Formation of oligomers in secondary organic aerosol

Particle-phase chemistry of secondary organic material: modeled compared to measured O:C and H:C elemental ratios provide constraints

Chemical mechanisms for the production of secondary organic material (SOM) are developed in focused laboratory studies but widely used in the complex modeling context of the atmosphere. Given this extrapolation, a stringent testing of the mechanisms is important. In addition to particle mass yield as a typical standard for model-measurement comparison, particle composition expressed as O:C and H:C elemental ratios can serve as a higher dimensional constraint. A paradigm for doing so is developed herein for SOM production from a C(5)-C(10)-C(15) terpene sequence, namely isoprene, α-pinene, and β-caryopyhllene. The model MCM-SIMPOL is introduced based on the Master Chemical Mechanism (MCM v3.2) and a group contribution method for vapor pressures (SIMPOL). The O:C and H:C ratios of the SOM are measured using an Aerosol Mass Spectrometer (AMS). Detailed SOM-specific AMS calibrations for the organic contribution to the H(2)O(+) and CO(+) ions indicate that published O:C and H:C ratios for SOM are systematically too low. Overall, the measurement-model gap was small for particle mass yield but significant for particle-average elemental composition. The implication is that a key chemical pathway is missing from the chemical mechanism. The data can be explained by the particle-phase homolytic decomposition of organic hydroperoxides and subsequent alkyl-radical-promoted oligomerization.

Higher-order mass defect analysis for mass spectra of complex organic mixtures

Higher-order mass defect analysis is introduced as a unique formula assignment and visualization method for the analysis of complex mass spectra. This approach is an extension of the concepts of Kendrick mass transformation widely used for identification of homologous compounds differing only by a number of base units (e.g., CH(2), H(2), O, CH(2)O, etc.) in complex mixtures. We present an iterative renormalization routine for defining higher-order homologous series and multidimensional clustering of mass spectral features. This approach greatly simplifies visualization of complex mass spectra and increases the number of chemical formulas that can be confidently assigned for given mass accuracy. The potential for using higher-order mass defects for data reduction and visualization is shown. Higher-order mass defect analysis is described and demonstrated through third-order analysis of a deisotoped high-resolution mass spectrum of crude oil containing nearly 13,000 peaks.

Formation of nitrogen-containing oligomers by methylglyoxal and amines in simulated evaporating cloud droplets

Reactions of methylglyoxal with amino acids, methylamine, and ammonium sulfate can take place in aqueous aerosol and evaporating cloud droplets. These processes are simulated by drying droplets and bulk solutions of these compounds (at low millimolar and 1 M concentrations, respectively) and analyzing the residuals by scanning mobility particle sizing, nuclear magnetic resonance, aerosol mass spectrometry (AMS), and electrospray ionization MS. The results are consistent with imine (but not diimine) formation on a time scale of seconds, followed by the formation of nitrogen-containing oligomers, methylimidazole, and dimethylimidazole products on a time scale of minutes to hours. Measured elemental ratios are consistent with imidazoles and oligomers being major reaction products, while effective aerosol densities suggest extensive reactions take place within minutes. These reactions may be a source of the light-absorbing, nitrogen-containing oligomers observed in urban and biomass-burning aerosol particles.

Molecular characterization of organic aerosols using nanospray-desorption/electrospray ionization-mass spectrometry

Nanospray desorption electrospray ionization (nano-DESI) combined with high-resolution mass spectrometry (HR-MS) is a promising approach for the detailed, molecular-level chemical characterization of atmospheric organic aerosols (OA) collected in laboratory and field experiments. The nano-DESI technique possesses distinct advantages of technical simplicity, enhanced sensitivity, and signal stability. In nano-DESI, analyte is desorbed into a solvent bridge formed between two capillaries and the analysis surface, which enables fast and efficient characterization of OA collected on substrates without sample preparation. Stable signals achieved using nano-DESI make it possible to obtain high-quality HR-MS data both for laboratory-generated and field-collected OA using only a small amount of material (<10 ng). Furthermore, nano-DESI enables the efficient detection of chemically labile compounds in OA, which is important for understanding chemical aging phenomena.

