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

Recent advances in mass spectrometry techniques for atmospheric chemistry research on molecular-level

The Earth's atmosphere is composed of an enormous variety of chemical species associated with trace gases and aerosol particles whose composition and chemistry have critical impacts on the Earth's climate, air quality, and human health. Mass spectrometry analysis as a powerful and popular analytical technique has been widely developed and applied in atmospheric chemistry for decades. Mass spectrometry allows for effective detection, identification, and quantification of a broad range of organic and inorganic chemical species with high sensitivity and resolution. In this review, we summarize recently developed mass spectrometry techniques, methods, and applications in atmospheric chemistry research in the past several years on molecular-level. Specifically, new developments of ion-molecule reactors, various soft ionization methods, and unique coupling with separation techniques are highlighted. The new mass spectrometry applications in laboratory studies and field measurements focused on improving the detection limits for traditional and emerging volatile organic compounds, characterizing multiphase highly oxygenated molecules, and monitoring particle bulk and surface compositions.

Prolonged Dark Chemical Processes in Secondary Organic Aerosols on Filters and in Aqueous Solution

Secondary organic aerosol (SOA) represents a large fraction of atmospheric aerosol particles that significantly affect both the Earth's climate and human health. Laboratory-generated SOA or ambient particles are routinely collected on filters for a detailed chemical analysis. Such filter sampling is prone to artifactual changes in composition during collection, storage, sample workup, and analysis. In this study, we investigate the chemical composition differences in SOA generated in the laboratory, kept at room temperature as aqueous extracts or on filters, and analyzed in detail after a storage time of a day and up to 4 weeks using liquid chromatography coupled to high-resolution mass spectrometry. We observe significantly different temporal concentration changes for monomers and oligomers in both extracts and on filters. In SOA aqueous extracts, many monomers increase in concentration over time, while many dimers decay at the same time. In contrast, on filters, we observe a strong and persistent concentration increase of many dimers and a decrease of many monomers. This study highlights artifacts arising from SOA chemistry occurring during storage, which should be considered when detailed organic aerosol compositions are studied. The particle-phase reactions on filters can also serve as a model system for atmospheric particle aging processes.

Enhanced Light Absorption and Elevated Viscosity of Atmospheric Brown Carbon through Evaporation of Volatile Components

Samples of brown carbon (BrC) material were collected from smoke emissions originating from wood pyrolysis experiments, serving as a proxy for BrC representative of biomass burning emissions. The acquired samples, referred to as "pyrolysis oil (PO1)," underwent subsequent processing by thermal evaporation of their volatile compounds, resulting in a set of three additional samples with volume reduction factors of 1.33, 2, and 3, denoted as PO1.33, PO2, and PO3. The chemical compositions of these POx samples and their BrC chromophore features were analyzed using a high-performance liquid chromatography instrument coupled with a photodiode array detector and a high-resolution mass spectrometer. The investigation revealed a noteworthy twofold enhancement of BrC light absorption observed for the progression of PO1 to PO3 samples, assessed across the spectral range of 300-500 nm. Concurrently, a decrease in the absorption Ångstrom exponent (AAE) from 11 to 7 was observed, indicating a weaker spectral dependence. The relative enhancement of BrC absorption at longer wavelengths was more significant, as exemplified by the increased mass absorption coefficient (MAC) measured at 405 nm from 0.1 to 0.5 m2/g. Molecular characterization further supports this darkening trend, manifesting as a depletion of small oxygenated, less absorbing monoaromatic compounds and the retention of relatively large, less polar, more absorbing constituents. Noteworthy alterations of the PO1 to PO3 mixtures included a reduction in the saturation vapor pressure of their components and an increase in viscosity. These changes were quantified by the mean values shifting from approximately 1.8 × 103 μg/m3 to 2.3 μg/m3 and from ∼103 Pa·s to ∼106 Pa·s, respectively. These results provide quantitative insights into the extent of BrC aerosol darkening during atmospheric aging through nonreactive evaporation. This new understanding will inform the refinement of atmospheric and chemical transport models.

Impact of ozonation on the formation of particulate nitrosodi-methylamine (NDMA) in atmosphere

The contribution of ozonation to the formation of particulate nitrosodi-methylamine (NDMA) in the aqueous aerosol phase was investigated using measurement data from 2018 in Seoul, Republic of Korea and a box model. The correlation between the NDMA concentration and aerosol liquid water content and box model results showed that aqueous aerosol phase reactions, including nitrosation and ozonation, might contribute to the formation of NDMA. The concentration of NDMA and the ratio of O3/dimethylamine exhibited a negative correlation, suggesting that the contribution of ozonation to NDMA formation may not be significant. Furthermore, when the daily concentration of NDMA exceeded 10 ng/m3, the pH was 3.96 ± 0.48, indicating that the impact of ozonation on NDMA concentration might not be significant. To quantitatively investigate the contribution of ozonation, the ozonation mechanism that forms NDMA was included in the box model developed in our previous study. The model results showed that the ozonation contributed to the ambient concentration of NDMA (7.9 ± 3.8% (winter); 1.9 ± 3.0% (spring); 10.0 ± 0.77% (summer); 3.6 ± 7.3% (autumn)). It is estimated that the relatively higher O3/NOx ratio in summer (1.63 ± 0.69; 0.64 ± 0.52 (winter); 1.14 ± 0.92 (spring); 0.52 ± 0.54 (autumn)) could enhance ozonation and that relatively lower pH in summer (2.2 ± 0.4; 5.3 ± 1.2 (winter); 3.9 ± 1.2 (spring); 3.9 ± 0.7 (autumn)) could hinder nitrosation compared to that in other seasons.

