(234s) Simulation of Atmospheric Cr Speciation in Droplets

The two chromium oxidation states found in ambient atmospheric particulate matter are trivalent [Cr(III)] and hexavalent [Cr(VI)] chromium. Cr(III) is a trace element essential for the proper function of living organisms. However, Cr(VI) is toxic and exposure to Cr(VI) may lead to cancer, asthma and bronchitis. Therefore, it is important to accurately discriminate between these two species in atmospheric particulate matter (PM). Little is known regarding atmospheric Cr chemistry. It was found that atmospheric Cr(III) is oxidized to Cr(VI) in the presence of dissolved ozone in liquid coated particles or droplets or even at dry conditions, while Cr(VI) is reduced to Cr(III) in the presence of organic carbons and metal ions such as Fe(II), As (III) and V(II) at high humidity. This inter-conversion between Cr(III) and Cr(VI) results in some biases in measuring atmospheric Cr concentrations. Therefore, it is critical to estimate the interconversion rate of Cr(III) and Cr(VI) to be able to quantify Cr(VI) concentrations in the ambient air. This study focuses on the simulation of interconversion of atmospheric chromium in the atmospheric droplets in the presence of PM matrix. Recent field measurements of concentrations of atmospheric Cr and other reductants and oxidants were used as model input. The simulation results demonstrated that the major form of Cr(VI) in the atmosphere is insoluble Cr(VI) which is in agreement with the literature findings. Insoluble Cr(VI) forms quickly within few seconds while the system reach steady state after approximately 15 hours. No insoluble form of Cr(III) was found when initial Cr(III) concentration was zero, and small amount of soluble Cr(III) was formed. However, the final form of Cr(III) can be influenced by the initial Cr(III) concentrations that may vary with the type of he source and the measurement location. This study is being continued for different pH of the solutions and initial Cr(VI) and Cr(III) concentrations.