Stabilized Criegee intermediates (SCI) are reactive oxygenated species formed in the ozonolysis of hydrocarbons. Their chemistry could influence the oxidative capacity of the atmosphere by affecting ...the HO
and NO
cycles, or by the formation of low-volatility oxygenates enhancing atmospheric aerosols known to have an important impact on climate. The concentration of SCI in the atmosphere has hitherto not been determined reliably, and very little is known about their speciation. Here we show that the concentration of biogenic SCI is strongly limited by their unimolecular decay, based on extensive theory-based structure-activity relationships (SARs) for the reaction rates for decomposition. Reaction with water vapor, H
O and (H
O)
molecules, is the second most important loss process; SARs are also proposed for these reactions. For SCI derived from the most common biogenic VOCs, we find that unimolecular decay is responsible for just over half of the loss, with reaction with water vapor the main remaining loss process. Reactions with SO
, NO
, or acids have negligible impact on the atmospheric SCI concentration. The ambient SCI concentrations are further characterized by analysis of field data with speciated hydrocarbon information, and by implementation of the chemistry in a global chemistry model. The results show a highly complex SCI speciation, with an atmospheric peak SCI concentrations below 1 × 10
molecule cm
, and annual average SCI concentrations less than 7 × 10
molecule cm
. We find that SCI have only a negligible impact on the global gas phase H
SO
formation or removal of oxygenates, though some contribution around the equatorial belt, and in select regions, cannot be excluded.
The reaction of Criegee intermediates (CI) with NO and RO(2) radicals is studied for the first time by theoretical methodologies; additionally, the reaction of CI with SO(2) molecules is re-examined. ...The reaction of CI with NO was found to be slow, with a distinct energy barrier. Their reaction with RO(2) radicals proceeds by the formation of a pre-reactive complex followed by addition of the RO(2) radical on the CI carbon over a submerged barrier, leading to a larger peroxy radical and opening the possibility for oligomer formation in agreement with experiment. The impact of singlet biradicals on the reaction of CI with SO(2) is examined, finding a different reaction mechanism compared to earlier work. For larger CI, the reaction with SO(2) at atmospheric pressures mainly yields thermalized sulfur-bearing secondary ozonides. The fate of the CI in the atmosphere is examined in detail, based on observed concentration of a multitude of coreactants in the atmosphere, and estimated rate coefficients available from literature data. The impact of SCI on tropospheric chemistry is discussed.
The reaction of Criegee intermediates with a number of coreactants is examined using theoretical methodologies, combining ROCCSD(T)//M06-2X quantum calculations with theoretical kinetic predictions ...of the rate coefficients. The reaction of CI with alkenes is found to depend strongly on the substitutions in the reactants, resulting in significant differences in the predicted rate coefficient as a function of the selected alkene and CI. Despite submerged barriers, these entropically disfavored reactions are not expected to affect CI chemistry. The reaction of H2COO + H2COO is found to be barrierless, with a rate coefficient nearing the collision limit, ≥4 × 10(-11) cm(3) molecule(-1) s(-1). The dominant reaction products are expected to be carbonyl compounds and an oxygen molecule, though chemically activated reactions may give rise to a plethora of different (per)acids and carbonyl compounds. CI + CI reactions are expected to be important only in laboratory environments with high CI concentrations. The reaction of H2COO with O3 was predicted to proceed through a pre-reactive complex and a submerged barrier, with a rate coefficient of 1 × 10(-12) cm(3) molecule(-1) s(-1). A study of the dominant CI reactions under experimental and atmospheric conditions shows that the latter reaction might affect CI chemistry.
