Flask Samples Research Articles

First observations, trends, and emissions of HCFC‐31 (CH2ClF) in the global atmosphere

AbstractWe report the first multiyear atmospheric record of HCFC‐31 (CH2ClF), based on flask samples and in situ analyses of air from both hemispheres. Although HCFC‐31 has never been produced in large amounts, observed mole fractions in the Northern Hemisphere increased from 2000 onward, reaching 170 ppq (parts per quadrillion, 10−15) in 2011–2012 before decreasing rapidly. By combining our observations with a two‐dimensional atmospheric chemistry‐transport model, we infer an increase in global emissions from 240 t yr−1 in 2000 to 840 t yr−1 in 2011–2012, followed by a relatively fast decline to 570 t yr−1 in 2014. Emissions of HCFC‐31 originate most probably from intermediate product release during the manufacturing process of HFC‐32 (CH2F2). The rapid decline in recent years could be due to changes in production methods rather than declines in diffusive sources such as landfills or HCFC‐31 contaminations in merchandised HFC‐32.

Long-lived atmospheric trace gases measurements in flask samples from three stations in India

Abstract. With the rapid growth in population and economic development, emissions of greenhouse gases (GHGs) from the Indian subcontinent have sharply increased during recent decades. However, evaluation of regional fluxes of GHGs and characterization of their spatial and temporal variations by atmospheric inversions remain uncertain due to a sparse regional atmospheric observation network. As a result of an Indo-French collaboration, three new atmospheric stations were established in India at Hanle (HLE), Pondicherry (PON) and Port Blair (PBL), with the objective of monitoring the atmospheric concentrations of GHGs and other trace gases. Here we present the results of the measurements of CO2, CH4, N2O, SF6, CO, and H2 from regular flask sampling at these three stations over the period 2007–2011. For each species, annual means, seasonal cycles and gradients between stations were calculated and related to variations in natural GHG fluxes, anthropogenic emissions, and monsoon circulations. Covariances between species at the synoptic scale were analyzed to investigate the likely source(s) of emissions. The flask measurements of various trace gases at the three stations have the potential to constrain the inversions of fluxes over southern and northeastern India. However, this network of ground stations needs further extension to other parts of India to better constrain the GHG budgets at regional and continental scales.

Atmospheric CO2 and its δ13C measurements from flask sampling at Lin'an regional background station in China

This study presents CO2 concentrations of air flask samples obtained at a regional background station Lin'an (LAN) in China from January 2009 to December 2013, as well as their stable isotope ratios (δ13C) from January 2011 to December 2013. Background data is selected to represent background characteristics of atmospheric CO2 and δ13C over Yangzte River Delta. The annual mean background CO2 concentrations vary from 395.0 ppm in 2009 to 407.6 ppm in 2013, with a growth rate of 2.8 ± 0.1 ppm yr−1 over the study period, which is higher than the global average level. The annual mean values of δ13C vary from −8.55‰ in 2011 to −8.73‰ in 2013, with a growth rate of −0.029 ± 0.010‰ yr−1. Seasonal cycles of CO2 concentrations and δ13C are observed to be obvious at LAN. The minimum and maximum values of mean seasonal CO2 cycle appear in August and January, respectively, with the peak-to-peak amplitude of 19.4 ppm. The sign of δ13C seasonal cycle is opposite to that of CO2, and the peak-to-peak amplitude is 0.85‰. The isotopic signature of CO2 sources/sinks (δs) is also discussed in this paper. The estimated δs values for heating season (December–February) is −25.4‰ and for vegetative season is −21.3‰ (March–November) suggest the significant impact of fossil fuel combustions during winter heating season and biological activities during vegetative season.

