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

Connecting the Light Absorption of Atmospheric Organic Aerosols with Oxidation State and Polarity

The light-absorbing organic aerosol (OA) constitutes an important fraction of absorbing components, counteracting major cooling effect of aerosols to climate. The mechanisms in linking the complex and changeable chemistry of OA with its absorbing properties remain to be elucidated. Here, by using solvent extraction, ambient OA from an urban environment was fractionated according to polarity, which was further nebulized and online characterized with compositions and absorbing properties. Water extracted high-polar compounds with a significantly higher oxygen to carbon ratio (O/C) than methanol extracts. A transition O/C of about 0.6 was found, below and above which the enhancement and reduction of OA absorptivity were observed with increasing O/C, occurring on the less polar and high polar compounds, respectively. In particular, the co-increase of nitrogen and oxygen elements suggests the important role of nitrogen-containing functional groups in enhancing the absorptivity of the less polar compounds (e.g., forming nitrogen-containing aromatics), while further oxidation (O/C > 0.6) on high-polar compounds likely led to fragmentation and bleaching chromophores. The results here may reconcile the previous observations about darkening or whitening chromophores of brown carbon, and the parametrization of O/C has the potential to link the changing chemistry of OA with its polarity and absorbing properties.

The Influence of Absolute Mass Loading of Secondary Organic Aerosols on Their Phase State

Absolute secondary organic aerosol (SOA) mass loading (CSOA) is a key parameter in determining partitioning of semi- and intermediate volatility compounds to the particle phase. Its impact on the phase state of SOA, however, has remained largely unexplored. In this study, systematic laboratory chamber measurements were performed to elucidate the influence of CSOA, ranging from 0.2 to 160 µg m−3, on the phase state of SOA formed by ozonolysis of various precursors, including α-pinene, limonene, cis-3-hexenyl acetate (CHA) and cis-3-hexen-1-ol (HXL). A previously established method to estimate SOA bounce factor (BF, a surrogate for particle viscosity) was utilized to infer particle viscosity as a function of CSOA. Results show that under nominally identical conditions, the maximum BF decreases by approximately 30% at higher CSOA, suggesting a more liquid phase state. With the exception of HXL-SOA (which acted as the negative control), the phase state for all studied SOA precursors varied as a function of CSOA. Furthermore, the BF was found to be the maximum when SOA particle distributions reached a geometric mean particle diameter of 50–60 nm. Experimental results indicate that CSOA is an important parameter impacting the phase state of SOA, reinforcing recent findings that extrapolation of experiments not conducted at atmospherically relevant SOA levels may not yield results that are relevant to the natural environment.

Optical Properties of Secondary Organic Aerosol from cis-3-Hexenol and cis-3-Hexenyl Acetate: Effect of Chemical Composition, Humidity, and Phase

Atmospheric aerosols play an important role in Earth's radiative balance directly, by scattering and absorbing radiation, and indirectly, by acting as cloud condensation nuclei (CCN). Atmospheric aerosol is dominated by secondary organic aerosol (SOA) formed by the oxidation of biogenic volatile organic compounds (BVOCs). Green leaf volatiles (GLVs) are a class of BVOCs that contribute to SOA, yet their role in the Earth's radiative budget is poorly understood. In this work we measured the scattering efficiency (at 450, 525, and 635 nm), absorption efficiency (between 190 and 900 nm), particle phase, bulk chemical properties (O:C, H:C), and molecular-level composition of SOA formed from the ozonolysis of two GLVs: cis-3-hexenol (HXL) and cis-3-hexenyl acetate (CHA). Both HXL and CHA produced SOA that was weakly absorbing, yet CHA-SOA was a more efficient absorber than HXL-SOA. The scatter efficiency of SOA from both systems was wavelength-dependent, with the stronger dependence exhibited by HXL-SOA, likely due to differences in particle size. HXL-SOA formed under both dry (10% RH) and wet (70% RH) conditions had the same bulk chemical properties (O:C), yet significantly different optical properties, which was attributed to differences in molecular-level composition. We have found that SOA derived from green leaf volatiles has the potential to affect the Earth's radiative budget, and also that bulk chemical properties can be insufficient to predict SOA optical properties.

Method for characterization of low molecular weight organic acids in atmospheric aerosols using ion chromatography mass spectrometry

The structural composition of PM2.5 monitored in the atmosphere is usually divided by the analysis of organic carbon, black (also called elemental) carbon, and inorganic salts. The characterization of the chemical composition of aerosols represents a significant challenge to analysts, and studies are frequently limited to determination of aerosol bulk properties. To better understand the potential health effects and combined interactions of components in aerosols, a variety of measurement techniques for individual analytes in PM2.5 need to be implemented. The method developed here for the measurement of organic acids achieves class separation of aliphatic monoacids, aliphatic diacids, aromatic acids, and polyacids. The selective ion monitoring capability of a triple quadropole mass analyzer was frequently capable of overcoming instances of incomplete separations. Standard Reference Material (SRM) 1649b Urban Dust was characterized; 34 organic acids were qualitatively identified, and 6 organic acids were quantified.

