenic sources and the ability of B isotopic ratios to decipher the origin of B in the atmosphere.