Changes in the levels and variability of halocarbons and the compliance with the Montreal Protocol from an urban view

Ambient levels and variability of major atmospheric halocarbons, i.e. CFC-12, CFC-11, CFC-113, CCl4, CH3CCl3, C2HCl3, and C2Cl4 in a major metropolis (Taipei, Taiwan) were re-investigated after fourteen years by flask sampling in 2012. Our data indicates that the variability expressed as standard deviations (SD) of CFC-113 and CCl4 remained small (2.0ppt and 1.9ppt, respectively) for the 10th–90th percentile range in both sampling periods; whereas the variability of CFC-12, CFC-11, C2HCl3, and C2Cl4 measured in 2012 became noticeably smaller than observed in 1998, suggesting their emissions were reduced over time. By comparing with the background data of a global network (NOAA/ESRL/GMD baseline observatories), the ambient levels and distribution of these major halocarbons in Taipei approximated those at a background site (Mauna Loa) in 2012, suggesting that the fingerprint of the major halocarbons in a used-to-be prominent source area has gradually approached to that of the background atmosphere.

First on-line isotopic characterization of N<sub>2</sub>O above intensively managed grassland

Abstract. The analysis of the four main isotopic N2O species (14N14N16O, 14N15N16O, 15N14N16O, 14N14N18O) and especially the intramolecular distribution of 15N ("site preference", SP) has been suggested as a tool to distinguish source processes and to help constrain the global N2O budget. However, current studies suffer from limited spatial and temporal resolution capabilities due to the combination of discrete flask sampling with subsequent laboratory-based mass-spectrometric analysis. Quantum cascade laser absorption spectroscopy (QCLAS) allows the selective high-precision analysis of N2O isotopic species at trace levels and is suitable for in situ measurements. Here, we present results from the first field campaign, conducted on an intensively managed grassland site in central Switzerland. N2O mole fractions and isotopic composition were determined in the atmospheric surface layer (at 2.2 m height) at a high temporal resolution with a modified state-of-the-art laser spectrometer connected to an automated N2O preconcentration unit. The analytical performance was determined from repeated measurements of a compressed air tank and resulted in measurement repeatability of 0.20, 0.12 and 0.11‰ for δ15Nα, δ15Nβ and δ18O, respectively. Simultaneous eddy-covariance N2O flux measurements were used to determine the flux-averaged isotopic signature of soil-emitted N2O. Our measurements indicate that, in general, nitrifier-denitrification and denitrification were the prevalent sources of N2O during the campaign and that variations in isotopic composition were due to alterations in the extent to which N2O was reduced to N2 rather than to other pathways, such as hydroxylamine oxidation. Management and rewetting events were characterized by low values of the intramolecular 15N site preference (SP), δ15Nbulk and δ18O, suggesting that nitrifier-denitrification and incomplete heterotrophic bacterial denitrification responded most strongly to the induced disturbances. The flux-averaged isotopic composition of N2O from intensively managed grassland was 6.9 ± 4.3, −17.4 ± 6.2 and 27.4 ± 3.6‰ for SP, δ15Nbulk and δ18O, respectively. The approach presented here is capable of providing long-term data sets also for other N2O-emitting ecosystems, which can be used to further constrain global N2O inventories.

First continuous measurements of δ<sup>18</sup>O-CO<sub>2</sub> in air with a Fourier transform infrared spectrometer

Abstract. The continuous in situ measurement of δ18O in atmospheric CO2 opens a new door to differentiating between CO2 source and sink components with high temporal resolution. Continuous 13C–CO2 measurement systems have already been commercially available for some time, but until now, only few instruments have been able to provide a continuous measurement of the oxygen isotope ratio in CO2. Besides precise 13C/12C observations, the Fourier transform infrared (FTIR) spectrometer is also able to measure the 18O / 16O ratio in CO2, but the precision and accuracy of the measurements have not yet been evaluated. Here we present a first analysis of δ18O-CO2 (and δ13C-CO2) measurements with the FTIR analyser in Heidelberg. We used Allan deviation to determine the repeatability of δ18O-CO2 measurements and found that it decreases from 0.25‰ for 10 min averages to about 0.1‰ after 2 h and remains at that value up to 24 h. We evaluated the measurement precision over a 10-month period (intermediate measurement precision) using daily working gas measurements and found that our spectrometer measured δ18O-CO2 to better than 0.3‰ at a temporal resolution of less than 10 min. The compatibility of our FTIR-spectrometric measurements to isotope-ratio mass-spectrometric (IRMS) measurements was determined by comparing FTIR measurements of cylinder gases and ambient air with IRMS measurements of flask samples, filled with gases of the same cylinders or collected from the same ambient air intake. Two-sample t tests revealed that, at the 0.01 significance level, the FTIR and the IRMS measurements do not differ significantly from each other and are thus compatible. We describe two weekly episodes of ambient air measurements, one in winter and one in summer, and discuss what potential insights and new challenges combined highly resolved CO2, δ13C-CO2 and δ18O-CO2 records may provide in terms of better understanding regional scale continental carbon exchange processes.

