Overview of the Alaskan Layered Pollution and Chemical Analysis (ALPACA) Field Experiment.

The Alaskan Layered Pollution And Chemical Analysis (ALPACA) field experiment was a collaborative study designed to improve understanding of pollution sources and chemical processes during winter (cold climate and low-photochemical activity), to investigate indoor pollution, and to study dispersion of pollution as affected by frequent temperature inversions. A number of the research goals were motivated by questions raised by residents of Fairbanks, Alaska, where the study was held. This paper describes the measurement strategies and the conditions encountered during the January and February 2022 field experiment, and reports early examples of how the measurements addressed research goals, particularly those of interest to the residents. Outdoor air measurements showed high concentrations of particulate matter and pollutant gases including volatile organic carbon species. During pollution events, low winds and extremely stable atmospheric conditions trapped pollution below 73 m, an extremely shallow vertical scale. Tethered-balloon-based measurements intercepted plumes aloft, which were associated with power plant point sources through transport modeling. Because cold climate residents spend much of their time indoors, the study included an indoor air quality component, where measurements were made inside and outside a house to study infiltration and indoor sources. In the absence of indoor activities such as cooking and/or heating with a pellet stove, indoor particulate matter concentrations were lower than outdoors; however, cooking and pellet stove burns often caused higher indoor particulate matter concentrations than outdoors. The mass-normalized particulate matter oxidative potential, a health-relevant property measured here by the reactivity with dithiothreiol, of indoor particles varied by source, with cooking particles having less oxidative potential per mass than pellet stove particles.

Geological evidence of extensive N-fixation by volcanic lightning during very large explosive eruptions.

Most of the nitrogen (N) accessible for life is trapped in dinitrogen (N2), the most stable atmospheric molecule. In order to be metabolized by living organisms, N2 has to be converted into biologically assimilable forms, so-called fixed N. Nowadays, nearly all the N-fixation is achieved through biological and anthropogenic processes. However, in early prebiotic environments of the Earth, N-fixation must have occurred via natural abiotic processes. One of the most invoked processes is electrical discharges, including from thunderstorms and lightning associated with volcanic eruptions. Despite the frequent occurrence of volcanic lightning during explosive eruptions and convincing laboratory experimentation, no evidence of substantial N-fixation has been found in any geological archive. Here, we report on the discovery of a significant amount of nitrate in volcanic deposits from Neogene caldera-forming eruptions, which are well correlated with the concentrations of species directly emitted by volcanoes (sulfur, chlorine). The multi-isotopic composition (δ18O, Δ17O) of the nitrates reveals that they originate from the atmospheric oxidation of nitrogen oxides formed by volcanic lightning. According to these first geological volcanic nitrate archive, we estimate that, on average, about 60 Tg of N can be fixed during a large explosive event. Our findings hint at a unique role potentially played by subaerial explosive eruptions in supplying essential ingredients for the emergence of life on Earth.

Discovering Nature's Fingerprints: Isotope Ratio Analysis on Bioanalytical Mass Spectrometers.

For a generation or more, the mass spectrometry that developed at the frontier of molecular biology was worlds apart from isotope ratio mass spectrometry, a label-free approach done on optimized gas-source magnetic sector instruments. Recent studies show that electrospray-ionization Orbitraps and other mass spectrometers widely used in the life sciences can be fine-tuned for high-precision isotope ratio analysis. Since isotope patterns form everywhere in nature based on well-understood principles, intramolecular isotope measurements allow unique insights into a fascinating range of research topics. This Perspective introduces a wider readership to current topics in stable isotope research with the aim of discussing how soft-ionization mass spectrometry coupled with ultrahigh mass resolution can enable long-envisioned progress. We highlight novel prospects of observing isotopes in intact polar compounds and speculate on future directions of this adventure into the overlapping realms of biology, chemistry, and geology.

Sunlight-driven nitrate loss records Antarctic surface mass balance.

Standard proxies for reconstructing surface mass balance (SMB) in Antarctic ice cores are often inaccurate or coarsely resolved when applied to more complicated environments away from dome summits. Here, we propose an alternative SMB proxy based on photolytic fractionation of nitrogen isotopes in nitrate observed at 114 sites throughout East Antarctica. Applying this proxy approach to nitrate in a shallow core drilled at a moderate SMB site (Aurora Basin North), we reconstruct 700 years of SMB changes that agree well with changes estimated from ice core density and upstream surface topography. For the under-sampled transition zones between dome summits and the coast, we show that this proxy can provide past and present SMB values that reflect the immediate local environment and are derived independently from existing techniques.

