Photo-Oxidation of α-Pinene Oxidation Products in Atmospheric Waters - pH- and Temperature-Dependent Kinetic Studies.

The atmospheric α-pinene oxidation leads to three carboxylic acids: norpinonic acid (NPA), pinic acid (PA), and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA). In this study, the OH radical kinetics in the aqueous phase of these carboxylic acids were investigated at different temperatures and pH values of solutions. Activation parameters and the corresponding atmospheric lifetimes of the acids in the troposphere were derived. The overall second-order rate constants for the individual speciation forms of the acids (AH and A- for NPA; AH2, AH- and A2- for PA; and AH3, AH2-, AH2- and A3- for MBTCA) were determined. At 298 K, the rate constants for reactions of protonated forms (AHx) of NPA, PA, and MBTCA with •OH, were (1.5 ± 0.2) × 109 L mol-1 s-1, (2.4 ± 0.1) × 109 L mol-1 s-1, and (4.1 ± 0.6) × 108 L mol-1 s-1, respectively. For the fully deprotonated forms (Ax-) of studied acids, the second-order rate constants were (2.2 ± 0.2) × 109 L mol-1 s-1, (2.8 ± 0.1) × 109 L mol-1 s-1, and (10.2 ± 0.7) × 108 L mol-1 s-1 at 298 K, respectively. It was found that the reactions of NPA and PA with OH radicals are faster than with MBTCA. For MBTCA, the reaction rate depends on pH more strongly at elevated temperatures (>298 K). The atmospheric lifetimes of the acids considered due to their reactivity with •OH were calculated for different model scenarios at a temperature of 283 K and pH = 2 in the aqueous phase. For this purpose, liquid water content (LWC) was used for aerosols and clouds under storm conditions and at various aqueous-phase concentrations of OH radicals. The lifetimes decreased with increasing LWC (from 10-12 m3 m-3 in aerosol to 10-5 m3 m-3 in storms), indicating that the acids undergo significant aqueous processing under realistic atmospheric conditions. Besides, the aerosol systems appeared less effective in removing PA and NPA, with lifetimes ranging from hundreds of days to tens and hundreds of hours, respectively. Clouds were more effective, with lifetimes ranging from tens of hours to a single second or less. MBTCA, which dissolves better in water, was effectively removed in all systems, with the longest lifetime of approximately 90 min.

Daytime Atmospheric Halogen Cycling through Aqueous-Phase Oxygen Atom Chemistry.

Halogen atoms are important atmospheric oxidants that have unidentified daytime sources from photochemical halide oxidation in sea salt aerosols. Here, we show that the photolysis of nitrate in aqueous chloride solutions generates nitryl chloride (ClNO2) in addition to Cl2 and HOCl. Experimental and modeling evidence suggests that O(3P) formed in the minor photolysis channel from nitrate oxidizes chloride to Cl2 and HOCl, which reacts with nitrite to form ClNO2. This chemistry is different than currently accepted mechanisms involving chloride oxidation by OH and could shift our understanding of daytime halogen cycling in the lower atmosphere.

Humic Substance Photosensitized Degradation of Phthalate Esters Characterized by 2H and 13C Isotope Fractionation.

The photosensitized transformation of organic chemicals is an important degradation mechanism in natural surface waters, aerosols, and water films on surfaces. Dissolved organic matter including humic-like substances (HS), acting as photosensitizers that participate in electron transfer reactions, can generate a variety of reactive species, such as OH radicals and excited triplet-state HS (3HS*), which promote the degradation of organic compounds. We use phthalate esters, which are important contaminants found in wastewaters, landfills, soils, rivers, lakes, groundwaters, and mine tailings. We use phthalate esters as probes to study the reactivity of HS irradiated with artificial sunlight. Phthalate esters with different side-chain lengths were used as probes for elucidation of reaction mechanisms using 2H and 13C isotope fractionation. Reference experiments with the artificial photosensitizers 4,5,6,7-tetrachloro-2',4',5',7'-tetraiodofluorescein (Rose Bengal), 3-methoxy-acetophenone (3-MAP), and 4-methoxybenzaldehyde (4-MBA) yielded characteristic fractionation factors (-4 ± 1, -4 ± 2, and -4 ± 1‰ for 2H; 0.7 ± 0.2, 1.0 ± 0.4, and 0.8 ± 0.2‰ for 13C), allowing interpretation of reaction mechanisms of humic substances with phthalate esters. The correlation of 2H and 13C fractions can be used diagnostically to determine photosensitized reactions in the environment and to differentiate among biodegradation, hydrolysis, and photosensitized HS reaction.