Alkylperoxy radicals are responsible for the formation of oxygenated primary organic aerosol

Organic aerosol (OA) is an air pollutant ubiquitous in urban atmospheres. Urban OA is usually apportioned into primary OA (POA), mostly emitted by mobile sources, and secondary OA (SOA), which forms in the atmosphere due to oxidation of gas-phase precursors from anthropogenic and biogenic sources. By performing coordinated measurements in the particle phase and the gas phase, we show that the alkylperoxy radical chemistry that is responsible for low-temperature ignition also leads to the formation of oxygenated POA (OxyPOA). OxyPOA is distinct from POA emitted during high-temperature ignition and is chemically similar to SOA. We present evidence for the prevalence of OxyPOA in emissions of a spark-ignition engine and a next-generation advanced compression-ignition engine, highlighting the importance of understanding OxyPOA for predicting urban air pollution patterns in current and future atmospheres.

Sensitivity Constraints of Extractive Electrospray for a Model System and Secondary Organic Aerosol

The quantification of an aerosol chemical composition is complicated by the uncertainty in the sensitivity of each species detected. Soft-ionization response factors can vary widely from molecule to molecule. Here, we have employed a method to separate molecules by their volatility through systematic evaporation with a thermal denuder (TD). The fraction remaining after evaporation is compared between an extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF) and a scanning mobility particle sizer (SMPS), which provides a comparison between a quantified mass loss by the SMPS and the signal loss in the EESI-TOF. The sensitivity of the EESI-TOF is determined for both a simplified complex mixture (PEG-300) and also for a complex mixture of α-pinene secondary organic aerosol (SOA). For PEG-300, separation is possible on a molecule-by-molecule level with the TD and provides insights into the molecule-dependent sensitivity of the EESI-TOF, showing a higher sensitivity toward the most volatile molecule. For α-pinene SOA, sensitivity determination for specific classes is possible because of the number of molecular formula observed by the EESI-TOF. These classes are separated by their volatility and are broken down into monomers (O3-5,6-7,8+), dimers (O4-7,8+), and higher order oligomers (e.g., trimers and tetramers). Here, we show that the EESI-TOF initially measures 60.1% monomers, 32.7% dimers, and 7.2% trimers and tetramers in α-pinene SOA, but after sensitivity correction, the distribution of SOA is 37.4% monomers, 56.1% dimers, and 6.4% trimers and tetramers. These results provide a path forward for the quantification of aerosol components with the EESI-TOF in other applications and potentially for atmospheric measurements.

Analytical methodologies for oxidized organic compounds in the atmosphere

Oxidized compounds in the atmosphere can occur as emitted primary compounds or as secondary products when volatile emitted precursors react with various oxidants. Due to the presence of polar functional groups, their vapor pressures decrease, and they condense onto small particles. Thereby, they have an effect on climate change by the formation of clouds and scattering solar radiation. The particles and oxidized compounds themselves can cause serious health problems when inhaled. Therefore, it is of utmost importance to study oxidized compounds in the atmosphere. Much ongoing research is focused on the discovery of new oxidized substances and on the evaluation of their sources and factors influencing their formation. Monitoring biogenic and anthropogenic primary oxidized compounds or secondary oxidized products in chamber experiments or field campaigns is common. New discoveries have been reported, including various oxidized compounds and a new group of compounds called highly oxidized organic molecules (HOMs). Analytics of HOMs are mainly focused on chromatography and high-resolution mass spectrometry employing chemical ionization for identifying and quantifying compounds at low concentrations. Oxidized compounds can also be monitored by spectrophotometric methods in which the determinations of total amounts are based on functional groups. This review highlights recent findings on oxidized organic compounds in the atmosphere and analytical methodologies used for their detection and quantification. The discussion includes gas and liquid chromatographic methods, sampling, extraction, concentration, and derivatization procedures involved, as well as mass spectrometric and spectrophotometric methods.

Molecular characterization of atmospheric particulate organosulfates in a port environment using ultrahigh resolution mass spectrometry: Identification of traffic emissions

Organosulfates (OSs) are an important component of atmospheric organic aerosol (OA) and are widespread in various environments. However, the OSs generated from anthropogenic emissions are poorly understood. In this study, the molecular compositions of OSs from atmospheric PM_2.5 samples collected during a winter measurement campaign (SEISO-Bohai) at Jingtang Harbor were characterized via ultrahigh resolution mass spectrometry (UHRMS). The changes of port OS compositions were observed in episodes of complete haze pollution. As the pollution aggravated, the relative abundances of OSs were apparently increased, and the molecule compositions became more complex, primarily driven by the oxidation and fragmentation processes. Potential OS precursors from traffic emissions were identified based on an optimized "OS precursor map" developed in the previous study. OSs characterized by high molecular weights and low degrees of both unsaturation and oxidization were suggested to mainly derive from secondary reactions of intermediate volatile organic compounds (IVOCs) emitted by traffic sources. These OSs were primarily detected in clean-day samples, followed by decreasing with the pollution process. In addition, our study also finds that ship emissions may further facilitated OS productions under haze pollution conditions.