Molecular composition of monoterpene secondary organic aerosol at low mass loading

The molecular composition of secondary organic aerosol (SOA) from the ozonolysis of monoterpenes (α-pinene and β-pinene) was studied by liquid chromatography mass spectrometry and high-resolution Fourier transform ion cyclotron resonance mass spectrometry techniques, both employing electrospray ionization (ESI). SOA particles were generated in a flow tube reactor with a reaction time of 23 s. A microsampling assembly in combination with ESI-FTICR analysis permitted SOA with a mass loading as low as 3.5 μg/m(3) to be characterized with high accuracy and precision mass analysis. Hundreds of product molecular formulas were identified that were common to all mass loadings; however the relative intensities changed significantly. In particular, a species with the (neutral molecule) formula C(17)H(26)O(8) increased substantially in intensity relative to other products as the mass loading decreased. Tandem mass spectrometry (MS(n)) of this species showed it to be a dimer of C(9)H(14)O(4) and C(8)H(12)O(4), most likely pinic acid and terpenylic acid, respectively. LCMS analysis showed different elution times for the dimer and monomer species, confirming that the dimer was not an artifact of ESI analysis. The particle number concentration increased linearly with ozone concentration (the limiting reactant in the experiment), arguing against gas phase dimerization as the rate limiting step in particle formation.

Evidence for high molecular weight nitrogen-containing organic salts in urban aerosols

High molecular weight (M(w)) species were observed at substantial intensities in the positive-ion mass spectra in urban Shanghai aerosols collected from a single-particle time-of-flight mass spectrometer (in the m/z range 250-500) during three separate periods over 2007-2009. These species correlate well with the CN(-) mass signal, suggesting that C-N bonds are prevalent and that the observed high-M(w) species are potentially nitrogen-containing organic salts. Anti-correlation with the ambient O(3) concentration suggests that photochemical oxidants are not involved directly in the formation of these species. The Mannich reaction, among amines (or ammonia), formaldehyde, and carbonyls with an adjacent, acidic proton, is proposed as a plausible pathway leading to these organic salts. Although the high-M(w) species observed in the single-particle mass spectra appear to be nitrogen-containing organics, further chemical confirmation is desired to verify if the proposed Mannich reaction can explain the formation of these high-M(w) species in regions where ammonia, amines, and carbonyls are prevalent.

The carbon versus mass diagram to visualize and exploit FTICR-MS data of natural organic matter

A two-dimensional diagram is proposed, in which the carbon number of each formula is plotted against its nominal mass, to visualize large sets of molecular formula data that can be derived from data generated by ultrahigh-resolution Fourier transform ion cyclotron resonance-MS. In such a carbon versus mass (CvM) diagram, each formula (C(c)H(h)O(o)) is unambiguously described by c, its (nominal) mass and the parameter i = c + o. Calculations of chemically allowable formulas illustrate that organic molecules occupy only certain spaces in such a diagram. The extension of these spaces increases with molecular mass in x-direction (hydrogenation) and y-direction (oxygenation). The data sets of molecules determined in natural organic matter(NOM) occupy only a certain range of the allowable space. The intensity of the mass spectrometric signals can be included as the third dimension into a CvM diagram. Separate CvM diagrams can be plotted for NOM molecules that include different heteroatoms. The benefits of the CvM diagram are illustrated by application onto data sets of fulvic acids from riverine and marine origin, of secondary organic aerosol, including organosulfates and organonitrates, as well as of ozonation of fulvic acids. The CvM diagram is a useful tool to visualize the elemental regularities in NOM isolates as well as the differences between isolates. It may also be applicable to large sets of molecular formula data generated in other disciplines such as petroleum biogeochemistry or metabolomics.

