Ozonolysis Of Alpha-pinene Research Articles

Vacuum ultraviolet free-electron laser photoionization mass spectrometry of alpha-pinene ozonolysis

α-pinene is the most abundant monoterpene that represents an important family of volatile organic compounds. Molecular identification of key transient compounds during the α-pinene ozonolysis has been proven to be a challenging experimental target because of a large number of intermediates and products involved. Here we exploit the recently developed hybrid instruments that integrate aerosol mass spectrometry with a vacuum ultraviolet free-electron laser to study the α-pinene ozonolysis. The experiments of α-pinene ozonolysis are performed in an indoor smog chamber, with reactor having a volume of 2 m3 which is made of fluorinated ethylene propylene film. Distinct mass spectral peaks provide direct experimental signatures of previously unseen compounds produced from the reaction of α-pinene with O3. With the aid of quantum chemical calculations, plausible mechanisms for the formation of these new compounds are proposed. These findings provide crucial information on fundamental understanding of the initial steps of α-pinene oxidation and the subsequent processes of new particle formation.

Effect of temperature on the formation of highly oxygenated organic molecules (HOMs) from alpha-pinene ozonolysis

Abstract. Highly oxygenated organic molecules (HOMs) are important contributors to secondary organic aerosol (SOA) and new-particle formation (NPF) in the boreal atmosphere. This newly discovered class of molecules is efficiently formed from atmospheric oxidation of biogenic volatile organic compounds (VOCs), such as monoterpenes, through a process called autoxidation. This process, in which peroxy-radical intermediates isomerize to allow addition of molecular oxygen, is expected to be highly temperature-dependent. Here, we studied the dynamics of HOM formation during α-pinene ozonolysis experiments performed at three different temperatures, 20, 0 and −15 ∘C, in the Aarhus University Research on Aerosol (AURA) chamber. We found that the HOM formation, under our experimental conditions (50 ppb α-pinene and 100 ppb ozone), decreased considerably at lower temperature, with molar yields dropping by around a factor of 50 when experiments were performed at 0 ∘C, compared to 20 ∘C. At −15 ∘C, the HOM signals were already close to the detection limit of the nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer used for measuring gas-phase HOMs. Surprisingly, comparing spectra measured at 0 and 20 ∘C, ratios between HOMs of different oxidation levels, e.g., the typical HOM products C10H14O7, C10H14O9, and C10H14O11, changed considerably less than the total HOM yields. More oxidized species have undergone more isomerization steps; yet, at lower temperature, they did not decrease more than the less oxidized species. One possible explanation is that the primary rate-limiting steps forming these HOMs occur before the products become oxygenated enough to be detected by our CI-APi-TOF (i.e., typically seven or more oxygen atoms). The strong temperature dependence of HOM formation was observed under temperatures highly relevant to the boreal forest, but the exact magnitude of this effect in the atmosphere will be much more complex: the fate of peroxy radicals is a competition between autoxidation (influenced by temperature and VOC type) and bimolecular termination pathways (influenced mainly by concentration of reaction partners). While the temperature influence is likely smaller in the boreal atmosphere than in our chamber, both the magnitude and complexity of this effect clearly deserve more consideration in future studies in order to estimate the ultimate role of HOMs on SOA and NPF under different atmospheric conditions.

The effect of sub-zero temperature on the formation and composition of secondary organic aerosol from ozonolysis of alpha-pinene.

