N2O Cycling Research Articles

More N2O is no laughing matter

Nitrogen Cycling Because N2O is a potent greenhouse gas, tracking its sources and sinks—including those from natural processes—is imperative. Babbin et al. developed an isotopic tracer method to measure biological N2O reduction rates directly in the Eastern Tropical North Pacific Ocean. Incomplete denitrification results in the rapid cycling and net accumulation of N2O. As oxygen minimum zones expand in the global ocean, more N2O may enter the atmosphere than previously expected. Science , this issue p. [1127][1] [1]: /lookup/doi/10.1126/science.aaa8380

Applying cavity ring-down spectroscopy for the measurement of dissolved nitrous oxide concentrations and bulk nitrogen isotopic composition in aquatic systems: Correcting for interferences and field application

Laser spectroscopy is an emerging technology for measuring nitrous oxide (N2O) dynamics in the environment, but most studies have focused on atmospheric applications. We have coupled a commercially available cavity ring-down spectroscope (CRDS) (Picarro G5101-I isotopic N2O analyzer) to an air/water gas equilibration device to collect continuous in situ dissolved N2O molar concentration and bulk nitrogen isotopic (δ15N-N2O) data. The δ15N-N2O values measured by the CRDS unit were found to be significantly affected by changes in the mixing ratios of O2, CO, CH4, and CO2. There was also an effect of N2O mixing ratio on δ15N-N2O. A series of equations was developed to correct for the matrix effect of O2 and the spectral interference by CH4. Chemical traps effectively prevented interferences by CO and CO2. The maximum corrections required for N2O mixing ratio and O2 matrix effects, were 1‰ (at a mixing ratio of 1.2 ppmv), and 11‰ (at 0% O2 content), respectively. The CH4 correction only became important at mixing ratios greater than 500 ppmv (>0.5‰). Measurements of N2O molar concentration and δ15N-N2O from the CRDS isotopic N2O analyzer were similar to those measured with isotope ratio mass spectrometry. We demonstrated the utility of the laser-based system with field deployments in three estuarine tidal creeks in subtropical Australia. Future work in this field should focus on the application of the laser-based system to the measurement of N2O isotopologues in aquatic habitats, allowing for further constraints to be placed on the pathways of N2O cycling in aquatic system.

From the ground up: global nitrous oxide sources are constrained by stable isotope values.

Rising concentrations of nitrous oxide (N2O) in the atmosphere are causing widespread concern because this trace gas plays a key role in the destruction of stratospheric ozone and it is a strong greenhouse gas. The successful mitigation of N2O emissions requires a solid understanding of the relative importance of all N2O sources and sinks. Stable isotope ratio measurements (δ15N-N2O and δ18O-N2O), including the intramolecular distribution of 15N (site preference), are one way to track different sources if they are isotopically distinct. ‘Top-down’ isotope mass-balance studies have had limited success balancing the global N2O budget thus far because the isotopic signatures of soil, freshwater, and marine sources are poorly constrained and a comprehensive analysis of global N2O stable isotope measurements has not been done. Here we used a robust analysis of all available in situ measurements to define key global N2O sources. We showed that the marine source is isotopically distinct from soil and freshwater N2O (the continental source). Further, the global average source (sum of all natural and anthropogenic sources) is largely controlled by soils and freshwaters. These findings substantiate past modelling studies that relied on several assumptions about the global N2O cycle. Finally, a two-box-model and a Bayesian isotope mixing model revealed marine and continental N2O sources have relative contributions of 24–26% and 74–76% to the total, respectively. Further, the Bayesian modeling exercise indicated the N2O flux from freshwaters may be much larger than currently thought.

Isotopic constraints on marine and terrestrial N2O emissions during the last deglaciation.