Temperature-Dependent Oxidation of Hydroxylated Aldehydes by •OH, SO4•-, and NO3• Radicals in the Atmospheric Aqueous Phase.

T-dependent aqueous-phase rate constants were determined for the oxidation of the hydroxy aldehydes, glyceraldehyde, glycolaldehyde, and lactaldehyde, by the hydroxyl radicals (•OH), the sulfate radicals (SO4•-), and the nitrate radicals (NO3•). The obtained Arrhenius expressions for the oxidation by the •OH radical are: k(T,GLYCERALDEHYDE+OH•) = (3.3 ± 0.1) × 1010 × exp((-960 ± 80 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+OH•) = (4.3 ± 0.1) × 1011 × exp((-1740 ± 50 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+OH•) = (1.6 ± 0.1) × 1011 × exp((-1410 ± 180 K)/T)/L mol-1 s-1; for the SO4•- radical: k(T,GLYCERALDEHYDE+SO4•-) = (4.3 ± 0.1) × 109 × exp((-1400 ± 50 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+SO4•-) = (10.3 ± 0.3) × 109 × exp((-1730 ± 190 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+SO4•-) = (2.2 ± 0.1) × 109 × exp((-1030 ± 230 K)/T)/L mol-1 s-1; and for the NO3• radical: k(T,GLYCERALDEHYDE+NO3•) = (3.4 ± 0.2) × 1011 × exp((-3470 ± 460 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+NO3•) = (7.8 ± 0.2) × 1011 × exp((-3820 ± 240 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+NO3•) = (4.3 ± 0.2) × 1010 × exp((-2750 ± 340 K)/T)/L mol-1 s-1, respectively. Targeted simulations of multiphase chemistry reveal that the oxidation by OH radicals in cloud droplets is important under remote and wildfire influenced continental conditions due to enhanced partitioning. There, the modeled average aqueous •OH concentration is 2.6 × 10-14 and 1.8 × 10-14 mol L-1, whereas it is 7.9 × 10-14 and 3.5 × 10-14 mol L-1 under wet particle conditions. During cloud periods, the aqueous-phase reactions by •OH contribute to the oxidation of glycolaldehyde, lactaldehyde, and glyceraldehyde by about 35 and 29%, 3 and 3%, and 47 and 37%, respectively.

Itch, sleep loss, depressive symptoms, fatigue, and productivity loss in patients with moderate-to-severe atopic dermatitis: Analyses of TREATgermany registry data.

BACKGROUND: TREATgermany is a multicenter registry including patients with moderate-to-severe atopic dermatitis (AD) from currently 74 study centers (university clinics, hospitals and practices) in Germany. As of August 31, 2021, 1,230 adult patients were enrolled. METHODS: In TREATgermany, patients and physicians fill in questionnaires pertaining to symptoms, disease severity, quality of life, depressiveness, and fatigue. In particular, limitations in work performance are assessed using the Work Limitations Questionnaire (WLQ). To assess associations between occupational performance/work limitations and symptoms, correlations and regression models were calculated. RESULTS: The examined sample of 228 employed patients reported an average of 6% at-work productivity loss within the past two weeks prior to enrolment in the registry. The WLQ productivity loss score was moderately associated with itch (r = 0.32) and sleep loss (r = 0.39) and strongly associated with depressive symptoms (r = 0.68) and fatigue (r = 0.60). CONCLUSIONS: The analyses of the registry data show that moderate-to-severe atopic dermatitis has a negative impact on the work productivity of the patients. The analyses further point out the relevant associations between work productivity, depressive symptoms, and fatigue highlighting the disease burden caused by the psychological components of AD.

