Step Change in Improving the Accuracy of Nitrous Oxide Reference Materials for Underpinning Atmospheric Composition Measurements.

We have employed a new approach to quantify the amount fraction of nitrous oxide in a synthetic air matrix gas used to prepare high-accuracy reference materials of the same component. Until now, this was the largest contributor to the measurement uncertainty of nitrous oxide in air reference materials at atmospheric amount fractions (∼330 nmol mol-1), as identified in a recent international comparison. A novel preconcentration method has resulted in a measurement of 363 pmol mol-1 of nitrous oxide in a synthetic air matrix gas with an expanded uncertainty of 27 pmol mol-1. This represents a significant breakthrough as using these developments with an optimized dilution hierarchy (to minimize the gravimetric uncertainty) promises to result in SI traceable reference materials with expanded uncertainties as low as 0.032% relative (k = 2). This supports the World Meteorological Organization-Global Atmosphere Watch network compatibility goal for underpinning atmospheric observations.

Boreas: A Sample Preparation-Coupled Laser Spectrometer System for Simultaneous High-Precision In Situ Analysis of δ13C and δ2H from Ambient Air Methane.

We present a new instrument, "Boreas", a cryogen-free methane (CH4) preconcentration system coupled to a dual-laser spectrometer for making simultaneous measurements of δ13C(CH4) and δ2H(CH4) in ambient air. Excluding isotope ratio scale uncertainty, we estimate a typical standard measurement uncertainty for an ambient air sample of 0.07‰ for δ13C(CH4) and 0.9‰ for δ2H(CH4), which are the lowest reported for a laser spectroscopy-based system and comparable to isotope ratio mass spectrometry. We trap CH4 (∼1.9 µmol mol-1) from ∼5 L of air onto the front end of a packed column, subsequently separating CH4 from interferences using a controlled temperature ramp with nitrogen (N2) as the carrier gas, before eluting CH4 at ∼550 µmol mol-1. This processed sample is then delivered to an infrared laser spectrometer for measuring the amount fractions of 12CH4, 13CH4, and 12CH3D isotopologues. We calibrate the instrument using a set of gravimetrically prepared amount fraction primary reference materials directly into the laser spectrometer that span a range of 500-626 µmol mol-1 (CH4 in N2) made from a single pure CH4 source that has been isotopically characterized for δ13C(CH4) by IRMS. Under the principle of identical treatment, a compressed ambient air sample is used as a working standard and measured between air samples, from which a final calibrated isotope ratio is calculated. Finally, we make automated measurements of both δ13C(CH4) and δ2H(CH4) in over 200 ambient air samples and demonstrate the application of Boreas for deployment to atmospheric monitoring sites.

Breakthrough in Negating the Impact of Adsorption in Gas Reference Materials.

We have shown that an exchange dilution preparation method reduces the impact of surface adsorption of the target component in high-pressure gas mixtures used for underpinning measurements of amount-of-substance fraction. Gas mixtures are diluted in the same cylinder by releasing an aliquot of the parent mixture. Additional matrix gas is then added to the cylinder. This differs from conventional methods where dilutions are achieved by transferring the parent mixture to another cylinder, which then stores the final reference material. The benefit of this revolutionary approach is that losses due to adsorption to the walls of the cylinder and the valve are reduced as the parent mixture pacifies the surface with only a negligible relative change in amount-of-substance fraction. This development allows for preparation of gas reference materials with unprecedented uncertainties beyond the existing state of the art. It has significant implications for the preparation of high accuracy gas reference materials which underpin a broad range of requirements, particularly in atmospheric monitoring of carbon dioxide, where understanding the adsorption effects is the major obstacle to advancing the measurement science. It has the potential to remove the reliance on proprietary surface pretreatments as the method provides an in situ and consistent alternative.

Stability of gaseous volatile organic compounds contained in gas cylinders with different internal wall treatments.