Molecular characterization of ultrafine particles using extractive electrospray time-of-flight mass spectrometry

Aerosol particles negatively affect human health while also having climatic relevance due to, for example, their ability to act as cloud condensation nuclei. Ultrafine particles (diameter D_p < 100 nm) typically comprise the largest fraction of the total number concentration, however, their chemical characterization is difficult because of their low mass. Using an extractive electrospray time-of-flight mass spectrometer (EESI-TOF), we characterize the molecular composition of freshly nucleated particles from naphthalene and β-caryophyllene oxidation products at the CLOUD chamber at CERN. We perform a detailed intercomparison of the organic aerosol chemical composition measured by the EESI-TOF and an iodide adduct chemical ionization mass spectrometer equipped with a filter inlet for gases and aerosols (FIGAERO-I-CIMS). We also use an aerosol growth model based on the condensation of organic vapors to show that the chemical composition measured by the EESI-TOF is consistent with the expected condensed oxidation products. This agreement could be further improved by constraining the EESI-TOF compound-specific sensitivity or considering condensed-phase processes. Our results show that the EESI-TOF can obtain the chemical composition of particles as small as 20 nm in diameter with mass loadings as low as hundreds of ng m⁻³ in real time. This was until now difficult to achieve, as other online instruments are often limited by size cutoffs, ionization/thermal fragmentation and/or semi-continuous sampling. Using real-time simultaneous gas- and particle-phase data, we discuss the condensation of naphthalene oxidation products on a molecular level.

Using real-time simultaneous gas- and particle-phase data, the condensation of naphthalene and β-caryophyllene oxidation products on a molecular level is discussed.

Indoor Illumination of Terpenes and Bleach Emissions Leads to Particle Formation and Growth

Application of chlorine bleach solution (major component sodium hypochlorite, NaOCl) in indoor environments leads to the emission of gaseous hypochlorous acid (HOCl) and chlorine (Cl₂), both of which are strong oxidants. In contrast to the outdoor atmosphere, where mixing ratios of HOCl and Cl₂ tend to be low (10s-100s of ppt), indoor HOCl and Cl₂ can reach high levels during cleaning activities (100s of ppb or higher). HOCl and Cl₂ may react with unsaturated organic compounds on indoor surfaces and in indoor air. In this study, we studied the reaction of limonene, one of the most common indoor volatile organic compounds (VOCs) arising from use of cleaning products, fragrance, and air fresheners, with HOCl and Cl₂ in an environmental chamber. A dark reaction was observed between limonene and HOCl/Cl₂ leading to gas-phase reaction products that were investigated using proton transfer reaction mass spectrometry (PTR-MS). With subsequent exposure to indoor fluorescent lights or diffuse sunlight through a nearby window, a substantial mass loading of secondary particles were formed with an averaged mass yield of 40% relative to the amount of limonene consumed. Aerosol mass spectrometry (AMS) measurements indicate a large contribution of particulate chlorine species. Electrospray ionization mass spectrometry (ESI-MS) analysis of filter-collected particles indicates the formation of high molecular weight products. This is the first study of the oxidation of limonene with HOCl and Cl₂, and it illustrates the potential for particle formation to occur with indoor lighting during the use of common cleaning products.

Bouncier Particles at Night: Biogenic Secondary Organic Aerosol Chemistry and Sulfate Drive Diel Variations in the Aerosol Phase in a Mixed Forest

Aerosol phase state is critical for quantifying aerosol effects on climate and air quality. However, significant challenges remain in our ability to predict and quantify phase state during its evolution in the atmosphere. Herein, we demonstrate that aerosol phase (liquid, semisolid, solid) exhibits a diel cycle in a mixed forest environment, oscillating between a viscous, semisolid phase state at night and liquid phase state with phase separation during the day. The viscous nighttime particles existed despite higher relative humidity and were independently confirmed by bounce factor measurements and atomic force microscopy. High-resolution mass spectrometry shows the more viscous phase state at night is impacted by the formation of terpene-derived and higher molecular weight secondary organic aerosol (SOA) and smaller inorganic sulfate mass fractions. Larger daytime particulate sulfate mass fractions, as well as a predominance of lower molecular weight isoprene-derived SOA, lead to the liquid state of the daytime particles and phase separation after greater uptake of liquid water, despite the lower daytime relative humidity. The observed diel cycle of aerosol phase should provoke rethinking of the SOA atmospheric lifecycle, as it suggests diurnal variability in gas-particle partitioning and mixing time scales, which influence aerosol multiphase chemistry, lifetime, and climate impacts.