Nitrous oxide (N2O) is an important greenhouse gas and ozone-depleting substance that has anthropogenic as well as natural marine and terrestrial sources. The tropospheric N2O concentrations have varied substantially in the past in concert with changing climate on glacial-interglacial and millennial timescales. It is not well understood, however, how N2O emissions from marine and terrestrial sources change in response to varying environmental conditions. The distinct isotopic compositions of marine and terrestrial N2O sources can help disentangle the relative changes in marine and terrestrial N2O emissions during past climate variations. Here we present N2O concentration and isotopic data for the last deglaciation, from 16,000 to 10,000 years before present, retrieved from air bubbles trapped in polar ice at Taylor Glacier, Antarctica. With the help of our data and a box model of the N2O cycle, we find a 30 per cent increase in total N2O emissions from the late glacial to the interglacial, with terrestrial and marine emissions contributing equally to the overall increase and generally evolving in parallel over the last deglaciation, even though there is no a priori connection between the drivers of the two sources. However, we find that terrestrial emissions dominated on centennial timescales, consistent with a state-of-the-art dynamic global vegetation and land surface process model that suggests that during the last deglaciation emission changes were strongly influenced by temperature and precipitation patterns over land surfaces. The results improve our understanding of the drivers of natural N2O emissions and are consistent with the idea that natural N2O emissions will probably increase in response to anthropogenic warming.

Efficiency of Cascade Refrigeration Cycle Using C3H8, N2O, and N2

This article presents a new natural gas liquefaction cycle that utilizes propane (C3H8), nitrogen monoxide (N2O), and nitrogen gas (N2) cycles. A liquefaction cycle with staged compression was designed and simulated using HYSYS software for improving cycle efficiency. This included a cascade cycle with a three-stage compression consisting of a C3H8, an N2O, and an N2 cycle. The new C3H8, N2O, and N2 liquefaction cycle is compared to the optimized cascade cycle using propane, ethylene, and methane. The compressor work, specific energy, and coefficient of performance (COP) of the cascade cycles were compared and analyzed. The COP of the new cascade cycle that utilizes three-stage compression process 3 is 25% higher than that of basic single-stage process 1. Also, the new liquefaction cycle requires less specific power for the same amount of liquefied natural gas (LNG) produced by optimized cascade cycle. For example, the specific power and COP of the new cascade cycle are respectively up to 16% less and 14% higher than those of the optimized cascade cycle under the same conditions of feed gas composition and liquefaction rate.

Distribution and air–sea exchange of nitrous oxide in the coastal Bay of Bengal during peak discharge period (southwest monsoon)

In order to examine the impact of river discharge from the Indian subcontinent on the concentration and air–sea exchange of nitrous oxide (N2O) a study was conducted during peak discharge period in the coastal Bay of Bengal, The study revealed that freshwater discharge exerts a dominant control on the N2O cycling in the surface waters of the coastal Bay of Bengal. The surface concentration of N2O in the southwestern (SW) coastal Bay of Bengal was high (7.4±1.6nM) and supersaturated (126±27%) whereas contrasting trend was found in the northwestern (NW) region (4.9±0.3nM and 81±6%). Such spatial differences in N2O concentration and saturation were resulted from variable characteristics of the discharged waters, and vertical stratification. The NW region of the coastal Bay of Bengal was under the influence of the discharge from the Ganges River having N2O below the saturation in the estuary (82±5%) while the SW region was under the influence of peninsular river discharges that were super-saturated (187±29%). The low N2O concentration at NW region resulted from low concentrations in the source water (Ganges) as these waters were formed by melting of the Himalayan glacier where low ammonium concentrations were observed due to less human settlement resulting in lower nitrification rates. Higher concentration of N2O in the SW region was attributed to the discharge from monsoonal rivers containing high N2O concentrations, high nitrification rates and mild coastal upwelling. The sea-to-air fluxes of N2O suggest that NW region is a sink for atmospheric N2O due to discharge of under saturated water from Ganges and strong stratification while SW region is a source caused by coastal upwelling and discharge of highly saturated water from monsoonal rivers.

