High Ozone Increases Soil Perchlorate but Does Not Affect Foliar Perchlorate Content

Ozone (O 3 ) is implicated in the natural source inventory of ClO 4 − , a hydrophilic salt that migrates to groundwater and interferes with the uptake of iodide in mammals, including humans. Tropospheric O 3 is elevated in many urban and some rural areas in the United States and globally. We previously showed that controlled O 3 exposure at near‐ambient concentrations (up to 114 nL L −1 , 12‐h mean) did not increase foliar ClO 4 − . Under laboratory conditions, O 3 has been shown to oxidize Cl − to ClO 4 − . Plant tissues contain Cl − and exhibit responses to O 3 invoking redox reactions. As higher levels of O 3 are associated with stratospheric incursion and with developing megacities, we have hypothesized that exposure of vegetation to such elevated O 3 may increase foliar ClO 4 − . This would contribute to ClO 4 − in environments without obvious point sources. At these high O 3 concentrations (up to 204 nL L −1 , 12‐h mean; 320 nL L −1 maximum), we demonstrated an increase in the ClO 4 − concentration in surface soil that was linearly related to the O 3 concentration. There was no relationship of foliar ClO 4 − with O 3 exposure or dose (stomatal uptake). Accumulation of ClO 4 − varied among species at low O 3 , but this was not related to soil surface ClO 4 − or to foliar ClO 4 − concentrations following exposure to O 3 . These data extend our previous conclusions to the highest levels of plausible O 3 exposure, that tropospheric O 3 contributes to environmental ClO 4 − through interaction with the soil but not through increased foliar ClO 4 − .