New application of direct analysis in real time high-resolution mass spectrometry for the untargeted analysis of fresh and aged secondary organic aerosols generated from monoterpenes

RATIONALE

Secondary organic aerosols (SOAs) represent a significant portion of total atmospheric aerosols. They are generated by the oxidation of volatile organic compounds (VOCs), and particularly biogenic VOCs (BVOCs). The analysis of such samples is usually performed by targeted methods that often require time-consuming preparation steps that can induce loss of compounds and/or sample contaminations.

METHODS

Recently, untargeted methods using high-resolution mass spectrometry (HRMS) have been successfully employed for a broad characterization of chemicals in SOAs. Herein we propose a new application of the direct analysis in real time (DART) ionization method combined with HRMS to quickly detect several hundred chemicals in SOAs collected on a quartz filter without sample preparation or separation techniques.

RESULTS

The reproducibility of measurements was good, with several hundred elemental compositions common to three different replicates. The relative standard deviations of the intensities of the chemical families ranged from 6% to 35%, with sufficient sensitivity to allow the unambiguous detection of 4 ng/mm² of pinic acid. The presence of oligomers and specific tracers was highlighted by MSⁿ (n ≤ 4) experiments, an achievement that is difficult to attain with other ultrahigh-resolution mass spectrometers. Contributions of this untargeted DART-HRMS method were illustrated by the analysis of fresh and aged SOAs from different gaseous precursors such as limonene, a β-pinene/limonene mixture or scots pines emissions.

CONCLUSIONS

The results show that it is possible to use DART-HRMS for the identification of tracers of specific aging reactions, or for the identification of aerosols from specific biogenic precursors.

Photochemistry of Solid Films of the Neonicotinoid Nitenpyram

The environmental fates of nitenpyram (NPM), a widely used neonicotinoid insecticide, are not well-known. A thin solid film of NPM deposited on a germanium attenuated total reflectance (ATR) crystal was exposed to radiation from a low-pressure mercury lamp at 254 nm, or from broadband low pressure mercury photolysis lamps centered at 350 or 313 nm. The loss during photolysis was followed in time using FTIR. The photolysis quantum yields (ϕ), defined as the number of NPM molecules lost per photon absorbed, were determined to be (9.4 ± 1.5) × 10^-4 at 350 nm, (1.0 ± 0.3) × 10^-3 at 313 nm, and (1.2 ± 0.4) × 10^-2 at 254 nm (±2σ). Imines, one with a carbonyl group, were detected as surface-bound products and gaseous N₂O was generated in low (11%) yield. The UV-vis absorption spectra of NPM in water was different from that in acetonitrile, dichloromethane, and methanol, or in a thin solid film. The photolytic lifetime of solid NPM at a solar zenith angle at 35° is calculated to be 36 min, while that for NPM in water is 269 min, assuming that the quantum yield is the same as in the solid. Thus, there may be a significant sensitivity to the medium for photolytic degradation and the lifetime of NPM in the environment.

Water-Soluble Organic Composition of the Arctic Sea Surface Microlayer and Association with Ice Nucleation Ability

Organic matter in the sea surface microlayer (SML) may be transferred to the atmosphere as sea spray and hence influence the composition and properties of marine aerosol. Recent work has demonstrated that the SML contains material capable of heterogeneously nucleating ice, but the nature of this material remains largely unknown. Water-soluble organic matter was extracted from SML and underlying seawater from the Arctic and analyzed using a combination of mass spectrometric approaches. High performance liquid chromatography-ion trap mass spectrometry (LC-IT-MS), and Fourier transform ion cyclotron resonance MS (FT-ICR-MS), showed seawater extracts to be compositionally similar across all stations, whereas microlayer extracts had a different and more variable composition. LC-IT-MS demonstrated the enrichment of particular ions in the microlayer. Ice nucleation ability (defined as the median droplet freezing temperature) appeared to be related to the relative abundances of some ions, although the extracts themselves did not retain this property. Molecular formulas were assigned using LC-quadrupole time-of-flight MS (LC-TOF-MS²) and FT-ICR-MS. The ice nucleation tracer ions were associated with elevated biogenic trace gases, and were also observed in atmospheric aerosol collected during the summer, but not early spring suggesting a biogenic source of ice nuclei in the Arctic microlayer.

Use of an Open Port Sampling Interface Coupled to Electrospray Ionization for the On-Line Analysis of Organic Aerosol Particles

A simple design for an open port sampling interface coupled to electrospray ionization (OPSI-ESI) is presented for the analysis of organic aerosols. The design uses minimal modifications to a Bruker electrospray (ESI) emitter to create a continuous flow, self-aspirating open port sampling interface. Considerations are presented for introducing aerosol to the open port sampling interface including aerosol gas flow and solvent flow rates. The device has been demonstrated for use with an aerosol of nicotine as well as aerosol formed in the pyrolysis of biomass. Upon comparison with extractive electrospray ionization (EESI), this device has similar sensitivity with increased reproducibility by nearly a factor of three. The device has the form factor of a standard Bruker/Agilent ESI emitter and can be used without any further instrument modifications. Graphical Abstract ᅟ.