This study presents a newly constructed temperature controlled cold-room smog chamber at Aarhus University, Denmark. The chamber is herein utilized to study the effect of sub-zero temperature on the formation and chemical composition of secondary organic aerosol (SOA) from ozone initiated oxidation of α-pinene. The chemical composition of α-pinene SOA formed from dark ozonolysis of α-pinene at 293 K and 258 K was investigated using High-Resolution Time-of-Flight Aerosol Mass Spectrometry (HR-ToF-AMS) and Ultra-High Performance Liquid Chromatography/Electrospray Ionization Quadrupole Time-of-Flight Mass Spectrometry (UHPLC/ESI-qToF-MS). For comparison, an OH-initiated oxidation experiment was performed at 293 K. In ozonolysis experiments it was found that oxygen-to-carbon (O : C) ratios were higher in the particles formed at 293 K compared to 258 K. A total of 16 different organic acids and 30 dimers esters were quantified in the collected particles composing up to 34% of the total α-pinene SOA mass with increased mass fraction of carboxylic acids in particles from α-pinene ozonolysis at 258 K compared to 293 K. In contrast, dimer esters showed suppressed formation at the sub-zero reaction temperature, thus contributing 3% to SOA mass at 258 K while contributing 9% at 293 K. SOA formed in the OH-initiated oxidation of α-pinene at 293 K resulted in low concentrations of dimer esters supporting Criegee intermediates as a possible pathway to dimer ester formation. Vapour pressure estimates of the identified carboxylic acids and dimer esters are presented and show how otherwise semi-volatile carboxylic acids at sufficiently low temperatures may classify as low or even extremely low volatile organic compounds (ELVOC), thus may add to an enhanced particle formation observed at the sub-zero temperature through gas-to-particle conversion. The change in chemical composition of the SOA particles with temperature is ascribed to a combination of effects: the decreased vapour pressures and hence increased condensation of carboxylic acids from the gas phase to the particle phase along with suppressed formation of the high molecular weight dimer esters and different gas and particle phase chemistry results in particles of different chemical composition as a consequence of low reaction temperatures.

A chemical analyzer for charged ultrafine particles

Abstract. New particle formation is a frequent phenomenon in the atmosphere and of major significance for the Earth's climate and human health. To date the mechanisms leading to the nucleation of particles as well as to aerosol growth are not completely understood. A lack of appropriate measurement equipment for online analysis of the chemical composition of freshly nucleated particles is one major limitation. We have developed a Chemical Analyzer for Charged Ultrafine Particles (CAChUP) capable of analyzing particles with diameters below 30 nm. A bulk of size-separated particles is collected electrostatically on a metal filament, resistively desorbed and subsequently analyzed for its molecular composition in a time of flight mass spectrometer. We report on technical details as well as characterization experiments performed with the CAChUP. Our instrument was tested in the laboratory for its detection performance as well as for its collection and desorption capabilities. The manual application of defined masses of camphene (C10H16) to the desorption filament resulted in a detection limit between 0.5 and 5 ng, and showed a linear response of the mass spectrometer. Flow tube experiments of 25 nm diameter secondary organic aerosol from ozonolysis of alpha-pinene also showed a linear relation between collection time and the mass spectrometer's signal intensity. The resulting mass spectra from the collection experiments are in good agreement with published work on particles generated by the ozonolysis of alpha-pinene. A sensitivity study shows that the current setup of CAChUP is ready for laboratory measurements and for the observation of new particle formation events in the field.

Kinetic limitations in gas-particle reactions arising from slow diffusion in secondary organic aerosol

The potential for aerosol physical properties, such as phase, morphology and viscosity/ diffusivity, to affect particle reactivity remains highly uncertain. We report here a study of the effect of bulk diffusivity of polycyclic aromatic hydrocarbons (PAHs) in secondary organic aerosol (SOA) on the kinetics of the heterogeneous reaction of particle-borne benzo[a]pyrene (BaP) with ozone. The experiments were performed by coating BaP-ammonium sulfate particles with multilayers of SOA formed from ozonolysis of alpha-pinene, and by subsequently investigating the kinetics of BaP loss via reaction with excess ozone using an aerosol flow tube coupled to an Aerodyne Aerosol Mass Spectrometer (AMS). All reactions exhibit pseudo-first order kinetics and are empirically well described by a Langmuir-Hinshelwood (L-H) mechanism. The results show that under dry conditions (RH < 5%) diffusion through the SOA coating can lead to significant mass transfer constraints on the kinetics, with behavior between that previously observed by our group for solid and liquid organic coats. The reactivity of BaP was enhanced at -50% relative humidity (RH) suggesting that water uptake lowers the viscosity of the SOA, hence lifting the mass transfer constraint to some degree. The kinetics for -70% RH were similar to results obtained without SOA coats, indicating that the SOA had sufficiently low viscosity and was sufficiently liquid-like that reactants could rapidly diffuse through the coat. A kinetic multi-layer model for aerosol surface and bulk chemistry was applied to simulate the kinetics, yielding estimates for the diffusion coefficients (in cm2 s(-1)) for BaP in alpha-pinene SOA of 2 x 10(-14), 8 x 10(-14) and > 1 x 10(-12) for dry (RH < 5%), 50% RH and 70% RH conditions, respectively. These results clearly indicate that slow diffusion of reactants through SOA coats under specific conditions can provide shielding from gas-phase oxidants, enabling the long-range atmospheric transport of toxic trace species, such as PAHs and persistent organic pollutants.

