Improved Atmospheric Sampling of Hexavalent Chromium

Hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) are the primary chromium oxidation states found in ambient atmospheric particulate matter. There is limited understanding of chromium chemistry in the atmosphere as well as after particles containing chromium are captured on filters during extended sampling intervals (24 hours). Thus, experiments were conducted to investigate the effect of deliquescence and pH as well as the role of ozone (4ppm) and particle-bound reactive oxygen species (ROS) (>700 nM of equivalent H2O2) on chromium speciation. For this study, the deliquescence relative humidity (DRH) and liquid water mass content determined in filter samples of airborne PM collected in the northeastern United States are in good agreement with previously reported values for ammonium sulfate and ammonium nitrate. This agreement in DRH value suggests that these two salts control the hygroscopic properties of the ambient particles in the northeastern United States. DRH of the collected airborne PM, sodium bicarbonate, K2Cr2O 7 and Cr (NO3)3 were found to be, 76, 91, 94 and 52%, respectively, in this study. The conversion of Cr(VI) to Cr(III) increased significantly up to 85% at acidic pH as Cr(III) becomes more stable. Under basic pH conditions, deliquescence increased the conversion of Cr(VI) such that up to a 33% reduction of Cr(VI) was observed at 96% relative humidity. No statistically significant difference was observed for Cr(VI) and Cr(III) interconversion over a range of ambient PM mass at basic pH. After 2 hours of exposure to ozone, reduction of Cr(VI) collected on a filter at 24°C, pH 4, and 10% RH decreased to 26 ± 3% ( p < 0.001) compared to 48± 3% in the absence of added ozone. In this study, ozone and ROS were found to be oxidants for the conversion of Cr(III) to Cr(VI) at low RH. Ozone increased the Cr(III) conversion to Cr(VI) from 3.0 ± 0.1% to 7.1 ± 1.3% (p < 0.001) after 120 minutes of exposure at 24°C, pH 4, and 10% RH, compared to the control experiments with laboratory air. Both ROS and ozone increased conversion of Cr(III) to Cr(VI) (4.5 times) compared to the control experiments (in the absence of ROS and ozone) at 24°C, pH 9 and 10% RH. There was a decrease in the oxidation of Cr(III) to Cr(VI) with ozone at 12°C, pH 4, and 10% RH relative to the conversion at 24°C.;In the present study, a new Cr(VI) sampler (Clarkson sampler) was designed, constructed, and field tested to improve the sampling of Cr(VI) in ambient air. The new Clarkson Cr(VI) sampler was based on the concept that deliquescence during sampling leads to aqueous phase reactions. Thus, the relative humidity of the sampled air was reduced below the deliquescence relative humidity (DRH) of the ambient particles. The new sampler was operated to collect Total Suspended Particles (TSP), and compared side-by-side with the current National Air Toxics Trends Stations (NATTS) Cr(VI) sampler that is utilized in the United States Environmental Protection Agency (USEPA) air toxics monitoring program. Side-by-side field testing of four samplers occurred in Elizabeth, NJ during the winter and summer of 2012. The average recovery values of Cr(VI) spikes after 24 hour sampling intervals during summer and winter sampling were 57% and 72%, respectively, for the Clarkson sampler, while the corresponding average values for NATTS samplers were 46% for both summer and winter sampling. Thus, drying the particulate matter containing Cr species reduces the conversion of Cr(VI) to Cr(III). Even dry particles can undergo some reactions, but reducing the temperature does slow the rates of these reactions. Hence, preventing the ambient particles collected on the filters from deliquescing at a reduced temperature, is a key to improving the sampling of Cr(VI). These results imply that a sampler that both helps to keep the particle collection filter cool and dry results in measurements that are more likely to reflect the true concentration of Cr(VI) in the ambient aerosol. This study reports the development and field testing of such a sampler. (Abstract shortened by UMI.).