Industrial-era decline in Arctic methanesulfonic acid is offset by increased biogenic sulfate aerosol.

Marine phytoplankton are primary producers in ocean ecosystems and emit dimethyl sulfide (DMS) into the atmosphere. DMS emissions are the largest biological source of atmospheric sulfur and are one of the largest uncertainties in global climate modeling. DMS is oxidized to methanesulfonic acid (MSA), sulfur dioxide, and hydroperoxymethyl thioformate, all of which can be oxidized to sulfate. Ice core records of MSA are used to investigate past DMS emissions but rely on the implicit assumption that the relative yield of oxidation products from DMS remains constant. However, this assumption is uncertain because there are no long-term records that compare MSA to other DMS oxidation products. Here, we share the first long-term record of both MSA and DMS-derived biogenic sulfate concentration in Greenland ice core samples from 1200 to 2006 CE. While MSA declines on average by 0.2 µg S kg-1 over the industrial era, biogenic sulfate from DMS increases by 0.8 µg S kg-1. This increasing biogenic sulfate contradicts previous assertions of declining North Atlantic primary productivity inferred from decreasing MSA concentrations in Greenland ice cores over the industrial era. The changing ratio of MSA to biogenic sulfate suggests that trends in MSA could be caused by time-varying atmospheric chemistry and that MSA concentrations alone should not be used to infer past primary productivity.

Rapid and precise measurement of carbonate clumped isotopes using laser spectroscopy.

Carbonate clumped isotope abundance is an important paleothermometer, but measurement is difficult, slow, and subject to cardinal mass (m/z) interferences using isotope ratio mass spectrometry (IRMS). Here, we describe an optical spectroscopic measurement of carbonate clumped isotopes. We have adapted a tunable infrared laser differential absorption spectrometer (TILDAS) system to measure the abundances of four CO2 isotopologues used for clumped isotope thermometry. TILDAS achieves the same precision (0.01‰ SE) as IRMS measurements rapidly (∼50 min per carbonate analysis) and using small samples (<2 mg of calcite), without making assumptions about 17O abundance in the sample. A temperature calibration based on 406 analyses of CO2 produced by digestion of 51 synthetic carbonates equilibrated at 6° to 1100°C is consistent with results for natural carbonates and previous calibrations. Our system results were indistinguishable from IRMS systems after replicating the InterCarb interlaboratory calibration. Measurement by TILDAS could change the landscape for clumped isotope analysis.

Solid organic-coated ammonium sulfate particles at high relative humidity in the summertime Arctic atmosphere.

SignificancePhysical and chemical properties of individual atmospheric particles determine their climate impacts. Hygroscopic inorganic salt particles mixed with trace amounts of organic material are predicted to be liquid under typical tropospheric conditions in the summertime Arctic. Yet, we unexpectedly observed a significant concentration of solid particles composed of ammonium sulfate with an organic coating under conditions of high relative humidity and low temperature. These particle properties are consistent with marine biogenic-derived new particle formation and growth, with particle collision hypothesized to result in the solid phase. This particle source is predicted to have increasing relevance in the context of declining Arctic sea ice and increasing open water, with impacts on clouds, and therefore climate.

Rapid and Precise Analysis of Carbon Dioxide Clumped Isotopic Composition by Tunable Infrared Laser Differential Spectroscopy.

The high precision measurement of doubly substituted ("clumped") isotopologues in CO2 is a topic of significant interest in isotope geochemistry. Here we describe the performance of a new isotope ratio laser spectrometer using tunable infrared laser differential absorption spectroscopy (TILDAS). The TILDAS instrument has two continuous-wave lasers to simultaneously measure the four isotopologues involved in the 12C16O2 + 13C16O18O ⇆ 13C16O2 + 12C16O18O exchange reaction. CO2 samples are trapped in a low volume (∼250 mL) optical multipass cell with a path length of 36 m. Each sample is compared to a reference gas, and clumped isotopologue precision of 0.01‰ (SE) is achieved within 20 min for 15 µmol samples. Similar precision is also achieved for bulk isotopic composition. The degree of rare isotope clumping in excess of strictly random distribution (Δ16O13C18O) measured by this TILDAS instrument varies linearly with theoretically calculated values and shows a very weak dependence on bulk isotopic composition.