Online deposition of nano-aerosol for matrix-assisted laser desorption/ionization mass spectrometry

An online nano-aerosol sample deposition method for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is described in which matrix and analyte particles between 50 and 500 nm are aerodynamically focused onto a tight spot, ca. 200 microm in diameter, on the target plate under vacuum. MALDI analysis of the target is performed without additional sample preparation. The method is evaluated with insulin as the analyte and alpha-cyano-4-hydroxycinnamic acid (CHCA) as the matrix. Two preparation modes are compared with conventional dried-droplet deposition: mixture deposition where a single layer is deposited consisting of particles that contain both matrix and analyte, and layered deposition where an underlayer of matrix particles and an overlayer of analyte particles are deposited separately. Desalting is performed by adding ammonium sulfate to the solution used to generate the matrix aerosol. With mixture deposition, the optimum matrix-to-analyte mole ratio is about 500:1 compared with 5000:1 for the conventional dried-droplet method. With layered deposition, the thicknesses of the matrix and analyte layers are more important determinants of the analyte signal intensity than the matrix-to-analyte mole ratio. Analyte signal intensities are independent of matrix layer thickness above 200 nm, and the optimum analyte signal is obtained with an analyte layer thickness of about 100 nm.

Composition domains in monoterpene secondary organic aerosol

The composition and structure of freshly formed oligomers in alpha- and beta- pinene SOA are studied with high performance mass spectrometry to provide insight into the SOA formation mechanism. Van Krevelen plots (H:C ratio vs O:C ratio) are interpreted in the context of distinct structural domains that correspond to separate oligomer formation routes. The domain containing most of the signal intensity encompasses elemental formulas that correspond to oligomerization reactions of intermediates and/or stable molecule monomers produced by ozonolysis of the precursor. While oligomers involving reactive intermediates from the hydroperoxide channel dominate the product distribution, products are also observed that uniquely map to the stable Criegee intermediate and/ or combinations of stable molecule monomers. A second domain encompasses molecules having lower H:C ratios but similar O:C ratios to the first domain. Many of the products observed in this domain have double bond equivalents greater than the maximum number possible when forming dimers by standard reaction mechanisms and are interpreted in the context of repeated self-reactions of alkoxy/peroxy radicals. A third domain encompasses molecules having very high H:C and O:C ratios consistent with polymerization of formaldehyde and/or acetaldehyde. These domains remain distinguishable from experiment to experiment and among different extraction solvents (50/50 methanol-water, 50/50 acetonitrile-water,100% water).

Influence of aerosol acidity on the formation of secondary organic aerosol from biogenic precursor hydrocarbons

Secondary organic carbon (SOC) concentrations in steady-state aerosol were measured in a series of alpha-pinene/NOx and one series of beta-caryophyllene/NOx irradiation experiments. The acidity of the inorganic seed aerosol was varied while the hydrocarbon and NOx concentrations were held constant in each series of experiments. Measurements were made for acidity levels and SOC concentrations much closer to ambient levels than had been previously achieved for alpha-pinene, while there are no previous measurements for SOC increases due to acidity for beta-caryophyllene. The observed enhancement in SOC concentration linearly increases with the measured hydrogen ion concentration in air for each system. For the conditions of these studies, SOC increased by 0.04% per nmol H+ m(-3) for alpha-pinene under two conditions where the organic carbon concentration differed by a factor of 5. For alpha-pinene, this level of response to acidic aerosol was a factor of 8 lower than was reported by Surratt et al. for similar series of experiments for SOC from the photooxidation of isoprene/NOx mixtures. By contrast, SOC from beta-caryophyllene showed an increase of 0.22% per nmol H+ m(-3), roughly two-thirds of the response in the isoprene system. Mass fractions for SOC particle-phase tracers for alpha-pinene decreased slightly with increasing aerosol acidity, although remaining within previously stated uncertainties. Below 200 nmol H+ m(-3), the mass fraction of beta-caryophyllenic acid, the only identified tracer for beta-caryophyllene SOC, was constant although beta-caryophyllenic acid showed a substantial decrease for acidities greater than 400 nmol H+ m(-3).