Energetic and exergetic analyses of a transcritical N2O heat pump system

In the present study, energetic and exergetic analyses of a transcritical nitrous oxide (N2O) heat pump cycle are carried out on the basis of the optimization of discharge pressure. A simulation code was developed to study cycle performances for the given design and operating parameters. This code was integrated with the thermodynamic property subroutine ‘N2OPROP’ to estimate the thermodynamic properties of N2O in subcritical and supercritical regions. Variation trends of optimal parameters for an N2O system are similar to those of a CO2 system. However, the N2O cycle exhibits higher cooling COP, lower compressor pressure ratio and lower discharge pressure and temperature, and higher second-law efficiency than those of CO2-based systems. An N2O system is inferior in terms of volumetric cooling capacity compared with a transcritical CO2 system. The maximum exergy loss is observed in the throttle valve (expansion valve) as in the case of a CO2 transcritical system due to low critical temperature.

An assessment of N-cycling and sources of N2O during a simulated rain event using natural abundance 15N

In order to accurately predict N2O emissions from agricultural soils and to develop effective management strategies, it is important to understand mechanisms underlying N2O emissions under field conditions. This involves identification of sources of N2O, which is currently methodologically challenging, especially under field conditions. We assessed the suitability of 15N tracers and natural abundance 15N to study N cycling and sources of N2O after a rainfall simulation in an annual cropping system in the Central Valley of California. Our natural abundance 15N approach differed from other studies due to a combination of emphasizing a per-event (e.g. rainfall simulation in this study) assessment of N2O emissions, applying high temporal sampling frequency during this event, determination of 15N of NH4+ and NO3− in addition to N2O, and data analysis using isotope models. In our study, the suitability of 15N tracers to assess N cycling and sources of N2O emissions was limited, likely due to a combination of a fine soil texture, the use of undisturbed soil cores, and a low 15N application rate. Based on natural abundance 15N, we were able to calculate gross NH4+ mineralization, NH4+ immobilization, nitrification and NO3− immobilization rates of 5.37±1.72, 2.70±1.72, 3.01±1.13 and 0.15±0.29μgNg−1soild−1, respectively. Natural abundance 15N was, however, a rather poor predictor of the contribution of nitrification versus denitrification to N2O production. Nevertheless, important trends in N2O reduction rates could be observed, showing a sharp increase from 48% to 78% in reduction of produced N2O between 2hours and 24hours after rainfall simulation, followed by a gradual decrease to 46% of reduction by the fifth day after rainfall simulation. We conclude that the natural abundance 15N approach is very promising to elucidate mechanisms driving N-cycling and N2O emissions during agricultural management or weather events, especially if isotope dynamics are incorporated in site-specific biogeochemical process models.

Trace metal requirements for microbial enzymes involved in the production and consumption of methane and nitrous oxide.

Fluxes of greenhouse gases to the atmosphere are heavily influenced by microbiological activity. Microbial enzymes involved in the production and consumption of greenhouse gases often contain metal cofactors. While extensive research has examined the influence of Fe bioavailability on microbial CO2 cycling, fewer studies have explored metal requirements for microbial production and consumption of the second- and third-most abundant greenhouse gases, methane (CH4), and nitrous oxide (N2O). Here we review the current state of biochemical, physiological, and environmental research on transition metal requirements for microbial CH4 and N2O cycling. Methanogenic archaea require large amounts of Fe, Ni, and Co (and some Mo/W and Zn). Low bioavailability of Fe, Ni, and Co limits methanogenesis in pure and mixed cultures and environmental studies. Anaerobic methane oxidation by anaerobic methanotrophic archaea (ANME) likely occurs via reverse methanogenesis since ANME possess most of the enzymes in the methanogenic pathway. Aerobic CH4 oxidation uses Cu or Fe for the first step depending on Cu availability, and additional Fe, Cu, and Mo for later steps. N2O production via classical anaerobic denitrification is primarily Fe-based, whereas aerobic pathways (nitrifier denitrification and archaeal ammonia oxidation) require Cu in addition to, or possibly in place of, Fe. Genes encoding the Cu-containing N2O reductase, the only known enzyme capable of microbial N2O conversion to N2, have only been found in classical denitrifiers. Accumulation of N2O due to low Cu has been observed in pure cultures and a lake ecosystem, but not in marine systems. Future research is needed on metalloenzymes involved in the production of N2O by enrichment cultures of ammonia oxidizing archaea, biological mechanisms for scavenging scarce metals, and possible links between metal bioavailability and greenhouse gas fluxes in anaerobic environments where metals may be limiting due to sulfide-metal scavenging.

