K. A. Walker

University of Toronto, Toronto, Ontario, Canada

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Publications (292)679.02 Total impact

  • Niall J. Ryan, Kaley A. Walker
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    ABSTRACT: A sensitivity study was performed to assess the impact that uncertainties in the spectroscopic parameters of atmospheric species have on the retrieval of gas concentrations using the 265–280 GHz region of the electromagnetic spectrum. Errors in the retrieval of O3, N2O, HNO3, and ClO from spectra measured by ground-based radiometers were investigated. The goal of the study was to identify the spectroscopic parameters of these target species, and other interfering species, available in the JPL and HITRAN 2008 catalogues, which contribute the largest error to retrieved atmospheric concentration profiles in order to provide recommendations for new laboratory measurements. The parameters investigated were the line position, line strength, broadening coefficients and their temperature dependence, and pressure shift. Uncertainties in the air broadening coefficients of gases tend to contribute the largest error to retrieved atmospheric concentration profiles. For O3 and N2O, gases with relatively strong spectral signatures, the retrieval is sensitive to uncertainties in the parameters of the main spectral line that is observed. For HNO3, the uncertainties in many closely spaced HNO3 lines can cause large errors in the retrieved profile, and for ClO, the error in the profile is dominated by uncertainties in nearby, stronger O3 lines. Fourteen spectroscopic parameters are identified, for which updated measurements would have the most impact on the accuracy of ground-based remote sensing of the target species at 265–280 GHz.
    Journal of Quantitative Spectroscopy and Radiative Transfer 08/2015; 161. DOI:10.1016/j.jqsrt.2015.03.012 · 2.29 Impact Factor
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    ABSTRACT: The MIPAS spectrometer onboard the Envisat platform observed infrared emission from the Earth's limb between 2002 and 2012. It recorded high-resolution spectra during day and night, from pole to pole and between 6 and 70 km altitude in the nominal measurement mode or up to 170 km in special measurement modes, producing daily more than 1000 vertical profiles of various trace gases. The operational Level-2 data are processed by ESA/DLR but there exist three other, independent research Level-2 processors that are hosted by ISAC-CNR/University of Bologna, Oxford University, and KIT IMK/IAA. All four Level-2 processors rely on the same Level-1b data provided by ESA but their retrieval schemes differ. As part of ESA's Ozone Climate Change Initiative project, an intercomparison of the four MIPAS processors took place, in which vertical ozone profiles retrieved by these four processors from MIPAS nominal mode measurements were compared for 2007 and 2008. We present the results of this comparison exercise, which consisted of five parts: an information content study of the vertical averaging kernels, an intercomparison of zonal seasonal means and spreads, a determination of biases through comparison to ozonesonde and lidar measurements, a comparison to other satellite records (bias estimation and precision assessment with respect to ACE-FTS and Aura-MLS data), and a geophysical validation of the provided error bars using MIPAS–MIPAS collocations.
    Remote Sensing of Environment 06/2015; 162. DOI:10.1016/j.rse.2014.12.013 · 4.77 Impact Factor
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    ABSTRACT: This paper contains a comprehensive investiga-tion of the sunset–sunrise difference (SSD, i.e., the sunset-minus-sunrise value) of the ozone mixing ratio in the lat-itude range of 10 • S–10 • N. SSD values were determined from solar occultation measurements based on data ob-tained from the Stratospheric Aerosol and Gas Experiment (SAGE) II, the Halogen Occultation Experiment (HALOE), and the Atmospheric Chemistry Experiment–Fourier trans-form spectrometer (ACE–FTS). The SSD was negative at al-titudes of 20–30 km (−0.1 ppmv at 25 km) and positive at 30–50 km (+0.2 ppmv at 40–45 km) for HALOE and ACE– FTS data. SAGE II data also showed a qualitatively simi-lar result, although the SSD in the upper stratosphere was 2 times larger than those derived from the other data sets. On the basis of an analysis of data from the Superconduct-ing Submillimeter-Wave Limb-Emission Sounder (SMILES) and a nudged chemical transport model (the specified dy-namics version of the Whole Atmosphere Community Cli-mate Model: SD–WACCM), we conclude that the SSD can be explained by diurnal variations in the ozone concentra-tion, particularly those caused by vertical transport by the at-mospheric tidal winds. All data sets showed significant sea-sonal variations in the SSD; the SSD in the upper strato-sphere is greatest from December through February, while that in the lower stratosphere reaches a maximum twice: dur-ing the periods March–April and September–October. Based on an analysis of SD–WACCM results, we found that these seasonal variations follow those associated with the tidal ver-tical winds.