Probing and Comparing the Photobromination and Photoiodination of Dissolved Organic Matter by Using Ultra-High-Resolution Mass Spectrometry

Photochemical halogenation of dissolved organic matter (DOM) may represent an important abiotic process for the formation of natural organobromine compounds (OBCs) and natural organoiodine compounds (OICs) within surface waters. Here we report the enhanced formation of OBCs and OICs by photohalogenating DOM in freshwater and seawater, as well as the noticeable difference in the distribution and composition pattern of newly formed OBCs and OICs. By using negative ion electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry, various OBCs and OICs were identified during the photohalogenation processes in sunlit waters. The respective number of OBCs and OICs formed in artificial seawater (ASW) under light radiation was higher than that in artificial freshwater (AFW), suggesting a possible role of the mixed reactive halogen species. OBCs were formed mainly via substitution reactions and addition reactions accompanied by other reactions and distributed into three classes: unsaturated hydrocarbons with relatively low oxygen content, unsaturated aliphatic compounds, and saturated fatty acids and carbohydrates with relatively high hydrogen content. Unlike the OBCs, OICs were located primarily in the region of carboxylic-rich alicyclic molecules composed of esterified phenolic, carboxylated, and fused alicyclic structures and were generated mainly through electrophilic substitution of the aromatic proton. Our findings call for further investigation on the exact structure and toxicity of the OBCs and OICs generated in the natural environment.

Methanol adducts leading to the identification of a reactive aldehyde metabolite of CPAQOP in human liver microsomes by ultra-high-performance liquid chromatography/mass spectrometry

RATIONALE

The incubation of CPAQOP (1-[(2R)-2-[[4-[3-chloro-4-(2-pyridyloxy)anilino]quinazolin-5-yl]oxymethyl]-1-piperidyl]-2-hydroxy) with human liver microsomes generated several metabolites that highlighted the hydroxyacetamide side chain was a major site of metabolism for the molecule. The metabolites were derived predominantly from oxidative biotransformations; however, two unexpected products were detected by liquid chromatography/ultraviolet/mass spectrometry (LC/UV/MS) and identified as methanol adducts. This observation prompted further LC/MS investigations into their formation.

METHODS

Three separate incubations of CPAQOP were conducted in human liver microsomes; Naïve, fortified with methoxyamine and fortified with glutathione. Separation was achieved via ultra-high-performance liquid chromatography with either methanol or acetonitrile gradients containing formic acid. MS analysis was conducted by electrospray ionisation LTQ Orbitrap mass spectrometry acquiring accurate mass full scan, data-dependent MS² and all ion fragmentation.

RESULTS

No methanol adducts were detected by MS when acetonitrile was used in the mobile phase instead of methanol, verifying that a metabolite was reacting with methanol on column. Although this reactive metabolite could not be isolated or structurally characterised by LC/MS directly, product ion spectra of the methanol adducts confirmed addition of methanol on the hydroxyacetamide side chain. Additional experiments using methoxyamine showed the disappearance of the two methanol adducts and appearance of a methoxyamine adduct, confirming the presence of an aldhyde. Product ion spectra of the methoxyamine adduct confirmed addition of methoxyamine to the hydroxyacetamide side chain.

CONCLUSIONS

The proposed bioactivation of CPAQOP occurred via the reactive aldehyde intermediate, which readily reacted with methanol in the mobile phase to form a pair of isomeric hemiacetal methanol adducts. In acidified methanol the equilibrium favoured the methanol adduct and in acidified acetonitrile it favoured the hydrate; therefore, the reactive aldehyde metabolite was not detected and could not be structurally characterised directly. Copyright © 2016 John Wiley & Sons, Ltd.

Chemical characterization of α-pinene secondary organic aerosol constituents using gas chromatography, liquid chromatography, and paper spray-based mass spectrometry techniques

RATIONALE

Despite ample research into the atmospheric oxidation of α-pinene, an important precursor to biogenic secondary organic aerosol formation, the identification of its reaction products, specifically organic nitrates, which impact atmospheric NOx concentrations, is still incomplete. This negatively impacts our understanding of α-pinene oxidation chemistry and its relation to air quality.

METHODS

Photochemical chamber experiments were conducted in conjunction with mass spectrometric techniques, including gas chromatography/mass spectrometry (GC/MS), high-performance liquid chromatography/time-of-flight (HPLC/TOF), and paper spray ionization MS, to investigate products from the OH radical initiated oxidation of α-pinene under high NOx conditions.

RESULTS

Over 30 compounds were tentatively identified, including those newly detected from photochemical chamber studies of α-pinene oxidation, pinocamphenol, fencholenic aldehyde, and α-pinene-derived nitrate isomers. α-Pinene-derived hydroxynitrate isomers were successfully detected using chromatographic methods, demonstrating, for the first time, the identification of individual first-generation organic nitrate products derived from α-pinene. The application of paper spray ionization to particle-phase compounds collected on filters represents a novel method for the direct analysis of filter samples at ambient pressure and temperature.

CONCLUSIONS

The use of HPLC/TOF and paper spray ionization methods to identify previously unobserved α-pinene-derived products helps lower the uncertainty in α-pinene oxidation chemistry and provides new platforms that can be used to identify and quantify important atmospheric compounds that relate to air quality in a complex sample matrix, such as ambient aerosol particles. Additionally, the use of paper spray ionization for direct filter analysis is a fast, relatively inexpensive sample preparation technique that can be used to reduce sample manipulation from solvent-induced reactions. Copyright © 2016 John Wiley & Sons, Ltd.

