Limonene Ozonolysis Research Articles

Secondary organic aerosol formation from multiphase oxidation of limonene by ozone: mechanistic constraints via two-dimensional heteronuclear NMR spectroscopy

Because it is doubly unsaturated, gaseous limonene and ozone reactions exhibit considerable potential for not only a large quantity of secondary organic aerosol (SOA), but also a diverse and complicated product mixture influenced by reactant conditions. We explore the chemistry of limonene ozonolysis and provide evidence that under low-NOx conditions, the endocyclic double bond is oxidized in the gas phase, while under excess ozone conditions, the residual exocyclic double bond in condensed-phase products is heterogeneously oxidized by ozone. We use regular and multinuclear-multidimensional NMR spectroscopy, in particular 1D 1H-NMR, 1H,1H-COSY correlation spectroscopy, and 1H,13C-HSQC (heteronuclear single quantum coherence). For structural assignments we simulate 1H and 13C NMR spectra with ACDLabs online, relying representative products consistent with the postulated reaction mechanism. The 1-D 1H-NMR data allow us to quantify the extent to which the residual unsaturation is oxidized with rising ozone, confirming our hypothesis that the residual unsaturation is oxidized via heterogeneous uptake of ozone to fresh SOA particles.

Time-resolved molecular characterization of limonene/ozone aerosol using high-resolution electrospray ionization mass spectrometry

Molecular composition of limonene/O3 secondary organic aerosol (SOA) was investigated using high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) as a function of reaction time. SOA was generated by ozonation of D-limonene in a reaction chamber and sampled at different time intervals using a cascade impactor. The SOA samples were extracted into acetonitrile and analyzed using a HR-ESI-MS instrument with a resolving power of 100,000 (m/Deltam). The resulting mass spectra provided detailed information about the extent of oxidation inferred from the O:C ratios, double bond equivalency (DBE) factors, and aromaticity index (AI) values in hundreds of identified individual SOA species. The chemical composition of SOA was approximately the same for all size-fractionated samples studied in this experiment (0.05 to 0.5 microm range). The SOA constituents quickly reached an average O:C ratio of 0.43, which grew to 0.46 after one hour of additional oxidation of particles by the excess ozone. The dominant mechanism of oligomerization, inferred from high resolution ESI-MS data, was reaction between Criegee intermediates and stable first-generation products of limonene ozonolysis. Although the SOA composition was dominated by various oxidized aliphatic compounds, a small fraction of products appeared to contain aromatic rings. SOA generation was also studied in the presence of UV radiation and at elevated relative humidity (RH). The presence of UV radiation had a negligible effect on the SOA composition. The presence of water vapor resulted in a slight redistribution of peak intensities in the mass spectrum likely arising from hydration of certain SOA constituents. The data are consistent with fast production of the first-generation SOA constituents, including oligomers, followed by very slow aging processes that have a relatively small effect on the average molecular composition on the timescale of our experiments.

The effect of temperature and water on secondary organic aerosol formation from ozonolysis of limonene, Δ<sup>3</sup>-carene and α-pinene

Abstract. The effect of reaction temperature and how water vapour influences the formation of secondary organic aerosol (SOA) in ozonolysis of limonene, Δ3-carene and α-pinene, both regarding number and mass of particles, has been investigated by using a laminar flow reactor (G-FROST). Experiments with cyclohexane and 2-butanol as OH scavengers were compared to experiments without any scavenger. The reactions were conducted in the temperature range between 298 and 243 K, and at relative humidities between <10 and 80%. Results showed that there is still a scavenger effect on number and mass concentrations at low temperatures between experiments with and without an addition of an OH scavenger. This shows that the OH chemistry is influencing the SOA formation also at these temperatures. The overall temperature dependence on SOA formation is not as strong as expected from partitioning theory. In some cases there is even a positive temperature dependence that must be related to changes in the chemical mechanism and/or reduced rates of secondary chemistry at low temperatures. The precursor's α-pinene and Δ3-carene exhibit a similar temperature dependence regarding both number and mass of particles formed, whereas limonene shows a different dependence. The water effect at low temperature could be explained by physical uptake and cluster stabilisation. At higher temperatures, only a physical explanation is not sufficient and the observations are in line with water changing the chemical mechanism or reaction rates. The data presented adds to the understanding of SOA contribution to new particle formation and atmospheric degradation mechanisms.