A High Arctic soil ecosystem resists long‐term environmental manipulations

AbstractWe evaluated above‐ and belowground ecosystem changes in a 16 year, combined fertilization and warming experiment in a High Arctic tundra deciduous shrub heath (Alexandra Fiord, Ellesmere Island, NU, Canada). Soil emissions of the three key greenhouse gases (GHGs) (carbon dioxide, methane, and nitrous oxide) were measured in mid‐July 2009 using soil respiration chambers attached to a FTIR system. Soil chemical and biochemical properties including Q10 values for CO2, CH4, and N2O, Bacteria and Archaea assemblage composition, and the diversity and prevalence of key nitrogen cycling genes including bacterial amoA, crenarchaeal amoA, and nosZ were measured. Warming and fertilization caused strong increases in plant community cover and height but had limited effects on GHG fluxes and no substantial effect on soil chemistry or biochemistry. Similarly, there was a surprising lack of directional shifts in the soil microbial community as a whole or any change at all in microbial functional groups associated with CH4 consumption or N2O cycling in any treatment. Thus, it appears that while warming and increased nutrient availability have strongly affected the plant community over the last 16 years, the belowground ecosystem has not yet responded. This resistance of the soil ecosystem has resulted in limited changes in GHG fluxes in response to the experimental treatments.

Coupled Cycles of Dissolved Oxygen and Nitrous Oxide in Rivers along a Trophic Gradient in Southern Ontario, Canada

Diel (24-h) cycling of dissolved O2 (DO) in rivers is well documented, but evidence for coupled diel changes in DO and nitrogen cycling has only been demonstrated in hypereutrophic systems where DO approaches zero at night. Here, we show diel changes in N2O and DO concentration at several sites across a trophic gradient. Nitrous oxide concentration increased at night at all but one site in spring and summer, even when gas exchange was rapid and minimum water column DO was well above hypoxic conditions. Diel N2O curves were not mirror images of DO curves and were not symmetrical about the mean. Although inter- and intrasite variation was high, N2O peaked around the time of lowest DO at most of the sites. These results suggest that N2O must be measured several times per diel period to characterize curve shape and timing. Nitrous oxide concentration was not significantly correlated with NO3- concentration, contrary to studies in agricultural streams and to the current United Nations Intergovernmental Panel for Climate Change protocols for N2O emission estimation. The strong negative correlation between N2O concentration and daily minimum DO concentration suggested that N2O production was limited by DO. This is consistent with N2O produced by nitrite reduction. The ubiquity of diel N2O cycling suggests that most DO and N2O sampling strategies used in rivers are insufficient to capture natural variability. Ecosystem-level effects of microbial processes, such as denitrification, are sensitive to small changes in redox conditions in the water column even in low-nutrient oxic rivers, suggesting diel cycling of redox-sensitive compounds may exist in many aquatic systems.