Characterization of Highly Oxidized Molecules in Fresh and Aged Biogenic Secondary Organic Aerosol

In this work, highly oxidized multifunctional molecules (HOMs) in fresh and aged secondary organic aerosol (SOA) derived from biogenic precursors are characterized with high-resolution mass spectrometry. Fresh SOA was generated by mixing ozone with a biogenic precursor (β-pinene, limonene, α-pinene) in a flow tube reactor. Aging was performed by passing the fresh SOA through a photochemical reactor where it reacted with hydroxyl radicals. Although these aerosols were as a whole not highly oxidized, molecular analysis identified a significant number of HOMs embedded within it. HOMs in fresh SOA consisted mostly of monomers and dimers, which is consistent with condensation of extremely low-volatility organic compounds (ELVOCs) that have been detected in the gas phase in previous studies and linked to SOA particle formation. Aging caused an increase in the average number of carbon atoms per molecule of the HOMs, which is consistent with particle phase oxidation of (less oxidized) oligomers already existing in fresh SOA. HOMs having different combinations of oxygen-to-carbon ratio, hydrogen-to-carbon ratio and average carbon oxidation state are discussed and compared to low volatility oxygenated organic aerosol (LVOOA), which has been identified in ambient aerosol based on average elemental composition but not fully understood at a molecular level. For the biogenic precursors and experimental conditions studied, HOMs in fresh biogenic SOA have molecular formulas more closely resembling LVOOA than HOMs in aged SOA, suggesting that aging of biogenic SOA is not a good surrogate for ambient LVOOA.

Real-Time Analysis of Ambient Organic Aerosols Using Aerosol Flowing Atmospheric-Pressure Afterglow Mass Spectrometry (AeroFAPA-MS)

Organic compounds contribute to a major fraction of atmospheric aerosols and have significant impacts on climate and human health. However, because of their chemical complexity, their measurement remains a major challenge for analytical instrumentation. Here we present the development and characterization of a new soft ionization technique that allows mass spectrometric real-time detection of organic compounds in aerosols. The aerosol flowing atmospheric-pressure afterglow (AeroFAPA) ion source is based on a helium glow discharge plasma, which generates excited helium species and primary reagent ions. Ionization of the analytes occurs in the afterglow region after thermal desorption and produces mainly intact quasimolecular ions, facilitating the interpretation of the acquired mass spectra. We illustrate that changes in aerosol composition and concentration are detected on the time scale of seconds and in the ng m(-3) range. Additionally, the successful application of AeroFAPA-MS during a field study in a mixed forest region is presented. In general, the observed compounds are in agreement with previous offline studies; however, the acquisition of chemical information and compound identification is much faster. The results demonstrate that AeroFAPA-MS is a suitable tool for organic aerosol analysis and reveal the potential of this technique to enable new insights into aerosol formation, growth, and transformation in the atmosphere.

Characterizing an extractive electrospray ionization (EESI) source for the online mass spectrometry analysis of organic aerosols

Organic compounds comprise a major fraction of tropospheric aerosol and understanding their chemical complexity is a key factor for determining their climate and health effects. We present and characterize here a new online technique for measuring the detailed chemical composition of organic aerosols, namely extractive electrospray ionization mass spectrometry (EESI-MS). Aerosol particles composed of soluble organic compounds were extracted into and ionized by a solvent electrospray, producing molecular ions from the aerosol with minimal fragmentation. We demonstrate here that the technique has a time resolution of seconds and is capable of making stable measurements over several hours. The ion signal in the MS was linearly correlated with the mass of aerosol delivered to the EESI source over the range tested (3-600 μg/m(3)) and was independent of particle size and liquid water content, suggesting that the entire particle bulk is extracted for analysis. Tandem MS measurements enabled detection of known analytes in the sub-μg/m(3) range. Proof-of-principle measurements of the ozonolysis of oleic acid aerosol (20 μg/m(3)) revealed the formation of a variety of oxidation products in good agreement with previous offline studies. This demonstrates the technique's potential for studying the product-resolved kinetics of aerosol-phase chemistry at a molecular level with high sensitivity and time resolution.

Molecular composition of boreal forest aerosol from Hyytiälä, Finland, using ultrahigh resolution mass spectrometry

Organic compounds are important constituents of fine particulate matter (PM) in the troposphere. In this study, we applied direct infusion nanoelectrospray (nanoESI) ultrahigh resolution mass spectrometry (UHR-MS) and liquid chromatography LC/ESI-UHR-MS for the analysis of the organic fraction of PM1 aerosol samples collected over a two week period at a boreal forest site (Hyytiälä), southern Finland. Elemental formulas (460-730 in total) were identified with nanoESI-UHR-MS in the negative ionization mode and attributed to organic compounds with a molecular weight below 400. Kendrick Mass Defect and Van Krevelen approaches were used to identify compound classes and mass distributions of the detected species. The molecular composition of the aerosols strongly varied between samples with different air mass histories. An increased number of nitrogen, sulfur, and highly oxygenated organic compounds was observed during the days associated with continental air masses. However, the samples with Atlantic air mass history were marked by a presence of homologous series of unsaturated and saturated C12-C20 fatty acids suggesting their marine origin. To our knowledge, we show for the first time that the highly detailed chemical composition obtained from UHR-MS analyses can be clearly linked to meteorological parameters and trace gases concentrations that are relevant to atmospheric oxidation processes. The additional LC/ESI-UHR-MS analysis revealed 29 species, which were mainly attributed to oxidation products of biogenic volatile compounds BVOCs (i.e., α,β-pinene, Δ3-carene, limonene, and isoprene) supporting the results from the direct infusion analysis.