T- and pH-dependent OH radical reaction kinetics with glycine, alanine, serine, and threonine in the aqueous phase.

Glycine, alanine, serine, and threonine are essential amino acids originating from biological activities. These substances can be emitted into the atmosphere directly. In the present study, the aqueous phase reaction kinetics of hydroxyl radicals (˙OH) with the four amino acids is investigated using the competition kinetics method under controlled temperature and pH conditions. The following T-dependent Arrhenius expressions are derived for the ˙OH reactions with glycine, k(T, H2A+) = (9.1 ± 0.3) × 109 × exp[(-2360 ± 230 K)/T], k(T, HA±) = (1.3 ± 0.1) × 1010 × exp[(-2040 ± 240 K)/T]; alanine, k(T, H2A+) = (1.4 ± 0.1) × 109 × exp[(-1120 ± 320 K)/T], k(T, HA±) = (5.5 ± 0.2) × 109 × exp[(-1300 ± 200 K)/T]; serine, k(T, H2A+) = (1.1 ± 0.1) × 109 × exp[(-470 ± 150 K)/T], k(T, HA±) = (3.9 ± 0.1) × 109 × exp[(-720 ± 130 K)/T]; and threonine, k(T, H2A+) = (5.0 ± 0.1) × 1010 × exp[(-1500 ± 100 K)/T], k(T, HA±) = (3.3 ± 0.1) × 1010 × exp[(-1320 ± 90 K)/T] (in units of L mol-1 s-1). The energy barriers of the ˙OH-induced H atom abstractions were simulated by the density functional theory (DFT) calculation performed with GAUSSIAN using the method of M06-2X and the basis set of 6-311++G(3df,2p). According to the calculation results, the -COOH and -NH3+ groups with strong negative inductive effects increase the energy barriers and thus decrease the ˙OH reaction rate constants. In contrast, the presence of a -OH or -CH3 group with weak negative or positive inductive effects can reduce energy barriers and hence increase the ˙OH reaction rate constants. To improve the previous structure-activity relationship, the contribution factors of -NH3+ at Cα-atom and Cß-atom are determined as 0.07 and 0.15, respectively. Aqueous phase ˙OH oxidation acts as an important sink of the amino acids in the atmosphere, and can be accurately described by the obtained Arrhenius expressions under atmospheric conditions.

Temperature- and pH- Dependent OH Radical Reaction Kinetics of Tartaric and Mucic Acids in the Aqueous Phase.

Tartaric acid and mucic acid are dicarboxylic acids (DCAs), a substance class often found in atmospheric aerosols and cloud droplets. The hydroxyl radical (•OH)-induced oxidation in the aqueous phase is known to be an important loss process of organic compounds such as DCAs. However, the study of •OH kinetics of DCAs in the aqueous phase is still incomplete. In the present study, the rate constants of the •OH reactions of tartaric acid and mucic acid in the aqueous phase were determined by the thiocyanate competition kinetics method as a function of temperature and pH. The following T-dependent Arrhenius expressions (in units of L mol-1 s-1) were first derived for the •OH reactions with tartaric acid─k(T, H2A) = (3.3 ± 0.1) × 1010 exp[(-1350 ± 110 K)/T], k(T, HA-) = (3.6 ± 0.1) × 1010 exp[(-580 ± 110 K)/T], and k(T, A2-) = (3.3 ± 0.1) × 1010 exp[(-1190 ± 170 K)/T]─as well as mucic acid─k(T, H2A) = (2.2 ± 0.1) × 1010 exp[(-1140 ± 150 K)/T], k(T, HA-) = (4.8 ± 0.1) × 1010 exp[(-1280 ± 170 K)/T], and k(T, A2-) = (2.1 ± 0.1) × 1010 exp[(-970 ± 70 K)/T]. A general trend of the •OH rate constant is found as kA2- > kHA- > kH2A. The pH- and temperature-dependent rate constants of the OH radical reactions allow an accurate description of the source and sink processes in the tropospheric aqueous phase.

