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In order to establish the role of the magnetic effect in the key stages of the autoxidation and initiated oxidation radical-chain reactions, the experimental data and kinetic analysis of the influence of a magnetic field on the oxidative transformations of adrenaline are presented in this work. In the case of autoxidation, the process is being controlled by the rate of adrenaline consumption in the gross process of quinoid oxidation. The analysis of the obtained results is estimative and is based on the assumption of the leading role of superoxide radical during the autoxidation. Superoxide radical concentration increases with the increase in the applied magnetic field strength, which leads to the decrease in the rate of initiation of the quinoid process. In the case of initiated oxidation, the results obtained are based on the known radical-chain mechanism, and they were interpreted using the theory of radical pairs. The observed magnetic effect is explained by the influence of a constant magnetic field on the mechanism of chain termination of radical-chain oxidation and/or initiation of the autoxidation process.

Nighttime NO3-initiated oxidation of biogenic volatile organic compounds (BVOCs) such as monoterpenes is important for the atmospheric formation and growth of secondary organic aerosol (SOA), which has significant impact on climate, air quality, and human health. In such SOA formation and growth, highly oxygenated organic molecules (HOM) may be crucial, but their formation pathways and role in aerosol formation have yet to be clarified. Among monoterpenes, limonene is of particular interest for its high emission globally and high SOA yield. In this work, HOM formation in the reaction of limonene with nitrate radical (NO3) was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). About 280 HOM products were identified, grouped into 19 monomer families, 11 dimer families, and 3 trimer families. Both closed-shell products and open-shell peroxy radicals (RO2⚫) were observed, and many of them have not been reported previously. Monomers and dimers accounted for 47 % and 47 % of HOM concentrations, respectively, with trimers making up the remaining 6 %. In the most abundant monomer families, C10H15−17NO6−14, carbonyl products outnumbered hydroxyl products, indicating the importance of RO2⚫ termination by unimolecular dissociation. Both RO2⚫ autoxidation and alkoxy–peroxy pathways were found to be important processes leading to HOM. Time-dependent concentration profiles of monomer products containing nitrogen showed mainly second-generation formation patterns. Dimers were likely formed via the accretion reaction of two monomer RO2⚫, and HOM-trimers via the accretion reaction between monomer RO2⚫ and dimer RO2⚫. Trimers are suggested to play an important role in new particle formation (NPF) observed in our experiment. A HOM yield of 1.5%-0.7%+1.7% was estimated considering only first-generation products. SOA mass growth could be reasonably explained by HOM condensation on particles assuming irreversible uptake of ultra-low volatility organic compounds (ULVOCs), extremely low volatility organic compounds (ELVOCs), and low volatility organic compounds (LVOCs). This work provides evidence for the important role of HOM formed via the limonene +NO3 reaction in NPF and growth of SOA particles.

Nitryl chloride (ClNO2) accumulation at night acts as a significant reservoir for active chlorine and impacts the following day's photochemistry when the chlorine atom is liberated at sunrise. Here, we report simultaneous measurements of N2O5 and a suite of inorganic halogens including ClNO2 and reactions of chloride with volatile organic compounds (Cl–VOCs) in the gas and particle phases utilising the Filter Inlet for Gas and AEROsols time-of-flight chemical ionisation mass spectrometer (FIGAERO-ToF-CIMS) during an intensive measurement campaign 40 km northwest of Beijing in May and June 2016. A maximum mixing ratio of 2900 ppt of ClNO2 was observed with a mean campaign nighttime mixing ratio of 487 ppt, appearing to have an anthropogenic source supported by correlation with SO2, CO and benzene, which often persisted at high levels after sunrise until midday. This was attributed to such high mixing ratios persisting after numerous e-folding times of the photolytic lifetime enabling the chlorine atom production to reach 2.3 × 105 molecules cm-3 from ClNO2 alone, peaking at 09:30 LT and up to 8.4 × 105 molecules cm-3 when including the supporting inorganic halogen measurements.Cl–VOCs were observed in the particle and gas phases for the first time at high time resolution and illustrate how the iodide ToF-CIMS can detect unique markers of chlorine atom chemistry in ambient air from both biogenic and anthropogenic sources. Their presence and abundance can be explained via time series of their measured and steady-state calculated precursors, enabling the assessment of competing OH and chlorine atom oxidation via measurements of products from both of these mechanisms and their relative contribution to secondary organic aerosol (SOA) formation.