Influence of OH Scavenger on the Water Effect on Secondary Organic Aerosol Formation from Ozonolysis of Limonene, Δ3-Carene, and α-Pinene

The effect of OH scavengers on how water vapor influences the formation of secondary organic aerosol (SOA) in ozonolysis of limonene, Delta3-carene, and alpha-pinene at low concentrations has been investigated by using a laminar flow reactor. Cyclohexane and 2-butanol (3-40 x 10(13) molecules cm(-3)) were used as scavengers and compared to experiments without any scavenger. The reactions were conducted at 298 K and at relative humidities between <10 and 80%. The yield of SOA decreased in the order "no scavenger" > 2-butanol > cyclohexane. The effect of water vapor was similar for 2-butanol and without a scavenger, with an increase in particle number and mass concentration with increasing relative humidity. The water effect for cyclohexane was more complex, depending on the terpene, scavenger concentration, and SOA concentration. The water effect seems to be influenced by the HO2/RO2 ratio. The results are discussed in relation to the currently suggested mechanism for alkene ozonolysis and to atmospheric importance. The results imply that the ozone-initiated oxidation of terpenes needs revision in order to fully account for the role of water in the chemical mechanism.

The formation of organic sulfate esters in the limonene ozonolysis secondary organic aerosol (SOA) under acidic conditions

Secondary organic aerosol (SOA) formation from the gas phase ozonolysis of limonene and the influence of particle acidity were investigated in a series of indoor aerosol chamber experiments. The particle acidity was adjusted in three stages using Na 2SO 4 (neutral), (NH 4) 2SO 4/H 2SO 4 (acidic) and H 2SO 4 (strongly acidic) seed particles. Detected low molecular weight carboxylic acids ( M w < 300 ) were found to account only for a smaller fraction of produced SOA mass at all particle acidities. Although SOA components with molecular weights between 300 and 900 were detected regardless of the seed particle acidity, the intensities of these compounds were much higher for acidic seed particle experiments. Compounds with mass to charge ratios ( m / z ) 281, 465 and 481 were only detected in the presence of sulfuric acid. MS n and TOFMS analyses showed a strong fragment of m / z 97 and elemental compositions of C 10 H 17 O 7 S - , C 20 H 33 O 10 S - and C 20 H 33 O 11 S - suggesting organic sulfate structures, possibly formed by a heterogeneous acid catalyzed reaction of limonene oxidation products and sulfuric acid in the particle phase. The high intensities of organic sulfate esters suggest that these compounds contribute at least as much as first and second generation oxidation products to the SOA mass.

Efflorescence Transitions of Ammonium Sulfate Particles Coated with Secondary Organic Aerosol

Ammonium sulfate particles were generated by atomization and introduced into a smog chamber where they were coated with secondary organic aerosol from ozonolysis of limonene or alpha-pinene. These mixed particles were then sampled with a humidified Tandem-DMA system where a monodisperse aerosol population was selected, humidified, and dried to observe the relative humidity (RH) at which the particles returned to the original dry diameter. The volume fraction of secondary organic aerosol (SOA) in the mixed particles ranged from 0.59 to 0.94 for limonene SOA and 0.54 to 0.72 for alpha-pinene SOA. Efflorescence RHs for our mixed aerosols were in the range of 28-34%, similar to our observation of 32% ERH for pure ammonium sulfate nanoparticles. These findings indicate that the effect of SOA on the ERH of inorganic salts in the atmosphere may be negligible.

Secondary organic aerosol from limona ketone: insights into terpene ozonolysis via synthesis of key intermediates

Limona ketone was synthesized to explore the secondary organic aerosol (SOA) formation mechanism from limonene ozonolysis and also to test group-additivity concepts describing the volatility distribution of ozonolysis products from similar precursors. Limona ketone SOA production is indistinguishable from alpha-pinene, confirming the expected similarity. However, limona ketone SOA production is significantly less intense than limonene SOA production. The very low vapor pressure of limonene ozonolysis products is consistent with full oxidation of both double bonds in limonene and furthermore with production of products other than ketones after oxidation of the exo double bond in limonene. Mass-balance constraints confirm that ketone products from exo double-bond ozonolysis have a minimal contribution to the ultimate product yield. These results serve as the foundation for an emerging framework to describe the effect on volatility of successive generations of organic compounds in the atmosphere.

Secondary Organic Aerosol Formation from Limonene Ozonolysis: Homogeneous and Heterogeneous Influences as a Function of NOx

Limonene has a high emission rate both from biogenic sources and from household solvents. Here we examine the limonene + ozone reaction as a source for secondary organic aerosol (SOA). Our data show that limonene has very high potential to form SOA and that NO(x) levels, O(3) levels, and UV radiation all influence SOA formation. High SOA formation is observed under conditions where both double bonds in limonene are oxidized, but those conditions depend strongly on NO(x). At low NO(x), heterogeneous oxidation of the terminal double bond follows the initial limonene ozonolysis (at the endocyclic double bond) almost immediately, making the initial reaction rate limiting. This requires a high uptake coefficient between ozone and the first-generation, unsaturated organic particles. However, at high NO(x), this heterogeneous processing is inhibited and gas-phase oxidation of the terminal double bond dominates. Although this chemistry is slower, it also yields products with low volatility. UV light suppresses production of the lowest volatility products, as we have shown in earlier studies of the alpha-pinene + ozone reaction.