Measurements of volatile organic compounds (VOCs) have been ongoing for decades to track growth rates and assist in curbing emissions of these compounds into the atmosphere. To accurately establish mole fraction trends and assess the role of these gas-phase compounds in atmospheric chemistry it is essential to have good calibration standards. A necessity and precursor to accurate VOC gas standards are the gas cylinders and the internal wall treatments that aid in maintaining the stability of the mixtures over long periods of time, measured in years. This paper will discuss the stability of VOC gas mixtures in different types of gas cylinders and internal wall treatments. Stability data will be given for 85 VOCs studied in gas mixtures by National Metrology Institutes and other agency laboratories. This evaluation of cylinder treatment materials is the outcome of an activity of the VOC Expert Group within the framework of the World Meteorological Organization (WMO) Global Atmospheric Watch (GAW) program.

Synthetic Zero Air Reference Material for High Accuracy Greenhouse Gas Measurements.

The purity analysis of zero air is a significant contributor to the uncertainty in preparing reference materials of high impact greenhouse gases, limiting progress toward coherent and comparable measurements required to assess climate trends. We have produced a commutable synthetic zero air reference material with an oxygen, nitrogen, and argon matrix closely matching atmospheric composition. This is the critical step in preventing systematic biases from pressure broadening effects when using these reference materials to calibrate instruments based on optical spectroscopy. The amount fractions of carbon dioxide, methane, and carbon monoxide, which are present as minor impurities in the zero air reference material, have been accurately quantified using a novel dilution device that can generate gas mixtures of these components at trace amount fractions. These developments will have a significant impact on advancing the state of the art in high accuracy reference materials and for baseline calibration of spectroscopic instrumentation.

Monoterpenes are the largest source of summertime organic aerosol in the southeastern United States.

The chemical complexity of atmospheric organic aerosol (OA) has caused substantial uncertainties in understanding its origins and environmental impacts. Here, we provide constraints on OA origins through compositional characterization with molecular-level details. Our results suggest that secondary OA (SOA) from monoterpene oxidation accounts for approximately half of summertime fine OA in Centreville, AL, a forested area in the southeastern United States influenced by anthropogenic pollution. We find that different chemical processes involving nitrogen oxides, during days and nights, play a central role in determining the mass of monoterpene SOA produced. These findings elucidate the strong anthropogenic-biogenic interaction affecting ambient aerosol in the southeastern United States and point out the importance of reducing anthropogenic emissions, especially under a changing climate, where biogenic emissions will likely keep increasing.

Influence of Pressure on the Composition of Gaseous Reference Materials.

We have shown that the amount fraction of carbon dioxide in a nitrogen or synthetic air matrix stored in cylinders increases as the pressure of the gas mixture reduces, while the amount fraction of methane remains unchanged. Our measurements show the initial amount fraction of carbon dioxide to be lower than the gravimetric value after preparation, which we attribute to the adsorption of a proportion of the molecules to active sites on the internal surface of the cylinder and the valve. As the mixture is consumed, the pressure in the cylinder reduces and the amount fraction of the component is observed to increase. The effect is less pronounced in the presence of water vapor. More dramatic effects have been observed for hydrogen chloride. These findings have significant implications for the preparation of high accuracy gaseous reference materials with unprecedented uncertainties which underpin a broad range of requirements, in particular atmospheric monitoring of high impact greenhouse gases.

Improved molecular level identification of organic compounds using comprehensive two-dimensional chromatography, dual ionization energies and high resolution mass spectrometry.

A new analytical methodology combining comprehensive two-dimensional gas chromatography (GC×GC), dual ionization energies and high resolution time of flight mass spectrometry has been developed that improves molecular level identification of organic compounds in complex environmental samples. GC×GC maximizes compound separation providing cleaner mass spectra by minimizing erroneous fragments from interferences and co-eluting peaks. Traditional electron ionization (EI, 70 eV) provides MS fragmentation patterns that can be matched to published EI MS libraries while vacuum ultraviolet photoionization (VUV, 10.5 eV) yields MS with reduced fragmentation enhancing the abundance of the molecular ion providing molecular formulas when combined with high resolution mass spectrometry. We demonstrate this new approach by applying it to a sample of organic aerosol. In this sample, 238 peaks were matched to EI MS library data with FM ≥ 800 but a fifth (42 compounds) were determined to be incorrectly identified because the molecular formula was not confirmed by the VUV MS data. This highlights the importance of using a complementary technique to confirm compound identifications even for peaks with very good matching statistics. In total, 171 compounds were identified by EI MS matching to library spectra with confirmation of the molecular formula from the high resolution VUV MS data and were not dependent on the matching statistics being above a threshold value. A large number of unidentified peaks were still observed with FM < 800, which in routine analysis would typically be neglected. Where possible, these peaks were assigned molecular formulas from the VUV MS data (211 in total). In total, the combination of EI and VUV MS data provides more than twice as much molecular level peak information than traditional approaches and improves confidence in the identification of individual organic compounds. The molecular formula data from the VUV MS data was used, in conjunction with GC×GC retention times and the observed EI MS, to generate a new, searchable EI MS library compatible with the standard NIST MS search program. This library is deliberately dynamic and editable so that other end users can add new entries and update existing entries as new information becomes available.