Liquid chromatography tandem mass spectrometry method for characterization of monoaromatic nitro-compounds in atmospheric particulate matter

Nitrogen-containing organic compounds in the atmosphere have drawn attention owing to their impact on aerosol chemistry and physics and their potential adverse effects on the biosphere. Among them, nitrocatechols and their homologs have recently been associated with biomass burning. In the present study, nitrocatechols, nitrophenols, nitroguaiacols and nitrosalicylic acids (NSAs) were simultaneously quantified for the first time by using a new analytical method based on liquid chromatography/tandem mass spectrometry, which was systematically optimized and validated. Several analyte specific issues regarding the sample preparation and chromatographic analysis were addressed in order to ensure method sensitivity, precision, and accuracy. Sample matrix effects were thoroughly investigated in order to ensure method specificity. The method was found to be sensitive with limits of detection ranging from 0.1 to 1.0 μg L(-1), and with accuracy generally between 90 and 104%. The relative standard deviations for repeatability and intermediate precision were better than 4% and 9%, respectively. The method was applied to the analysis of winter and summer PM(10) samples from the city of Ljubljana, Slovenia. Aerosol concentrations as high as 152 and 134 ng m(-3) were obtained for the major aerosol nitro-aromatics: 4-nitrocatechol (4NC) and methyl-nitrocatechols (MNCs), respectively. Up to 500-times higher concentrations of 4NC and MNCs were found in winter compared to summer aerosols. The correlation analysis for winter samples showed that 4NC, MNCs, and NSAs are strongly inter-correlated (R(2)=0.84-0.96). Significant correlations between these analytes and anhydrosugars support their proposed origin from biomass burning. The studied nitro-aromatics were found to constitute a non-negligible fraction (around 1%) of the organic carbon.

Applications of high-resolution electrospray ionization mass spectrometry to measurements of average oxygen to carbon ratios in secondary organic aerosols

The applicability of high-resolution electrospray ionization mass spectrometry (HR ESI-MS) to measurements of the average oxygen to carbon ratio (O/C) in secondary organic aerosols (SOAs) was investigated. Solutions with known average O/C containing up to 10 standard compounds representative of low-molecular-weight SOA constituents were analyzed and the corresponding electrospray ionization efficiencies were quantified. The assumption of equal ionization efficiency commonly used in estimating O/C ratios of SOAs was found to be reasonably accurate. We found that the accuracy of the measured O/C ratios increases by averaging the values obtained from both the posive and negative modes. A correlation was found between the ratio of the ionization efficiencies in the positive (+) and negative (-) ESI modes and the octanol-water partition constant and, more importantly, the compound's O/C. To demonstrate the utility of this correlation for estimating average O/C values of unknown mixtures, we analyzed the ESI (+) and ESI (-) data for SOAs produced by oxidation of limonene and isoprene and compared them online to O/C measurements using an aerosol mass spectrometer (AMS). This work demonstrates that the accuracy of the HR ESI-MS method is comparable to that of the AMS with the added benefit of molecular identification of the aerosol constituents.

Organic constituents on the surfaces of aerosol particles from Southern Finland, Amazonia, and California studied by vibrational sum frequency generation

This article summarizes and compares the analysis of the surfaces of natural aerosol particles from three different forest environments by vibrational sum frequency generation. The experiments were carried out directly on filter and impactor substrates, without the need for sample preconcentration, manipulation, or destruction. We discuss the important first steps leading to secondary organic aerosol (SOA) particle nucleation and growth from terpene oxidation by showing that, as viewed by coherent vibrational spectroscopy, the chemical composition of the surface region of aerosol particles having sizes of 1 μm and lower appears to be close to size-invariant. We also discuss the concept of molecular chirality as a chemical marker that could be useful for quantifying how chemical constituents in the SOA gas phase and the SOA particle phase are related in time. Finally, we describe how the combination of multiple disciplines, such as aerosol science, advanced vibrational spectroscopy, meteorology, and chemistry can be highly informative when studying particles collected during atmospheric chemistry field campaigns, such as those carried out during HUMPPA-COPEC-2010, AMAZE-08, or BEARPEX-2009, and when they are compared to results from synthetic model systems such as particles from the Harvard Environmental Chamber (HEC). Discussions regarding the future of SOA chemical analysis approaches are given in the context of providing a path toward detailed spectroscopic assignments of SOA particle precursors and constituents and to fast-forward, in terms of mechanistic studies, through the SOA particle formation process.

