Anthropogenic inputs of boron into urban atmosphere: Evidence from boron isotopes of precipitations in Guiyang City, China

Boron (B) concentrations and isotopic compositions were measured in the precipitations of Guiyang, China for one year. Most precipitation samples have boron concentrations of from 2.1 to 4.8 ng ml−1, and δ11B values of from +2.0‰ to +30.0‰. Boron concentrations and δ11B values of heavy rain samples are generally higher than those of light rain and snow samples. Anthropogenic inputs provided most of the SO42− and NO3−, which were predominant ions in the precipitation. The major cation Ca2+ in the precipitation was mainly originated from local dust.The total boron in precipitation from Guiyang is explained by the mixing model of three boron sources. Assuming a δ11B value of +45‰ for the seawater component, contributions of marine source, organic matter and biomass combustion, and coal combustion were estimated to be 32%, 49%, and 19%, respectively to the total boron in Guiyang precipitations. The coal combustion and biomass (and/or organic matter) combustion showed different contributions of boron to the rainwaters in different seasons, the former in cold season while the latter in summer season had a more marked influence on the chemical and isotopic composition of the rainwater. The largest contribution of seawater-originated boron was observed for the heavy rain samples, which was up to 68%. This study indicates that the atmospheric environment of Guiyang city was strongly influenced by human activities, and boron isotopic composition is of great sensitivity to anthropogenic sources and can be a powerful technique to trace various sources of atmospheric emissions and even their origins.