(42h) Simulation of Atmospheric Chromium Chemistry

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) samples. Since 1995 when first study on the simulation of atmospheric chromium chemistry was published by Seigneur and Constantinou (Environ. Sci. Technol, 29, 222-231), no such study has been added to the literature to improve our understanding on atmospheric chromium speciation. The simulation results published by Seigneur and Constantinou showed how fast Cr(III) and Cr(VI) inter conversion would occur in the atmosphere. However, the impact of chromium reactions with atmospheric gases and organic carbon, on the overall direction of chromium speciation was not studied. More importantly, the model did not employed experimental or field data as input or for validation. Since 1995, few experimental data on atmospheric chromium chemistry have been published by Amouei Torkmahalleh et al. (Atmospheric Environment (2013), 71, 92-94; Aerosol Science and Technology (2012), 46(6), 690-696) and other researchers which can be employed together with atmospheric chromium concentration data to improve the proposed model by Seigneur and Constantinou. The current study performs a computer simulation of atmospheric chromium speciation. We found that for liquid-coated particles with pH of 1 the half life of chromium is around 1 minute, and it increases to almost 3 minutes if lack of some heavy metals such as Pb²⁺, Cu²⁺ and Zn²⁺ exists in the solution. The system attains steady state after approximately 15 minutes. The effects of pH and liquid water content which is controlled by the deliquescence of ambient PM were found to be significant on chromium speciation.