Robust Evidence of 14C, 13C, and 15N Analyses Indicating Fossil Fuel Sources for Total Carbon and Ammonium in Fine Aerosols in Seoul Megacity.

Carbon- and nitrogen-containing aerosols are ubiquitous in urban atmospheres and play important roles in air quality and climate change. We determined the 14C fraction modern (fM) and δ13C of total carbon (TC) and δ15N of NH4+ in the PM2.5 collected in Seoul megacity during April 2018 to December 2019. The seasonal mean δ13C values were similar to -25.1‰ ± 2.0‰ in warm and -24.2‰ ± 0.82‰ in cold seasons. Mean δ15N values were higher in warm (16.4‰ ± 2.8‰) than in cold seasons (4.0‰ ± 6.1‰), highlighting the temperature effects on atmospheric NH3 levels and phase-equilibrium isotopic exchange during the conversion of NH3 to NH4+. While 37% ± 10% of TC was apportioned to fossil-fuel sources on the basis of fM values, δ15N indicated a higher contribution of emissions from vehicle exhausts and electricity generating units (power-plant NH3 slip) to NH3: 60% ± 26% in warm season and 66% ± 22% in cold season, based on a Bayesian isotope-mixing model. The collective evidence of multiple isotope analysis reasonably supports the major contribution of fossil-fuel-combustion sources to NH4+, in conjunction with TC, and an increased contribution from vehicle emissions during the severe PM2.5 pollution episodes. These findings demonstrate the efficacy of a multiple-isotope approach in providing better insight into the major sources of PM2.5 in the urban atmosphere.

Sulfur-isotope anomalies recorded in Antarctic ice cores as a potential proxy for tracing past ozone layer depletion events.

Changes in the cosmic-ray background of the Earth can impact the ozone layer. High-energy cosmic events [e.g. supernova (SN)] or rapid changes in the Earth's magnetic field [e.g. geomagnetic Excursion (GE)] can lead to a cascade of cosmic rays. Ensuing chemical reactions can then cause thinning/destruction of the ozone layer-leading to enhanced penetration of harmful ultraviolet (UV) radiation toward the Earth's surface. However, observational evidence for such UV "windows" is still lacking. Here, we conduct a pilot study and investigate this notion during two well-known events: the multiple SN event (≈10 kBP) and the Laschamp GE event (≈41 kBP). We hypothesize that ice-core-Δ33S records-originally used as volcanic fingerprints-can reveal UV-induced background-tropospheric-photochemical imprints during such events. Indeed, we find nonvolcanic S-isotopic anomalies (Δ33S ≠ 0‰) in background Antarctic ice-core sulfate during GE/SN periods, thereby confirming our hypothesis. This suggests that ice-core-Δ33S records can serve as a proxy for past ozone-layer-depletion events.

Isotopic evidence for acidity-driven enhancement of sulfate formation after SO2 emission control.

After the 1980s, atmospheric sulfate reduction is slower than the dramatic reductions in sulfur dioxide (SO2) emissions. However, a lack of observational evidence has hindered the identification of causal feedback mechanisms. Here, we report an increase in the oxygen isotopic composition of sulfate ([Formula: see text]) in a Greenland ice core, implying an enhanced role of acidity-dependent in-cloud oxidation by ozone (up to 17 to 27%) in sulfate production since the 1960s. A global chemical transport model reproduces the magnitude of the increase in observed [Formula: see text] with a 10 to 15% enhancement in the conversion efficiency from SO2 to sulfate in Eastern North America and Western Europe. With an expected continued decrease in atmospheric acidity, this feedback will continue in the future and partially hinder air quality improvements.

Homogeneous sulfur isotope signature in East Antarctica and implication for sulfur source shifts through the last glacial-interglacial cycle.

Sulfate aerosol (SO42-) preserved in Antarctic ice cores is discussed in the light of interactions between marine biological activity and climate since it is mainly sourced from biogenic emissions from the surface ocean and scatters solar radiation during traveling in the atmosphere. However, there has been a paradox between the ice core record and the marine sediment record; the former shows constant non-sea-salt (nss-) SO42- flux throughout the glacial-interglacial changes, and the latter shows a decrease in biogenic productivity during glacial periods compared to interglacial periods. Here, by ensuring the homogeneity of sulfur isotopic compositions of atmospheric nss-SO42- (δ34Snss) over East Antarctica, we established the applicability of the signature as a robust tool for distinguishing marine biogenic and nonmarine biogenic SO42-. Our findings, in conjunction with existing records of nss-SO42- flux and δ34Snss in Antarctic ice cores, provide an estimate of the relative importance of marine biogenic SO42- during the last glacial period to be 48 ± 10% of nss-SO42-, slightly lower than 59 ± 11% during the interglacial periods. Thus, our results tend to reconcile the ice core and sediment records, with both suggesting the decrease in marine productivity around Southern Ocean under the cold climate.