Time Scales for Gas-Particle Partitioning Equilibration of Secondary Organic Aerosol Formed from Alpha-Pinene Ozonolysis

Most chemical transport models assume instantaneous equilibrium to represent gas-particle partitioning of semivolatile organic aerosol. This approach has been challenged by recent studies suggesting that secondary organic aerosol (SOA) cannot reach equilibrium within atmospheric time scales. The emergent hypothesis is that gas-particle partitioning rates are limited by diffusion within the condensed phase, which is thought to be "glassy." Here, we investigate the equilibration time scales of SOA formed from α-pinene ozonolysis by measuring the dynamic response to a modest step-change in temperature. Upon heating, equilibrium is disturbed, and the particles evaporate to restore equilibrium at the new temperature, which is attained when evaporation ceases. The SOA was formed at 10 °C and then heated to near room temperature (30 °C) so that the phase state (viscosity) of the condensed-phase after heating is similar to how it would be in the atmosphere. Experiments were performed in both a thermodenuder, with SOA loading of 350 μg/m(3), and in a smog chamber, with SOA loading of 2-12 μg/m(3). Both experiments show, contrary to previous findings, that the SOA achieves equilibrium with dynamic responses consistent with a mass accommodation coefficient of order 0.1. For typical atmospheric conditions, this translates into equilibration time scales on the order of minutes to tens of minutes, supporting the use of equilibrium partitioning in chemical transport models.

Atmospheric Solids Analysis Probe Mass Spectrometry: A New Approach for Airborne Particle Analysis

Secondary organic aerosols (SOA) formed in the atmosphere from the condensation of semivolatile oxidation products are a significant component of airborne particles which have deleterious effects on health, visibility, and climate. In this study, atmospheric solids analysis probe mass spectrometry (ASAP-MS) is applied for the first time to the identification of organics in particles from laboratory systems as well as from ambient air. SOA were generated in the laboratory from the ozonolysis of alpha-pinene and isoprene, as well as from NO(3) oxidation of alpha-pinene, and ambient air was sampled at forested and suburban sites. Particles were collected by impaction on ZnSe disks, analyzed by Fourier transform-infrared spectroscopy (FT-IR) and then transferred to an ASAP-MS probe for further analysis. ASAP-MS data for the laboratory-generated samples show peaks from well-known products of these reactions, and higher molecular weight oligomers are present in both laboratory and ambient samples. Oligomeric products are shown to be present in the NO(3) reaction products for the first time. A major advantage of this technique is that minimal sample preparation is required, and complementary information from nondestructive techniques such as FT-IR can be obtained on the same samples. In addition, a dedicated instrument is not required for particle analysis. This work establishes that ASAP-MS will be useful for identification of organic components of SOA in a variety of field and laboratory studies.

An important pathway for ozonolysis of alpha-pinene and beta-pinene in aqueous phase and its atmospheric implications

The aqueous ozonolysis of α-pinene and β-pinene was conducted under simulated tropospheric conditions at different pHs and temperatures. Three kinds of products, peroxides, carbonyl compounds, and organic acids, were well characterized, and the detection of these products provides effective evidence for understanding the atmospheric aqueous reaction pathway. We have two interesting findings: (1) the unexpected formation of methacrolein (MACR), with a yield of ∼40%, in the α-pinene–O 3 aqueous reaction indicates a potentially new SOA formation pathway, because MACR is one of the important precursors of SOA; and (2) the surprisingly high yields of H 2O 2, ∼60% for the α-pinene–O 3 reaction and ∼100% for the β-pinene–O 3 reaction, indicates that H 2O 2 can be a significant contributor to the origin and transformation of oxidants in the atmosphere, especially in the humid regions. Moreover, we have determined the rate constant for aqueous reaction between MACR and H 2O 2 in pH 2 to 7 and obtained its upper limit as 0.13 M L −1 s −1. A mechanism concerning the formation of the species mentioned above is proposed, and it differs from that in the gas-phase reaction. We suggest that water plays a key role in the mechanism, by participating in the reactions as a direct reactant and by removing the excess energy of intermediates formed in the reactions.

