Interconversion of Chromium Species During Air Sampling: Effects of O₃, NO₂, SO₂, Particle Matrices, Temperature, and Humidity

The interconversion between Cr­(VI), a pulmonary carcinogen, and Cr­(III), an essential human nutrient, poses challenges to the measurement of Cr­(VI) in airborne particles. Chamber and field tests were conducted to identify the factors affecting Cr­(VI)–Cr­(III) interconversion in the basic filter medium under typical sampling conditions. In the chamber tests, isotopically enriched ⁵³Cr­(VI) and ⁵⁰Cr­(III) were spiked on diesel particulate matter (DPM) and secondary organic aerosol (SOA) that were precollected on a basic MCE filter. The filter samples were then exposed to clean air or the air containing SO₂ (50 and 160 ppb), 100 ppb O₃, or 150 ppb NO₂ for 24 h at 16.7 LPM flow rate at designated temperature (20 and 31 °C) and RH (40% and 70%) conditions. Exposure to 160 ppb SO₂ had the greatest effect on ⁵³Cr­(VI) reduction, with ⁵³Cr­(VI) recovery of 31.7 ± 15.8% (DPM) and 42.0 ± 7.9% (SOA). DPM and SOA matrix induced ⁵³Cr­(VI) reduction when exposed to clean air while reactive oxygen species in SOA could promote ⁵⁰Cr­(III) oxidation. Deliquescence when RH increased from 40% to 70% led to conversion of Cr­(III) in SOA, whereas oxidized organics in DPM and SOA enhanced hygroscopicity and thus facilitated Cr­(VI) reduction. Field tests showed seasonal variation of Cr­(VI)–Cr­(III) interconversion during sampling. Correction of the interconversion using USEPA method 6800 is recommended to improve accuracy of ambient Cr­(VI) measurements