Identification of organic hydroperoxides and hydroperoxy acids in secondary organic aerosol formed during the ozonolysis of different monoterpenes and sesquiterpenes by on-line analysis using atmospheric pressure chemical ionization ion trap mass spectrometry

On-line ion trap mass spectrometry (ITMS) enables the real-time characterization of reaction products of secondary organic aerosol (SOA). The analysis was conducted by directly introducing the aerosol particles into the ion source. Positive-ion chemical ionization at atmospheric pressure (APCI(+)) ITMS was used for the characterization of constituents of biogenic SOA produced in reaction-chamber experiments. APCI in the positive-ion mode usually enables the detection of [M+H](+) ions of the individual SOA components. In this paper the identification of organic peroxides from biogenic volatile organic compounds (VOCs) by on-line APCI-ITMS is presented. Organic peroxides containing a hydroperoxy group, generated by gas-phase ozonolysis of monoterpenes (alpha-pinene and beta-pinene) and sesquiterpenes (alpha-cedrene and alpha-copaene), could be detected via on-line APCI(+)-MS/MS experiments. A characteristic neutral loss of 34 Da (hydrogen peroxide, H(2)O(2)) in the on-line MS/MS spectra is a clear indication for the existence of an organic peroxide, containing a hydroperoxy functional group.

Molecular characterization of nitrogen-containing organic compounds in biomass burning aerosols using high-resolution mass spectrometry

Although nitrogen-containing organic compounds (NOC) are important components of atmospheric aerosols, little is known about their chemical composition. Here we present detailed characterization of the NOC constituents of biomass burning aerosol (BBA) samples using high-resolution electrospray ionization mass spectrometry (ESI/MS). Accurate mass measurements combined with MS/MS fragmentation experiments of selected ions were used to assign molecular structures to individual NOC species. Our results indicate that N-heterocyclic alkaloid compounds (species naturally produced by plants and living organisms) comprise a substantial fraction of NOC in BBA samples collected from test burns of five biomass fuels. High abundance of alkaloids in test burns of ponderosa pine (a widespread tree in the western U.S. areas frequently affected by large scale fires) suggests that N-heterocyclic alkaloids in BBA may play a significant role in dry and wet deposition of fixed nitrogen in this region.

Characterization of oligomeric compounds in secondary organic aerosol using liquid chromatography coupled to electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

The components of secondary organic aerosols (SOAs) generated from the gas-phase ozonolysis of two C(10)H(16)-terpenes (alpha-pinene; sabinene) and a cyclic C(6)H(10) alkene (cyclohexene) were characterized by the use of a Fourier transform ion cyclotron mass spectrometer equipped with an electrospray ionization source operated in the negative ion mode. Reversed-phase high-performance liquid chromatography was used to achieve chromatographic separation of highly oxidized organic compounds. In addition to the well-known group of low molecular weight oxidation products (monomers; e.g. dicarboxylic acids), higher molecular weight compounds (dimers) were also detected and their exact elemental compositions were determined. In order to provide additional information for the structural elucidation of these compounds, collision-induced dissociation was applied. Based on the MS/MS spectra, two higher molecular weight products are proposed to be an ester and a peroxide. Molecular formulae calculated from the exact masses show that the SOA-compounds are heavily oxidized and this information creates the background to a discussion of potential reaction pathways for the formation of higher molecular weight compounds.

Assessing the potential for diol and hydroxy sulfate ester formation from the reaction of epoxides in tropospheric aerosols

Polyols and sulfate esters have recently been identified in the secondary organic aerosol (SOA) formed in the photooxidation of biogenic hydrocarbons both in the laboratory and under ambient atmospheric conditions. In the present study, the potential role of the reactions of epoxides in SOA to form diols and hydroxy sulfate esters is explored. Nuclear magnetic resonance methods were used to monitor the bulk reaction kinetics of the epoxide hydrolysis reactions for a number of simple epoxides. The experiments were carried out at various acid concentrations in order to confirm the acid-catalysis rate order and to determine the second-order rate constants for such reactions in aerosols under the previously studied laboratory conditions and under ambient atmospheric conditions. The measured rate constants depended systematically on the carbon substitution nature of the epoxide ring, with the tertiary epoxides characterized by the largest rate constants. The hydroxy sulfate yield was observed to depend linearly on the total sulfate concentration, with yields as high as 30% observed at high sulfate concentrations. Due to the large values of the observed rate constants, these reactions are expected to be efficient even for mostly neutralized tropospheric SOA, let alone the much more acidic SOA particles previously studied in laboratory experiments. Therefore, the epoxide hydrolysis mechanism appears to be a kinetically feasible route to the formation of the diols and hydroxy sulfate esters observed in the SOA resulting from the photooxidation of biogenic hydrocarbons.