Kinetics and Mechanisms of Aqueous-Phase Reactions of Triplet-State Imidazole-2-carboxaldehyde and 3,4-Dimethoxybenzaldehyde with α,ß-Unsaturated Carbonyl Compounds.

Reactions in the atmospheric aqueous phase are an important source of secondary organic aerosols (SOA). Within the present study, the reactions of triplet-state imidazole-2-carboxaldehyde (32-IC*) with methyl vinyl ketone (MVK, R1), methacrolein (MACR, R2), and methacrylic acid (MAA, R3), as well as the reaction of triplet-state 3,4-dimethoxybenzaldehyde (3DMB*) with the unsaturated compounds (MVK, R4), (MACR, R5), and (MAA, R6), in the aqueous phase were investigated using laser flash excitation-laser long path absorption and ultraperformance liquid chromatography coupled with high definition electrospray ionization spectrometry. The second-order reaction constants for 32-IC* were determined to be k1 = (1.0 ± 0.1) × 109 L mol-1 s-1 at pH 4-5 and 9, k2 = (1.4 ± 0.4) × 109 L mol-1 s-1 and (1.5 ± 0.1) × 109 L mol-1 s-1 at pH 4-5 and 9, and k3 = (1.4 ± 0.4) × 109 L mol-1 s-1 and (1.1 ± 0.4) × 108 L mol-1 s-1 at pH 4-5 and 9, respectively. The main products of the [2 + 2] photocycloaddition reactions of 32-IC* with both monomer and dimer of MVK as well as MACR were characterized. Similarly, the [2 + 2] photocycloaddition of the carbonyl of the excited triplet state of 3,4-dimethoxybenzaldehyde (3DMB*) with MVK was observed. The second order rate constants for the reactions of 3DMB* were determined: k4 = (1.5 ± 0.2) × 108 L mol-1 s-1, k5 = (2.8 ± 0.5) × 108 L mol-1 s-1, and k6 = (5.2 ± 1.2) × 106 L mol-1 s-1 at pH 9. The studied reactions show that different triplet photosensitizers react with strongly varying rate constants. Advanced CAPRAM process model studies show that active photosensitizers such as 3DMB* can quickly react with unsaturated organic compounds under deliquesced aerosol conditions modifying SOA, while the quenching with oxygen dominates the excited photosensitizer loss under cloud conditions.

Reaction Kinetics of Green Leaf Volatiles with Sulfate, Hydroxyl, and Nitrate Radicals in Tropospheric Aqueous Phase.

Green plants exposed to abiotic or biotic stress release C-5 and C-6 unsaturated oxygenated hydrocarbons called Green Leaf Volatiles (GLVs). GLVs partition into tropospheric waters and react to form secondary organic aerosol (SOA). We explored the kinetics of aqueous-phase reactions of 1-penten-3-ol (PENTOL), (Z)-2-hexen-1-ol (HEXOL), and (E)-2-hexen-1-al (HEXAL) with SO4•-, •OH, and NO3•. At 298 K, the rate constants for reactions of PENTOL, HEXOL, and HEXAL with SO4•- were, respectively, (9.4 ± 1.0) × 108 L mol-1 s-1, (2.5 ± 0.3) × 109 L mol-1 s-1, and (4.8 ± 0.2) × 108 L mol-1 s-1; with •OH - (6.3 ± 0.1) × 109 L mol-1 s-1, (6.7 ± 0.3) × 109 L mol-1 s-1, and (4.8 ± 0.3) × 109 L mol-1 s-1; and with NO3• - (1.5 ± 0.15) × 108 L mol-1 s-1, (8.4 ± 2.3) × 108 L mol-1 s-1, and (3.0 ± 0.7) × 107 L mol-1 s-1. The rate constants increased weakly with temperatures ranging from 278 to 318 K. The diffusional limitations of the rate constants appeared significant only for the GLV-•OH reactions. The aqueous-phase reactions appeared negligible in deliquescent aerosol and haze water but not in clouds and rains. The atmospheric lifetimes of GLVs decreased from many days to hours with increasing liquid water content and radicals' concentration.

