Abstract. The rate of consumption of dithiothreitol (DTT) is increasingly used to measure the oxidative potential of particulate matter (PM), which has been linked to the adverse health effects of PM. While several quinones are known to be very reactive in the DTT assay, it is unclear what other chemical species might contribute to the loss of DTT in PM extracts. To address this question, we quantify the rate of DTT loss from individual redox-active species that are common in ambient particulate matter. While most past research has indicated that the DTT assay is not sensitive to metals, our results show that seven out of the ten transition metals tested do oxidize DTT, as do three out of the five quinones tested. While metals are less efficient at oxidizing DTT compared to the most reactive quinones, concentrations of soluble transition metals in fine particulate matter are generally much higher than those of quinones. The net result is that metals appear to dominate the DTT response for typical ambient PM 2.5 samples. Based on particulate concentrations of quinones and soluble metals from the literature, and our measured DTT responses for these species, we estimate that for typical PM 2.5 samples approximately 80 % of DTT loss is from transition metals (especially copper and manganese), while quinones account for approximately 20 %. We find a similar result for DTT loss measured in a small set of PM 2.5 samples from the San Joaquin Valley of California. Because of the important contribution from metals, we also tested how the DTT assay is affected by EDTA, a chelator that is sometimes used in the assay. EDTA significantly suppresses the response from both metals and quinones; we therefore recommend that EDTA should not be included in the DTT assay.

Photolysis of nitrate (NO) produces reactive nitrogen and oxygen species via three different channels, forming: (1) nitrogen dioxide (NO) and hydroxyl radical (OH), (2) nitrite (NO) and oxygen atom (O(P)), and (3) peroxynitrite (ONOO). These photoproducts are important oxidants and reactants in surface waters, atmospheric drops, and snowpacks. While the efficiency of the first channel, to form NO, is well documented, a large range of values have been reported for the second channel, nitrite, above 300 nm. In part, this disagreement reflects secondary chemistry that can produce or destroy nitrite. In this study, we examine factors that influence nitrite production and find that pH, nitrate concentration, and the presence of an OH scavenger can be important. We measure an average nitrite quantum yield (Φ(NO)) of (1.1 ± 0.2)% (313 nm, 50 μM nitrate, pH ≥ 5), which is at the upper end of past measurements and an order of magnitude above the smallest-and most commonly cited-value reported for this channel. Nitrite production is often considered a very minor channel in nitrate photolysis, but our results indicate it is as important as the NO channel. In contrast, at 313 nm we observe no formation of peroxynitrite, corresponding to Φ(ONOO) < 0.26%.

The rate of consumption of dithiothreitol (DTT) is increasingly used to measure the oxidative potential of particulate matter (PM), which has been linked to the adverse health effects of PM. While several quinones are known to be very reactive in the DTT assay, it is unclear what other chemical species might contribute to the loss of DTT in PM extracts. To address this question, we quantify the rate of DTT loss from individual redox-active species that are common in ambient particulate matter. While most past research has indicated that the DTT assay is not sensitive to metals, our results show that seven out of the ten transition metals tested do oxidize DTT, as do three out of the five quinones tested. While metals are less efficient at oxidizing DTT compared to the most reactive quinones, concentrations of soluble transition metals in fine particulate matter are generally much higher than those of quinones. The net result is that metals appear to dominate the DTT response for typical ambient PM2.5 samples. Based on particulate concentrations of quinones and soluble metals from the literature, and our measured DTT responses for these species, we estimate that for typical PM2.5 samples approximately 80 % of DTT loss is from transition metals (especially copper and manganese), while quinones account for approximately 20 %. We find a similar result for DTT loss measured in a small set of PM2.5 samples from the San Joaquin Valley of California. Because of the important contribution from metals, we also tested how the DTT assay is affected by EDTA, a chelator that is sometimes used in the assay. EDTA significantly suppresses the response from both metals and quinones; we therefore recommend that EDTA should not be included in the DTT assay.

Inhalation of ambient particulate matter causes morbidity and mortality in humans. One hypothesized mechanism of toxicity is the particle-induced formation of reactive oxygen species (ROS) – including the highly damaging hydroxyl radical (·OH) – followed by inflammation and a variety of diseases. While past studies have found correlations between ROS formation and a variety of metals, there are no quantitative measurements of ·OH formation from transition metals at concentrations relevant to 24-hour ambient particulate exposure. This research reports specific and quantitative measurements of ·OH formation from 10 individual transition metals (and several mixtures) in a cell-free surrogate lung fluid (SLF) with four antioxidants: ascorbate, citrate, glutathione, and uric acid. We find that Fe and Cu can produce ·OH under all antioxidant conditions as long as ascorbate is present and that mixtures of the two metals synergistically increase ·OH production. Manganese and vanadium can also produce ·OH under some conditions, but given that their ambient levels are typically very low, these metals are not likely to chemically produce significant levels of ·OH in the lung fluid. Cobalt, chromium, nickel, zinc, lead, and cadmium do not produce ·OH under any of our experimental conditions. The antioxidant composition of our SLF significantly affects ·OH production from Fe and Cu: ascorbate is required for ·OH formation, citrate increases ·OH production from Fe, and both citrate and glutathione suppress ·OH production from Cu. MINTEQ ligand speciation modeling indicates that citrate and glutathione affect ·OH production by changing metal speciation, altering the reactivity of the metals. In the most realistic SLF (i.e., with all four antioxidants), Fe generates approximately six times more ·OH than does the equivalent amount of Cu. Since levels of soluble Fe in PM are typically higher than those of Cu, our results suggest that Fe dominates the chemical generation of ·OH from deposited particles in the lungs.

While the photolysis of nitrite is an important source of hydroxyl radical (*OH) in some natural waters, its wavelength and temperature dependence have not been fully described in solution. In addition, there are no studies of this reaction on ice, although there is evidence of nitrite production in snow. To address these gaps, we have measured the wavelength and temperature dependence of the quantum yields of *OH from the photolysis of frozen and aqueous NO2-. From our solution and ice results, we derive a master equation that describes the *OH quantum yield from NO2 photolysis as a function of both temperature (240-295 K) and illumination wavelength (302-390 nm): phi(NO1- -->OH*)(T,lamda) = (Y0 + a/(1 + exp((lamda-c)/b)))exp-(((e lamda) + f)/R) x (1/295 - 1/T)) where Y0 = 0.0204 +/- 0.0010, a = 0.0506 +/- 0.0022, b = 11.2 +/- 1.2, c = 332 +/- 1, e = 20.5 +/- 3.2, f = 7553 +/- 1204, uncertainties represent 1 standard error, Tis the temperature (K), Ris the gas constant (8.314 J mol(-1) K(-1)), and lamda is the wavelength (nm). Using these results we predict the pseudo-steady-state concentrations of nitrite on sunlit polar snow grains and compare the relative importance of the photolysis of nitrite, nitrate, and hydrogen peroxide as sources of snow-grain *0H.