Ozonolysis of unsaturated hydrocarbons (VOCs) is one of the main oxidation processes in the atmosphere. The stabilized Criegee intermediates (SCI) formed are highly reactive oxygenated species that ...potentially influence the HOx, NOx and SOx cycles, and affect aerosol formation by yielding low-volatility oxygenated compounds. The current knowledge spans mostly SCI formed from primary emitted VOCs, but little is known about the reactivity of oxygenated SCI. In this work we present a theoretical kinetic study of a large number of unsaturated and oxygenated SCI, covering C&z.dbd;C, OH, OR, OOH, OOOH, COOH, COOR, and ONO
2
functionalities at various stereo- and site-specific substitutions relative to the SCI carbonyl oxide moiety. Several novel reaction types are covered, the most important of which are fast intramolecular insertion reactions in OH, OOH and COOH groups, or secondary ozonide formation with a COOH group, forming cyclic oxygenated species; these reaction classes are reminiscent of the analogous bimolecular reactions. The reaction with H
2
O molecules was likewise studied, finding that these cyclisation reactions can be catalysed, with predicted rate coefficients nearing the collision limit. The theoretical data is used to extend the structure-activity relationships (SARs) proposed by Vereecken
et al.
(2017), predicting the dominant unimolecular reaction class and rate, and the rates for reaction with H
2
O and (H
2
O)
2
. The SARs cover over 300 SCI categories with over 40 substituent categories. The validation of these SARs is discussed, and an outlook is given for further improvement. The generally short lifetime of oxygenated SCI suggests that ozonolysis of secondary, oxygenated VOCs is unlikely to yield ambient concentrations of SCI exceeding 10
4
cm
−3
but will contribute strongly to the
in situ
formation of oxygenated VOCs.
Theoretical kinetic data is used to update structure-activity relationships for Criegee intermediates, including fast reactions for intramolecular insertion in oxygenated SCI.
The syn-CH3CHOO Criegee intermediate formed from the ozonolysis of propene and (E)-2-butene was detected via unimolecular decomposition and subsequent detection of OH radicals by a LIF-FAGE ...instrument. An observed time dependent OH concentration profile was analysed using a detailed model focusing on the speciated chemistry of Criegee intermediates based on the recent literature. The absolute OH concentration was found to depend on the steady state concentration of syn-CH3CHOO at the injection point while the time dependence of the OH concentration profile was influenced by the sum of the rates of unimolecular decomposition of syn-CH3CHOO and wall loss. By varying the most relevant parameters influencing the SCI chemistry in the model and based on the temporal OH concentration profile, the unimolecular decomposition rate k (293 K) of syn-CH3CHOO was shown to lie within the range 3-30 s(-1), where a value of 20 ± 10 s(-1) yields the best agreement with the CI chemistry literature.
Raman spectrometry appears to be an opportunity to perform rapid tests in microbiological diagnostics as it provides phenotype-related information from single bacterial cells thus holding the promise ...of direct analysis of clinical specimens without any time-consuming growth phase. Here, we demonstrate the feasibility of a rapid antibiotic-susceptibility determination based on the use of Raman spectra acquired on single bacterial cells. After a two-hour preculture step, one susceptible and two resistant E. coli strains were incubated, for only two hours, in the presence of different bactericidal antibiotics (gentamicin, ciprofloxacin, amoxicillin) in a range of concentrations that included the clinical breakpoints used as references in microbial diagnostic. Spectra were acquired and processed to isolate spectral modifications associated with the antibiotic effect. We evidenced an "antibiotic effect signature" which is expressed with specific Raman peaks and the coexistence of three spectral populations in the presence of antibiotic. We devised an algorithm and a test procedure that overcome single-cell heterogeneities to estimate the MIC and determinate the susceptibility phenotype of the tested bacteria using only a few single-cell spectra in four hours only if including the preculture step.