Long-term urban carbon dioxide observations reveal spatial and temporal dynamics related to urban characteristics and growth.

Cities are concentrated areas of CO2 emissions and have become the foci of policies for mitigation actions. However, atmospheric measurement networks suitable for evaluating urban emissions over time are scarce. Here we present a unique long-term (decadal) record of CO2 mole fractions from five sites across Utah's metropolitan Salt Lake Valley. We examine "excess" CO2 above background conditions resulting from local emissions and meteorological conditions. We ascribe CO2 trends to changes in emissions, since we did not find long-term trends in atmospheric mixing proxies. Three contrasting CO2 trends emerged across urban types: negative trends at a residential-industrial site, positive trends at a site surrounded by rapid suburban growth, and relatively constant CO2 over time at multiple sites in the established, residential, and commercial urban core. Analysis of population within the atmospheric footprints of the different sites reveals approximately equal increases in population influencing the observed CO2, implying a nonlinear relationship with CO2 emissions: Population growth in rural areas that experienced suburban development was associated with increasing emissions while population growth in the developed urban core was associated with stable emissions. Four state-of-the-art global-scale emission inventories also have a nonlinear relationship with population density across the city; however, in contrast to our observations, they all have nearly constant emissions over time. Our results indicate that decadal scale changes in urban CO2 emissions are detectable through monitoring networks and constitute a valuable approach to evaluate emission inventories and studies of urban carbon cycles.

Isotopic evidence of multiple controls on atmospheric oxidants over climate transitions.

The abundance of tropospheric oxidants, such as ozone (O3) and hydroxyl (OH) and peroxy radicals (HO2 + RO2), determines the lifetimes of reduced trace gases such as methane and the production of particulate matter important for climate and human health. The response of tropospheric oxidants to climate change is poorly constrained owing to large uncertainties in the degree to which processes that influence oxidants may change with climate and owing to a lack of palaeo-records with which to constrain levels of atmospheric oxidants during past climate transitions. At present, it is thought that temperature-dependent emissions of tropospheric O3 precursors and water vapour abundance determine the climate response of oxidants, resulting in lower tropospheric O3 in cold climates while HOx (= OH + HO2 + RO2) remains relatively buffered. Here we report observations of oxygen-17 excess of nitrate (a proxy for the relative abundance of atmospheric O3 and HOx) from a Greenland ice core over the most recent glacial-interglacial cycle and for two Dansgaard-Oeschger events. We find that tropospheric oxidants are sensitive to climate change with an increase in the O3/HOx ratio in cold climates, the opposite of current expectations. We hypothesize that the observed increase in O3/HOx in cold climates is driven by enhanced stratosphere-to-troposphere transport of O3, and that reactive halogen chemistry is also enhanced in cold climates. Reactive halogens influence the oxidative capacity of the troposphere directly as oxidants themselves and indirectly via their influence on O3 and HOx. The strength of stratosphere-to-troposphere transport is largely controlled by the Brewer-Dobson circulation, which may be enhanced in colder climates owing to a stronger meridional gradient of sea surface temperatures, with implications for the response of tropospheric oxidants and stratospheric thermal and mass balance. These two processes may represent important, yet relatively unexplored, climate feedback mechanisms during major climate transitions.

Choice of 17 O correction affects clumped isotope (Δ47 ) values of CO2 measured with mass spectrometry.