Impact of Humidity on the Ozone Initiated Oxidation of Limonene, Δ3-Carene, and α-Pinene

The effect of water on the initial secondary organic aerosol (SOA) formation from gas-phase ozonolysis of limonene, delta3-carene, and alpha-pinene (-3 and approximately 1.5 x 10(11) molecule cm(-3) reacted) has been investigated in a flow reactor at controlled relative humidity (RH), temperature (298 +/- 0.4 K), and reaction time (270 +/- 2 s). Low amounts of terpene converted minimize the impact of secondary reactions. A comparison of the SOA formation from the three terpenes was made for initial rate of reactions being around 7.5 x 10(8) and 15 x 10(8) molecule cm(-3) s(-1). The most efficient species in producing SOA was limonene, while alpha-pinene was the least efficient. The results showed that an enhancement in water vapor concentration (<2-85% RH) caused an increase in both integrated mass (M10-300nm) and total number (N10-300nm). The effect on number and mass were a factor of 2-3 and 4-8, respectively. Physical water up-take can partly explain the increase in mass, but not the observed increase in number. Therefore it was concluded that the increase in water concentration must, by a gas-phase reaction, produce more low volatility product(s).

A study of the gas-phase ozonolysis of terpenes: the impact of radicals formed during the reaction

The gas-phase ozonolysis of α-pinene, Δ 3-carene and limonene was investigated at ppb levels and the impact of the ozone, relative air humidity (RH), and time was studied using experimental design. The amounts of terpene reacted varied in the different settings and were as high as 8.1% for α-pinene, 10.9% for Δ 3-carene and 23.4% for limonene. The designs were able to describe almost all the variation in the experimental data and were also successful in predicting omitted values. The results described the effects of time and ozone and also showed that RH did not have a statistically significant effect on the ozonolysis. The results also showed that all three terpenes were affected by an additional oxidation of OH radicals and/or other reactive species. The results from the designs states that this additional oxidation was responsible for 40% of the total amount of α-pinene reacted, 33% of the total amount of Δ 3-carene reacted and 41% of the total amount of limonene reacted at the settings 20 ppb terpene, 75 ppb ozone, 20% RH and a reaction time of 213 s. Additional experiments with 2-butanol as OH radical scavenger showed that the reaction with OH radicals was responsible for 37% of the total α-pinene reacted and 39% of the total Δ 3-carene reacted at the same settings. The scavenger experiments also showed that there were no significant amounts of OH radicals formed during the ozonolysis of limonene. The results from the designs were also compared to a mathematical model in order to evaluate further the data.

Formation of new particles in the gas-phase ozonolysis of monoterpenes

The formation of organic acids and secondary organic aerosol in the gas-phase ozonolysis was investigated by laboratory experiments at 295±2 K in the absence of seed aerosol for a series of monoterpenes ( β-pinene, sabinene, α-pinene, Δ 3-carene, limonene, terpinolene) and methylene- cyclo-hexane and methyl- cyclo-hexene as model compounds. In the filter samples of the aerosol produced by ozonolysis series of organic acids were identified as methyl ester using GC/MS. In the ozonolysis of β-pinene, sabinene, α-pinene, Δ 3-carene and limonene the corresponding C 9-dicarboxylic acids were found as main products of the organic acid fraction. In case of terpinolene, methylene- cyclo-hexane and methyl- cyclo-hexene C 7- and C 6-dicarboxylic acids, respectively, were detected. The yields of these dicarboxylic acids were determined to range between 1 and 5 mol% using ion chromatography. Particle formation was observed with a 10 nm condensation nuclei counter after the consumption of (6.1±0.3)×10 10 molecule cm −3 of β-pinene, sabinene, α-pinene, Δ 3-carene and limonene, respectively. In case of terpinolene, methylene- cyclo-hexane and methyl- cyclo-hexene (1.8±0.1)×10 11 molecule cm −3 of the reactants were converted. Upper limits for the partial vapor pressures of the dicarboxylic acids in the aerosol were determined to be (5.6±4.0)×10 −8 Torr for the C 9-dicarboxylic acids and (1.7±1.2)×10 −7 Torr for the C 7- and C 6-dicarboxylic acids. The formation of secondary organic aerosol by ozonolysis of terpenes under suitable atmospheric conditions has most likely to be taken into account.