Oxidative Potential and Chemical Characteristics of Ambient PM2.5 in Guangzhou, China

The dithiothreitol (DTT) assay is widely used to characterize the Oxidation Potential (OP) of atmospheric particulate matter (PM), which can cause adverse effects on human health. However, it’s under debate which chemical species determines the consumption rate of DTT. During January and April 2018, we measured the improved DTT assay of daily PM_2.5 samples collected in Guangzhou, China with complimentary measurements of water-soluble ions, organic/elemental carbon (OC/EC) and metal elements. The average sampled air volume normalized consumption rate of DTT (DTT_v) was 4.67 ±1.06 and 4.45 ± 1.02 nmol min^-1 m^-3, in January and April, respectively while the average PM_2.5 mass normalized consumption rate of DTT (DTT_m) was 13.47 ± 3.86 and 14.66 ± 4.49 pmol min^-1 μg^-1. Good correlations were found between DTT_v and concentration of PM_2.5, OC, and EC while no correlation was found between DTT_m and concentrations of water-soluble ions, OC, EC or metal element, which is consistent with most early observations. We also evaluated the contribution of soluble metals to DTT assay by addition of EDTA, a strong metal chelator. We found that nearly 90% of DTT_v and DTT_m were reduced by EDTA, suggesting a dominant role of soluble metals in determining the response of DTT to ambient PM_2.5. Based on responses of DTT to soluble metals in literature, we found that Cu(II) and Mn(II) are the major contributors to OP of PM_2.5 in Guangzhou. The correlation coefficient between DTT_m and OC shows a clear increase after addition of EDTA, implying that the response of DTT to quinones is not strongly suppressed by EDTA.