Aqueous-phase OH-oxidation Research Articles

Water Significantly Changes the Ring-Cleavage Process During Aqueous Photooxidation of Toluene.

As a major constituent of aromatic compounds, toluene exists widely in environmental aqueous phases. This research investigated the aqueous-phase OH oxidation of toluene to determine how liquid water changes the radical chemistry of ring-cleavage pathways. Results show that ring-cleavage pathways through the C7 bicyclic peroxy radical (BPR) account for about 70% of total aqueous-phase oxidation pathways, which is similar to that in the gas-phase oxidation. However, detailed ring-cleavage pathways in the aqueous phase change significantly compared with those in the gas phase as shown by the decreased production of glyoxal and methylglyoxal and the enhanced production of formic acid and acetic acid as primary products, which can be attributed to the presence of liquid water. Water facilitates the formation of the BPR whose structure is different from that in the gas-phase oxidation and results in different ring-cleavage pathways through hydrogen-shift reactions. Furthermore, water helps the hydration of acyl radicals formed by the BPR to produce organic acids. With the suggested ring-cleavage mechanisms, a box model can simulate aqueous-phase product distributions better than that with the classical ring-cleavage mechanisms. Given the influence of water on reaction mechanisms, aqueous-phase oxidation of hydrophobic organic compounds may be more important than previously assumed.

Photo-oxidation of pinic acid in the aqueous phase: a mechanistic investigation under acidic and basic pH conditions

The mechanism of aqueous-phase OH-oxidation of pinic acid was investigated for the first time, and norpinic acid and 3-methyl-1,2,3-butanetricarboxylic acid were identified as major products. The mechanism is pH-dependent, with enhanced yields under basic conditions.

PH Dependence of the OH Reactivity of Organic Acids in the Aqueous Phase.

Photochemical processing taking place in atmospheric aqueous phases serves as both a source and a sink of organic compounds. In aqueous environments, acid-base chemistry and, by extension, aqueous-phase pH, are an important yet often neglected factors to consider when investigating the kinetics of organic compounds. We have investigated the aqueous-phase OH-oxidation of pinic acid, cis-pinonic acid, limononic acid, and formic acid (FA) as a function of pH. We have also extended our studies to other organic acids (OAs) present in the water-soluble fraction of secondary organic aerosol (SOA) arising from the ozonolysis of α-pinene. Although all the OAs exhibited larger OH reactivities at pH 10, the pH dependence was dramatically different between FA, the smallest OA, and those that contained more than eight carbons. A kinetic box model was also employed to characterize our photoreactor and to provide confidence to our results. Our finding shows that the atmospheric lifetimes of small OAs (e.g., FA) are highly sensitive to cloud water pH. However, those of larger OAs and many other OAs in α-pinene SOA are affected to a much less extent. These results are of great importance for the simplification of cloud water chemistry models.

Rapid Aqueous-Phase Photooxidation of Dimers in the α-Pinene Secondary Organic Aerosol

Chemical complexity significantly hinders our understanding of the formation and evolution of secondary organic aerosol (SOA), which is known to have impacts on air quality and global climate. Dimeric substances present in SOA comprise a major fraction of extremely low-volatile organic compounds that are especially poorly characterized. Using online mass spectrometry, we have investigated the aqueous-phase OH oxidation of dimers present in the water-soluble fraction of SOA arising from ozonolysis of α-pinene. This study highlights very rapid OH oxidation of dimeric compounds. In particular, using pinonic acid as a reference compound, we obtained second-order rate constants for the loss of 12 dimers, with an average value of (1.3 ± 0.5) × 109 M–1 s–1 at room temperature. For the first time, this study demonstrates that rapid loss of dimeric compounds will occur in cloudwater and potentially also in aqueous aerosols.

Kinetics, Mechanism, and Secondary Organic Aerosol Yield of Aqueous Phase Photo-oxidation of α-Pinene Oxidation Products.

