Assessment of oxidative potential by hydrophilic and hydrophobic fractions of water-soluble PM2.5 and their mixture effects

Transition metals (TMs) (e.g. copper (Cu) and iron (Fe)) and certain organic compounds are known active constituents causing oxidative potential (OP) by inhaled ambient fine particulate matter (PM2.5) in lung fluid. Humic-like substances (HULIS), isolated from atmospheric PM2.5, are largely metal-free and contain mixtures of organics that are capable of complexing TMs. TMs and HULIS co-exist in the waterextractable part of PM2.5. In this work, we used a solid phase extraction procedure to isolate the water-soluble TMs in the hydrophilic fraction (HPI) and HULIS in the hydrophobic fraction (HPO) and carried out this isolation procedure to a set of 32 real-world PM2.5 samples collected in Beijing and Hong Kong, China. We quantified two OP endpoints, namely hydroxyl radical formation (denoted as OP center dot OH) and ascorbic acid depletion (denoted as OPAA), by the two fractions separately and combined, as well as by the bulk water-soluble aerosols. OP center dot OH and OPAA were well-correlated in both separate fractions and their combined mixtures or bulk water-soluble aerosols. OP by HPI far exceeded that by HPO. On a per unit PM2.5 mass basis, the Hong Kong samples on average had a higher OPAA and OP center dot OH than the Beijing samples due to more water-soluble Cu. For HPI, Cu was a dominant OP *center dot OH and OPAA contributor (>80%), although water-soluble Fe was present at a concentration approximately one order of magnitude higher. Suppression effects on OP center dot OH were observed through comparing the OP of the bulk water-soluble aerosol with that of HPI. Our work reveals the importance of monitoring PM2.5 chemical compositions (especially water-soluble redox active metals). Furthermore, we demonstrate the need to consider metal-organic interactions when evaluating the aggregate OP by PM2.5 from individual components or apportioning OP by PM2.5 to specific chemical components. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study reveals that oxidative potential (OP) in inhaled ambient fine particulate matter (PM2.5) is mainly caused by transition metals (TMs) and humic-like substances (HULIS), with water-soluble copper (Cu) in Hong Kong samples leading to higher OP than in Beijing samples. Cu is a dominant contributor to hydroxyl radical formation and ascorbic acid depletion, despite higher levels of water-soluble iron (Fe) present.