Research Program Area: Emissions Monitoring & Control
A three-phase study was conducted to determine the fate of hexavalent chromium [Cr(VI)] in the atmosphere. Components of the study included the following:
* Identification of the most suitable sampling and analytical methods for Cr(VI)
* Determination, through a series of laboratory reaction chamber tests, the most likely chemical pathways and rate of conversion of hexavalent chromium in ambient air to trivalent chromium [Cr(III)] under simulated atmospheric conditions.
* A field test, conducted in California, to support those findings from the laboratory studies.
Two general analytical methods were evaluated, a complexation/atomic absorption technique currently utilized by the California Air Resources Board (CARB) and an ion chromatographic method. The atomic absorption method was found to provide the required sensitivity for measurement of atmospheric Cr(VI) [3-10 percent coefficient of variation (C. V.)] and was reasonably precise in the determination of Cr (VI) corresponding to concentrations varying from 0.7 to 5.5 nanograms per cubic meter (ng/m3).
The ion chromatographic technique was more precise, sensitive, and rapid. The detection limit was 0.1 ng/m3) with a precision of better than five percent C.V. A modification of this method provided an even greater detection limit (0.1 ng/m3 and lower) but was more time consuming and complex.
A sampling method currently employed by CARB utilizing polyvinyl-chloride (PVC) membranes was found to provide low estimates of the true Cr (VI) atmospheric concentrations. Air concentrations approximately ten times higher were measured using a n impinger method that provided greater stability of the Cr(VI) species during sampling.
Laboratory reaction chamber tests were conducted that utilized PVC filters spiked with Cr(VI) compounds and exposed to atmopsheric reactants typical of the Southern California region. The results indicated that under low atmospheric pH condition (measurable atmospheric HNO3, etc.) greater than 50% of Cr (VI) species will be reduced in the presence of oxidizable species, such as unsaturated and oxygenated organic compounds, and mono and divalent vanadium [V(I, II)] species over a 24-hour period.
Finally, a two-phase field test conducted in the Los Angeles basin area indicated the following:
* Cr(VI) species exist at measurable levels at distances of up to 0.5 miles or greater from chromium emission sources.
* The rates of reduction of Cr(VI) under true ambient air conditions parallel those seen in the laboratory.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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