Mechanisms for the formation of organic acids in the gas-phase ozonolysis of 3-carene

This paper describes experimental studies aimed at elucidating mechanisms for the formation of low-volatility organic acids in the gas-phase ozonolysis of 3-carene. Experiments were carried out in a static chamber under 'OH-free' conditions. A range of multifunctional acids-which are analogous to those observed from alpha-pinene ozonolysis-were identified in the condensed phase using gas chromatography coupled to mass spectrometry after derivation. Product yields were determined as a function of different OH radical scavengers and relative humidities to give mechanistic information about their routes of formation. Furthermore, an enone and an enal derived from 3-carene were ozonised in order to probe the early mechanistic steps in the reaction and, in particular, which of the two initially formed Criegee intermediates gives rise to which products. Branching ratios for the formation of the two Criegee Intermediates are determined. Similarities and differences in product formation from 3-carene and alpha-pinene ozonolysis are discussed and possible mechanisms-supported by experimental evidence-are developed for all acids investigated.

A New Real-Time Method for Determining Particles’ Sphericity and Density: Application to Secondary Organic Aerosol Formed by Ozonolysis of α-Pinene

Particle volumes are most often obtained by measuring particle mobility size distributions and assuming that the particles are spherical. Particle volumes are then converted to mass loads by using particle densities that are commonly estimated from measured mobility and vacuum aerodynamic diameters, assuming that the particles are spherical. For aspherical particles, these assumptions can introduce significant errors. We present in this work a new method that can be applied to any particle system to determine in real time whether the particles are spherical or not. We use our second-generation single particle mass spectrometer (SPLAT II) to measure with extremely high precision the vacuum aerodynamic size distributions of particles that are classified by differential mobility analyzer and demonstrate that the line shape of these vacuum aerodynamic size distributions provide a way to unambiguously distinguish between spherical and aspherical particles. Moreover, the very same experimental system is used to obtain the size, density, composition, and dynamic shape factors of individual particles. We present an application of this method to secondary organic aerosols that are formed as a result of ozonolysis of alpha-pinene in the presence and absence of an OH scavenger and find these particles to be spherical with densities of 1.198 +/- 0.004 and 1.213 +/- 0.003 g cm(-3), respectively.

Multifunctional acid formation from the gas-phase ozonolysis of β-pinene

The gas-phase ozonolysis of beta-pinene was studied in static chamber experiments, using gas chromatography coupled to mass spectrometric and flame ionisation detection to separate and detect products. A range of multifunctional organic acids-including pinic acid, norpinic acid, pinalic-3-acid, pinalic-4-acid, norpinalic acid and OH-pinalic acid-were identified in the condensed phase after derivatisation. Formation yields for these products under systematically varying reaction conditions (by adding different OH radical scavengers and Criegee intermediate scavengers) were investigated and compared with those observed from alpha-pinene ozonolysis, allowing detailed information on product formation mechanisms to be elucidated. In addition, branching ratios for the initial steps of the reaction were inferred from quantitative measurements of primary carbonyl formation. Atmospheric implications of this work are discussed.

Mechanisms for the formation of secondary organic aerosol components from the gas-phase ozonolysis of α-pinene

Gas-phase ozonolysis of alpha-pinene was studied in static chamber experiments under 'OH-free' conditions. A range of multifunctional products-in particular low-volatility carboxylic acids-were identified in the condensed phase using gas chromatography coupled to mass spectrometry after derivatisation. The dependence of product yields on reaction conditions (humidity, choice of OH radical scavengers, added Criegee intermediate scavengers, NO(2)etc.) was investigated to probe the mechanisms of formation of these products; additional information was obtained by studying the ozonolysis of an enal and an enone derived from alpha-pinene. On the basis of experimental findings, previously suggested mechanisms were evaluated and detailed gas-phase mechanisms were developed to explain the observed product formation. Atmospheric implications of this work are discussed.