Foliar uptake of atmospheric nitrate by two dominant subalpine plants: insights from in situ triple-isotope analysis.

The significance of foliar uptake of nitrogen (N) compounds in natural conditions is not well understood, despite growing evidence of its importance to plant nutrition. In subalpine meadows, N-limitation fosters the dominance of specific subalpine plant species, which in turn ensures the provision of essential ecosystems services. Understanding how these plants absorb N and from which sources is important in predicting ecological consequences of increasing N deposition. Here, we investigate the sources of N to plants from subalpine meadows with distinct land-use history in the French Alps, using the triple isotopes (Δ17 O, δ18 O, and δ15 N) of plant tissue nitrate (NO3- ). We use this approach to evaluate the significance of foliar uptake of atmospheric NO3- (NO3-atm ). The foliar uptake of NO3-atm accounted for 4-16% of the leaf NO3- content, and contributed more to the leaf NO3- pool after peak biomass. Additionally, the gradual 15 N enrichment of NO3- from the soil to the leaves reflected the contribution of NO3-atm assimilation to plants' metabolism. The present study confirms that foliar uptake is a potentially important pathway for NO3-atm into subalpine plants. This is of major significance as N emissions (and deposition) are predicted to increase globally in the future.

Tracing the Fate of Atmospheric Nitrate in a Subalpine Watershed Using Δ17O.

Nitrogen is an essential nutrient for life on Earth, but in excess, it can lead to environmental issues (e.g., N saturation, loss of biodiversity, acidification of lakes, etc.). Understanding the nitrogen budget (i.e., inputs and outputs) is essential to evaluate the prospective decay of the ecosystem services (e.g., freshwater quality, erosion control, loss of high patrimonial-value plant species, etc.) that subalpine headwater catchments provide, especially as these ecosystems experience high atmospheric nitrogen deposition. Here, we use a multi-isotopic tracer (Δ17O, δ15N and δ18O) of nitrate in aerosols, snow, and streams to assess the fate of atmospherically deposited nitrate in the subalpine watershed of the Lautaret Pass (French Alps). We show that atmospheric N deposition contributes significantly to stream nitrate pool year-round, either by direct inputs (up to 35%) or by in situ nitrification of atmospheric ammonium (up to 35%). Snowmelt in particular leads to high exports of atmospheric nitrate, most likely fast enough to impede assimilation by surrounding ecosystems. Yet, in a context of climate change, with shorter snow seasons, and increasing nitrogen emissions, our results hint at possibly stronger ecological consequences of nitrogen atmospheric deposition in the close future.

Atmospheric nitrate export in streams along a montane to urban gradient.

Nitrogen (N) emissions associated with urbanization exacerbate the atmospheric N influx to remote ecosystems - like mountains -, leading to well-documented detrimental effects on ecosystems (e.g., soil acidification, pollution of freshwaters). Here, the importance and fate of N deposition in a watershed was evaluated along a montane to urban gradient, using a multi-isotopic tracers approach (Δ17O, δ15N, δ18O of nitrate, δ2H and δ18O of water). In this setting, the montane streams had higher proportions of atmospheric nitrate compared to urban streams, and exported more atmospheric nitrate on a yearly basis (0.35 vs 0.10 kg-Nha-1yr-1). In urban areas, nitrate exports were driven by groundwater, whereas in the catchment head nitrate exports were dominated by surface runoff. The main sources of nitrate to the montane streams were microbial nitrification and atmospheric deposition, whereas microbial nitrification and sewage leakage contributed most to urban streams. Based on the measurement of δ15N and δ18O-NO3-, biological processes such as denitrification or N assimilation were not predominant in any streams in this study. The observed low δ15N and δ18O range of terrestrial nitrate (i.e., nitrate not coming from atmospheric deposition) in surface water compared to literature suggests that atmospheric deposition may be underestimated as a direct source of N.

A simple and reliable method reducing sulfate to sulfide for multiple sulfur isotope analysis.