RATIONALE: The clumped isotope composition of CO2 (Δ47 ) derived from carbonate is widely used as a paleotemperature proxy with broad applications in geoscience. Its accuracy is, however, limited by inter-laboratory discrepancies of reference materials and disagreement among carbonate geothermometer calibrations. Here we show how the correction for the abundance of 17 O influences these discrepancies. METHODS: We used CO2 -H2 O equilibration at known temperatures and phosphoric acid digested carbonates to generate CO2 samples with a wide range in 13 C and 18 O compositions. All samples were purified using an offline vacuum line. We used a Thermo MAT 253 isotope ratio mass spectrometer with a Faraday collector array for m/z 44-49, to measure δ13 C, δ18 O, δ47 , and Δ47 values. Data were compiled using a traditional correction ('Santrock') for the 17 O interference in m/z 45 as well as a more recently proposed correction ('Brand') for 17 O interference. Two reference frames using CO2 with distinct 13 C compositions were constructed to simulate an inter-laboratory comparison. RESULTS: The traditional Santrock 17 O correction leads to a simulated Δ47 inter-laboratory comparison offset of 0.06 ‰, and a 0.1 ‰ Δ47 range in CO2 -H2 O 23°C equilibrations that is dependent on the 13 C composition. The more recent Brand 17 O correction removes these discrepancies. The traditional 17 O correction yields distinct temperature-Δ47 calibration curves for synthetic carbonates precipitated using different methods to degas CO2 , while the more recent 17 O correction collapses all calibration data onto a single curve. CONCLUSIONS: The 17 O correction strategy employed by CO2 and carbonate clumped-isotope researchers can have a large effect on the accuracy of Δ47 values. Use of the traditional 17 O correction may have caused errors in published studies as large as 0.1 ‰ and may account for Δ47 differences among laboratories and disagreement among previously published carbonate clumped isotope thermometry calibrations. Copyright © 2016 John Wiley & Sons, Ltd.

Routine high-precision analysis of triple water-isotope ratios using cavity ring-down spectroscopy.

RATIONALE: Water isotope analysis for δ(2) H and δ(18) O values via laser spectroscopy is routine for many laboratories. While recent work has added the δ(17) O value to the high-precision suite, it does not follow that researchers will routinely obtain high precision (17) O excess (Δ(17) O). We demonstrate the routine acquisition of high-precision δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values using a commercially available laser spectroscopy instrument. METHODS: We use a Picarro L2140-i cavity ring-down spectroscopy analyzer with discrete liquid injections into an A0211 vaporization module by a Leap Technologies LC PAL autosampler. The instrument is run in two modes: (1) as recommended by the manufacturer (default mode) and (2) after modifying select default settings and using alternative data types (advanced mode). Reference waters analyzed over the course of 15 months while running unknown samples are used to assess system performance. RESULTS: The default mode provides precision for δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values that may be sufficient for many applications. When using the advanced mode, we reach a higher level of precision for δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values (0.4 mUr, 0.04 mUr, 0.07 mUr, 0.5 mUr, and 8 µUr, respectively, where mUr = 0.001 = ‰, and µUr = 10(-6) ) in a shorter amount of time and with fewer syringe actuations than in the default mode. The improved performance results from an increase in the total integration time for each injected water pulse. CONCLUSIONS: Our recommended approach for routine δ(2) H, δ(17) O, δ(18) O, d and Δ(17) O measurements with the Picarro L2140-i is to make use of conditioning vials, use fewer injections (5 per vial) with greater pulse duration (520 seconds (s) per injection) and use only the first 120 s for δ(2) H measurements and all 520 s for δ(17) O and δ(18) O measurements. Although the sample throughput is 10 unknowns per day, our optimal approach reduces the number of syringe actuations, the effect of memory, and the total analysis time, while improving precision relative to the default approach. Copyright © 2016 John Wiley & Sons, Ltd.

The production of nitric oxide by marine ammonia-oxidizing archaea and inhibition of archaeal ammonia oxidation by a nitric oxide scavenger.

Nitrification is a critical process for the balance of reduced and oxidized nitrogen pools in nature, linking mineralization to the nitrogen loss processes of denitrification and anammox. Recent studies indicate a significant contribution of ammonia-oxidizing archaea (AOA) to nitrification. However, quantification of the relative contributions of AOA and ammonia-oxidizing bacteria (AOB) to in situ ammonia oxidation remains challenging. We show here the production of nitric oxide (NO) by Nitrosopumilus maritimus SCM1. Activity of SCM1 was always associated with the release of NO with quasi-steady state concentrations between 0.05 and 0.08 µM. NO production and metabolic activity were inhibited by the nitrogen free radical scavenger 2-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (PTIO). Comparison of marine and terrestrial AOB strains with SCM1 and the recently isolated marine AOA strain HCA1 demonstrated a differential sensitivity of AOB and AOA to PTIO and allylthiourea (ATU). Similar to the investigated AOA strains, bulk water column nitrification at coastal and open ocean sites with sub-micromolar ammonia/ammonium concentrations was inhibited by PTIO and insensitive to ATU. These experiments support predictions from kinetic, molecular and biogeochemical studies, indicating that marine nitrification at low ammonia/ammonium concentrations is largely driven by archaea and suggest an important role of NO in the archaeal metabolism.

