Abstract
Abstract. Chamber secondary organic aerosol (SOA) from low-NOx photooxidation of naphthalene by hydroxyl radical was examined with respect to its redox cycling behaviour using the dithiothreitol (DTT) assay. Naphthalene SOA was highly redox-active, consuming DTT at an average rate of 118 ± 14 pmol per minute per μg of SOA material. Measured particle-phase masses of the major previously identified redox active products, 1,2- and 1,4-naphthoquinone, accounted for only 21 ± 3% of the observed redox cycling activity. The redox-active 5-hydroxy-1,4-naphthoquinone was identified as a new minor product of naphthalene oxidation, and including this species in redox activity predictions increased the predicted DTT reactivity to 30 ± 5% of observations. These results suggest that there are substantial unidentified redox-active SOA constituents beyond the small quinones that may be important toxic components of these particles. A gas-to-SOA particle partitioning coefficient was calculated to be (7.0 ± 2.5) × 10−4 m3 μg−1 for 1,4-naphthoquinone at 25 °C. This value suggests that under typical warm conditions, 1,4-naphthoquinone is unlikely to contribute strongly to redox behaviour of ambient particles, although further work is needed to determine the potential impact under conditions such as low temperatures where partitioning to the particle is more favourable. Also, higher order oxidation products that likely account for a substantial fraction of the redox cycling capability of the naphthalene SOA are likely to partition much more strongly to the particle phase.
Highlights
Exposure to particulate matter (PM) is known to be a concern to human health
The concentration of 1,4-naphthoquinone in the gas phase was calculated from the proton transfer reaction mass spectrometer (PTR-MS) signal at m/z 159 ([M + H]+), and the concentration in the particle phase was determined from the aerosol mass spectrometer (AMS) signal at m/z 158 (M+)
We are interested in determining how well we can predict the redox activity of secondary organic aerosol particles using molecular-level information on the activities and loadings of known quinones
Summary
Exposure to particulate matter (PM) is known to be a concern to human health. Oxidative stress induction is believed to be a major toxicological mechanism of inhaled particles. 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. As we further examine the photochemical processes that generate redox activity in particles, we can better predict the possible influences of aerosol aging on particle toxicity
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