Chemolithoautotrophic production mediating the cycling of the greenhouse gases N<sub>2</sub>O and CH<sub>4</sub> in an upwelling ecosystem

Abstract. The high availability of electron donors occurring in coastal upwelling ecosystems with marked oxyclines favours chemoautotrophy, in turn leading to high N2O and CH4 cycling associated with aerobic NH4+ (AAO) and CH4 oxidation (AMO). This is the case of the highly productive coastal upwelling area off central Chile (36° S), where we evaluated the importance of total chemolithoautotrophic vs. photoautotrophic production, the specific contributions of AAO and AMO to chemosynthesis and their role in gas cycling. Chemolithoautotrophy was studied at a time-series station during monthly (2007–2009) and seasonal cruises (January 2008, September 2008, January 2009) and was assessed in terms of the natural C isotopic ratio of particulate organic carbon (δ13POC), total and specific (associated with AAO and AMO) dark carbon assimilation (CA), and N2O and CH4 cycling experiments. At the oxycline, δ13POC averaged −22.2‰; this was significantly lighter compared to the surface (−19.7‰) and bottom layers (−20.7‰). Total integrated dark CA in the whole water column fluctuated between 19.4 and 2.924 mg C m−2 d−1, was higher during active upwelling, and contributed 0.7 to 49.7% of the total integrated autotrophic CA (photo plus chemoautotrophy), which ranged from 135 to 7.626 mg C m−2 d−1, and averaged 20.3% for the whole sampling period. Dark CA was reduced by 27 to 48% after adding a specific AAO inhibitor (ATU) and by 24 to 76% with GC7, a specific archaea inhibitor. This indicates that AAO and AMO microbes (most of them archaea) were performing dark CA through the oxidation of NH4+ and CH4. Net N2O cycling rates varied between 8.88 and 43 nM d−1, whereas net CH4 cycling rates ranged from −0.41 to −26.8 nM d−1. The addition of both ATU and GC7 reduced N2O accumulation and increased CH4 consumption, suggesting that AAO and AMO were responsible, in part, for the cycling of these gases. These findings show that chemically driven chemolithoautotrophy (with NH4+ and CH4 acting as electron donors) could be more important than previously thought in upwelling ecosystems, raising new questions concerning its relevance in the future ocean.

Denitrification and nitrous oxide cycling within the upper oxycline of the eastern tropical South Pacific oxygen minimum zone

One of the shallowest, most intense oxygen minimum zones (OMZs) is found in the eastern tropical South Pacific, off northern Chile and southern Peru. It has a strong oxygen gradient (upper oxycline) and high N2O accumulation. N2O cycling by heterotrophic denitrification along the upper oxycline was studied by measuring N2O production and consumption rates using an improved acetylene blockage method. Dissolved N2O and its isotope (15N:14N ratio in N2O or δ15N) and isotopomer composition (intramolecular distribution of 15N in the N2O or δ15Nα and δ15Nβ), dissolved O2, nutrients, and other oceanographic variables were also measured. Strong N2O accumulation (up to 86 nmol L−1) was observed in the upper oxycline followed by a decline (around 8-12 nmol L−1) toward the OMZ core. N2O production rates by denitrification (NO2− reduction to N2O) were 2.25 to 50.0 nmol L−1 d−1, whereas N2O consumption rates (N2O reduction to N2) were 2.73 and 70.8 nmol L−1 d−1. δ15N in N2O increased from 8.57% in the middle oxycline (50-m depth) to 14.87% toward the OMZ core (100-m depth), indicating the progressive use of N2O as an electron acceptor by denitrifying organisms. Isotopomer signals of N2O (δ15Nα and δ15Nβ) showed an abrupt change at the middle oxycline, indicating different mechanisms of N2O production and consumption in this layer. Thus, partial denitrification along with aerobic ammonium oxidation appears to be responsible for N2O accumulation in the upper oxycline, where O2 levels fluctuate widely; N2O reduction, on the other hand, is an important pathway for N2 production. As a result, the proportion of N2O consumption relative to its production increased as O2 decreased toward the OMZ core. A N2O mass balance in the subsurface layer indicates that only a small amount of the gas could be effluxed into the atmosphere (12.7-30.7 µmol m−2 d−1) and that most N2O is used as an electron acceptor during denitrification (107-468 µmol m−2 d−1).