This work presents the results from a flow reactor study on the formation of carboxylic acids from limonene oxidation in the presence of ozone under NOx-free conditions in the dark. A High-Resolution Time-of-Flight acetate Chemical Ionisation Mass Spectrometer (HR-ToF-CIMS) was used in combination with a Filter Inlet for Gases and AEROsols (FIGAERO) to measure the carboxylic acids in the gas and particle phases. The results revealed that limonene oxidation produced large amounts of carboxylic acids which are important contributors to secondary organic aerosol (SOA) formation. The highest 10 acids contributed 56 %–91 % to the total gas-phase signal, and the dominant gas-phase species in most experiments were C8H12O4, C9H14O4, C7H10O4 and C10H16O3. The particle-phase composition was generally more complex than the gas-phase composition, and the highest 10 acids contributed 47 %–92 % to the total signal. The dominant species in the particle phase were C8H12O5, C9H14O5, C9H12O5 and C10H16O4. The measured concentration of dimers bearing at least one carboxylic acid function in the particle phase was very low, indicating that acidic dimers play a minor role in SOA formation via ozone (O3)/hydroxyl (OH) oxidation of limonene. Based on the various experimental conditions, the acidic compositions for all experiments were modelled using descriptions from the Master Chemical Mechanism (MCM). The experiment and model provided a yield of large (C7–C10) carboxylic acid of the order of 10 % (2 %–23 % and 10 %–15 %, respectively). Significant concentrations of 11 acids, from a total of 16 acids, included in the MCM were measured with the CIMS. However, the model predictions were, in some cases, inconsistent with the measurement results, especially regarding the OH dependence. Reaction mechanisms are suggested to fill-in the knowledge gaps. Using the additional mechanisms proposed in this work, nearly 75 % of the observed gas-phase signal in our lowest concentration experiment (8.4 ppb converted, ca. 23 % acid yield) carried out under humid conditions can be understood.

The photooxidation of the most abundant monoterpene, α-pinene, by the hydroxyl radical (OH) was investigated at atmospheric concentrations in the atmospheric simulation chamber SAPHIR. Concentrations of nitric oxide (NO) were below 120 pptv. Yields of organic oxidation products are determined from measured time series giving values of 0.11±0.05, 0.19±0.06, and 0.05±0.03 for formaldehyde, acetone, and pinonaldehyde, respectively. The pinonaldehyde yield is at the low side of yields measured in previous laboratory studies, ranging from 0.06 to 0.87. These studies were mostly performed at reactant concentrations much higher than observed in the atmosphere. Time series of measured radical and trace-gas concentrations are compared to results from model calculations applying the Master Chemical Mechanism (MCM) 3.3.1. The model predicts pinonaldehyde mixing ratios that are at least a factor of 4 higher than measured values. At the same time, modeled hydroxyl and hydroperoxy (HO2) radical concentrations are approximately 25 % lower than measured values. Vereecken et al. (2007) suggested a shift of the initial organic peroxy radical (RO2) distribution towards RO2 species that do not yield pinonaldehyde but produce other organic products. Implementing these modifications reduces the model–measurement gap of pinonaldehyde by 20 % and also improves the agreement in modeled and measured radical concentrations by 10 %. However, the chemical oxidation mechanism needs further adjustment to explain observed radical and pinonaldehyde concentrations. This could be achieved by adjusting the initial RO2 distribution, but could also be done by implementing alternative reaction channels of RO2 species that currently lead to the formation of pinonaldehyde in the model.