Comprehensive Chemical Characterization of Hydrocarbons in NIST Standard Reference Material 2779 Gulf of Mexico Crude Oil.

Comprehensive chemical information is needed to understand the environmental fate and impact of hydrocarbons released during oil spills. However, chemical information remains incomplete because of the limitations of current analytical techniques and the inherent chemical complexity of crude oils. In this work, gas chromatography (GC)-amenable C9-C33 hydrocarbons were comprehensively characterized from the National Institute of Standards and Technology Standard Reference Material (NIST SRM) 2779 Gulf of Mexico crude oil by GC coupled to vacuum ultraviolet photoionization mass spectrometry (GC/VUV-MS), with a mass balance of 68 ± 22%. This technique overcomes one important limitation faced by traditional GC and even comprehensive 2D gas chromatography (GC×GC): the necessity for individual compounds to be chromatographically resolved from one another in order to be characterized. VUV photoionization minimizes fragmentation of the molecular ions, facilitating the characterization of the observed hydrocarbons as a function of molecular weight (carbon number, NC), structure (number of double bond equivalents, NDBE), and mass fraction (mg kg(-1)), which represent important metrics for understanding their fate and environmental impacts. Linear alkanes (8 ± 1%), branched alkanes (11 ± 2%), and cycloalkanes (37 ± 12%) dominated the mass with the largest contribution from cycloalkanes containing one or two rings and one or more alkyl side chains (27 ± 9%). Linearity and good agreement with previous work for a subset of >100 components and for the sum of compound classes provided confidence in our measurements and represents the first independent assessment of our analytical approach and calibration methodology. Another crude oil collected from the Marlin platform (35 km northeast of the Macondo well) was shown to be chemically identical within experimental errors to NIST SRM 2779, demonstrating that Marlin crude is an appropriate surrogate oil for researchers conducting laboratory research into impacts of the DeepWater Horizon disaster.

Fundamental Time Scales Governing Organic Aerosol Multiphase Partitioning and Oxidative Aging.

Traditional descriptions of gas-particle partitioning of organic aerosols (OA) rely solely on thermodynamic properties (e.g., volatility). Under realistic conditions where phase partitioning is dynamic rather than static, the transformation of OA involves the interplay of multiphase partitioning with oxidative aging. A key challenge remains in quantifying the fundamental time scales for evaporation and oxidation of semivolatile OA. In this paper, we use isomer-resolved product measurements of a series of normal-alkanes (C18, C20, C22, and C24) to distinguish between gas-phase and heterogeneous oxidation products formed by reaction with hydroxyl radicals (OH). The product isomer distributions when combined with kinetics measurements of evaporation and oxidation enable a quantitative description of the multiphase time scales to be simulated using a single-particle kinetic model. Multiphase partitioning and oxidative transformation of semivolatile normal-alkanes under laboratory conditions is largely controlled by the particle phase state, since the time scales of heterogeneous oxidation and evaporation are found to occur on competing time scales (on the order of 10(-1) h). This is in contrast to atmospheric conditions where heterogeneous oxidation time scales are expected to be much longer (on the order of 10(2) h), with gas-phase oxidation being the dominant process regardless of the evaporation kinetics. Our results demonstrate the dynamic nature of OA multiphase partitioning and oxidative aging and reveal that the fundamental time scales of these processes are crucial for reliably extending laboratory measurements of OA phase partitioning and aging to the atmosphere.

Isomeric product detection in the heterogeneous reaction of hydroxyl radicals with aerosol composed of branched and linear unsaturated organic molecules.