Particle-Phase Photoreactions of HULIS and TMIs Establish a Strong Source of H2O2 and Particulate Sulfate in the Winter North China Plain.

During haze periods in the North China Plain, extremely high NO concentrations have been observed, commonly exceeding 1 ppbv, preventing the classical gas-phase H2O2 formation through HO2 recombination. Surprisingly, H2O2 mixing ratios of about 1 ppbv were observed repeatedly in winter 2017. Combined field observations and chamber experiments reveal a photochemical in-particle formation of H2O2, driven by transition metal ions (TMIs) and humic-like substances (HULIS). In chamber experiments, steady-state H2O2 mixing ratios of 116 ± 83 pptv were observed upon the irradiation of TMI- and HULIS-containing particles. Correspondingly, H2O2 formation rates of about 0.2 ppbv h-1 during the initial irradiation periods are consistent with the H2O2 rates observed in the field. A novel chemical mechanism was developed explaining the in-particle H2O2 formation through a sequence of elementary photochemical reactions involving HULIS and TMIs. Dedicated box model studies of measurement periods with relative humidity >50% and PM2.5 ≥ 75 µg m-3 agree with the observed H2O2 concentrations and time courses. The modeling results suggest about 90% of the particulate sulfate to be produced from the SO2 reaction with OH and HSO3- oxidation by H2O2. Overall, under high pollution, the H2O2-caused sulfate formation rate is above 250 ng m-3 h-1, contributing to the sulfate formation by more than 70%.

Aromatic Carbonyl and Nitro Compounds as Photosensitizers and Their Photophysical Properties in the Tropospheric Aqueous Phase.

Secondary organic aerosol formation in the atmospheric aqueous/particulate phase by photosensitized reactions is currently subject to uncertainties. To understand the impact of photosensitized reactions, photophysical and -chemical properties of photosensitizers, kinetic data, and reaction mechanisms of these processes are required. The photophysical properties of acetophenones, benzaldehydes, benzophenones, and naphthalenes were investigated in aqueous solution using laser flash excitation. Quantum yields of excited photosensitizers were determined giving values between 0.06-0.80 at 298 K and pH = 5. Molar absorption coefficients (εmax(3PS*) = (0.8-13) × 104 L mol-1 cm-1), decay rate constants in water (k1st = (9.4 ± 0.5) × 102 to (2.2 ± 0.1) × 105 s-1), and quenching rate constants with oxygen (kq(O2) = (1.7 ± 0.1-4.4 ± 0.4) × 109 L mol-1 s-1) of the excited triplet states were determined at 298 K and pH = 5. Photosensitized reactions of carboxylic acids and alkenes show second-order rate constants in the range of (37 ± 7.0-0.55 ± 0.1) × 104 and (27 ± 5.0-0.04 ± 0.01) × 108 L mol-1 s-1. The results show that different compound classes act differently as a photosensitizer and can be a sink for certain organic compounds in the atmospheric aqueous phase.

Short halt in vaping modifies cardiorespiratory parameters and urine metabolome: a randomized trial.

