Hydroxyl radicals (. OH) are key players in chemistry in surface waters, clouds and aerosols. Additionally,. OH may contribute to the inflammation underlying adverse health outcomes associated with particulate matter exposure. Terephthalate is a particularly sensitive probe for hydroxyl radicals, with a detection limit as low as 2 nM. However, there is uncertainty in. OH quantification using this method, and potential for interference from fluorescent compounds and from some transition metals. Terephthalate reacts with. OH to form a fluorescent product, 2-hydroxylterephthalic acid (hTA), with a moderate dependence on pH and temperature. However, there 1 is disagreement in the literature on the yield of the fluorescent product (Y hTA), which introduces a large uncertainty in the quantification of OH. Additionally, TA and similar organic probes are known to complex Cu(II) at high concentrations, thus if this reaction is important at lower concentrations, Cu(II) could reduce apparent hTA formation, and reduce activity of Cu(II) in target samples. Using a pH 3.5 dark ferrous Fenton system to generate. OH radicals, we find that Y hTA = 31.5 ± 7%. This is about double the recent literature value measured, but in excellent agreement with earlier measurements. Additionally, we find that interactions between Cu(II) and hTA are small enough to be ignored at Cu(II) concentrations below ~50 µM.

Reactive oxygen species, including hydroxyl radicals generated by particles, play a role in both aerosol aging and PM2.5 mediated health effects. We assess the impacts of switching marine vessels from conventional diesel to renewable fuel on the ability of particles to generate hydroxyl radical when extracted in a simulated lung lining fluid or in water at pH 3.5, for samples of engine emissions from a research vessel when operating on ultra-low sulfur diesel (ULSD) and hydrogenation-derived renewable diesel (HDRD). Samples were collected during dedicated cruises in 2014 and 2015, including aged samples collected by re-intercepting the ship plume. After normalizing to particle mass, particles generated from HDRD combustion had slightly to significantly (5-50%) higher OH generation activity than those from ULSD, a difference that was statistically significant for some permutations of year/fuel/engine speed. Water soluble trace metal concentrations and fuel metal concentrations were similar, and compared to urban Los Angeles samples lower in soluble iron and manganese, but similar for most other trace metals. Because PM mass emissions were higher for HDRD, normalizing to fuel increased this difference. Freshly emitted PM had lower activity than the "plume chase" samples, and samples collected on the ship had lower activity than the urban reference. The differences in OH production correlated reasonably well with redox-active transition metals, most strongly with soluble manganese, with roles for vanadium and likely copper and iron. The results also suggest that atmospheric processing of fresh combustion particles rapidly increases metal solubility, which in turn increases OH production.

Hydroxyl radical (OH) reactions in cloud water play a key role in secondary organic aerosol formation and sulfur oxidation. We collected aerosol samples (PM 4 ) during summer at an urban receptor site in Southern California. The site mostly receives air from the urban area in the morning, photochemically processed air arriving from the urban area and a commercial ports area in the afternoon, and a largely unpopulated mountainous area overnight. Filters were extracted in small quantities of water at pH 3.5 (adjusted with H 2 SO 4 ), simulating cloud water formation. Samples were analyzed for particle mass, OH generation in the presence of near UV light, soluble trace metals (filtered through a 0.22 μm filter, measured with inductively coupled plasma mass spectrometry, ICP−MS), soluble Fe(II) and Fe(III) (measured with the ferrozine assay, Fe fzn ), and quinones. Soluble speciated iron was about equally divided into Fe(II) fzn and Fe(III) fzn and accounted for only 22 ± 7% of the soluble Fe measured with ICP−MS. The highest concentrations of Fe fzn came from the urban area; high Fe ICP came both from city and mountains, and the mountains were the dominant source of Cu. OH formation was characterized by an initial spike in formation lasting for 1−3 min with a formation rate at ∼(0.2−1) × 10 −8 M•s −1 , followed by a second much slower phase of (0.1−10) × 10 −11 M•s −1 . OH formation activity was strongly correlated with mass and soluble (ICP) Fe and Cu; it did not correlate with ferrozine iron. Quinone concentrations were too low to contribute much to OH formation. The initial burst of OH formation is large enough to contribute substantial OH to cloud and fog droplets.