Soil N2O and NO emissions from an arid, urban ecosystem

We measured soil nitrogen (N) cycling and fluxes of N2O and NO in three land‐use types across the metropolitan area of Phoenix, Arizona. Urbanization increased N2O emissions compared to native landscapes, primarily due to the expansion of fertilized and irrigated lawns. Fluxes of N2O from lawns ranged from 18 to 80 μg N m−2 h−1 and were significantly larger than managed xeric landscapes (2.5–22 μg N m−2 h−1) and remnant desert sites within the urban core (3.7–14 μg N m−2 h−1). In contrast, average NO fluxes from lawns were not significantly different from native desert when dry (6–80 μg N m−2 h−1 lawn; 5–16 μg N m−2 h−1 desert) and were lower than fluxes from deserts after wetting events. Furthermore, urbanization has significantly altered the temporal dynamics of NO emissions by replacing pulse‐driven desert ecosystems with year‐round irrigated, managed lawns. Short‐term, pulse‐driven emissions of NO from wetting of dry desert soils may reach 35% of anthropogenic emissions within a day after summer monsoon storms. If regional O3 production is NOx‐limited during the monsoon season, fluxes from warm, recently wet arid soils may contribute to summer O3 episodes.

Seasonal cycle of N2O: Analysis of data

We carried out a systematic study of the seasonal cycle and its latitudinal variation in the nitrous oxide (N2O) data collected by National Oceanic and Atmospheric Administration–Global Monitoring Division (NOAA‐GMD) and the Advanced Global Atmospheric Gases Experiment (AGAGE). In order to confirm the weak seasonal signal in the observations, we applied the multitaper method for the spectrum analysis and studied the stations with significant seasonal cycle. In addition, the measurement errors must be small compared with the seasonal cycle. The N2O seasonal cycles from seven stations satisfied these criteria and were analyzed in detail. The stations are Alert (82°N, 62°W), Barrow (71°N, 157°W), Mace Head (53°N, 10°W), Cape Kumukahi (19°N, 155°W), Cape Matatula (14°S, 171°W), Cape Grim (41°S, 145°E) and South Pole (90°S, 102°W). The amplitude (peak to peak) of the seasonal cycle of N2O varies from 0.29 ppb (parts‐per‐billion by mole fraction in dry air) at the South Pole to 1.15 ppb at Alert. The month at which the seasonal cycle is at a minimum varies monotonically from April (South Pole) to September (Alert). The seasonal cycle in the Northern Hemisphere shows the influence of the stratosphere; the seasonal cycle of N2O in the Southern Hemisphere suggests greater influence from surface sources. Preliminary estimates are obtained for the magnitude of the seasonally varying sources needed to account for the observations.

Biogeochemical simulation of nitrous oxide cycle based on the major nitrogen processes

We modeled the biogeochemical nitrogen cycle with specific emphasis on N2O behavior to describe the global N2O cycle system quantitatively. Here all the major nitrogen cycle processes are included plus the processes directly related to N2O. The model includes 24 nitrogen reservoirs: 3 in the stratosphere (N2O, N2, and NOx), 4 in the troposphere (N2O, N2, NH3, and NOx), 7 on land (biospheric, N2O, NOx, PON, DON, NH4+, and NO3−), 6 in surface‐ocean (biospheric, PON, DON, N2O, NH4+, and NO3−), 3 in the deep ocean (N2O, NH4+, NO3−), and sedimentary organic nitrogen. The exploration of the steady state flux combination, which produces the tropospheric N2O concentration profile consistent with observations, provided the most likely estimates of the N2O emissions from soils, rivers, and oceans, including contributions from nitrification and denitrification. Full incorporation of nitrogen species facilitated an analysis of the influences of various nitrogen cycle processes on tropospheric N2O concentration. Our sensitivity analyses revealed that the tropospheric N2O concentration was linked closely to other nitrogen species and was especially sensitive to terrestrial biological activities via nitrogen fixation, nitrogen assimilation, and ammonification. This result implies that the N2O budget in the troposphere can be constrained well by these terrestrial processes.