Propylene glycol and glycerol are e-cigarette constituents that facilitate liquid vaporization and nicotine transport. As these small hydrophilic molecules quickly cross the lung epithelium, we hypothesized that short-term cessation of vaping in regular users would completely clear aerosol deposit from the lungs and reverse vaping-induced cardiorespiratory toxicity. We aimed to assess the acute effects of vaping and their reversibility on biological/clinical cardiorespiratory parameters [serum/urine pneumoproteins, hemodynamic parameters, lung-function test and diffusing capacities, transcutaneous gas tensions (primary outcome), and skin microcirculatory blood flow]. Regular e-cigarette users were enrolled in this randomized, investigator-blinded, three-period crossover study. The periods consisted of nicotine-vaping (nicotine-session), nicotine-free vaping (nicotine-free-session), and complete cessation of vaping (stop-session), all maintained for 5 days before the session began. Multiparametric metabolomic analyses were used to verify subjects' protocol compliance. Biological/clinical cardiorespiratory parameters were assessed at the beginning of each session (baseline) and after acute vaping exposure. Compared with the nicotine- and nicotine-free-sessions, a specific metabolomic signature characterized the stop-session. Baseline serum club cell protein-16 was higher during the stop-session than the other sessions (P < 0.01), and heart rate was higher in the nicotine-session (P < 0.001). Compared with acute sham-vaping in the stop-session, acute nicotine-vaping (nicotine-session) and acute nicotine-free vaping (nicotine-free-session) slightly decreased skin oxygen tension (P < 0.05). In regular e-cigarette-users, short-term vaping cessation seemed to shift baseline urine metabolome and increased serum club cell protein-16 concentration, suggesting a decrease in lung inflammation. Additionally, acute vaping with and without nicotine decreased slightly transcutaneous oxygen tension, likely as a result of lung gas exchanges disturbances.

Organosulfates in Ambient Aerosol: State of Knowledge and Future Research Directions on Formation, Abundance, Fate, and Importance.

Organosulfates (OSs), also referred to as organic sulfate esters, are well-known and ubiquitous constituents of atmospheric aerosol particles. Commonly, they are assumed to form upon mixing of air masses of biogenic and anthropogenic origin, that is, through multiphase reactions between organic compounds and acidic sulfate particles. However, in contrast to this simplified picture, recent studies suggest that OSs may also originate from purely anthropogenic precursors or even directly from biomass and fossil fuel burning. Moreover, besides classical OS formation pathways, several alternative routes have been discovered, suggesting that OS formation possibly occurs through a wider variety of formation mechanisms in the atmosphere than initially expected. During the past decade, OSs have reached a constantly growing attention within the atmospheric science community with evermore studies reporting on large numbers of OS species in ambient aerosol. Nonetheless, estimates on OS concentrations and implications on atmospheric physicochemical processes are still connected to large uncertainties, calling for combined field, laboratory, and modeling studies. In this Critical Review, we summarize the current state of knowledge in atmospheric OS research, discuss unresolved questions, and outline future research needs, also in view of reductions of anthropogenic sulfur dioxide (SO2) emissions. Particularly, we focus on (1) field measurements of OSs and measurement techniques, (2) formation pathways of OSs and their atmospheric relevance, (3) transformation, reactivity, and fate of OSs in atmospheric particles, and (4) modeling efforts of OS formation and their global abundance.

OH radicals reactivity towards phenol-related pollutants in water: temperature dependence of the rate constants and novel insights into the [OH-phenol]˙ adduct formation.