Formation of secondary organic aerosol (SOA) involves atmospheric oxidation of volatile organic compounds (VOCs), the majority of which are emitted from biogenic sources. Oxidation can occur not only in the gas-phase but also in atmospheric aqueous phases such as cloudwater and aerosol liquid water. This study explores for the first time the aqueous-phase OH oxidation chemistry of oxidation products of α-pinene, a major biogenic VOC species emitted to the atmosphere. The kinetics, reaction mechanisms, and formation of SOA compounds in the aqueous phase of two model compounds, cis-pinonic acid (PIN) and tricarballylic acid (TCA), were investigated in the laboratory; TCA was used as a surrogate for 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), a known α-pinene oxidation product. Aerosol time-of-flight chemical ionization mass spectrometry (Aerosol-ToF-CIMS) was used to follow the kinetics and reaction mechanisms at the molecular level. Room-temperature second-order rate constants of PIN and TCA were determined to be 3.3 (± 0.5) × 10(9) and 3.1 (± 0.2) × 10(8) M(-1) s(-1), respectively, from which were estimated their condensed-phase atmospheric lifetimes. Aerosol-ToF-CIMS detected a large number of products leading to detailed reaction mechanisms for PIN and MBTCA. By monitoring the particle size distribution after drying, the amount of SOA material remaining in the particle phase was determined. An aqueous SOA yield of 40 to 60% was determined for PIN OH oxidation. Although recent laboratory studies have focused primarily on aqueous-phase processing of isoprene-related compounds, we demonstrate that aqueous formation of SOA materials also occurs from monoterpene oxidation products, thus representing an additional source of biogenically driven aerosol formation.

Factors controlling the ice nucleating abilities of α‐pinene SOA particles

AbstractThe ice nucleation abilities of fresh, water‐soluble, internally mixed, and photochemically oxidized α‐pinene secondary organic aerosol (SOA) particles were investigated at cirrus cloud temperatures in a continuous flow diffusion chamber. SOA sampled from a flow tube (SOA‐fresh‐FT) mimicked freshly generated particles, while the water‐soluble organic compound fraction from a FT and smog chamber (SOA‐WSOC‐FT, SOA‐WSOC‐SC) mimicked cloud‐processed particles. SOA‐fresh‐FT, SOA‐WSOC‐FT, and SOA‐WSOC‐SC particles were not highly active at nucleating ice between 233 K and 213 K, with activation onsets (i.e., 0.1% of particles forming ice) at or slightly above the theoretical homogeneous freezing line. A significant increase in the O/C of SOA‐WSOC‐SC via aqueous phase OH oxidation did not modify the ice nucleation abilities, indicating that the detailed composition of the particles is not of paramount importance to their ice nucleating abilities. Instead, precooling the SOA‐WSOC‐FT and SOA‐WSOC‐SC particles to 233 K dropped their ice nucleation onsets by up to 20% relative humidity with respect to ice, with lower temperatures likely driving the particles to be more viscous and solid‐like. However, it is possible that preactivation contributed to the reduction of the ice nucleation onsets. Particles composed of both SOA‐WSOC and ammonium sulfate (AS) were significantly less active in the deposition nucleation mode than pure, solid AS particles.

Radical mechanisms of methyl vinyl ketone oligomerization through aqueous phase OH-oxidation: on the paradoxical role of dissolved molecular oxygen

Abstract. It is now accepted that one of the important pathways of secondary organic aerosol (SOA) formation occurs through aqueous phase chemistry in the atmosphere. However, the chemical mechanisms leading to macromolecules are still not well understood. It was recently shown that oligomer production by OH radical oxidation in the aerosol aqueous phase from α-dicarbonyl precursors, such as methylglyoxal and glyoxal, is irreversible and fast. Methyl vinyl ketone (MVK) was chosen in the present study as it is an α,β-unsaturated carbonyl that can undergo radical oligomerization in the aerosol aqueous phase. We present here experiments on the aqueous phase OH-oxidation of MVK, performed under various conditions. Using NMR and UV absorption spectroscopy, high and ultra-high resolution mass spectrometry, we show that the fast formation of oligomers up to 1800 Da is due to radical oligomerization of MVK, and 13 series of oligomers (out of a total of 26 series) are identified. The influence of atmospherically relevant parameters such as temperature, initial concentrations of MVK and dissolved oxygen are presented and discussed. In agreement with the experimental observations, we propose a chemical mechanism of OH-oxidation of MVK in the aqueous phase that proceeds via radical oligomerization of MVK on the olefin part of the molecule. This mechanism highlights in our experiments the paradoxical role of dissolved O2: while it inhibits oligomerization reactions, it contributes to produce oligomerization initiator radicals, which rapidly consume O2, thus leading to the dominance of oligomerization reactions after several minutes of reaction. These processes, together with the large range of initial concentrations investigated show the fundamental role that radical oligomerization processes likely play in polluted fogs and atmospheric aerosol.