Soft ionization chemical analysis of secondary organic aerosol from green leaf volatiles emitted by turf grass

Globally, biogenic volatile organic compound (BVOC) emissions contribute 90% of the overall VOC emissions. Green leaf volatiles (GLVs) are an important component of plant-derived BVOCs, including cis-3-hexenylacetate (CHA) and cis-3-hexen-1-ol (HXL), which are emitted by cut grass. In this study we describe secondary organic aerosol (SOA) formation from the ozonolysis of dominant GLVs, their mixtures and grass clippings. Near-infrared laser desorption/ionization aerosol mass spectrometry (NIR-LDI-AMS) was used for chemical analysis of the aerosol. The chemical profile of SOA generated from grass clippings was correlated with that from chemical standards of CHA and HXL. We found that SOA derived from HXL most closely approximated SOA from turf grass, in spite of the approximately 5× lower emission rate of HXL as compared to CHA. Ozonolysis of HXL results in formation of low volatility, higher molecular weight compounds, such as oligomers, and formation of ester-type linkages. This is in contrast to CHA, where the hydroperoxide channel is the dominant oxidation pathway, as oligomer formation is inhibited by the acetate functionality.

New insights into secondary organic aerosol from the ozonolysis of α-pinene from combined infrared spectroscopy and mass spectrometry measurements

Thermograms of desorbing species from size-fractionated SOA.

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.

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.

Nitrogen-containing organic compounds and oligomers in secondary organic aerosol formed by photooxidation of isoprene

Electrospray ionization high-resolution mass spectrometry (ESI HR-MS) was used to probe molecular structures of oligomers in secondary organic aerosol (SOA) generated in laboratory experiments on isoprene photooxidation at low- and high-NO(x) conditions. Approximately 80-90% of the observed products are oligomers and up to 33% by number are nitrogen-containing organic compounds (NOC). We observe oligomers with maximum 8 monomer units in length. Tandem mass spectrometry (MS(n)) confirms NOC compounds are organic nitrates and elucidates plausible chemical building blocks contributing to oligomer formation. Most organic nitrates are comprised of methylglyceric acid units. Other important multifunctional C(2)-C(5) monomer units are identified including methylglyoxal, hydroxyacetone, hydroxyacetic acid, and glycolaldehyde. Although the molar fraction of NOC in the high-NO(x) SOA is high, the majority of the NOC oligomers contain only one nitrate moiety resulting in a low average N:C ratio of 0.019. Average O:C ratios of the detected SOA compounds are 0.54 under the low-NO(x) conditions and 0.83 under the high-NO(x) conditions. Our results underscore the importance of isoprene photooxidation as a source of NOC in organic particulate matter.

The Gas-phase Ozonolysis of 1-Penten-3-ol, (Z)-2-Penten-1-ol and 1-Penten-3-one: Kinetics, Products and Secondary Organic Aerosol Formation

The gas-phase ozonolysis of the biogenic unsaturated compounds 1-penten-3-ol, (Z)-2-penten-1-ol and 1-penten-3-one has been investigated in two atmospheric simulation chambers. The following rate coefficients (in units of 10−17 cm3 molecule−1 s−1) were determined at atmospheric pressure and 293±2 K using an absolute rate method: 1-penten-3-ol, (1.64±0.15); (Z)-2-penten-1-ol, (11.5±0.66); 1-penten-3-one, (1.17±0.15). Reaction products were identified by in situ FTIR spectroscopy and gas chromatography – mass spectrometry (GC-MS). The major products and their average molar yields in the presence of a radical scavenger at relative humidity < 1% were: formaldehyde (0.49±0.02), 2-hydroxybutanal (0.46±0.03) and propanal (0.15±0.02) from 1-penten-3-ol; propanal (0.39±0.03) and glycolaldehyde (0.43±0.04) from (Z)-2-penten-1-ol; formaldehyde (0.37±0.02) and 2-oxobutanal (0.49±0.03) from 1-penten-3-one. The formation of secondary organic aerosol was also observed with yields ranging from 0.13–0.17 for the unsaturated alcohols. Significantly lower yields of around 0.03 were measured for 1-penten-3-one. The results of this work are used to determine atmospheric lifetimes and reaction mechanisms for the gas-phase ozonolysis of 1-penten-3-ol, (Z)-2-penten-1-ol and 1-penten-3-one. The broader atmospheric implications of this work are also discussed.

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.