Substituted phenols are known to readily react with the hydroxyl radical (OH˙), which is the most powerful atmospheric oxidant and is also most often used in advanced oxidation processes (AOP) for wastewater treatment. We report temperature-dependent (278.15-318.15 K) second order kinetic rate constants for the aqueous-phase reactions of OH˙ with phenol and four substituted phenols: catechol, phloroglucinol, pyrogallol and 3-methylcatechol, with the last two measured for the first time. The constructed Hammett plots for mono- and di-substituted phenols have the potential to be further applied for predicting the reaction rate constants of other substituted phenols at 298.15 K. This will significantly facilitate the optimization of AOP and improve the predictive capabilities of atmospheric multiphase models in the future. Moreover, an advancement in the understanding of the underlying mechanism, i.e. OH˙ addition to the aromatic ring is made by theoretical calculations at the M06-2X level. We demonstrate that the position of substituents on the aromatic ring is important for the [OH-phenol]˙ adduct formation, which is supported by the experiment and theoretical calculations. Adjacent and nonadjacent electron donor/acceptor substituents differently impact the interplay between the activation energy and entropy. We also show that explicit solvation has to be accounted for in theoretical models in order to explicitly describe the formation of the transition state.

Five-Membered Heterocycles as Potential Photosensitizers in the Tropospheric Aqueous Phase: Photophysical Properties of Imidazole-2-carboxaldehyde, 2-Furaldehyde, and 2-Acetylfuran.

Photosensitized reactions of organic compounds in the atmospheric aqueous and particle phase might be potential sources for secondary organic aerosol (SOA) formation, addressed as aqueous SOA. However, data regarding the photophysical properties of photosensitizers, their kinetics, as well as reaction mechanisms of such processes in the aqueous/particle phase are scarce. The present study investigates the determination of the photophysical properties of imidazole-2-carboxaldehyde, 2-furaldehyde, and 2-acetylfuran as potential photosensitizers using laser flash excitation in aqueous solution. Quantum yields of the formation of the excited photosensitizers were obtained by a scavenging method with thiocyanate, resulting in values between 0.86 and 0.96 at 298 K and pH = 5. The time-resolved absorbance spectra of the excited photosensitizers were measured, and their molar attenuation coefficients were determined ranging between (0.30 and 1.4) × 104 L mol-1 cm-1 at their absorbance maxima (λmax = 335-440 nm). Additionally, the excited photosensitizers are quenched by water and molecular oxygen, resulting in quenching rate constants of k1st = (1.0 ± 0.2-1.8 ± 0.2) × 105 s-1 and kq(O2) = (2.1 ± 0.2-2.7 ± 0.2) × 109 L mol-1 s-1, respectively.

Aqueous-Phase Oxidation of cis-ß-Isoprene Epoxydiol by Hydroxyl Radicals and Its Impact on Atmospheric Isoprene Processing.

Isoprene epoxydiols are well-known intermediates of the isoprene oxidation in the gas phase under pristine conditions. According to their Henry's Law constants, they are well water-soluble and will undergo a phase transfer to the tropospheric aqueous phase to be further processed there. In the present study, the aqueous-phase oxidation by hydroxyl radicals (OH) of the isomer cis-ß-isoprene epoxydiol (cis-ß-IEPOX) was investigated with regards to its kinetics and reaction product distribution. For the reaction of cis-ß-IEPOX with hydroxyl radicals, a second-order rate constant of (1.4 ± 0.1) × 109 L mol-1 s-1 was determined at 298 K. Short chain carbonyls, such as glycolaldehyde with a maximum molar fraction of 6.9%, hydroxyacetone (7.4%), methylglyoxal (10.3%), and glyoxal (6.6%), as well as acetic acid (13.9%), were observed as major oxidation products. A total molar recovery of 75% was obtained in the oxidation experiments. The overall contribution of the aqueous-phase oxidation of cis-ß-IEPOX was estimated to range from 29.7 to 21.3% to the atmospheric processing depending on the scenario.

OH-Initiated Oxidation of Imidazoles in Tropospheric Aqueous-Phase Chemistry.