Secondary organic aerosol from sesquiterpene and monoterpene emissions in the United States

Emissions of volatile organic compounds (VOC) from vegetation are believed to be a major source of secondary organic aerosol (SOA), which in turn comprises a large fraction of fine particulate matter in many areas. Sesquiterpenes are a class of biogenic VOC with high chemical reactivity and SOA yields. Sesquiterpenes have only recently been quantified in emissions from a wide variety of plants. In this study, a new sesquiterpene emission inventory is used to provide input to the Models-3 Community Multiscale Air Quality (CMAQ) model. CMAQ is used to estimate the contribution of sesquiterpenes and monoterpenes to SOA concentrations over the contiguous United States. The gas-particle partitioning module of CMAQ was modified to include condensable products of sesquiterpene oxidation and to update values of the enthalpy of vaporization. The resulting model predicts July monthly average surface concentrations of total SOA in the eastern U.S. ranging from about 0.2-0.8 microg m(-3). This is roughly double the amount of SOA produced in this region when sesquiterpenes are not included. Even with sesquiterpenes included, however, the model significantly underpredicts surface concentrations of particle-phase organic matter compared to observed values. Treating all SOA as capable of undergoing polymerization increases predicted monthly average surface concentrations in July to 0.4-1.2 microg m(-3), in closer agreement with observations. Using the original enthalpy of vaporization value in CMAQ in place of the values estimated from the recent literature results in predicted SOA concentrations of about 0.3-1.3 microg m(-3).

The effect of solvent on the analysis of secondary organic aerosol using electrospray ionization mass spectrometry

This study examined the effect of solvent on the analysis of organic aerosol extracts using electrospray ionization mass spectrometry (ESI-MS). Secondary organic aerosol (SOA) produced by ozonation of d-limonene, as well as several organic molecules with functional groups typical for OA constituents, were extracted in methanol, d3-methanol, acetonitrile, and d3-acetonitrile to investigate the extent and relative rates of reactions between analyte and solvent. High resolution ESI-MS showed that reactions of carbonyls with methanol produce significant amounts of hemiacetals and acetals on time scales ranging from several minutes to several days, with the reaction rates increasing in acidified solutions. Carboxylic acid groups were observed to react with methanol resulting in the formation of esters. In contrast acetonitrile extracts showed no evidence of reactions with analyte molecules, suggesting that acetonitrile is the preferred solvent for SOA extraction. The use of solvent-analyte reactivity as a tool for the improved characterization of functional groups in complex organic mixtures was demonstrated. Direct comparison between mass spectra of the same SOA samples extracted in methanol versus acetonitrile was used to estimate the lower limits for the relative fractions of carbonyls (> or = 42%) and carboxylic acids (> or = 55%) in d-limonene SOA.

NMR determination of total carbonyls and carboxyls: a tool for tracing the evolution of atmospheric oxidized organic aerosols

Nuclear magnetic resonance (NMR) spectroscopy is used to investigate the chemical composition of organic aerosol in terms of functional group distribution with a special focus on secondary organic aerosol (SOA) formation. The knowledge of the functional group composition is a benchmark for understanding how SOA components partition into the particulate phase and undergo chemical transformation. The paper presents a new chemical derivatization procedure coupled to proton NMR (1H NMR) analysis for the specific determination of total carbonylic groups in atmospheric aerosol samples, which couples with the procedure for determination of total carboxylic acid groups described in a previous work. A first deployment of the combined techniques for the analysis of PM10 samples collected in the Po Valley in the warm season shows that the concentration in the particulate phase of total carbonyls varies and covaries with respect to those of carboxylic acids and of less-oxidized functional groups. The proposed methodology provides the breakdown of the oxygenated fraction of the organic aerosol into major functional groups through well-established chemical methods and can be used to benchmark the more sensitive and widely used aerosol mass spectrometric techniques.

