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 Cr-53(VI) and Cr-50(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 O-3, or 150 ppb NO2 for 24 h at 16.7 LPM flow rate at designated temperature (20 and 31 degrees C) and RH (40% and 70%) conditions. Exposure to 160 ppb SO2 had the greatest effect on Cr-53(VI) reduction, with Cr-53(VI) recovery of 31.7 +/- 15.8% (DPM) and 42.0 +/- 7.9% (SOA). DPM and SOA matrix induced Cr-53(VI) reduction when exposed to clean air while reactive oxygen species in SOA could promote Cr-50(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.