The influence of molecular structure (branched vs linear) on product formation in the heterogeneous oxidation of unsaturated organic aerosol is investigated. Particle phase product isomers formed from the reaction of squalene (C30H50, a branched alkene with six C═C double bonds) and linolenic acid (C18H30O2, a linear carboxylic acid with three C═C double bonds) with OH radicals are identified and quantified using two-dimensional gas chromatography-mass spectrometry. The reactions are measured at low and high [O2] (∼1% vs 10% [O2]) to understand the roles of hydroxyalkyl and hydroxyperoxy radical intermediates in product formation. A key reaction step is OH addition to a C═C double bond to form a hydroxyalkyl radical. In addition, allylic alkyl radicals, formed from H atom abstraction reactions by hydroxyalkyl or OH radicals play important roles in the chemistry of product formation. Functionalization products dominate the squalene reaction at ∼1% [O2], with the total abundance of observed functionalization products being approximately equal to the fragmentation products at 10% [O2]. The large abundance of squalene fragmentation products at 10% [O2] is attributed to the formation and dissociation of tertiary hydroxyalkoxy radical intermediates. For linolenic acid aerosol, the formation of functionalization products dominates the reaction at both ∼1% and 10% [O2], suggesting that the formation and dissociation of secondary hydroxyalkoxy radicals are minor reaction channels for linear molecules. The distribution of linolenic acid functionalization products depends upon [O2], indicating that O2 controls the reaction pathways of the secondary hydroxyalkyl radical. For both reactions, alcohols are formed in favor of carbonyl functional groups, suggesting that there are some key differences between heterogeneous reactions involving allylic radical intermediates and those reactions of OH radicals with simple saturated hydrocarbons.

Secondary Organic Aerosol Formation via 2-Methyl-3-buten-2-ol Photooxidation: Evidence of Acid-Catalyzed Reactive Uptake of Epoxides.

Secondary organic aerosol (SOA) formation from 2-methyl-3-buten-2-ol (MBO) photooxidation has recently been observed in both field and laboratory studies. Similar to the level of isoprene, the level of MBO-derived SOA increases with elevated aerosol acidity in the absence of nitric oxide; therefore, an epoxide intermediate, (3,3-dimethyloxiran-2-yl)methanol (MBO epoxide), was synthesized and tentatively proposed to explain this enhancement. In this study, the potential of the synthetic MBO epoxide to form SOA via reactive uptake was systematically examined. SOA was observed only in the presence of acidic aerosol. Major SOA constituents, 2,3-dihydroxyisopentanol and MBO-derived organosulfate isomers, were chemically characterized in both laboratory-generated SOA and in ambient fine aerosol collected from the BEACHON-RoMBAS field campaign during the summer of 2011, where MBO emissions are substantial. Our results support the idea that epoxides are potential products of MBO photooxidation leading to the formation of atmospheric SOA and suggest that reactive uptake of epoxides may explain acid enhancement of SOA observed from other biogenic hydrocarbons.

Lubricating oil dominates primary organic aerosol emissions from motor vehicles.

Motor vehicles are major sources of primary organic aerosol (POA), which is a mixture of a large number of organic compounds that have not been comprehensively characterized. In this work, we apply a recently developed gas chromatography mass spectrometry approach utilizing "soft" vacuum ultraviolet photoionization to achieve unprecedented chemical characterization of motor vehicle POA emissions in a roadway tunnel with a mass closure of >60%. The observed POA was characterized by number of carbon atoms (NC), number of double bond equivalents (NDBE) and degree of molecular branching. Vehicular POA was observed to predominantly contain cycloalkanes with one or more rings and one or more branched alkyl side chains (≥80%) with low abundances of n-alkanes and aromatics (<5%), similar to "fresh" lubricating oil. The gas chromatography retention time data indicates that the cycloalkane ring structures are most likely dominated by cyclohexane and cyclopentane rings and not larger cycloalkanes. High molecular weight combustion byproducts, that is, alkenes, oxygenates, and aromatics, were not present in significant amounts. The observed carbon number and chemical composition of motor vehicle POA was consistent with lubricating oil being the dominant source from both gasoline and diesel-powered vehicles, with an additional smaller contribution from unburned diesel fuel and a negligible contribution from unburned gasoline.