Characterisation of solvent extractable organic constituents in atmospheric particulate matter: an overview

In spite of accounting for 10-70% of the atmospheric aerosol mass, particulate-phase organic compounds are not well characterised, and many aspects of aerosol formation and evolution are still unknown. The growing awareness of the impact of particulate aerosols on climate, and the incompletely recognised but serious effects of anthropogenic constituents on air quality and human health, have conducted to several scientific studies. These investigations have provided information about the behaviour of atmospheric particulate matter and the description of the character of its carbonaceous content. The compilation of such results is important as they append to the emergent global-wide dataset of the organic composition of atmospheric aerosols. The contribution of the major emission sources to regional particulate pollution can be diagnosed by using specific molecular markers. This overview is mainly focused on results obtained with gas chromatography coupled with mass spectrometry, since it is the analytical method of choice in elucidating the solvent-extractable organic compounds in atmospheric particulate matter. A synopsis of the selection of organic tracers and the application of geochemical parameters to the analysis of organic constituents as a tool for source apportionment is shown here. Besides the assessment of current knowledge, this paper also presents the identification of further areas of concern.

Characterization of polar compounds and oligomers in secondary organic aerosol using liquid chromatography coupled to mass spectrometry

A generic method has been developed for the analysis of polar compounds and oligomers in secondary organic aerosol (SOA) formed during atmospheric simulation chamber experiments. The technique has been successfully applied to SOA formed in a variety of systems, ranging from ozonolysis of biogenic volatile organic compounds to aromatic photooxidation. An example application of the method is described for the SOA produced from the reaction of ozone with cis-3-hexenyl acetate, an important biogenic precursor. A range of solvents were tested as extraction media, and water was found to yield the highest recovery. Extracts were analyzed using reversed-phase liquid chromatography coupled to ion trap mass spectrometry. In order to determine correct molecular weight assignments and increase sensitivity for less polar species, a series of low-concentration mobile-phase additives were used (NaCl, LiBr, NH4OH). Lithium bromide produced better fragmentation patterns, with more structural information than in the other cases with no reduction in sensitivity. The main reaction products identified in the particle-phase were 3-acetoxypropanal, 3-acetoxypropanoic acid, and 3-acetoxypropane peroxoic acid and a series of dimers and trimers up to 500 Da. Structural identification of oligomers indicates the presence of linear polyesters possibly formed via esterfication reactions or decomposition of peroxyhemiacetals.

Evidence for the existence of organosulfates from beta-pinene ozonolysis in ambient secondary organic aerosol

The formation of organosulfates from the gas-phase ozonolysis of beta-pinene in the presence of neutral or acidic sulfate particles was investigated in a series of indoor aerosol chamber experiments. The organosulfates were analyzed using high-performance liquid chromatography (LC) coupled to electrospray ionization-time-of-flight mass spectrometry (MS) in parallel to ion trap MS. Organosulfates were only found in secondary organic aerosol from beta-pinene ozonolysis in the presence of acidic sulfate seed particles. One of the detected organosulfates also occurred in ambient aerosol samples that were collected at a forest site in northeastern Bavaria, Germany. beta-Pinene oxide, an oxidation product in beta-pinene/O3 and beta-pinene/NO3 reactions, is identified as a possible precursor for the beta-pinene-derived organosulfate. Furthermore, several nitroxy-organosulfates originating from monoterpenes were found in the ambient samples. These nitroxy-organosulfates were only detected in the nighttime samples, suggesting a role for nighttime chemistry in their formation. Their LC/MS chromatographic peak intensities suggest that they represent an important fraction of the organic mass in ambient aerosols, especially at night.