Southern Ocean ventilation inferred from seasonal cycles of atmospheric N2O and O2/N2 at Cape Grim, Tasmania

The seasonal cycle of atmospheric N2O is derived from a 10-yr observational record at Cape Grim, Tasmania (41°S, 145°E). After correcting for thermal and stratospheric influences, the observed atmospheric seasonal cycle is consistent with the seasonal outgassing of microbially produced N2O from the Southern Ocean, as predicted by an ocean biogeochemistry model coupled to an atmospheric transport model (ATM). The model—observation comparison suggests a Southern Ocean N2O source of ~0.9 Tg N yr−1 and is the first study to reproduce observed atmospheric seasonal cycles in N2O using specified surface sources in forward ATM runs. However, these results are sensitive to the thermal and stratospheric corrections applied to the atmospheric N2O data. The correlation in subsurface waters between apparent oxygen utilization (AOU) and N2O production (approximated as the concentration in excess of atmospheric equilibrium ΔN2O) is exploited to infer the atmospheric seasonal cycle in O2/N2 due to ventilation of O2-depleted subsurface waters. Subtracting this cycle from the observed, thermally corrected seasonal cycle in atmospheric O2/N2 allows the residual O2/N2 signal from surface net community production to be inferred. Because N2O is only produced in subsurface ocean waters, where it is correlated to O2 consumption, atmospheric N2O observations provide a methodology for distinguishing the surface production and subsurface ventilation signals in atmospheric O2/N2, which have previously been inseparable.

N2O cycling at the core of the oxygen minimum zone off northern Chile

Northern Chile's oxygen minimum zone (OMZ) is considered to be an important site of N 2 O production and efflux into the atmosphere, with a potentially global impact. Seawater samples from the OMZ core were used to determine how different O 2 levels and electron donor/acceptor availability affect N 2 O cycling. N 2 O production by denitrification (N 2 Opd; acetylene treatment) and nitrification (N 2 Opn; allylthiourea [ATU] treatment), and N 2 O consumption by denitrification (N 2 Ocd), were determined with in situ O 2 , anoxic (0 μM O 2 ), hypoxic (∼22.3 μM O 2 ), and potential (added substrate) experimental conditions. Under in situ O 2 levels (∼4.6 pM), total N 2 O production (N 2 Opd + N 2 Opn) was ∼2.62 μM d -1 . Denitrification was responsible for over 92% of the total N 2 Opd and nitrification for less than 8%. Nearly 100% of the N 2 O produced was, however, consumed by denitrification. NO 3 - was reduced twice as rapidly as NO 2 - . Under anoxia, N 2 Opd and N 2 Ocd rates decreased by over 90%. The NO 3 - reduction was similar to that observed with in situ O 2 , whereas a high rate of NO 2 - accumulation was observed. Conversely, increasing O 2 levels (∼22.3 μM) doubled N 2 Opd. Consequently, N 2 Opd or NOW reduction seems to be the process most sensitive to O 2 fluctuations. Adding organic carbon and NO 3 - increased N 2 Opd and N 2 Ocd slightly, whereas additional N 2 O increased N 2 Ocd abruptly. The fate of reduced NO 3 - in the OMZ core was controlled mainly by O 2 concentrations and indirectly by available organic carbon. Both variables are susceptible to the changes experienced in the eastern South Pacific during the El Nino Southern Oscillation cycle.