Oligomer and SOA formation through aqueous phase photooxidation of methacrolein and methyl vinyl ketone

Abstract This work investigates the ability of methacrolein (MACR) and methyl vinyl ketone (MVK) (the two main gas phase atmospheric oxidation products of isoprene) to form oligomers and secondary organic aerosol (SOA) upon aqueous phase OH-oxidation and subsequent water evaporation. For the two precursors, electrospray mass spectrometry (in infusion and coupled to liquid chromatography) analysis of the reacting solutions brought clear evidence for the formation of oligomer systems having a mass range of up to 1400 Da. More than 11 series of oligomers were found. For MVK, the intensity and masses of oligomers became increasingly important as MVK initial concentrations increased from 0.2 to 20 mM. For both precursors, the oligomers were responsible for the SOA formation during nebulization experiments. The evaluated SOA mass yield ranged from 3.9 to 9.9% for MVK. These yields were time dependent and were in good agreement with the range (1.6–11.7%) obtained for MACR under the same conditions by El Haddad et al. (2009) .

Aqueous-Phase OH Oxidation of Glyoxal: Application of a Novel Analytical Approach Employing Aerosol Mass Spectrometry and Complementary Off-Line Techniques

Aqueous-phase chemistry of glyoxal may play an important role in the formation of highly oxidized secondary organic aerosol (SOA) in the atmosphere. In this work, we use a novel design of photochemical reactor that allows for simultaneous photo-oxidation and atomization of a bulk solution to study the aqueous-phase OH oxidation of glyoxal. By employing both online aerosol mass spectrometry (AMS) and offline ion chromatography (IC) measurements, glyoxal and some major products including formic acid, glyoxylic acid, and oxalic acid in the reacting solution were simultaneously quantified. This is the first attempt to use AMS in kinetics studies of this type. The results illustrate the formation of highly oxidized products that likely coexist with traditional SOA materials, thus, potentially improving model predictions of organic aerosol mass loading and degree of oxidation. Formic acid is the major volatile species identified, but the atmospheric relevance of its formation chemistry needs to be further investigated. While successfully quantifying low molecular weight organic oxygenates and tentatively identifying a reaction product formed directly from glyoxal and hydrogen peroxide, comparison of the results to the offline total organic carbon (TOC) analysis clearly shows that the AMS is not able to quantitatively monitor all dissolved organics in the bulk solution. This is likely due to their high volatility or low stability in the evaporated solution droplets. This experimental approach simulates atmospheric aqueous phase processing by conducting oxidation in the bulk phase, followed by evaporation of water and volatile organics to form SOA.

In-cloud processes of methacrolein under simulated conditions – Part 3: Hygroscopic and volatility properties of the formed secondary organic aerosol

Abstract. The hygroscopic and volatility properties of secondary organic aerosol (SOA) produced from the nebulization of solutions after aqueous phase photooxidation of methacrolein was experimentally studied in a laboratory, using a Volatility-Hygroscopicity Tandem DMA (VHTDMA). The obtained SOA were 80% 100°C-volatile after 5 h of reaction and only 20% 100°C-volatile after 22 h of reaction. The Hygroscopic Growth Factor (HGF) of the SOA produced from the nebulization of solutions after aqueous-phase photooxidation of methacrolein is 1.34–1.43, which is significantly higher than the HGF of SOA formed by gas-phase photooxidation of terpenes, usually found almost hydrophobic. These hygroscopic properties were confirmed for SOA formed by the nebulization of the same solutions where NaCl was added. The hygroscopic properties of the cloud droplet residuals decrease with the reaction time, in parallel with the formation of more refractory compounds. This decrease was mainly attributed to the 250°C-refractive fraction (presumably representative of the highest molecular weight compounds), which evolved from moderately hygroscopic (HGF of 1.52) to less hygroscopic (HGF of 1.36). Oligomerization is suggested as a process responsible for the decrease of both volatility and hygroscopicity with time. The NaCl seeded experiments enabled us to show that 19±4 mg L−1 of SOA was produced after 9.5 h of reaction and 41±9 mg L−1 after 22 h of in-cloud reaction. Because more and more SOA is formed as the reaction time increases, our results show that the reaction products formed during the aqueous-phase OH-oxidation of methacrolein may play a major role in the properties of residual particles upon the droplet's evaporation. Therefore, the specific physical properties of SOA produced during cloud processes should be taken into account for a global estimation of SOA and their atmospheric impacts.

