Variations in atmospheric concentrations and isotopic compositions of gaseous and particulate boron in Shizuoka City, Japan

- The concentrations and δ11B of gaseous and particulate B were measured concurrently.
- There was a strong temperature dependence on the phase partitioning of B.
- A positive correlation was observed between the δ11B of gaseous and particulate B.
- Particulate B is enriched in 10B relative to gaseous B due to isotopic fractionation.
- Gaseous B may be transformed to particulate B through the precipitation of borates.

To clarify the partitioning and isotopic fractionation of boron (B) into the gas and particle phases in the atmosphere, the concentrations and isotopic compositions of gaseous and particulate B were measured concurrently for more than one year at a site in Shizuoka City, Japan. This area has few anthropogenic sources of B, such as coal combustion facilities. Gaseous B concentration showed clearly a seasonal variation, increasing during summer and decreasing during winter. Conversely, particulate B concentration tended to decrease during the warm season and increase during winter. The increase in gaseous B concentration during summer is attributable to the enhanced emissions of B from sea-salt degassing owing to higher temperatures and the predominance of winds from the Pacific Ocean. Moreover, the decrease in gaseous B concentration and the increase in particulate B concentration during winter is probably due to the enhanced condensation of gaseous B on atmospheric particles. The δ11B values of gaseous and particulate B varied largely, and did not indicate a distinctive seasonal variation. A positive correlation was observed between the δ11B values of gaseous and particulate B (R2 = 0.518, P < 0.001). Moreover, the δ11B values of particulate B were approximately 0-20‰ lower than those of gaseous B. There is an isotopic fractionation (ΔB(OH)4−−B(OH)3) of about −20‰ between B(OH)3 and B(OH)4− species in solution (Kakihana et al., 1977). This tends to support the hypotheses that gaseous B is transformed to particulate B through the reaction of condensed B(OH)3 with chemical constituents on particles to precipitate borates, and that the condensed B(OH)3 remaining on particles is unstable and evaporates.