Holocene history of ENSO variance and asymmetry in the eastern tropical Pacific.

Understanding the response of the El Niño-Southern Oscillation (ENSO) to global warming requires quantitative data on ENSO under different climate regimes. Here, we present a reconstruction of ENSO in the eastern tropical Pacific spanning the past 10,000 years derived from oxygen isotopes in fossil mollusk shells from Peru. We found that ENSO variance was close to the modern level in the early Holocene and severely damped ~4000 to 5000 years ago. In addition, ENSO variability was skewed toward cold events along coastal Peru 6700 to 7500 years ago owing to a shift of warm anomalies toward the Central Pacific. The modern ENSO regime was established ~3000 to 4500 years ago. We conclude that ENSO was sensitive to changes in climate boundary conditions during the Holocene, including but not limited to insolation.

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.

Measurement of SLAP2 and GISP δ17O and proposed VSMOW-SLAP normalization for δ17O and 17O(excess).

RATIONALE: The absence of an agreed-upon δ(17)O value for the primary reference water SLAP leads to significant discrepancies in the reported values of δ(17)O and the parameter (17)O(excess). The accuracy of δ(17)O and (17)O(excess) values is significantly improved if the measurements are normalized using a two-point calibration, following the convention for δ(2)H and δ(18)O values. METHODS: New measurements of the δ(17)O values of SLAP2 and GISP are presented and compared with published data. Water samples were fluorinated with CoF(3). Helium carried the O(2) product to a 5A (4.2 to 4.4 Å) molecular sieve trap submerged in liquid nitrogen. The O(2) sample was introduced into a dual-inlet ThermoFinnigan MAT 253 isotope ratio mass spectrometer for measurement of m/z 32, 33, and 34. The δ(18)O and δ(17) values were calculated after 90 comparisons with an O(2) reference gas. RESULTS: We propose that the accepted δ(17)O value of SLAP be defined in terms of δ(18) O = -55.5 ‰ and (17)O(excess) = 0, yielding a δ(17)O value of approximately -29.6986 ‰ [corrected]. Using this definition for SLAP and the recommended normalization procedure, the δ(17)O value of GISP is -13.16 ± 0.05 ‰ and the (17)O(excess) value of GISP is 22 ± 11 per meg. Correcting previous published values of GISP δ(17)O to both VSMOW and SLAP improves the inter-laboratory precision by about 10 per meg. CONCLUSIONS: The data generated here and compiled from previous studies provide a substantial volume of evidence to evaluate the various normalization techniques currently used for triple oxygen isotope measurements. We recommend that reported δ(17) O and (17)O(excess) values be normalized to the VSMOW-SLAP scale, using a definition of SLAP such that its (17)O(excess) is exactly zero.

Oxygen isotope exchange with quartz during pyrolysis of silver sulfate and silver nitrate.

RATIONALE: Triple oxygen isotopes of sulfate and nitrate are useful metrics for the chemistry of their formation. Existing measurement methods, however, do not account for oxygen atom exchange with quartz during the thermal decomposition of sulfate. We present evidence for oxygen atom exchange, a simple modification to prevent exchange, and a correction for previous measurements. METHODS: Silver sulfates and silver nitrates with excess (17)O were thermally decomposed in quartz and gold (for sulfate) and quartz and silver (for nitrate) sample containers to O(2) and byproducts in a modified Temperature Conversion/Elemental Analyzer (TC/EA). Helium carries O(2) through purification for isotope-ratio analysis of the three isotopes of oxygen in a Finnigan MAT253 isotope ratio mass spectrometer. RESULTS: The Δ(17)O results show clear oxygen atom exchange from non-zero (17)O-excess reference materials to zero (17)O-excess quartz cup sample containers. Quartz sample containers lower the Δ(17)O values of designer sulfate reference materials and USGS35 nitrate by 15% relative to gold or silver sample containers for quantities of 2-10 µmol O(2). CONCLUSIONS: Previous Δ(17)O measurements of sulfate that rely on pyrolysis in a quartz cup have been affected by oxygen exchange. These previous results can be corrected using a simple linear equation (Δ(17)O(gold) = Δ(17)O(quartz) * 1.14 + 0.06). Future pyrolysis of silver sulfate should be conducted in gold capsules or corrected to data obtained from gold capsules to avoid obtaining oxygen isotope exchange-affected data.