Structure-activity relationship for the estimation of OH-oxidation rate constants of aliphatic organic compounds in the aqueous phase: alkanes, alcohols, organic acids and bases

A robust estimation method of aqueous phase OH-oxidation rate constants of organic compounds would be of high benefit to a wide community relevant to atmospheric chemistry. Here, we propose an aqueous phase structure-reactivity relationship (SAR) for aliphatic organic compounds including alkanes, alcohols, organic acids, bases and polyfunctional compounds containing at least two of these functions. The methodology used is based on Atkinson's group-addivity SAR [Atkinson, R., 1987. A structure-activity relationship for the estimation of rate constants for the gas-phase reactions of OH radicals with organic compounds. International Journal of Chemical Kinetics 19, 799–828] for the gas phase reactions, with slight differences. It is constructed using an automatic solver, and using a wide database which includes 72 aliphatic compounds, and 7 functions, for reactions studied at room temperature in the aqueous phase. The results show a satisfactory efficiency of the proposed SAR as 60% of the estimated values were found within the range of 80% of the experimental values. The obtained parameters are discussed, and the performances of the method are compared to other estimation methods.

Kinetics of OH-initiated oxidation of oxygenated organic compounds in the aqueous phase: new rate constants, structure–activity relationships and atmospheric implications

The kinetics of OH oxidation of several organic compounds of atmospheric relevance were measured in the aqueous phase. Relative kinetics were performed using various organic references and OH sources. After validation of the protocol, temperature-dependent rate constants for the reactions of OH radical with ethyl ter-butyl ether ( k 297 K = 1.5 ( ± 1.7 ) × 1 0 9 M - 1 s - 1 , E a / R = 580 (±560) K), n-butyl acetate ( k 297 K = 1.8 (±0.4)×10 9 M −1 s −1, E a / R = 1000 (±200) K), acetone ( k 298 K = 0.11 (±0.05)×10 9 M −1 s −1, E a / R = 1400 (±500) K), methyl ethyl ketone ( k 298 K = 0.81 ( ± 0.18 ) × 1 0 9 M - 1 s - 1 , E a / R = 1200 (±200) K), methyl iso-butyl ketone ( k 298 K = 2.1 ( ± 0.5 ) × 1 0 9 M - 1 s - 1 , E a / R = 1200 (±300) K) and methylglyoxal ( k 298 K = 0.53 ( ± 0.04 ) × 1 0 9 M - 1 s - 1 , E a / R = 1100 (±300) K) were determined. A non-Arrhenius behavior was found for phenol, in good agreement with the contribution of an OH addition to the mechanism, which also includes H-abstraction by OH radicals. Global rate constants of acetaldehyde, propionaldehyde, butyraldehyde and valeraldehyde were studied at 298 K only, as these compounds partly hydrate in the aqueous phase. All the obtained data (except those of phenol) complemented by literature data were used to investigate three methods to estimate rate constants for H-abstraction reactions of OH radicals in aqueous solutions when measured data were not available: Evans-Polanyi-type correlations, comparisons with gas-phase data, structure activity relationships (SAR). The results show that the SAR method is promising; however, the data set is currently too small to extend this method to temperatures other than 298 K. The atmospheric impact of aqueous phase OH oxidation of water-soluble organic compounds is discussed with the determination of their global atmospheric lifetimes, taking into account both gas- and aqueous-phase reactivities. The results show that atmospheric droplets can act as powerful photoreactors to eliminate soluble organic compounds from the atmosphere.

Oxidation of methanol by hydroxyl radicals in aqueous solution under simulated cloud droplet conditions

The results of a detailed mechanistic study of aqueous-phase OH-oxidation of methanol are presented. Analysis of reaction products by specific chromatographic methods revealed that hydrated formaldehyde is not the only stable primary reaction product. Formic acid and/or formate ion are also stable primary molecular reaction products of methanol OH-oxidation. The branching ratios for their formation are highly pH dependent. At pH=7, hydrated formaldehyde is the dominant molecular reaction product (ratio 4.5 : 1 for hydrated formaldehyde : formate ion), whereas at pH=2, formic acid is the dominant product (ratio 3.7 : 1 for formic acid : hydrated formaldehyde). At all pH studied, the sum of the primary stable products represents 49 (±11)% of methanol removal, in agreement with the amount of OOCH 2OH radicals formed relative to methanol removal 48(±2)%. The formation of primary formic acid at pH=2 is attributed to OOCH 2OH self-reaction, and the strong pH effect is attributed to the base-catalyzed decomposition of OOCH 2OH leading to the formation of hydrated formaldehyde. Evaporation and/or an addition reaction between CH 2OH and HO 2 radicals leading to the formation of hydroxymethyl hydroperoxide is proposed to explain the missing yields. The implications of this mechanism to atmospheric chemistry are discussed.