    Atmospheric Chemistry and Physics 01/2015; 15(2):829-843. DOI:10.5194/acp-15-829-2015 · 4.88 Impact Factor
  • Atmospheric Chemistry and Physics 01/2015; 15(5):2487-2488. DOI:10.5194/acp-15-2487-2015 · 5.51 Impact Factor
  • Niall J. Ryan, Kaley A. Walker
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    ABSTRACT: A preparatory performance and error characterization was carried out for a ground-based millimeter wave instrument designed for high Arctic atmospheric research. The instrument is a radiometer to measure rotational emission spectra of O3, ClO, HNO3, and N2O, between 265 and 280 GHz, using a sensitive superconductor–insulator–superconductor detector. Forward and inverse modeling tests have been performed to assess the instrument/inversion system and to determine the sources of the most significant errors in the retrieval of each trace gas. The altitude ranges over which retrievals of concentrations can be made were found to be ~13–62 km for O3, ~12.5–39 km for N2O, ~12–36 km for HNO3, and ~18–46 km for ClO. For each target species the measurement and smoothing errors calculated with an optimal estimation method (OEM) were compared to the errors calculated from inversions of 500 simulated spectra. The absolute error from these inversions agreed well the OEM results, but there were systematic differences that are attributed to nonlinearities in the forward model. The results of these nonlinearities can cause biases of the order of 5–10% of the a priori profile if they are not accounted for when averaging concentration profiles or when analyzing trends in concentration. The techniques used here can be applied to any ground-based remote sounder.
    Journal of Quantitative Spectroscopy and Radiative Transfer 01/2015; 151:26–37. DOI:10.1016/j.jqsrt.2014.09.010 · 2.29 Impact Factor
  • Atmospheric Chemistry and Physics 01/2015; 15(4):4383-4426. DOI:10.5194/acpd-15-4383-2015 · 4.88 Impact Factor
  • 01/2015; 8(4):3697-3728. DOI:10.5194/amtd-8-3697-2015
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    ABSTRACT: Products from the Measurements Of Pollution In The Troposphere (MOPITT) instrument are regularly validated using in situ airborne measurements. However, few of these measurements reach into the upper troposphere, thus hindering MOPITT validation in that region. Here we evaluate upper tropospheric (~500 hPa to the tropopause) MOPITT CO profiles by comparing them to satellite Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) retrievals and to measurements from the High-performance Instrumented Airborne Platform for Environmental Research Pole to Pole Observations (HIPPO) Quantum Cascade Laser Spectrometer (QCLS). Direct comparison of co-located v5 MOPITT thermal-infrared-only retrievals, v3.0 ACE-FTS retrievals, and HIPPO-QCLS measurements show a slight positive MOPITT CO bias within its 10% accuracy requirement with respect to the other two datasets. Direct comparison of co-located ACE-FTS and HIPPO-QCLS measurements results in a small number of samples, due to the large disparity in sampling pattern and density of these datasets. Thus, two additional indirect techniques for comparison of non-coincident datasets have been applied: tracer-tracer (CO-O3) correlation analysis and analysis of profiles in tropopause coordinates. These techniques suggest a negative bias of ACE-FTS with respect to HIPPO-QCLS; this could be caused by differences in resolution (horizontal, vertical) or by deficiencies in the ACE-FTS CO retrievals below ~20 km of altitude, among others. We also investigate the temporal stability of MOPITT and ACE-FTS data, which provide unique global CO records and are thus important in climate analysis. Our results indicate that the relative bias between the two datasets has remained generally stable during the 2004–2010 period.
    12/2014; 119(24). DOI:10.1002/2014JD022397
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    ABSTRACT: We present the results of an extensive validation program of the most recent version of ozone vertical pro-files retrieved with the IMK/IAA (Institute for Meteorol-ogy and Climate Research/Instituto de Astrofísica de An-dalucía) MIPAS (Michelson Interferometer for Passive At-mospheric Sounding) research level 2 processor from ver-sion 5 spectral level 1 data. The time period covered corre-sponds to the reduced spectral resolution period of the MI-PAS instrument, i.e., January 2005–April 2012. The compar-ison with satellite instruments includes all post-2005 satellite limb and occultation sensors that have measured the vertical profiles of tropospheric and stratospheric ozone: ACE-FTS, GOMOS, HALOE, HIRDLS, MLS, OSIRIS, POAM, SAGE II, SCIAMACHY, SMILES, and SMR. In addition, balloon-borne MkIV solar occultation measurements and ground-based Umkehr measurements have been included, as well as two nadir sensors: IASI and SBUV. For each reference data set, bias determination and precision assessment are per-formed. Better agreement with reference instruments than for the previous data version, V5R_O3_220 (Laeng et al., 2014), Published by Copernicus Publications on behalf of the European Geosciences Union. 3972 A. Laeng et al.: Validation of MIPAS IMK/IAA V5R_O3_224 ozone profiles is found: the known high bias around the ozone vmr (vol-ume mixing ratio) peak is significantly reduced and the verti-cal resolution at 35 km has been improved. The agreement with limb and solar occultation reference instruments that have a known small bias vs. ozonesondes is within 7 % in the lower and middle stratosphere and 5 % in the upper tro-posphere. Around the ozone vmr peak, the agreement with most of the satellite reference instruments is within 5 %; this bias is as low as 3 % for ACE-FTS, MLS, OSIRIS, POAM and SBUV.