Experimental results on the effect of phospholipids ( l -α-phosphatidylcholine, dipal-mitoylphosphatidylcholine, egg yolk lecithin; soybean lecithin) on the inhibitory efficiency of 20 antioxidants (AOs) are summarized depending on the structure and kinetic parameters of AOs in reactions with free peroxyl radicals and the composition of phos-pholipids. The reactivity of AOs and their mixtures with phospholipids was evaluated using three model systems: the initiated oxidation of ethylbenzene and methyl oleate (kinetic oxidation region) and the autooxidation of neat methyl oleate in a free-air exposure system (diffusive oxidation region). For the most part, the addition of phospholipids leads to a change in the antiradical and antioxidant activity of AOs and in the initial oxidation rate due to the complexation between AOs and phospholipids (confirmed by UV and IR spectroscopy) and the effect of AOs on the ability of phospholipids to self-aggregate (demonstrated by dynamic light scattering). This effect depends on the structure of AOs, their ability to be involved in side reactions, the phospholipid composition, the polarity of the medium, and the intensity of oxidation processes. The obtained data and the literature analysis demonstrate that it is necessary to investigate in detail the mechanism of interrelations between AOs and phospholipids, which are the main components of biological membranes, in order to obtain an adequate preliminary evaluation of the most promising newly synthesized compounds for practical use.

Highly oxygenated organic molecules (HOMs) play a pivotal role in the formation of secondary organic aerosol (SOA). Therefore, the distribution and yields of HOMs are fundamental to understand their fate and chemical evolution in the atmosphere, and it is conducive to ultimately assess the impact of SOA on air quality and climate change. In this study, gas-phase HOMs formed from the reaction of limonene with OH radicals in photooxidation were investigated with SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction chamber), using a time-of-flight chemical ionization mass spectrometer with nitrate reagent ion (NO3--CIMS). A large number of HOMs, including monomers (C9–10) and dimers (C17–20), were detected and classified into various families. Both closed-shell products and open-shell peroxy radicals (RO2) were identified under low NO (0.06–0.1 ppb) and high NO conditions (17 ppb). C10 monomers are the most abundant HOM products and account for over 80 % total HOMs. Closed-shell C10 monomers were formed from a two peroxy radical family, C10H15Ox⚫ (x=6–15) and C10H17Ox⚫ (x=6–15), and their respective termination reactions with NO, RO2, and HO2. While C10H17Ox⚫ is likely formed by OH addition to C10H16, the dominant initial step of limonene plus OH, C10H15Ox⚫, is likely formed via H abstraction by OH. C10H15Ox⚫ and related products contributed 41 % and 42 % of C10 HOMs at low and high NO, demonstrating that the H-abstraction pathways play a significant role in HOM formation in the reaction of limonene plus OH. Combining theoretical kinetic calculations, structure–activity relationships (SARs), data from the literature, and the observed RO2 intensities, we proposed tentative mechanisms of HOM formation from both pathways. We further estimated the molar yields of HOMs to be 1.97-1.06+2.52 % and 0.29-0.16+0.38 % at low and high NO, respectively. Our study highlights the importance of H abstraction by OH and provides the yield and tentative pathways in the OH oxidation of limonene to simulate the HOM formation and assess the role of HOMs in SOA formation.

It has been established that the effect of lecithin on the oxidation of methyl oleate depends on the rate of the free radical initiation: lecithin does not affect the reaction rate at high rates of radical initiation (the initiated oxidation) and accelerates the oxidation at low rates (autooxidation), causing the reduction of the induction period. The inhibitory action of mixtures of lecithin and the natural antioxidant α-tocopherol depends on the concentrations of both α-tocopherol and lecithin at different rates of oxidation. The inhibitory efficiency of mixtures with low concentrations of α-tocopherol is close to additive. Increasing the initial concentration of α-tocopherol in mixtures causes a gain in the synergistic effect. The synergistic effect depends extremally on lecithin concentration at high concentrations of α-tocopherol. The mechanism of the synergistic effect for the mixtures of α-tocopherol and lecithin is discussed.