OH-initiated heterogeneous oxidation of cholestane: a model system for understanding the photochemical aging of cyclic alkane aerosols.

Aerosols containing aliphatic hydrocarbons play a substantial role in the urban atmosphere. Cyclic alkanes constitute a large fraction of aliphatic hydrocarbon emissions originating from incomplete combustion of diesel fuel and motor oil. In the present study, cholestane (C27H48) is used as a model system to examine the OH-initiated heterogeneous oxidation pathways of cyclic alkanes in a photochemical flow tube reactor. Oxidation products are collected on filters and analyzed by a novel soft ionization two-dimensional gas chromatography/mass spectrometry technique. The analysis reveals that the first-generation functionalization products (cholestanones, cholestanals, and cholestanols) are the dominant reaction products that account for up to 70% by mass of the total speciated compounds. The ratio of first-generation carbonyls to alcohols is near unity at every oxidation level. Among the cholestanones/cholestanals, 55% are found to have the carbonyl group on the rings of the androstane skeleton, while 74% of cholestanols have the hydroxyl group on the rings. Particle-phase oxidation products with carbon numbers less than 27 (i.e., "fragmentation products") and higher-generation functionalization products are much less abundant. Carbon bond cleavage was found to occur only on the side chain. Tertiary-carbon alkoxy radicals are suggested to play an important role in governing both the distribution of functionalization products (via alkoxy radical isomerization and reaction with oxygen) and the fragmentation products (via alkoxy radical decomposition). These results provide new insights into the oxidation mechanism of cyclic alkanes.

Chemical composition of gas-phase organic carbon emissions from motor vehicles and implications for ozone production.

Motor vehicles are major sources of gas-phase organic carbon, which includes volatile organic compounds (VOCs) and other compounds with lower vapor pressures. These emissions react in the atmosphere, leading to the formation of ozone and secondary organic aerosol (SOA). With more chemical detail than previous studies, we report emission factors for over 230 compounds from gasoline and diesel vehicles via two methods. First we use speciated measurements of exhaust emissions from on-road vehicles in summer 2010. Second, we use a fuel composition-based approach to quantify uncombusted fuel components in exhaust using the emission factor for total uncombusted fuel in exhaust together with detailed chemical characterization of liquid fuel samples. There is good agreement between the two methods except for products of incomplete combustion, which are not present in uncombusted fuels and comprise 32 ± 2% of gasoline exhaust and 26 ± 1% of diesel exhaust by mass. We calculate and compare ozone production potentials of diesel exhaust, gasoline exhaust, and nontailpipe gasoline emissions. Per mass emitted, the gas-phase organic compounds in gasoline exhaust have the largest potential impact on ozone production with over half of the ozone formation due to products of incomplete combustion (e.g., alkenes and oxygenated VOCs). When combined with data on gasoline and diesel fuel sales in the U.S., these results indicate that gasoline sources are responsible for 69-96% of emissions and 79-97% of the ozone formation potential from gas-phase organic carbon emitted by motor vehicles.

Observational insights into aerosol formation from isoprene.

Atmospheric photooxidation of isoprene is an important source of secondary organic aerosol (SOA) and there is increasing evidence that anthropogenic oxidant emissions can enhance this SOA formation. In this work, we use ambient observations of organosulfates formed from isoprene epoxydiols (IEPOX) and methacrylic acid epoxide (MAE) and a broad suite of chemical measurements to investigate the relative importance of nitrogen oxide (NO/NO2) and hydroperoxyl (HO2) SOA formation pathways from isoprene at a forested site in California. In contrast to IEPOX, the calculated production rate of MAE was observed to be independent of temperature. This is the result of the very fast thermolysis of MPAN at high temperatures that affects the distribution of the MPAN reservoir (MPAN / MPA radical) reducing the fraction that can react with OH to form MAE and subsequently SOA (F(MAE formation)). The strong temperature dependence of F(MAE formation) helps to explain our observations of similar concentrations of IEPOX-derived organosulfates (IEPOX-OS; ~1 ng m(-3)) and MAE-derived organosulfates (MAE-OS; ~1 ng m(-3)) under cooler conditions (lower isoprene concentrations) and much higher IEPOX-OS (~20 ng m(-3)) relative to MAE-OS (<0.0005 ng m(-3)) at higher temperatures (higher isoprene concentrations). A kinetic model of IEPOX and MAE loss showed that MAE forms 10-100 times more ring-opening products than IEPOX and that both are strongly dependent on aerosol water content when aerosol pH is constant. However, the higher fraction of MAE ring opening products does not compensate for the lower MAE production under warmer conditions (higher isoprene concentrations) resulting in lower formation of MAE-derived products relative to IEPOX at the surface. In regions of high NOx, high isoprene emissions and strong vertical mixing the slower MPAN thermolysis rate aloft could increase the fraction of MPAN that forms MAE resulting in a vertically varying isoprene SOA source.