A denuder-filter sampling technique for the detection of gas and particle phase carbonyl compounds

A denuder-filter sampling technique for the simultaneous collection of gas and particle phase carbonyl compounds has been developed and tested. The denuder was coated with XAD-4 resin and the derivatizing agent O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) to enable on-tube conversion of gas-phase carbonyls to their oxime derivatives which were extracted and identified by GC-MS. The performance of the PFBHA-coated denuder was tested on a range of carbonyls, dicarbonyls, aromatic aldehydes, carbonyl-containing furans, and benzoquinones. The collection efficiency of the PFBHA-coated denuder was over 90% for sampling times of 10 min and significantly higher than when using only XAD-4 as the sorbent. The collection efficiency and rate of on-tube derivatization was highest for aldehydes and lowest for ketones, consistent with the expected reactivity of carbonyls with PFBHA. The method was used to separate the gas and particle phase carbonyl products formed during the photooxidation of toluene in order to assess its potential for application to simulation chamber experiments of VOC oxidation. The results indicate that when compared to a conventional filter sampling setup, the PFBHA-coated denuder-filter reduced the extent of gas phase adsorption onto the filter by at least a factor of 2 for each of the carbonyls formed. Further potential applications of the denuder-filter sampling method are discussed.

Oligomers in the early stage of biogenic secondary organic aerosol formation and growth

The formation of secondary organic aerosol (SOA) by reaction of ozone with monoterpenes (beta-pinene, delta3-carene, limonene, and sabinene) was studied on a short time scale of 3-22 s with a flow tube reactor. Online chemical analysis was performed with the Photoionization Aerosol Mass Spectrometer (PIAMS) to obtain molecular composition and the Nanoaerosol Mass Spectrometer (NAMS) to obtain elemental composition. Molecular composition data showed that dimers and higher order oligomers are formed within seconds after the onset of reaction, indicating that there is no intrinsic kinetic barrier to oligomer formation. Because oligomer formation is fast, it is unlikely that a large number of steps are involved in their formation. Therefore, ion distributions in the PIAMS spectra were interpreted through reactions of intermediates postulated in previous studies with monomer end products or other intermediates. Based on ion signal intensities in the mass spectra, organic peroxides appear to comprise a greater fraction of the aerosol than secondary ozonides. This conclusion is supported by elemental composition data from NAMS that gave C:O ratios in the 2.2-2.7 range.

Chemical composition of secondary organic aerosol formed from the photooxidation of isoprene

Recent work in our laboratory has shown that the photooxidation of isoprene (2-methyl-1,3-butadiene, C(5)H(8)) leads to the formation of secondary organic aerosol (SOA). In the current study, the chemical composition of SOA from the photooxidation of isoprene over the full range of NO(x) conditions is investigated through a series of controlled laboratory chamber experiments. SOA composition is studied using a wide range of experimental techniques: electrospray ionization-mass spectrometry, matrix-assisted laser desorption ionization-mass spectrometry, high-resolution mass spectrometry, online aerosol mass spectrometry, gas chromatography/mass spectrometry, and an iodometric-spectroscopic method. Oligomerization was observed to be an important SOA formation pathway in all cases; however, the nature of the oligomers depends strongly on the NO(x) level, with acidic products formed under high-NO(x) conditions only. We present, to our knowledge, the first evidence of particle-phase esterification reactions in SOA, where the further oxidation of the isoprene oxidation product methacrolein under high-NO(x) conditions produces polyesters involving 2-methylglyceric acid as a key monomeric unit. These oligomers comprise approximately 22-34% of the high-NO(x) SOA mass. Under low-NO(x) conditions, organic peroxides contribute significantly to the low-NO(x) SOA mass (approximately 61% when SOA forms by nucleation and approximately 25-30% in the presence of seed particles). The contribution of organic peroxides in the SOA decreases with time, indicating photochemical aging. Hemiacetal dimers are found to form from C(5) alkene triols and 2-methyltetrols under low-NO(x) conditions; these compounds are also found in aerosol collected from the Amazonian rainforest, demonstrating the atmospheric relevance of these low-NO(x) chamber experiments.