ENSO-related variability in latitudinal distribution of annual mean atmospheric potential oxygen (APO) in the equatorial Western Pacific

We examined temporal variations in the latitudinal distribution of annual mean atmospheric potential oxygen (APO=O2+1.1×CO2), a useful tracer for studying ocean biogeochemical processes. To compute APO, we used atmospheric CO2 and O2 concentrations from flask samples and in-situ measurements onboard commercial cargo ships sailing between Japan and Australia/New Zealand. Most of the observed latitudinal distributions of the annual mean APO for the years 2002–2012 showed equatorial bulges, indicating tropical APO outgassing fluxes. However, the equatorial bulge was noticeably absent during the 2009/2010 El Niño period, especially in the Southern Hemisphere. The temporal variation in the 25–0°S latitudinal APO gradient correlated significantly with the El Niño/Southern Oscillation (ENSO); the equatorward APO gradients decreased (increased) during the El Niño (La Niña) period with a variability of about ±0.1 per meg/degree. Simulated APO based on an atmospheric transport model driven by climatological/constant flux fields and reanalysis meteorological data reproduced the overall characteristic of the observed temporal variation in the APO gradients well, suggesting that the atmospheric transport contributed substantially to the observed interannual variation in the global APO distributions. However, the model simulation underestimated the variability in the APO gradients by about 25%, compared to the observations. These discrepancies suggest a possibility of the existence of additional APO flux variability in the tropical Pacific, enhancing the ENSO-related variability in the observed APO gradients.

Quantification and source apportionment of the methane emission flux from the city of Indianapolis

Abstract We report the CH4 emission flux from the city of Indianapolis, IN, the site of the Indianapolis Flux Experiment (INFLUX) project for developing, assessing, and improving top-down and bottom-up approaches for quantifying urban greenhouse gas emissions. Using an aircraft-based mass balance approach, we find that the average CH4 emission rate from five flight experiments in 2011 is 135 ± 58 (1σ) moles s-1 (7800 ± 3300 kg hr-1). The effective per capita CH4 emission rate for Indianapolis is 77 kg CH4 person-1 yr-1, a figure that is less than the national anthropogenic CH4 emission (∼91 kg CH4 person-1 yr-1) but considerably larger than the global figure (∼48 kg CH4 person-1 yr-1). We consistently observed elevated CH4 concentrations at specific coordinates along our flight transects downwind of the city. Inflight investigations as well as back trajectories using measured wind directions showed that the elevated concentrations originated from the southwest side of the city where a landfill and a natural gas transmission regulating station (TRS) are located. Street level mobile measurements downwind of the landfill and the TRS supported the results of aircraft-based data, and were used to quantify the relative contributions from the two sources. We find that the CH4 emission from the TRS was negligible relative to the landfill, which was responsible for 33 ± 10% of the citywide emission flux. A regression of propane versus methane from aircraft flask samples suggests that the remaining citywide CH4 emissions (∼67%) derive from the natural gas distribution system. We discuss the combination of surface mobile observations and aircraft city-wide flux measurements to determine the total flux and apportionment to important sources.

