Cr speciation changes in the presence of ozone and reactive oxygen species at low relative humidity

There is limited understanding of chromium chemistry in the atmosphere as well as after particles containing chromium are captured on filters during extended sampling intervals (24 h). Thus, experiments were conducted to investigate the role of ozone (4 ppm) and particle-bound reactive oxygen species (ROS) (>700 nM of equivalent H₂O₂) on chromium speciation. For Cr(VI) collected on a filter, reduction to Cr(III) at 24 °C, pH 4 and 10% RH decreased from 48 ± 3% to 26 ± 3% (p < 0.001) after 2 h of exposure, compared to the absence of added ozone (control experiments). Ozone and ROS led to some oxidation of Cr(III) to Cr(VI) (up to 7% for ozone at pH 4, up to 4.5 times for ROS and ozone at pH 9) with a steady-state concentration being reached after few hours. Ozone and ROS can compensate to some extent for Cr(VI) reduction occurring on PM-laden filters. Oxidation of Cr(III) by ozone slowed as the temperature decreased, implying that Cr(VI) sampling in a cool environment might be less likely biased by the oxidation of Cr(III) by atmospheric oxidants.