Three years of trace gas observations over the EuroSiberian domain derived from aircraft sampling — a concerted action

A three-year trace gas climatology of CO2 and its stable isotopic ratios, as well as CH4, N2O and SF6, derived from regular vertical aircraft sampling over the Eurasian continent is presented. The four sampling sites range from about 1°E to 89°E in the latitude belt from 48°N to 62°N. The most prominent features of the CO2 observations are an increase of the seasonal cycle amplitudes of CO2 andδ13C–CO2 in the free troposphere (at 3000 m a.s.l.) by more than 60% from Western Europe to Western and Central Siberia. δ18O–CO2 shows an even larger increase of the seasonal cycle amplitude by a factor of two from Western Europe towards the Ural mountains, which decreases again towards the most eastern site, Zotino. These data reflect a strong influence of carbon exchange fluxes with the continental biosphere. In particular, during autumn and winter δ18O–CO2 shows a decrease by more than 0.5% from Orléans (Western Europe) to Syktyvkar (Ural mountains) and Zotino (West Siberia), mainly caused by soil respiration fluxes depleted inδ18O with respect to atmospheric CO2. CH4 mixing ratios in the free troposphere at 3000 m over Western Siberia are higher by about 20–30 ppb if compared to Western Europe. Wetland emissions seem to be particularly visible in July–September, with largest signals at Zotino in 1998. Annual mean CH4 mixing ratios decrease slightly from 1998 to 1999 at all Russian sites. In contrast to CO2 and CH4, which show significant vertical gradients between 2000 and 3000 m a.s.l., N2O mixing ratios are vertically very homogeneous and show no significant logitudinal gradient between the Ural mountains and Western Siberia, indicating insignificant emissions of this trace gas from boreal forest ecosystems in Western Siberia. The growth rate of N2O (1.2–1.3 ppb yr−1) and the seasonal amplitude (0.5–1.1 ppb) are similar at both aircraft sites, Syktyvkar and Zotino. For SF6 an annual increase of 5% is observed, together with a small seasonal cycle which is in phase with the N2O cycle, indicating that the seasonality of both trace gases are most probably caused by atmospheric transport processes with a possible contribution from stratosphere–troposphere exchange.

Temperature dependence of isotope fractionation in N2O photolysis

Stratospheric ultraviolet (UV) photolysis is the dominant sink reaction and main origin of isotopic enrichment for atmospheric nitrous oxide (N2O). To a large extent, the flux of isotopically heavy N2O from the stratosphere is responsible for the enrichment of tropospheric N2O relative to its sources at the Earth's surface. In order to simulate the stratospheric enrichments quantitatively in atmospheric models and to examine the global N2O cycle using isotope measurements, knowledge of the fractionation constants is required. However, to date, all experimental studies of isotopic enrichment in N2O photolysis have been performed at room temperature only. Here we report the first temperature-dependent (193 < T/K < 295) measurements of 18O and position-dependent 15N fractionation constants obtained by broadband photolysis at wavelengths of relevance to the stratospheric “UV window”. For a given extent of reaction, we find higher enrichments at lower temperatures, qualitatively in agreement with theoretical predictions. The relative changes are in the order 14N15NO > N218O > 15N14NO, similar to the absolute values. If temperature was the only parameter of influence, not only the fractionation constants themselves, but also the ratio of fractionation constants at the central to terminal nitrogen sites, η = 15e2/15e1, should decrease along the vertical stratospheric temperature gradient. These temperature effects do not help to explain the lower η values observed in the lower stratosphere, but they are nevertheless essential ingredients for models of atmospheric isotope chemistry. We also investigate a hitherto unexplained artefact in laboratory measurements of N2O photolysis: At high degrees of conversion, N2O loss by the reaction with O(1D) becomes important, presumably due to the photochemical production and subsequent photolysis of NO2 in the reaction cell. The effect gains importance with increasing concentration and in the present study, it caused decreases in the measured fractionation constants requiring correction for initial N2O mixing ratios of 4 mmol mol−1.