Abstract. We assessed the oxidative potential (OP) of both water-soluble and
methanol-soluble fractions of ambient fine particulate matter (PM2.5)
in the Midwestern United States. A large set of PM2.5 samples (N=241) was collected from five sites set up in different environments, i.e.,
urban, rural, and roadside, in Illinois, Indiana, and Missouri during May 2018–May 2019. Five acellular OP endpoints, including the consumption rate of
ascorbic acid and glutathione in a surrogate lung fluid (SLF) (OPAA and
OPGSH, respectively), dithiothreitol (DTT) depletion rate (OPDTT),
and ⚫OH generation rate in SLF and DTT (OPOH−SLF and
OPOH−DTT, respectively), were measured for all PM2.5 samples.
PM2.5 mass concentrations in the Midwestern US as obtained from these
samples were spatially homogeneously distributed, while most OP endpoints
showed significant spatiotemporal heterogeneity. Seasonally, higher
activities occurred in summer for most OP endpoints for both water- and
methanol-soluble extracts. Spatially, the roadside site showed the highest
activities for most OP endpoints in the water-soluble extracts, while only
occasional peaks were observed at urban sites in the methanol-soluble OP.
Most OP endpoints showed similar spatiotemporal trends between mass- and
volume-normalized activities across different sites and seasons. Comparisons
between two solvents (i.e., water and methanol) showed that methanol-soluble
OP generally had higher activity levels than corresponding water-soluble OP.
Site-to-site comparisons of OP showed stronger correlations for
methanol-soluble OP compared to water-soluble OP, indicating a better
extraction of water-insoluble redox-active compounds from various emission
sources into methanol. We found a weak correlation and inconsistent slope
values between PM2.5 mass and most OP endpoints. Moreover, the
poor to moderate intercorrelations among different OP endpoints indicate
different mechanisms of OP represented by these endpoints and thus
demonstrate the rationale for analyzing multiple acellular endpoints for a
better and more comprehensive assessment of OP.
Supplementary material to "Sources and oxidative potential of water-soluble humic-like substances (HULIS<sub>WS</sub>) in fine particulate matter (PM<sub>2.5</sub>) in Beijing"
