(58n) Simulation of Atmospheric Cr Chemistry during Air Sampling (pH 9)

Cr(III) is crucial microelement for living organisms and exhibit important role in control of lipids, glucose and proteins metabolism. In comparison, Cr(VI) is highly toxic and US EPA considered as ‘Group A carcinogens’ that leads to asthma, lung cancer, bronchitis, pneumonitis, nasal damage. Thus, it is important to be able to distinguish between these two oxidation states in the atmosphere¹. Cr is produced mainly from anthropogenic sources including Cr alloy processing, ferrochrome industry, welding, etc. When particles absorb water (deliquescence), a liquid layer forms around the particles where soluble species including soluble Cr dissolves in the liquid film. Consequently, the inter-conversion between Cr(III) and Cr(VI) can take place with atmospheric reactants and oxidants under atmospheric PM (Seigneur Christian and Elpida Constatinou, 1995). It was found that atmospheric Cr(III) is oxidized to Cr(VI) in the presence of Mn(III) at atmospheric conditions, while Cr(VI) is reduced to Cr(III) in the presence of atmospheric metal ions such as Fe(II), VO²⁺ and As(III) (Amouei Torkmahalleh et al., 2013). This inter-conversion between Cr(III) and Cr(VI) results in some biases in sampling and analytical measuring at pH=9 atmospheric Cr concentrations (Lihui Huang, 2013). The objective of this study was to develop a conceptual model to better understand the inter-conversion for soluble and insoluble forms of Cr compounds in the presence of PM matrix and dissolved gases such SO₂, O₃ and NO₂ during the sampling at pH=9 with Cellulose filters using bicarbonate buffer. In particular, Cr(VI) chemistry on sampling filters is simulated. Initial concentrations for conceptual model validation are obtained from field measurements of atmospheric Cr performed in New Jersey (Amouei Torkmahalleh et al., 2013). In the present study, it was found that the dominant form of Cr(VI) in the solution is CrO₄^2-(Figure 1). Moreover, CrO₄^2- was produced by the dissolution of Na₂CrO₄(s) and K₂CrO₄(s) available in the solid core, but a considerable portion of the CrO₄^2- precipitated as (NH₄)₂CrO₄(s), CaCrO₄(s), BaCrO₄(s) and PbCrO₄(s). The HCrO₄^- and Cr₂O₇^2- fractions of soluble Cr (VI) in the aqueous layer were significantly low which is almost negligible. In general, conversion of Cr(VI) to Cr(IIII) was higher than the conversion of Cr(III) to Cr(VI). The major reductant was found to be VO²⁺ (Figure 2). The simulation results agree with the field measurements near Cr industries. The results of this study provide new insights into the solubility of Cr(VI) species, and helps to better design sampling filters and analytical methods to quantify Cr species.