The Impact of Deliquescence and pH on Cr Speciation in Ambient PM Samples

The 2 chromium oxidation states found in ambient atmospheric particulate matter (PM) 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, nasal damage, asthma, bronchitis, pneumonitis, inflammation, dermatitis, and skin allergies. Therefore, it is important to accurately discriminate between these 2 species in atmospheric PM samples. This work focuses on the effect of deliquescence and pH on chromium speciation in filter samples of airborne PM collected in the northeastern USA. The deliquescence relative humidity (DRH) and liquid water mass content determined for the ambient particle samples are in good agreement with previously reported values for ammonium sulfate and ammonium nitrate suggesting that these 2 salts control the hygroscopic properties of the ambient particles in the northeastern USA. The loss of Cr(VI) increases significantly up to 85% at acidic pH as Cr(III) becomes more stable. Under basic pH conditions, deliquescence increases the loss of Cr(VI) such that up to 33% reduction was observed at 96% relative humidity (RH). No statistically significant difference was observed for Cr(VI) and Cr(III) interconversion over a range of ambient PM mass. Because of the effect of deliquescence on chromium speciation at basic pH, a new design criterion for Cr(VI) samplers can be defined to preserve the collected Cr(VI). DRH of sodium bicarbonate, K2Cr2O7, and Cr(NO3)(3) was found to be 91%, 94%, and 52%, respectively.