Hydroxyl radicals (OH) are the most important reagent for the oxidation of trace gases in the atmosphere. OH concentrations measured during recent field campaigns in isoprene-rich environments were unexpectedly large. A number of studies showed that unimolecular reactions of organic peroxy radicals (RO2) formed in the initial reaction step of isoprene with OH play an important role for the OH budget in the atmosphere at low mixing ratios of nitrogen monoxide (NO) of less than 100 pptv. It has also been suggested that similar reactions potentially play an important role for RO2 from other compounds. Here, we investigate the oxidation of methacrolein (MACR), one major oxidation product of isoprene, by OH in experiments in the simulation chamber SAPHIR under controlled atmospheric conditions. The experiments show that measured OH concentrations are approximately 50% larger than calculated by the Master Chemical Mechanism (MCM) for conditions of the experiments (NO mixing ratio of 90 pptv). The analysis of the OH budget reveals an OH source that is not accounted for in MCM, which is correlated with the production rate of RO2 radicals from MACR. In order to balance the measured OH destruction rate, 0.77 OH radicals (1σ error: ± 0.31) need to be additionally reformed from each reaction of OH with MACR. The strong correlation of the missing OH source with the production of RO2 radicals is consistent with the concept of OH formation from unimolecular isomerization and decomposition reactions of RO2. The comparison of observations with model calculations gives a lower limit of 0.03 s−1 for the reaction rate constant if the OH source is attributed to an isomerization reaction of MACR-1-OH-2-OO and MACR-2-OH-2-OO formed in the MACR + OH reaction as suggested in the literature (Crounse et al., 2012). This fast isomerization reaction would be a competitor to the reaction of this RO2 species with a minimum of 150 pptv NO. The isomerization reaction would be the dominant reaction pathway for this specific RO2 radical in forested regions, where NO mixing ratios are typically much smaller.

Polyacrylamide hydrogels are widely used in biomedical applications due to their tunable mechanical properties and charge neutrality. Our recent tribological investigations of polyacrylamide gels have revealed tunable and pH-dependent friction behavior. To determine the origins of this pH-responsiveness, we prepared polyacrylamide hydrogels with two different initiating chemistries: a reduction–oxidation (redox)-initiated system using ammonium persulfate (APS) and N,N,N ′ N ′ - tetramethylethylenediamine (TEMED) and a UV-initiated system with 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959). Hydrogel swelling, mechanical properties, and tribological behavior were investigated in response to solution pH (ranging from ≈ 0.34 to 13.5). For polyacrylamide hydrogels in sliding contact with glass hemispherical probes, friction coefficients decreased from µ  = 0.07 ± 0.02 to µ  = 0.002 ± 0.002 (redox-initiated) and from µ  = 0.05 ± 0.03 to µ  = 0.003 ± 0.003 (UV-initiated) with increasing solution pH. With hemispherical polytetrafluoroethylene (PTFE) probes, friction coefficients of redox-initiated hydrogels similarly decreased from µ  = 0.06 ± 0.01 to µ  = 0.002 ± 0.001 with increasing pH. Raman spectroscopy measurements demonstrated hydrolysis and the conversion of amide groups to carboxylic acid in basic conditions. We therefore propose that the mechanism for pH-responsive friction in polyacrylamide hydrogels may be credited to hydrolysis-driven swelling through the conversion of side chain amide groups into carboxylic groups and/or crosslinker degradation. Our results could assist in the rational design of hydrogel-based tribological pairs for biomedical applications from acidic to alkaline conditions. Graphical abstract