Atmospheric CO2, δ(O2/N2), APO and oxidative ratios from aircraft flask samples over Fyodorovskoye, Western Russia

We present atmospheric CO2 and δ(O2/N2) from flask samples taken on board aircraft over Fyodorovskoye (56°27′N, 32°55′E) at heights of 3000 m and 100 m between 1998 and 2008. The long-term trends for CO2 and δ(O2/N2) are similar for both sampling heights, and also similar to observations from marine background station Mace Head (Ireland) and coastal station Lutjewad (the Netherlands). The seasonal CO2 amplitude at 100 m was almost twice as large as at 3000 m and a phase shift in the seasonality of about two weeks between both sampling heights was observed. This indicates a dominant influence on CO2 in the boundary layer from the regional biosphere which is confirmed by analysis of the δ(O2/N2) to CO2 oxidative ratio (OR). Together with simulations with the TM3 model, our data suggest that the observed OR of −1.7 ± 0.2 in the free troposphere is mainly driven by exchange processes with the ocean. Within the boundary layer an OR of −0.89 ± 0.12 was observed which supports the results of other recent studies suggesting the commonly used value of −1.1 for biospheric OR is likely too low.

High-precision quasi-continuous atmospheric greenhouse gas measurements at Trainou tower (Orléans forest, France)

Abstract. Results from the Trainou tall tower measurement station installed in 2006 are presented for atmospheric measurements of CO2, CH4, N2O, SF6, CO, H2 mole fractions and radon-222 activity. Air is sampled from four sampling heights (180, 100, 50 and 5 m) of the Trainou 200 m television tower in the Orléans forest in France (47°57'53" N, 2°06'45" E, 131 m a.s.l.). The station is equipped with a custom-built CO2 analyser (CARIBOU), which is based on a commercial non-dispersive, infrared (NDIR) analyser (Licor 6252), and a coupled gas chromatography (GC) system equipped with an electron capture detector (ECD) and a flame ionization detector (FID) (HP6890N, Agilent) and a reduction gas detector (PP1, Peak Performer). Air intakes, pumping and air drying system are shared between the CARIBOU and the GC systems. The ultimately achieved short-term repeatability (1 sigma, over several days) for the GC system is 0.05 ppm for CO2, 1.4 ppb for CH4, 0.25 ppb for N2O, 0.08 ppb for SF6, 0.88 ppb for CO and 3.8 for H2. The repeatability of the CARIBOU CO2 analyser is 0.06 ppm. In addition to the in situ measurements, weekly flask sampling is performed, and flask air samples are analysed at the Laboratoire des Sciences du Climat et de l'Environnement (LSCE) central laboratory for the same species as well for stable isotopes of CO2. The comparison between in situ measurements and the flask sampling showed averaged differences of 0.08 ± 1.40 ppm for CO2, 0.7 ± 7.3 ppb for CH4, 0.6 ± 0.6 ppb for N2O, 0.01 ± 0.10 ppt for SF6, 1.5± 5.3 ppb for CO and 4.8± 6.9 ppb for H2 for the years 2008–2012. At Trainou station, the mean annual increase rates from 2007 to 2011 at the 180 m sampling height were 2.2 ppm yr−1 for CO2, 4 ppb yr−1 for CH4, 0.78 ppb yr−1 for N2O and 0.29 ppt yr−1 for SF6. For all species, the 180 m sampling level showed the smallest diurnal variation. Mean diurnal gradients between the 50 m and the 180 m sampling level reached up to 30 ppm CO2, 15 ppm CH4 or 0.5 ppb N2O during nighttime whereas the mean gradients are smaller than 0.5 ppm for CO2 and 1.5 ppb for CH4 during afternoon.