Elemental composition of HULIS in the Pearl River Delta Region, China: results inferred from positive and negative electrospray high resolution mass spectrometric data

The HUmic-LIke Substances (HULIS) fraction isolated from aerosol samples collected at a rural location of the Pearl River Delta Region (PRD), China, during the harvest season was analyzed by both positive and negative mode electrospray ionization (ESI) coupled with an ultrahigh resolution mass spectrometer (UHRMS). With the remarkable resolving power and mass accuracy of ESI-UHRMS, thousands of elemental formulas were identified. Formulas detected in the positive (ESI+) and the negative (ESI-) mode complement each other due to differences in the ionization mechanism, and the use of both provides a more complete characterization of HULIS. Compounds composed of C, H, and O atoms were preferentially detected in ESI- by deprotonation, implying their acidic properties. Tandem MS and Kendrick Mass Defect analysis implies that carboxyl groups are abundant in the CHO compounds. This feature is similar to those of natural fulvic acids, but relatively smaller molecular weights are observed in the HULIS samples. A greater number of reduced nitrogen organic compounds were observed in the ESI+ compared to ESI-. Compounds with biomass burning origin including alkaloids, amino acids, and their derivatives are their probable constituents. Sulfur-containing species were dominantly detected in ESI-. The presence of sulfate fragments in the MS/MS spectra of these species and their high O/S ratios implies that they are mainly organosulfates. Organosulfates and nitrooxy-organosulfates were often the most intensive peaks in the ESI- spectra. They are believed to be products of reactive uptake of photooxidation products of reactive volatile organic compounds by acidic sulfate particles. The elemental compositions deduced from the UHRMS analysis confirm the conclusion from our previous study that biomass burning and SOA formation are both important sources of HULIS in the PRD region.

COBRA: a computational brewing application for predicting the molecular composition of organic aerosols

Atmospheric organic aerosols (OA) represent a significant fraction of airborne particulate matter and can impact climate, visibility, and human health. These mixtures are difficult to characterize experimentally due to their complex and dynamic chemical composition. We introduce a novel Computational Brewing Application (COBRA) and apply it to modeling oligomerization chemistry stemming from condensation and addition reactions in OA formed by photooxidation of isoprene. COBRA uses two lists as input: a list of chemical structures comprising the molecular starting pool and a list of rules defining potential reactions between molecules. Reactions are performed iteratively, with products of all previous iterations serving as reactants for the next. The simulation generated thousands of structures in the mass range of 120-500 Da and correctly predicted ∼70% of the individual OA constituents observed by high-resolution mass spectrometry. Select predicted structures were confirmed with tandem mass spectrometry. Esterification was shown to play the most significant role in oligomer formation, with hemiacetal formation less important, and aldol condensation insignificant. COBRA is not limited to atmospheric aerosol chemistry; it should be applicable to the prediction of reaction products in other complex mixtures for which reasonable reaction mechanisms and seed molecules can be supplied by experimental or theoretical methods.

Fragmentation analysis of water-soluble atmospheric organic matter using ultrahigh-resolution FT-ICR mass spectrometry

Isolated water-soluble atmospheric organic matter (AOM) analytes extracted from radiation fogwater samples were analyzed using collision induced dissociation with ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Tandem mass analysis was performed on several mass ranges between 100 and 400 Da to characterize the functional groups of AOM species. Compounds containing nitrogen and/or sulfur were targeted because of the high number of oxygen atoms contained in their molecular formulas. Due to the large number of isobaric ions in the precursor isolation ranges, large numbers of product ions resulted from collision induced dissociation. Common neutral losses were assigned by matching the molecular formulas of the expected product ions with the detected product ions within the appropriate mass spectra. Since polar functional groups are expected to affect the hygroscopic properties of aerosols, the losses of H(2)O, CO(2), CH(3)OH, HNO(3), CH(3)NO(3), SO(3), SO(4) and combinations of these were specifically targeted. Among the 421 compounds studied, the most frequently observed neutral losses were CO(2) (54%), H(2)O (43%) and CH(3)OH (40%). HNO(3) losses were observed for 63% of the studied nitrogen containing compounds and 33% of the studied compounds containing both nitrogen and sulfur. SO(3) losses were observed for 85% of the studied sulfur containing compounds and 42% of studied compounds containing both nitrogen and sulfur. A number of molecular formulas matching those of monoterpene ozonolysis SOA were observed; they include organonitrates, organosulfates, and nitroxy-organosulfates. Overall, the results of fragmentation analysis of 400+ individual molecular precursors elucidate the complexity and multifunctional nature of the isolated water-soluble AOM.

Mass spectrometry of atmospheric aerosols--recent developments and applications. Part I: Off-line mass spectrometry techniques

Many of the significant advances in our understanding of atmospheric particles can be attributed to the application of mass spectrometry. Mass spectrometry provides high sensitivity with a fast response time to probe chemically complex particles. This review focuses on recent developments and applications in the field of mass spectrometry of atmospheric aerosols. In Part I of this two-part review, we concentrate on off-line mass spectrometry techniques, which require sample collection on filters but can provide detailed molecular speciation. In particular, off-line mass spectrometry techniques utilizing tandem mass spectrometry experiments and high resolution mass analyzers provide improved insight into secondary organic aerosol formation and heterogeneous reaction pathways through detailed structural elucidation at the molecular level.

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.

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).