RATIONALE: Precise analysis of four sulfur isotopes of sulfate in geological and environmental samples provides the means to extract unique information in wide geological contexts. Reduction of sulfate to sulfide is the first step to access such information. The conventional reduction method suffers from a cumbersome distillation system, long reaction time and large volume of the reducing solution. We present a new and simple method enabling the process of multiple samples at one time with a much reduced volume of reducing solution. METHODS: One mL of reducing solution made of HI and NaH2 PO2 was added to a septum glass tube with dry sulfate. The tube was heated at 124°C and the produced H2 S was purged with inert gas (He or N2 ) through gas-washing tubes and then collected by NaOH solution. The collected H2 S was converted into Ag2 S by adding AgNO3 solution and the co-precipitated Ag2 O was removed by adding a few drops of concentrated HNO3 . RESULTS: Within 2-3 h, a 100% yield was observed for samples with 0.2-2.5 µmol Na2 SO4 . The reduction rate was much slower for BaSO4 and a complete reduction was not observed. International sulfur reference materials, NBS-127, SO-5 and SO-6, were processed with this method, and the measured against accepted δ34 S values yielded a linear regression line which had a slope of 0.99 ± 0.01 and a R2 value of 0.998. CONCLUSIONS: The new methodology is easy to handle and allows us to process multiple samples at a time. It has also demonstrated good reproducibility in terms of H2 S yield and for further isotope analysis. It is thus a good alternative to the conventional manual method, especially when processing samples with limited amount of sulfate available.

Automated system measuring triple oxygen and nitrogen isotope ratios in nitrate using the bacterial method and N2 O decomposition by microwave discharge.

RATIONALE: Triple oxygen and nitrogen isotope ratios in nitrate are powerful tools for assessing atmospheric nitrate formation pathways and their contribution to ecosystems. N2 O decomposition using microwave-induced plasma (MIP) has been used only for measurements of oxygen isotopes to date, but it is also possible to measure nitrogen isotopes during the same analytical run. METHODS: The main improvements to a previous system are (i) an automated distribution system of nitrate to the bacterial medium, (ii) N2 O separation by gas chromatography before N2 O decomposition using the MIP, (iii) use of a corundum tube for microwave discharge, and (iv) development of an automated system for isotopic measurements. Three nitrate standards with sample sizes of 60, 80, 100, and 120 nmol were measured to investigate the sample size dependence of the isotope measurements. RESULTS: The δ17 O, δ18 O, and Δ17 O values increased with increasing sample size, although the δ15 N value showed no significant size dependency. Different calibration slopes and intercepts were obtained with different sample amounts. The slopes and intercepts for the regression lines in different sample amounts were dependent on sample size, indicating that the extent of oxygen exchange is also dependent on sample size. The sample-size-dependent slopes and intercepts were fitted using natural log (ln) regression curves, and the slopes and intercepts can be estimated to apply to any sample size corrections. When using 100 nmol samples, the standard deviations of residuals from the regression lines for this system were 0.5‰, 0.3‰, and 0.1‰, respectively, for the δ18 O, Δ17 O, and δ15 N values, results that are not inferior to those from other systems using gold tube or gold wire. CONCLUSIONS: An automated system was developed to measure triple oxygen and nitrogen isotopes in nitrate using N2 O decomposition by MIP. This system enables us to measure both triple oxygen and nitrogen isotopes in nitrate with comparable precision and sample throughput (23 min per sample on average), and minimal manual treatment. Copyright © 2016 John Wiley & Sons, Ltd.

A new feature in the internal heavy isotope distribution in ozone.

Ozone produced by discharge or photolysis of oxygen has unusually heavy isotopic composition ((18)O/(16)O and (17)O/(16)O ratio) which does not follow normal mass fractionation rule: δ(17)O ∼ 0.52(*)δ(18)O, expressed as an anomaly Δ(17)O = δ(17)O - 0.52(*)δ(18)O. Ozone molecule being an open isosceles triangle can have the heavy isotope located either in its apex or symmetric (s) position or the base or asymmetric (as) position. Correspondingly, one can define positional isotopic enrichment, written as δ(18)O (s) or δ(18)O (as) (and similarly for δ(17)O) as well as position dependent isotope anomaly Δ(17)O (s) and Δ(17)O (as). Marcus and co-workers have proposed a semi-empirical model based in principle on the RRKM model of uni-molecular dissociation but with slight modification (departure from statistical randomness assumption for symmetrical molecules) which explains many features of ozone isotopic enrichment. This model predicts that the bulk isotope anomaly is contained wholly in the asymmetric position and the Δ(17)O (s) is zero. Consequently, Δ(17)O (as) = 1.5 (*) Δ(17)O (bulk) (named here simply as the "1.5 rule") which has been experimentally confirmed over a range of isotopic enrichment. We now show that a critical re-analysis of the earlier experimental data demonstrates a small but significant departure from this 1.5 rule at the highest and lowest levels of enrichments. This departure provides the first experimental proof that the dynamics of ozone formation differs from a statistical model constrained only by restriction of symmetry. We speculate over some possible causes for the departure.