Atmospheric measurement of point source fossil CO<sub>2</sub> emissions

Abstract. We use the Kapuni Gas Treatment Plant to examine methodologies for atmospheric monitoring of point source fossil fuel CO2 (CO2ff) emissions. The Kapuni plant, located in rural New Zealand, removes CO2 from locally extracted natural gas and vents that CO2 to the atmosphere, at a rate of ~0.1 Tg carbon per year. The plant is located in a rural dairy farming area, with no other significant CO2ff sources nearby, but large, diurnally varying, biospheric CO2 fluxes from the surrounding highly productive agricultural grassland. We made flask measurements of CO2 and 14CO2 (from which we derive the CO2ff component) and in situ measurements of CO2 downwind of the Kapuni plant, using a Helikite to sample transects across the emission plume from the surface up to 100 m above ground level. We also determined the surface CO2ff content averaged over several weeks from the 14C content of grass samples collected from the surrounding area. We use the WindTrax plume dispersion model to compare the atmospheric observations with the emissions reported by the Kapuni plant, and to determine how well atmospheric measurements can constrain the emissions. The model has difficulty accurately capturing the fluctuations and short-term variability in the Helikite samples, but does quite well in representing the observed CO2ff in 15 min averaged surface flask samples and in ~ one week integrated CO2ff averages from grass samples. In this pilot study, we found that using grass samples, the modeled and observed CO2ff emissions averaged over one week agreed to within 30%. The results imply that greater verification accuracy may be achieved by including more detailed meteorological observations and refining 14C sampling strategies.

Tunable diode laser in-situ CH<sub>4</sub> measurements aboard the CARIBIC passenger aircraft: instrument performance assessment

Abstract. A laser spectrometer for automated monthly measurements of methane (CH4) mixing ratios aboard the CARIBIC passenger aircraft is presented. The instrument is based on a commercial Fast Greenhouse Gas Analyser (FGGA, Los Gatos Res.), which was adapted to meet the requirements imposed by unattended airborne operation. It was characterised in the laboratory with respect to instrument stability, precision, cross sensitivity to H2O, and accuracy. For airborne operation, a calibration strategy is described that utilises CH4 measurements obtained from flask samples taken during the same flights. The precision of airborne measurements is 2 ppb for 10 s averages. The accuracy at aircraft cruising altitude is 3.85 ppb. During aircraft ascent and descent, where no flask samples were obtained, instrumental drifts can be less accurately determined and the uncertainty is estimated to be 12.4 ppb. A linear humidity bias correction was applied to the CH4 measurements, which was most important in the lower troposphere. On average, the correction bias was around 6.5 ppb at an altitude of 2 km, and negligible at cruising flight level. Observations from 103 long-distance flights are presented that span a large part of the northern hemispheric upper troposphere and lowermost stratosphere (UT/LMS), with occasional crossing of the tropics on flights to southern Africa. These accurate data mark the largest UT/LMS in-situ CH4 dataset worldwide. An example of a tracer-tracer correlation study with ozone is given, highlighting the possibility for accurate cross-tropopause transport analyses.

Characteristics of atmospheric carbon monoxide at a high-mountain background station in East Asia

Atmospheric CO were monitored at the Lulin Atmospheric Background Station (LABS) with an elevation of 2862 m AMSL from April 2006 to April 2011 by the in-situ non-dispersive infrared (NDIR) spectrometer and weekly flask sample collections via collaboration with NOAA/ESRL/GMD. In general very coherent results were observed between the two datasets, despite a slight difference between the two. A distinct seasonal pattern of CO was noticed at the LABS with a springtime maximum and a summertime minimum, which was predominately shaped by the long-range transport of biomass burning air masses from Southeast Asia and oceanic influences from the Pacific, respectively. Diurnal cycles were also observed at the LABS, with a maximum in late afternoon and a minimum in early morning. The daytime CO maximum was most likely caused by the up-slope transport of lower elevation air. After filtering out the possibly polluted data points from the entire dataset with a mathematic procedure, the mean background CO level at the LABS was assessed as 129.3 ± 46.6 ppb, compared to 149.0 ± 72.2 ppb prior to the filtering.The cluster analysis of the backward trajectories revealed six possible source regions, which shows that air masses originating from the Westerly Wind Zone were dominated in spring and winter resulting in higher CO concentrations. As a contrast, the oceanic influences from the Pacific were found mostly in summer, contributing a lower seasonal CO concentration throughout a year.