The chemistry of nitrated alkoxy radicals, and its impact on RO
2
measurements using the laser induced fluorescence (LIF) technique, is examined by a combined theoretical and experimental study. ...Quantum chemical and theoretical kinetic calculations show that the decomposition of β-nitrate-alkoxy radicals is much slower than β-OH-substituted alkoxy radicals, and that the spontaneous fragmentation of the α-nitrate-alkyl radical product to a carbonyl product + NO
2
prevents other β-substituents from efficiently reducing the energy barrier. The systematic series of calculations is summarized as an update to the structure-activity relationship (SAR) by Vereecken and Peeters (2009), and shows increasing decomposition rates with higher degrees of substitution, as in the series ethene to 2,3-dimethyl-butene, and dominant H-migration for sufficiently large alkoxy radicals such as those formed from 1-pentene or longer alkenes. The slow decomposition allows other reactions to become competitive, including epoxidation in unsaturated nitrate-alkoxy radicals; the decomposition SAR is likewise updated for β-epoxy substituents. A set of experiments investigating the NO
3
-initiated oxidation of ethene, propene,
cis
-2-butene, 2,3-dimethyl-butene, 1-pentene, and
trans
-2-hexene, were performed in the atmospheric simulation chamber SAPHIR with measurements of HO
2
and RO
2
radicals performed with a LIF instrument. Comparisons between modelled and measured HO
2
radicals in all experiments, performed in excess of carbon monoxide to avoid OH radical chemistry, suggest that the reaction of HO
2
with β-nitrate alkylperoxy radicals has a channel forming OH and an alkoxy radical in yields of 15-65%, compatible with earlier literature data on nitrated isoprene and α-pinene radicals. Model concentrations of RO
2
radicals when including the results of the theoretical calculations described here, agreed within 10% with the measured RO
2
radicals for all species investigated when the alkene oxidation is dominated by NO
3
radicals. The formation of NO
2
in the decomposition of β-nitrate alkoxy radicals prevents detection of the parent RO
2
radical in a LIF instrument, as it relies on formation of HO
2
. The implications for measurements of RO
2
in ambient and experimental conditions, such as for the NO
3
-dominated chemistry during nighttime, is discussed. The current results appear in disagreement with an earlier indirect experimental study by Yeh
et al.
on pentadecene.
The chemistry of nitrated alkoxy radicals, and its impact on RO
2
measurements using the laser induced fluorescence (LIF) technique, is examined by a combined theoretical and experimental study.
Nitrite and nitrate are known to be involved in photochemical processes occurring in natural waters. In this study we have investigated the role played by these photosensitizers towards the ...transformation of xenobiotic organic matter in marine water, with the goal of assessing the typical transformation routes induced in seawater by irradiated nitrite/nitrate. For this purpose, phenol was chosen as model molecule. Phenol transformation was investigated under simulated solar radiation in the presence of nitrite (in the range of 1×10−5–1×10−2M) or nitrate ions, in pure water at pH 8, in artificial seawater (containing same dissolved salts as seawater but no organic matter), and in natural seawater. In all experiments, phenol degradation rate and formation of intermediates were assessed. As expected, phenol disappearance rate decreased with decreasing nitrite concentration and was slightly reduced by the presence of chloride. Other salts present in artificial seawater (e.g. HCO3−, CO32− and Br−) had a more marked effect on phenol transformation. Analysis of intermediates formed in the different matrices under study showed generation of hydroxyl-, nitro- and chloroderivatives of phenol, to a different extent depending on experimental conditions. 1,4-Benzoquinone prevailed in all cases, nitroderivatives were only formed with nitrite but were not detected in nitrate-spiked solutions. Competition was observed between halogenation and nitration of phenol, with variable outcome depending on nitrite concentration. The most likely reason is competition between nitrating and halogenating species for reaction with the phenoxyl radical. A kinetic model able to justify the occurrence of different intermediates under the adopted conditions is presented and discussed.
► Nitrite and nitrate-mediated solar-driven transformations of pollutant in seawater were studied. ► Phenol degradation rate and formation of intermediates were assessed. ► Competition was observed between halogenation and nitration of phenol. ► A kinetic model is presented able to justify the experimental results.
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