Imidazoles formed via the reaction of dicarbonyls with nitrogen containing compounds in the atmospheric particle phase can be expected to initiate secondary organic aerosol (SOA) growth due to their potential to act as photosensitizers. Recent field studies quantified imidazoles for the first time in ambient aerosol samples from Europe and China. However, knowledge about the kinetics and mechanisms of particle-phase reactions involving imidazoles is still very limited. In the present study, the radical-driven aqueous-phase oxidative degradation reactions of the hydroxyl radical (·OH) with imidazoles were investigated. For the imidazoles, the following rate constants at 298 K and acidic conditions are obtained as k(imidazole-2-carboxaldehyde) = (1.8 ± 0.1) × 109 L mol-1 s-1, k(1-methylimidazole) = (2.3 ± 0.1) × 109 L mol-1 s-1, k(2-methylimidazole) = (3.9 ± 0.1) × 109 L mol-1 s-1, k(4-methylimidazole) = (3.2 ± 0.2) × 109 L mol-1 s-1, k(1-ethylimidazole) = (2.6 ± 0.1) × 109 L mol-1 s1, and k(2-ethylimidazole) = (3.9 ± 0.1) × 109 L mol-1 s-1. Temperature and pH dependencies of the reactions as well as the activation parameters have been determined and discussed. The possible implications and restrictions for imidazoles acting as photosensitizers in tropospheric particles have been considered.

Kinetic and Theoretical Study of the Atmospheric Aqueous-Phase Reactions of OH Radicals with Methoxyphenolic Compounds.

Methoxyphenols, which are emitted through biomass burning, are an important species in atmospheric chemistry. In the present study, temperature-dependent aqueous-phase OH radical reactions of six methoxyphenols and two related phenols have been investigated through laser flash photolysis and the density functional theory. The rate constants obtained were in a range of (1.1-1.9) × 1010 L mol-1 s-1 with k(3-MC) > k(Cre) ≈ k(Syr) ≈ k(MEP) > k(Res) > k(3-MP) > k(2-EP) ≈ k(2-MP). We derived the parameters of these reactions from the obtained T-dependent rate constants and found a mean Arrhenius activation energy of 16.9 kJ mol-1. The diffusion rate constants were calculated for each case and compared to the measured ones. Generally, the rate constants are found to be close to fully diffusion-controlled (kdiff = (1.4-1.5) × 1010 L mol-1 s-1 for all reactions). A structure-function relationship was established through the measurement result, which could be used for predicting unknown rate constants of other phenolic compounds. All of these findings are expected to enhance the predictive capabilities of models, such as the chemical aqueous-phase radical mechanism.

Aqueous-Phase Oxidation of Terpene-Derived Acids by Atmospherically Relevant Radicals.

Terpene-derived acids formed through the atmospheric gas-phase oxidation of terpenes are able to efficiently undergo a phase transfer into the aqueous phase. The subsequent aqueous-phase oxidation of such compounds has not been intensely studied. Accordingly, the aqueous-phase second-order rate constants of the oxidation reactions of cis-pinonic acid (CPA) and (+)-camphoric acid (+CA) with hydroxyl radicals (•OH), nitrate radicals (NO3•), and sulfate radicals (SO4•-) were investigated as a function of temperature and pH in the present study. For CPA and +CA the following •OH reaction rate constants at T = 298 K are determined: ksecond(CPA, pH<2) = (2.8 ± 0.1) × 109 L mol-1 s-1, ksecond(CPA, pH>8) = (2.7 ± 0.3) × 109 L mol-1 s-1, ksecond(+CA, pH<2) = (2.1 ± 0.1) × 109 L mol-1 s-1, ksecond(+CA, pH=5.3) = (2.7 ± 0.3) × 109 L mol-1 s-1, ksecond(+CA, pH>8) = (2.7 ± 0.1) × 109 L mol-1 s-1. In order to assess the atmospheric impact of the aqueous-phase oxidation of such compounds, atmospheric aqueous-phase lifetimes were calculated for two model scenarios based on CAPRAM 3.0i. The aqueous-phase oxidation under remote conditions emerges to be the most favored pathway with lifetimes of 5 ± 1 h.