Stabilized Criegee intermediates react with organic acids in the gas phase and at the air-water interface to form a class of ester hydroperoxides, α-acyloxyalkyl hydroperoxides (αAAHPs). A number of recent studies have proposed the importance of αAAHPs to the formation and growth of secondary organic aerosol (SOA). The chemistry of αAAHPs has not been investigated due to a lack of commercially available chemical standards. In this work, the behavior of αAAHPs in condensed phases is investigated for the first time. Experiments were performed with two synthesized αAAHP species. αAAHPs decomposed rapidly in the aqueous phase, with the rate highly dependent on the solvent, temperature, solution pH, and other compounds present in the solution. The measured 1-order decomposition rate coefficient varied between 10 and 10 s under the conditions examined in this work. Elucidation of the reaction mechanism is complicated by byproducts arising from the synthetic procedure, but observations are consistent with a base-catalyzed hydrolysis of αAAHPs. The rapid hydrolysis of αAAHPs observed in this work implies their short lifetimes in ambient cloud and fog waters. Decomposition of αAAHPs likely gives rise to smaller peroxides, such as HO. The loss of αAAHPs is also relevant to filter extraction, which is commonly practiced in laboratory experiments, potentially explaining contradictory results reported in the existing literature regarding the importance of αAAHPs in SOA.

Aerosol particles and their interactions with clouds are one of the most uncertain aspects of the climate system. Aerosol processing by clouds contributes to this uncertainty, altering size distributions, chemical composition, and radiative properties. Many changes are limited by the availability of hydroxyl radicals in the droplets. We suggest an unrecognized potentially substantial source of OH formation in cloud droplets. During the first few minutes following cloud droplet formation, the material in aerosols produces a near-UV light–dependent burst of hydroxyl radicals, resulting in concentrations of 0.1 to 3.5 micromolar aqueous OH ([OH]aq). The source of this burst is previously unrecognized chemistry between iron(II) and peracids. The contribution of the “OH burst” to total OH in droplets varies widely, but it ranges up to a factor of 5 larger than previously known sources. Thus, this new process will substantially enhance the impact of clouds on aerosol properties.

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.

Adverse health effects of ambient PM 2.5 (d p < 2.5 μm) can be associated with the production of reactive oxygen species (ROS), among which hydroxyl radical (•OH) is the most reactive. However, •OH generated by PM 2.5 has not been quantified and studied in the North China Plain (NCP), which has suffered from heavy air pollution in recent years. In this study, PM 2.5 samples were collected at an urban site (Beijing) and a suburban site (Wangdu), extracted in a cell-free surrogate lung fluid (SLF), and •OH generated in the extracts were quantified. The results show that more •OH is produced on heavily polluted days than that on clean days (2.0 and 1.6 times higher in Beijing and Wangdu, respectively). The production of •OH per unit mass (ng/μg• PM 2.5 ) decreases with the increase of ambient PM 2.5 concentration because SO 4 2− , NO 3 − , and NH 4 + dominate the increased PM 2.5 , while these secondary inorganic components do not contribute to the generation of •OH. Trace metals (e.g., Fe, Cu, Se) and carbonaceous species (organic carbon and elemental carbon) correlate well with the •OH production, indicating that particles from combustion sources including coal combustion, vehicle exhaust, and industry contribute more to •OH generation.

Gas and particle emissions from R/V Robert Gordon Sproul were measured for ultra low sulfur diesel (ULSD) and hydrogenation derived renewable diesel (HDRD) during dedicated aerosol measurement cruises in 2014 (29 September-3 October) and 2015 (4-7 and 26-28 September). CO, CO 2 , and NO X were measured directly from the starboard stack from the 2-stroke, small bore, high speed engine, while number and mass size distributions for both particles and black carbon (BC) were measured by intercepting the ship plume. Measurements at constant engine speeds (1600 rpm, 1300 rpm, 1000 rpm, and 700 rpm) had emission factors of CO (EF CO ) and NO X EF NOX ð Þ that were lower by 20% and 13%, respectively, for HDRD compared to ULSD at 700 rpm. However, at 1600 rpm, EF CO and EF NOX were within one standard deviation for both ULSD (EF CO : 4.0 § 0.1 g [kg-fuel] ¡1 ; EF NOX : 51 § 0.8 g [kg-fuel] ¡1 ) and HDRD (EF CO : 3.9 § 0.2 g [kg-fuel] ¡1 ; EF NOX : 51 § 2 g [kg-fuel] ¡1 ). HDRD emission factors of particle number and mass concentrations were higher than ULSD by 46% to 107% and 36% to 150%, respectively, at 1600, 1300, and 1000 rpm, but the differences were smaller than the cycle-to-cycle variability at 700 rpm. BC mass emission factors were nearly 200% larger for 700, 1000, and 1300 rpm for HDRD compared to ULSD, but the mass differences were smaller than cycle-to-cycle variability at 1600 rpm. BC mass size distributions showed that the peak diameter of the BC mass mode for ULSD (»120 nm) is about 20 nm larger than for HDRD (»100 nm), even though the particle mass and number size distributions are quite similar.

Vapor phase absorption spectra and integrated band intensities of the OH stretching fundamental as well as first and second overtones (2ν(OH) and 3ν(OH)) in peroxyacetic acid (PAA) have been measured using a combination of FT-IR and photoacoustic spectroscopy. In addition, ab initio calculations have been carried out to examine the low energy stable conformers of the molecule. Spectral assignment of the primary features appearing in the region of the 2ν(OH) and 3ν(OH) overtone bands are made with the aid of isotopic substitution and anharmonic vibrational frequency calculations carried out at the MP2/aug-cc-pVDZ level. Apart from features associated with the zeroth-order OH stretch, the overtone spectra are dominated by features assigned to combination bands composed of the respective OH stretching overtone and vibrations involving the collective motion of several atoms in the molecule resulting from excitation of the internal hydrogen bonding coordinate. Integrated absorption cross section measurements reveal that internal hydrogen bonding, the strength of which is estimated to be ∼20 kJ/mol in PAA, does not result in a enhanced oscillator strength for the OH stretching fundamental of the molecule, as is often expected for hydrogen bonded systems, but does cause a precipitous drop in the oscillator strength of its 2ν(OH) and 3ν(OH) overtone bands, reducing them, respectively, by a factor of 165 and 7020 relative to the OH stretching fundamental.

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.