The influence of molecular structure and aerosol phase on the heterogeneous oxidation of normal and branched alkanes by OH.

Insights into the influence of molecular structure and thermodynamic phase on the chemical mechanisms of hydroxyl radical-initiated heterogeneous oxidation are obtained by identifying reaction products of submicrometer particles composed of either n-octacosane (C28H58, a linear alkane) or squalane (C30H62, a highly branched alkane) and OH. A common pattern is observed in the positional isomers of octacosanone and octacosanol, with functionalization enhanced toward the end of the molecule. This suggests that relatively large linear alkanes are structured in submicrometer particles such that their ends are oriented toward the surface. For squalane, positional isomers of first-generation ketones and alcohols also form in distinct patterns. Ketones are favored on carbons adjacent to tertiary carbons, while hydroxyl groups are primarily found on tertiary carbons but also tend to form toward the end of the molecule. Some first-generation products, viz., hydroxycarbonyls and diols, contain two oxygen atoms. These results suggest that alkoxy radicals are important intermediates and undergo both intramolecular (isomerization) and intermolecular (chain propagation) hydrogen abstraction reactions. Oxidation products with carbon number less than the parent alkane's are observed to a much greater extent for squalane than for n-octacosane oxidation and can be explained by the preferential cleavage of bonds involving tertiary carbons.

Insights into secondary organic aerosol formation mechanisms from measured gas/particle partitioning of specific organic tracer compounds.

In situ measurements of organic compounds in both gas and particle phases were made with a thermal desorption aerosol gas chromatography (TAG) instrument. The gas/particle partitioning of phthalic acid, pinonaldehyde, and 6,10,14-trimethyl-2-pentadecanone is discussed in detail to explore secondary organic aerosol (SOA) formation mechanisms. Measured fractions in the particle phase (f(part)) of 6,10,14-trimethyl-2-pentadecanone were similar to those expected from the absorptive gas/particle partitioning theory, suggesting that its partitioning is dominated by absorption processes. However, f(part) of phthalic acid and pinonaldehyde were substantially higher than predicted. The formation of low-volatility products from reactions of phthalic acid with ammonia is proposed as one possible mechanism to explain the high f(part) of phthalic acid. The observations of particle-phase pinonaldehyde when inorganic acids were fully neutralized indicate that inorganic acids are not required for the occurrence of reactive uptake of pinonaldehyde on particles. The observed relationship between f(part) of pinonaldehyde and relative humidity suggests that the aerosol water plays a significant role in the formation of particle-phase pinonaldehyde. Our results clearly show it is necessary to include multiple gas/particle partitioning pathways in models to predict SOA and multiple SOA tracers in source apportionment models to reconstruct SOA.

Elucidating secondary organic aerosol from diesel and gasoline vehicles through detailed characterization of organic carbon emissions.

Emissions from gasoline and diesel vehicles are predominant anthropogenic sources of reactive gas-phase organic carbon and key precursors to secondary organic aerosol (SOA) in urban areas. Their relative importance for aerosol formation is a controversial issue with implications for air quality control policy and public health. We characterize the chemical composition, mass distribution, and organic aerosol formation potential of emissions from gasoline and diesel vehicles, and find diesel exhaust is seven times more efficient at forming aerosol than gasoline exhaust. However, both sources are important for air quality; depending on a region's fuel use, diesel is responsible for 65% to 90% of vehicular-derived SOA, with substantial contributions from aromatic and aliphatic hydrocarbons. Including these insights on source characterization and SOA formation will improve regional pollution control policies, fuel regulations, and methodologies for future measurement, laboratory, and modeling studies.