High ozone increases soil perchlorate but does not affect foliar perchlorate content

Ozone (O₃) is implicated in the natural source inventory of ClO₄^-, a hydrophilic salt that migrates to groundwater and interferes with the uptake of iodide in mammals, including humans. Tropospheric O₃ is elevated in many urban and some rural areas in the United States and globally. We previously showed that controlled O₃ exposure at near-ambient concentrations (up to 114 nL L^-1, 12-h mean) did not increase foliar ClO₄^-. Under laboratory conditions, O₃ has been shown to oxidize Cl^- to ClO₄^-. Plant tissues contain Cl^- and exhibit responses to O₃ invoking redox reactions. As higher levels of O₃ are associated with stratospheric incursion and with developing megacities, we have hypothesized that exposure of vegetation to such elevated O₃ may increase foliar ClO₄^-. This would contribute to ClO₄^- in environments without obvious point sources. At these high O₃ concentrations (up to 204 nL L^-1, 12-h mean; 320 nL L^-1 maximum), we demonstrated an increase in the ClO₄^- concentration in surface soil that was linearly related to the O₃ concentration. There was no relationship of foliar ClO₄^- with O₃ exposure or dose (stomatal uptake). Accumulation of ClO₄^- varied among species at low O₃, but this was not related to soil surface ClO₄^- or to foliar ClO₄^- concentrations following exposure to O₃. These data extend our previous conclusions to the highest levels of plausible O₃ exposure, that tropospheric O₃ contributes to environmental ClO₄^- through interaction with the soil but not through increased foliar ClO₄^-.