Inter-conversion of Chromium Species During Air Sampling: Effects of O3 NO2 SO2 Particle Matrices Temperature and Humidity

摘要

The inter-conversion 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) inter-conversion 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 pre-collected 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 hours 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) inter-conversion during sampling. Correction of the inter-conversion using USEPA method 6800 is recommended to improve accuracy of ambient Cr(VI) measurements.