Atmospheric Chemistry and Physics, vol. 13, issue 19 (2013) pp. 9731-9744
Chamber secondary organic aerosol (SOA) from low-NO x photooxidation of naphthalene by hydroxyl radi-cal 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 re-dox 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 iden-tified 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 m 3 µg −1 for 1,4-naphthoquinone at 25 • C. This value suggests that under typical warm condi-tions, 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 con-ditions 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.
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