Organic acid formation in the gas-phase ozonolysis of α-pinene

The mechanism of formation of pinonic and norpinonic acids from alpha-pinene ozonolysis has been investigated by studying the products of the ozonolysis of an enone derived from alpha-pinene using gas chromatography coupled to mass spectrometry.

Investigation of α-Pinene + Ozone Secondary Organic Aerosol Formation at Low Total Aerosol Mass

We present a method for measuring secondary organic aerosol (SOA) production at low total organic mass concentration (COA) using proton-transfer reaction mass spectrometry (PTR-MS). PTR-MS provides high time resolution measurements of gas-phase organic species and, coupled with particle measurements, allows for the determination of aerosol yield in real time. This approach facilitates the measurement of aerosol production at low COA; in fact aerosol mass fractions can be measured during alpha-pinene consumption as opposed to only at the completion of gas-phase chemistry. The high time resolution data are consistent with both the partitioning theory of Pankow (Atmos. Environ. 1994, 28,185 and 189) and the previous experimental measurements. Experiments including the effect of UV illumination and NOx reveal additional features of alpha-pinene + ozone product photochemistry and volatility. The high time resolution data also elucidate aerosol production from alpha-pinene ozonolysis at COA < 10 microg m(-3) and show that extrapolations of current partitioning models to conditions of low COA significantly underestimate SOA production under dark, low-NOx conditions. However, extrapolations of current models overestimate SOA production under illuminated, higher-NOx conditions typical of polluted regional air masses.

SOA Formation from Partitioning and Heterogeneous Reactions: Model Study in the Presence of Inorganic Species

A predictive model for secondary organic aerosol (SOA) formation by both partitioning and heterogeneous reactions was developed for SOA created from ozonolysis of alpha-pinene in the presence of preexisting inorganic seed aerosols. SOA was created in a 2 m3 polytetrafluoroethylene film indoor chamber under darkness. Extensive sets of SOA experiments were conducted varying humidity, inorganic seed compositions comprising of ammonium sulfate and sulfuric acid, and amounts of inorganic seed mass. SOA mass was decoupled into partitioning (OM(P)) and heterogeneous aerosol production (OM(H)). The reaction rate constant for OM(H) production was subdivided into three categories (fast, medium, and slow) to consider different reactivity of organic products for the particle phase heterogeneous reactions. The influence of particle acidity on reaction rates was treated in a previous semiempirical model. Model OM(H) was developed with medium and strong acidic seed aerosols, and then extrapolated to OM(H) in weak acidic conditions, which are more relevant to atmospheric aerosols. To demonstrate the effects of preexisting glyoxal derivatives (e.g., glyoxal hydrate and dimer) on OM(H), SOA was created with a seed mixture comprising of aqueous glyoxal and inorganic species. Our results show that heterogeneous SOA formation was also influenced by preexisting reactive glyoxal derivatives.

Chemistry of Particle Inception and Growth during α-Pinene Ozonolysis

A flow-tube reactor was used to study the formation of particles from alpha-pinene ozonation. Particle phase products formed within the first 3-22 s of reaction were analyzed online using a scanning mobility particle sizer and two particle mass spectrometers. The first, a photoionization aerosol mass spectrometer (PIAMS), was used to determine the molecular composition of nascent particles between 30 and 50 nm in diameter. The second, a nano-aerosol mass spectrometer (NAMS), was used to determine the elemental composition of individual particles from 50 nm to below 10 nm in diameter. Molecular composition measurements with PIAMS confirm that both the stabilized Criegee intermediate and hydroperoxide channels of alpha-pinene ozonolysis are operative. However, these channels alone cannot explain the high oxygen content of the particles measured with NAMS. The carbon-to-oxygen mole ratios of suspected nucleating agents are in the range of 2.25-4.0, while the measured ratios are from 1.9 for 9 nm particles to 2.5 and 2.7 for 30 and 50 nm particles, respectively. The large oxygen content may arise by cocondensation of small oxygenated molecules such as water or multistep reactions with ozone, water, or other species that produce highly oxygenated macromolecules. In either case, the increasing ratio with increasing particle size suggests that the aerosol becomes less polar with time.