Atmospheric CO<sub>2</sub>, δ(O<sub>2</sub>/N<sub>2</sub>) and δ<sup>13</sup>CO<sub>2</sub> measurements at Jungfraujoch, Switzerland: results from a flask sampling intercomparison program

Abstract. We present results from an intercomparison program of CO2, δ(O2/N2) and δ13CO2 measurements from atmospheric flask samples. Flask samples are collected on a bi-weekly basis at the High Altitude Research Station Jungfraujoch in Switzerland for three European laboratories: the University of Bern, Switzerland, the University of Groningen, the Netherlands and the Max Planck Institute for Biogeochemistry in Jena, Germany. Almost 4 years of measurements of CO2, δ(O2/N2) and δ13CO2 are compared in this paper to assess the measurement compatibility of the three laboratories. While the average difference for the CO2 measurements between the laboratories in Bern and Jena meets the required compatibility goal as defined by the World Meteorological Organization, the standard deviation of the average differences between all laboratories is not within the required goal. However, the obtained annual trend and seasonalities are the same within their estimated uncertainties. For δ(O2/N2) significant differences are observed between the three laboratories. The comparison for δ13CO2 yields the least compatible results and the required goals are not met between the three laboratories. Our study shows the importance of regular intercomparison exercises to identify potential biases between laboratories and the need to improve the quality of atmospheric measurements.

N2O as a tracer of mixing stratospheric and tropospheric air based on CARIBIC data with applications for CO2

Stratosphere–troposphere exchange (STE) affects distributions of trace gases, with ozone (O3) and CO being commonly used to study mixing process in the UTLS (Upper Troposphere and Lower Stratosphere) region. Here we explore the application of N2O as a tracer of stratosphere to troposphere flux for CO2 and its isotopes, using N2O and CO2 mixing ratios including a set of CO2 isotope data for CARIBIC aircraft samples and NOAA-Carbon Cycle flask samples. A frequency distribution of CARIBIC N2O data (mostly UTLS at mid- and high-latitudes) reveals a narrow distribution around a tropospheric maximum at nearly the same N2O mixing ratio as the distribution peak for the station Mauna Loa (MLO) however with a skewing due to STE. We demonstrate that upper-tropospheric and STE-affected air can be distinguished using a threshold value based on the N2O distribution width at MLO. A comparison with the use of O3 is given. N2O is discussed to be a robust and linear (season and latitude independent) tracer of STE mixing proportions. We propose that CARIBIC data coupled with CO2 isotopic data published for the stratosphere and data of NOAA stations can be used as a frame of reference for δ18O(CO2) and Δ17O(CO2) STE fluxes. The Δ17O(CO2) flux is of particular interest as it is thought to constrain estimates of biosphere productivity, atmospheric oxygen cycle and global gross CO2 fluxes. CARBIC data can also help tracing STE flux by deep stratospheric intrusions and a data analysis is given for the Walliguan (WLG) observatory on the Tibetan Plateau.

Evaluation of a new JMA aircraft flask sampling system and laboratory trace gas analysis system