Large sulfur-isotope anomaly in nonvolcanic sulfate aerosol and its implications for the Archean atmosphere.

Sulfur-isotopic anomalies have been used to trace the evolution of oxygen in the Precambrian atmosphere and to document past volcanic eruptions. High-precision sulfur quadruple isotope measurements of sulfate aerosols extracted from a snow pit at the South Pole (1984-2001) showed the highest S-isotopic anomalies (Δ(33)S = +1.66‰ and Δ(36)S = +2‰) in a nonvolcanic (1998-1999) period, similar in magnitude to Pinatubo and Agung, the largest volcanic eruptions of the 20th century. The highest isotopic anomaly may be produced from a combination of different stratospheric sources (sulfur dioxide and carbonyl sulfide) via SOx photochemistry, including photoexcitation and photodissociation. The source of anomaly is linked to super El Niño Southern Oscillation (ENSO) (1997-1998)-induced changes in troposphere-stratosphere chemistry and dynamics. The data possess recurring negative S-isotope anomalies (Δ(36)S = -0.6 ± 0.2‰) in nonvolcanic and non-ENSO years, thus requiring a second source that may be tropospheric. The generation of nonvolcanic S-isotopic anomalies in an oxidizing atmosphere has implications for interpreting Archean sulfur deposits used to determine the redox state of the paleoatmosphere.

Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry.

Post-depositional processes alter nitrate concentration and nitrate isotopic composition in the top layers of snow at sites with low snow accumulation rates, such as Dome C, Antarctica. Available nitrate ice core records can provide input for studying past atmospheres and climate if such processes are understood. It has been shown that photolysis of nitrate in the snowpack plays a major role in nitrate loss and that the photolysis products have a significant influence on the local troposphere as well as on other species in the snow. Reported quantum yields for the main reaction spans orders of magnitude - apparently a result of whether nitrate is located at the air-ice interface or in the ice matrix - constituting the largest uncertainty in models of snowpack NOx emissions. Here, a laboratory study is presented that uses snow from Dome C and minimizes effects of desorption and recombination by flushing the snow during irradiation with UV light. A selection of UV filters allowed examination of the effects of the 200 and 305 nm absorption bands of nitrate. Nitrate concentration and photon flux were measured in the snow. The quantum yield for loss of nitrate was observed to decrease from 0.44 to 0.003 within what corresponds to days of UV exposure in Antarctica. The superposition of photolysis in two photochemical domains of nitrate in snow is proposed: one of photolabile nitrate, and one of buried nitrate. The difference lies in the ability of reaction products to escape the snow crystal, versus undergoing secondary (recombination) chemistry. Modeled NOx emissions may increase significantly above measured values due to the observed quantum yield in this study. The apparent quantum yield in the 200 nm band was found to be ∼1%, much lower than reported for aqueous chemistry. A companion paper presents an analysis of the change in isotopic composition of snowpack nitrate based on the same samples as in this study.

Laboratory study of nitrate photolysis in Antarctic snow. II. Isotopic effects and wavelength dependence.

Atmospheric nitrate is preserved in Antarctic snow firn and ice. However, at low snow accumulation sites, post-depositional processes induced by sunlight obscure its interpretation. The goal of these studies (see also Paper I by Meusinger et al. ["Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry," J. Chem. Phys. 140, 244305 (2014)]) is to characterize nitrate photochemistry and improve the interpretation of the nitrate ice core record. Naturally occurring stable isotopes in nitrate ((15)N, (17)O, and (18)O) provide additional information concerning post-depositional processes. Here, we present results from studies of the wavelength-dependent isotope effects from photolysis of nitrate in a matrix of natural snow. Snow from Dome C, Antarctica was irradiated in selected wavelength regions using a Xe UV lamp and filters. The irradiated snow was sampled and analyzed for nitrate concentration and isotopic composition (δ(15)N, δ(18)O, and Δ(17)O). From these measurements an average photolytic isotopic fractionation of (15)ɛ = (-15 ± 1.2)‰ was found for broadband Xe lamp photolysis. These results are due in part to excitation of the intense absorption band of nitrate around 200 nm in addition to the weaker band centered at 305 nm followed by photodissociation. An experiment with a filter blocking wavelengths shorter than 320 nm, approximating the actinic flux spectrum at Dome C, yielded a photolytic isotopic fractionation of (15)ɛ = (-47.9 ± 6.8)‰, in good agreement with fractionations determined by previous studies for the East Antarctic Plateau which range from -40 to -74.3‰. We describe a new semi-empirical zero point energy shift model used to derive the absorption cross sections of (14)NO3 (-) and (15)NO3 (-) in snow at a chosen temperature. The nitrogen isotopic fractionations obtained by applying this model under the experimental temperature as well as considering the shift in width and center well reproduced the values obtained in the laboratory study. These cross sections can be used in isotopic models to reproduce the stable isotopic composition of nitrate found in Antarctic snow profiles.

Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change.

Nitrogen stable isotope ratio (δ(15)N) in Greenland snow nitrate and in North American remote lake sediments has decreased gradually beginning as early as ∼1850 Christian Era. This decrease was attributed to increasing atmospheric deposition of anthropogenic nitrate, reflecting an anthropogenic impact on the global nitrogen cycle, and the impact was thought to be amplified ∼1970. However, our subannually resolved ice core records of δ(15)N and major ions (e.g., NO3(-), SO4(2-)) over the last ∼200 y show that the decrease in δ(15)N is not always associated with increasing NO3(-) concentrations, and the decreasing trend actually leveled off ∼1970. Correlation of δ(15)N with H(+), NO3(-), and HNO3 concentrations, combined with nitrogen isotope fractionation models, suggests that the δ(15)N decrease from ∼1850-1970 was mainly caused by an anthropogenic-driven increase in atmospheric acidity through alteration of the gas-particle partitioning of atmospheric nitrate. The concentrations of NO3(-) and SO4(2-) also leveled off ∼1970, reflecting the effect of air pollution mitigation strategies in North America on anthropogenic NO(x) and SO2 emissions. The consequent atmospheric acidity change, as reflected in the ice core record of H(+) concentrations, is likely responsible for the leveling off of δ(15)N ∼1970, which, together with the leveling off of NO3(-) concentrations, suggests a regional mitigation of anthropogenic impact on the nitrogen cycle. Our results highlight the importance of atmospheric processes in controlling δ(15)N of nitrate and should be considered when using δ(15)N as a source indicator to study atmospheric flux of nitrate to land surface/ecosystems.

Analysis of oxygen-17 excess of nitrate and sulfate at sub-micromole levels using the pyrolysis method.

RATIONALE: The oxygen-17 excess (Δ(17)O) of nitrate and sulfate contains valuable information regarding their atmospheric formation pathways. However, the current pyrolysis method to measure Δ(17)O requires large sample amounts (>4 µmol for nitrate and >1 µmol for sulfate). We present a new approach employing a Gas Bench interface which cryofocuses O2 produced from sample pyrolysis, enabling the analysis of sub-micromole size samples. METHODS: Silver nitrate or sulfate at sub-micromole levels in a sample container was thermally decomposed to O2 and byproducts in a modified Temperature Conversion/Elemental Analyzer (TC/EA). Byproducts (mainly NO2 for silver nitrate and SO2 for silver sulfate) were removed in a liquid nitrogen trap and the sample O2 was carried by ultra-pure helium (He) gas to a Gas Bench II interface where it was cryofocused prior to entering an isotope ratio mass spectrometer. RESULTS: Analysis of the international nitrate reference material USGS35 (Δ(17)O = 21.6‰) within the size range of 300-1000 nmol O2 gave a mean Δ(17)O value of (21.6 ± 0.69) ‰ (mean ±1σ). Three inter-laboratory calibrated sulfate reference materials, Sulf-α, Sulf-ß and Sulf-ε, each within the size range of 180-1000 nmol O2, were analyzed and shown to possess mean Δ(17)O values of (0.9 ± 0.10)‰, (2.1 ± 0.25)‰ and (7.0 ± 0.63)‰, respectively. CONCLUSIONS: The analyses of nitrate and sulfate reference materials at sub-micromole levels gave Δ(17)O values consistent with their accepted values. This new approach of employing the Gas Bench to cryofocus O2 after the pyrolysis of AgNO3 and Ag2SO4 particularly benefits the effort of measuring Δ(17)O in sample types with a low abundance of nitrate and sulfate such as ice cores.