Secondary Organic Aerosol Production from Terpene Ozonolysis. 2. Effect of NOx Concentration

We report secondary organic aerosol (SOA) yields from the ozonolysis of alpha-pinene in the presence of NO and NO2. Experimental conditions are characterized by the [VOC]0/ [NOx]0 ratio (ppbC/ppb), which varies from approximately 1 to approximately 300. SOA yield is constant for [VOC]0/[NOx]0 > approximately 15 and decreases dramatically (by more than a factor of 4) as [VOC]0/[NOx]0 decreases. Aerosol production is completely suppressed in the presence of NO for [VOC]0/[NOx]0 < or = 4.5. Fouriertransform IR analysis of filter samples reveals that nitrate-containing species contribute significantly to the total aerosol mass at low [VOC]0/[NOx]0. Yield reduction is a result of the formation of a more volatile product distribution as [VOC]0/[NOx]0 decreases; we propose that the change in the product distribution is driven by changes in the gas-phase chemistry as NOx concentration increases. We also present two-product model parameters to describe aerosol production from the alpha-pinene/0/NOx system under both high- and low-NOx conditions.

Representation of Secondary Organic Aerosol Laboratory Chamber Data for the Interpretation of Mechanisms of Particle Growth

Absorptive models of gas-particle partitioning have been shown to be successful in describing the formation and growth of secondary organic aerosol (SOA). Here the expression for particle growth derived by Odum et al. (Odum, J. R.; Hoffmann, T.; Bowman, F.; Collins, D.; Flagan, R. C.; Seinfeld, J. H. Gas/particle partitioning and secondary organic aerosol yields. Environ. Sci. Technol. 1996, 30, 2580-2585) is extended to facilitate interpretation of SOA growth data measured in the laboratory in terms of the underlying chemistry, even when details of the reactions are not well-constrained. A simple (one-component) expression for aerosol growth (deltaM) as a function of the amount of hydrocarbon reacted (deltaHC) is derived, and the effects of changes to three key parameters, stoichiometric yield of condensable species, gas-particle partitioning coefficient, and concentration of preexisting aerosol, are discussed. Two sets of laboratory chamber data on SOA growth are examined in this context: the ozonolysis of alpha-pinene and the OH-initiated photooxidation of aromatic compounds. Even though these two systems have a number of significant differences, both are described well within this framework. From the shapes of the deltaM versus deltaHC curves in each case, the importance of poorly constrained chemistry such as heterogeneous reactions and gas-phase reactions of oxidation products is examined.

On-line characterization of organic aerosols formed from biogenic precursors using atmospheric pressure chemical ionization mass spectrometry.

A method to investigate the chemical composition of organic aerosols formed from biogenic hydrocarbon oxidation using atmospheric pressure chemical ionization mass spectrometry (APCI/MS) is described. The method involves the direct introduction of aerosol particles into the ion source of the mass spectrometer. Using this technique, reaction monitoring experiments of alpha-pinene ozonolysis show the formation of hetero- and homomolecular cluster anions (dimers) of the primary oxidation products (multifunctional carboxylic acids). Since the formation of dimers plays a profound role in new particle formation processes by homogeneous nucleation in the atmosphere and, at the same time, is an intrinsic feature of APCI, it is essential to differentiate between both processes when on-line APCI/MS is applied. In this paper, we compare the results from the investigations of organic aerosols and artificially generated dimer cluster ions of the same compounds using identical ionization conditions. The clusters and their formation processes are characterized by varying the analyte concentration, investigating the thermal stability of dimers, and studying collisional activation properties of both ion species. The investigations show a significant difference in ion stability: dimer anions measured on-line have an estimated stability that is 20 kJ mol(-1) higher than that of the corresponding artificially generated cluster ions. Hence, the technique provides the possibility to accurately characterize dimers as ionized reaction products from biogenic hydrocarbon oxidation and allows an insight into the process of new-particle formation by homogeneous nucleation.