Abstract. We established and evaluated a flask air sampling system on a cargo C-130H aircraft, as well as a trace gas measurement system for the flask samples, as part of a new operational monitoring program of the Japan Meteorological Agency (JMA). Air samples were collected during each flight, between Kanagawa Prefecture (near Tokyo) and Minamitorishima (an island located nearly 2000 km southeast of Tokyo), from the air-conditioning system on the aircraft. Prior to the operational employment of the sampling system, a quality assurance test of the sampled air was made by specially coordinated flights at a low altitude of 1000 ft over Minamitorishima and comparing the flask values with those obtained at the surface. Based on our storage tests, the flask samples remained nearly stable until analyses. The trace gas measurement system has, in addition to the nondispersive infrared (NDIR) and vacuum ultraviolet resonance fluorescence (VURF) analyzers, two laser-based analyzers using wavelength-scanned cavity ring-down spectroscopy (WS-CRDS) and off-axis integrated cavity output spectroscopy (ICOS). Laboratory tests of the laser-based analyzers for measuring flask samples indicated relatively high reproducibility with overall precisions of less than ±0.06 ppm for CO2, ±0.68 ppb for CH4, ±0.36 ppb for CO, and ±0.03 ppb for N2O. Flask air sample measurements, conducted concurrently on different analyzers were compared. These comparisons showed a negligible bias in the averaged measurements between the laser-based measurement techniques and the other methods currently in use. We also estimated that there are no significant isotope effects for CH4, CO and N2O using standard gases with industrial isotopic compositions to calibrate the laser-based analyzers, but CO2 was found to possess isotope effects larger than its analytical precision.

Accurate measurements of carbon monoxide in humid air using the cavity ring-down spectroscopy (CRDS) technique

Abstract. Accurate measurements of carbon monoxide (CO) in humid air have been made using the cavity ring-down spectroscopy (CRDS) technique. The measurements of CO mole fractions are determined from the strength of its spectral absorption in the near-infrared region (~1.57 μm) after removing interferences from adjacent carbon dioxide (CO2) and water vapor (H2O) absorption lines. Water correction functions that account for the dilution and pressure-broadening effects as well as absorption line interferences from adjacent CO2 and H2O lines have been derived for CO2 mole fractions between 360–390 ppm and for reported H2O mole fractions between 0–4%. The line interference corrections are independent of CO mole fractions. The dependence of the line interference correction on CO2 abundance is estimated to be approximately −0.3 ppb/100 ppm CO2 for dry mole fractions of CO. Comparisons of water correction functions from different analyzers of the same type show significant differences, making it necessary to perform instrument-specific water tests for each individual analyzer. The CRDS analyzer was flown on an aircraft in Alaska from April to November in 2011, and the accuracy of the CO measurements by the CRDS analyzer has been validated against discrete NOAA/ESRL flask sample measurements made on board the same aircraft, with a mean difference between integrated in situ and flask measurements of −0.6 ppb and a standard deviation of 2.8 ppb. Preliminary testing of CRDS instrumentation that employs improved spectroscopic model functions for CO2, H2O, and CO to fit the raw spectral data (available since the beginning of 2012) indicates a smaller water vapor dependence than the models discussed here, but more work is necessary to fully validate the performance. The CRDS technique provides an accurate and low-maintenance method of monitoring the atmospheric dry mole fractions of CO in humid air streams.

A statistical approach to quantify uncertainty in carbon monoxide measurements at the Izaña global GAW station: 2008–2011

Abstract. Atmospheric CO in situ measurements are carried out at the Izaña (Tenerife) global GAW (Global Atmosphere Watch Programme of the World Meteorological Organization – WMO) mountain station using a Reduction Gas Analyser (RGA). In situ measurements at Izaña are representative of the subtropical Northeast Atlantic free troposphere, especially during nighttime. We present the measurement system configuration, the response function, the calibration scheme, the data processing, the Izaña 2008–2011 CO nocturnal time series, and the mean diurnal cycle by months. We have developed a rigorous uncertainty analysis for carbon monoxide measurements carried out at the Izaña station, which could be applied to other GAW stations. We determine the combined standard measurement uncertainty taking into consideration four contributing components: uncertainty of the WMO standard gases interpolated over the range of measurement, the uncertainty that takes into account the agreement between the standard gases and the response function used, the uncertainty due to the repeatability of the injections, and the propagated uncertainty related to the temporal consistency of the response function parameters (which also takes into account the covariance between the parameters). The mean value of the combined standard uncertainty decreased significantly after March 2009, from 2.37 nmol mol−1 to 1.66 nmol mol−1, due to improvements in the measurement system. A fifth type of uncertainty we call representation uncertainty is considered when some of the data necessary to compute the temporal mean are absent. Any computed mean has also a propagated uncertainty arising from the uncertainties of the data used to compute the mean. The law of propagation depends on the type of uncertainty component (random or systematic). In situ hourly means are compared with simultaneous and collocated NOAA flask samples. The uncertainty of the differences is computed and used to determine whether the differences are significant. For 2009–2011, only 24.5% of the differences are significant, and 68% of the differences are between −2.39 and 2.5 nmol mol−1. Total and annual mean differences are computed using conventional expressions but also expressions with weights based on the minimum variance method. The annual mean differences for 2009–2011 are well within the ±2 nmol mol−1 compatibility goal of GAW.

