Contribution of Water-Soluble and Insoluble Components and Their Hydrophobic/Hydrophilic Subfractions to the Reactive Oxygen Species-Generating Potential of Fine Ambient Aerosols

Abstract

Relative contributions of water- and methanol-soluble compounds and their hydrophobic/hydrophilic subfractions to the ROS (reactive oxygen species)-generating potential of ambient fine aerosols (D(p) < 2.5 μm) are assessed. ROS-generating (or oxidative) potential of the particulate matter (PM) was measured by the dithiothreitol (DTT) assay. Particles were collected on quartz filters (N = 8) at an urban site near central Atlanta during January-February 2012 using a PM(2.5) high-volume sampler. Filter punches were extracted separately in both water and methanol. Hydrophobic and hydrophilic fractions were then subsequently segregated via a C-18 solid phase extraction column. The DTT assay response was significantly higher for the methanol extract, and for both extracts a substantial fraction of PM oxidative potential was associated with the hydrophobic compounds as evident from a substantial attenuation in DTT response after passing PM extracts through the C-18 column (64% for water and 83% for methanol extract; both median values). The DTT activities of water and methanol extracts were correlated with the water-soluble (R = 0.86) and water-insoluble organic carbon (R = 0.94) contents of the PM, respectively. Brown carbon (BrC), which predominantly represents the hydrophobic organic fraction (referred to as humic-like substances, HULIS), was also correlated with DTT activity in both the water (R = 0.78) and methanol extracts (R = 0.83). Oxidative potential was not correlated with any metals measured in the extracts. These findings suggest that the hydrophobic components of both water-soluble and insoluble organic aerosols substantially contribute to the oxidative properties of ambient PM. Further investigation of these hydrophobic organic compounds could help identify sources of a significant fraction of ambient aerosol toxicity.

Inflammatory responses to secondary organic aerosols (SOA) generated from biogenic and anthropogenic precursors

“…For instance, concentration addition may not apply as ambient aerosol is formed in the presence of multiple precursors and the SOA produced may induce response levels completely different from those observed for single-precursor SOA systems that comprise the mixture. Interactions between organic components and metal species have also been suggested in previous studies (Verma et al, 2012;Tuet et al, 2016) and may influence responses significantly. While these interactions were not considered in the current study, there may be evidence to support the plausibility of mixture effects as ambient PM samples produced lower levels of ROS/RNS than that of any single SOA system investigated.…”

“…Previous studies have also suggested the possibility of metalorganic complexes. For instance, Verma et al (2012) showed that certain metals were retained on a C18 column, which is utilized to remove hydrophobic components, suggesting that these metals were likely complexed and removed in the process. Further chamber studies involving photochemically generated SOA and metals may elucidate these interactions.…”

Naphthalene SOA: redox activity and naphthoquinone gas–particle partitioning

“…The missing activity could arise from highly functionalised products, which are likely to have low vapour pressures and could contribute to redox cycling under ambient particle loading; structural activity relationships predict the vapour pressure of 5-hydroxy-1,4-naphthoquinone to be about an order of magnitude lower than that of 1,4-naphthoquinone (US EPA, 2013). In terms of importance, oxidation of phenanthrene to produce phenanthrenequinone (e.g., Wang et al, 2007;Lee and Lane, 2010), which is of much lower volatility than either 1,2-or 1,4-naphthoquinone and is more highly redox-active, may be a greater source of particle-phase redox-active organic species and has been shown to be an atmospherically relevant species (EigurenFernandez et al, 2008b). Certainly, previous apportioning of DTT activity to chemical species has shown phenanthrenequinone to be a much more substantial contributor to redox activity than 1,2-and 1,4-naphthoquinone (Chung et al, 2006;Charrier and Anastasio, 2012).…”

“…The catalytic capacity of DTT to transfer these electrons to molecular oxygen can be observed by measuring the DTT rate of decay, which has been correlated to oxidative stress markers in cellular systems (Li et al, 2003b;Koike and Kobayashi, 2006). Constituents of PM that have been shown as active redox cycling catalysts include black carbon from diesel particles (Shinyashiki et al, 2009) transition metals (Netto and Stadtman, 1996;Charrier and Anastasio, 2012), humic-like substances (Lin and Yu, 2011;Verma et al, 2012), and quinones (Kumagai et al, 2002). Quinones are a class of organic molecules derived from aromatic species, containing two carbonyl functionalities on a larger conjugated hydrocarbon ring.…”

“…They may also be produced as secondary products of polycyclic aromatic hydrocarbon (PAH) oxidation. Naphthalene (Bunce et al, 1997;Chan et al, 2009;Kautzman et al, 2010;Lee and Lane, 2009;Sasaki et al, 1997), phenanthrene (Lee and Lane, 2010;Wang et al, 2007), and anthracene (Kwamena et al, 2006) all generate their corresponding quinone species (naphthoquinone, phenanthrenequinone, and anthraquinone, respectively) via reaction with gas-phase oxidants. Formation of these species from PAH precursors in the atmosphere could increase the redox activity of ambient particles during photochemical aging in locations with PAH emission sources.…”