Alteration of micromilled carbonate δ18O during Kiel Device analysis.

Isotopic analysis of carbonate material has been greatly facilitated by the development of autosampling devices such as the Kiel III Carbonate Device, allowing rapid automated analysis of small sample sizes. This analysis is facilitated by holding samples and acid at temperatures around 70 °C prior to reaction. In most situations this has no measurable effect on sample powders on practical timeframes, but, when analyzing exceptionally fine-grained material produced by micromilling, the δ(18)O of both aragonite and calcite is altered by -0.1 ‰/day. Laboratories that use this technique should thoroughly test and correct for this phenomenon and avoid storing pre-weighed materials within the Kiel Device or similar drying oven prior to analysis.

An injection method for measuring the carbon isotope content of soil carbon dioxide and soil respiration with a tunable diode laser absorption spectrometer.

We present a novel technique in which the carbon isotope ratio (delta(13)C) of soil CO(2) is measured from small gas samples (<5 mL) injected into a stream of CO(2)-free air flowing into a tunable diode laser absorption spectrometer (TDL). This new method extends the dynamic range of the TDL to measure CO(2) mole fractions ranging from ambient to pure CO(2), reduces the volume of sample required to a few mL, and does not require field deployment of the instrument. The measurement precision of samples stored for up to 60 days was 0.23 per thousand. The new TDL method was applied with a simple gas well sampling technique to obtain and measure gas samples from shallow soil depth increments for CO(2) mole fraction and delta(13)C analysis, and subsequent determination of the delta(13)C of soil-respired CO(2). The method was tested using an artificial soil system containing a controlled CO(2) source and compared with an independent method using the TDL and an open soil chamber. The profile and chamber estimates of delta(13)C of an artificially produced CO(2) flux were consistent and converged to the delta(13)C of the CO(2) source at steady state, indicating the accuracy of both methods under controlled conditions. The new TDL method, in which a small pulse of sample is measured on a carrier gas stream, is analogous for the TDL technique to the development of continuous-flow configurations for isotope ratio mass spectrometry. While the applications presented here are focused on soil CO(2), this new TDL method could be applied in a number of situations requiring measurement of delta(13)C of CO(2) in small gas samples with ambient to high CO(2) mole fractions.

An automated system for stable isotope and concentration analyses of CO2 from small atmospheric samples.

We have developed an automated, continuous-flow isotope ratio mass spectrometry (CF-IRMS) system for the analysis of delta(13)C, delta(18)O, and CO(2) concentration (micromol mol(-1)) ([CO(2)]) from 2 mL of atmospheric air. Two replicate 1 mL aliquots of atmospheric air are sequentially sampled from fifteen 100 mL flasks. The atmospheric sample is inserted into a helium stream and sent through a gas chromatograph for separation of the gases and subsequent IRMS analysis. Two delta(13)C and delta(18)O standards and five [CO(2)] standards are run with each set of fifteen samples. We obtained a precision of 0.06 per thousand, 0.11 per thousand, and 0.48 micromol mol(-1) for delta(13)C, delta(18)O, and [CO(2)], respectively, by analyzing fifty 100 mL samples filled from five cylinders with a [CO(2)] range of 275 micromol mol(-1). Accuracy was determined by comparison with established methods (dual-inlet IRMS, and nondispersive infrared gas analysis) and found to have a mean offset of 0.00 per thousand, -0.09 per thousand, and -0.26 micromol mol(-1) for delta(13)C and delta(18)O, and [CO(2)], respectively.