Long-term greenhouse gas measurements from aircraft

Abstract. In March 2009 the NOAA/ESRL/GMD Carbon Cycle and Greenhouse Gases Group collaborated with the US Coast Guard (USCG) to establish the Alaska Coast Guard (ACG) sampling site, a unique addition to NOAA's atmospheric monitoring network. This collaboration takes advantage of USCG bi-weekly Arctic Domain Awareness (ADA) flights, conducted with Hercules C-130 aircraft from March to November each year. Flights typically last 8 h and cover a large area, traveling from Kodiak up to Barrow, Alaska, with altitude profiles near the coast and in the interior. NOAA instrumentation on each flight includes a flask sampling system, a continuous cavity ring-down spectroscopy (CRDS) carbon dioxide (CO2)/methane (CH4)/carbon monoxide (CO)/water vapor (H2O) analyzer, a continuous ozone analyzer, and an ambient temperature and humidity sensor. Air samples collected in flight are analyzed at NOAA/ESRL for the major greenhouse gases and a variety of halocarbons and hydrocarbons that influence climate, stratospheric ozone, and air quality. We describe the overall system for making accurate greenhouse gas measurements using a CRDS analyzer on an aircraft with minimal operator interaction and present an assessment of analyzer performance over a three-year period. Overall analytical uncertainty of CRDS measurements in 2011 is estimated to be 0.15 ppm, 1.4 ppb, and 5 ppb for CO2, CH4, and CO, respectively, considering short-term precision, calibration uncertainties, and water vapor correction uncertainty. The stability of the CRDS analyzer over a seven-month deployment period is better than 0.15 ppm, 2 ppb, and 4 ppb for CO2, CH4, and CO, respectively, based on differences of on-board reference tank measurements from a laboratory calibration performed prior to deployment. This stability is not affected by variation in pressure or temperature during flight. We conclude that the uncertainty reported for our measurements would not be significantly affected if the measurements were made without in-flight calibrations, provided ground calibrations and testing were performed regularly. Comparisons between in situ CRDS measurements and flask measurements are consistent with expected measurement uncertainties for CH4 and CO, but differences are larger than expected for CO2. Biases and standard deviations of comparisons with flask samples suggest that atmospheric variability, flask-to-flask variability, and possible flask sampling biases may be driving the observed flask versus in situ CO2 differences rather than the CRDS measurements.

Long-term continuous atmospheric CO<sub>2</sub> measurements at Baring Head, New Zealand

Abstract. We present descriptions of the in situ instrumentation, calibration procedures, intercomparison efforts, and data filtering methods used in a 39-yr record of continuous atmospheric carbon dioxide (CO2) observations made at Baring Head, New Zealand. Located on the southern coast of the North Island, Baring Head is exposed to extended periods of strong air flow from the south with minimal terrestrial influence resulting in low CO2 variability. The site is therefore well suited for sampling air masses that are representative of the Southern Ocean region. Instrumental precision is better than 0.015 ppm (1-σ) on 1-Hz values. Comparisons to over 600 co-located flask samples, as well as laboratory based flask and cylinder comparison exercises, suggest that over recent decades compatibility with respect to the Scripps Institution of Oceanography (SIO) and World Meteorological Organisation (WMO) CO2 scales has been 0.3 ppm or better.