    Atmospheric Measurement Techniques 11/2014; 7(11):3971-3987. DOI:10.5194/amt-7-3971-2014 · 3.21 Impact Factor
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    ABSTRACT: The abundance of chlorine in the Earth's atmosphere increased considerably during the 1970s to 1990s, following large emissions of anthropogenic long-lived chlorine-containing source gases, notably the chlorofluorocarbons. The chemical inertness of chlorofluorocarbons allows their transport and mixing throughout the troposphere on a global scale, before they reach the stratosphere where they release chlorine atoms that cause ozone depletion. The large ozone loss over Antarctica was the key observation that stimulated the definition and signing in 1987 of the Montreal Protocol, an international treaty establishing a schedule to reduce the production of the major chlorine- and bromine-containing halocarbons. Owing to its implementation, the near-surface total chlorine concentration showed a maximum in 1993, followed by a decrease of half a per cent to one per cent per year, in line with expectations. Remote-sensing data have revealed a peak in stratospheric chlorine after 1996, then a decrease of close to one per cent per year, in agreement with the surface observations of the chlorine source gases and model calculations. Here we present ground-based and satellite data that show a recent and significant increase, at the 2σ level, in hydrogen chloride (HCl), the main stratospheric chlorine reservoir, starting around 2007 in the lower stratosphere of the Northern Hemisphere, in contrast with the ongoing monotonic decrease of near-surface source gases. Using model simulations, we attribute this trend anomaly to a slowdown in the Northern Hemisphere atmospheric circulation, occurring over several consecutive years, transporting more aged air to the lower stratosphere, and characterized by a larger relative conversion of source gases to HCl. This short-term dynamical variability will also affect other stratospheric tracers and needs to be accounted for when studying the evolution of the stratospheric ozone layer.
    Nature 11/2014; 515(7525):104-107. DOI:10.1038/nature13857 · 42.35 Impact Factor
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  • Nature Geoscience 08/2014; 7(10):768-776. DOI:10.1038/ngeo2236 · 11.67 Impact Factor
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    ABSTRACT: We present measurements of a long-range smoke transport event recorded on 20–21 July 2011 over Halifax, Nova Scotia, Canada, during the Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites (BORTAS-B) campaign. Ground-based Fourier transform spectrometers and photometers detected air masses associated with large wild-land fires burning in eastern Manitoba and western Ontario. We investigate a plume with high trace gas amounts but low amounts of particles that preceded and overlapped at the Halifax site with a second plume with high trace gas loadings and significant amounts of particulate material. We show that the first plume experienced a meteorological scavenging event, but the second plume had not been similarly scavenged. This points to the necessity to account carefully for the plume history when considering long-range transport since simultaneous or near-simultaneous times of arrival are not necessarily indicative of either similar trajectories or meteorological history. We investigate the origin of the scavenged plume, and the possibility of an aerosol wet deposition event occurring in the plume ∼ 24 h prior to the measurements over Halifax. The region of lofting and scavenging is only monitored on an intermittent basis by the present ob-serving network, and thus we must consider many different pieces of evidence in an effort to understand the early dynamics of the plume. Through this discussion we also demonstrate the value of having many simultaneous remote-sensing measurements in order to understand the physical and chemical behaviour of biomass burning plumes.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 08/2014; 14:8449-8460. DOI:10.5194/acp-14-8449-2014 · 5.30 Impact Factor
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    ABSTRACT: We present the first comprehensive intercomparison of currently available satellite ozone climatologies in the upper troposphere / lower stratosphere (UTLS) (300-70 hPa) as part of the Stratosphere-troposphere Processes and their Role in Climate (SPARC) Data Initiative. The Tropospheric Emission Spectrometer (TES) instrument is the only nadir-viewing instrument in this initiative, as well as the only instrument with a focus on tropospheric composition. We apply the TES observational operator to ozone climatologies from the more highly vertically resolved limb-viewing instruments. This minimizes the impact of differences in vertical resolution among the instruments and allows identification of systematic differences in the large-scale structure and variability of UTLS ozone. We find that the climatologies from most of the limb-viewing instruments show positive differences (ranging from 5 to 75%) with respect to TES in the tropical UTLS, and comparison to a “zonal mean” ozonesonde climatology indicates that these differences likely represent a positive bias for p ≤ 100 hPa. In the extratropics, there is good agreement among the climatologies regarding the timing and magnitude of the ozone seasonal cycle (differences in the peak-to-peak amplitude of <15%) when the TES observational operator is applied, as well as very consistent midlatitude interannual variability. The discrepancies in ozone temporal variability are larger in the tropics, with differences between the datasets of up to 55% in the seasonal cycle amplitude. However, the differences among the climatologies are everywhere much smaller than the range produced by current chemistry-climate models, indicating that the multiple-instrument ensemble is useful for quantitatively evaluating these models.
    06/2014; 119(11). DOI:10.1002/2013JD020822
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    ABSTRACT: Drifts, trends and periodic variations were calculated from monthly zonally averaged ozone profiles. The ozone profiles were derived from level-1b data of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) by means of the scientific level-2 processor run by the Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK). All trend and drift analyses were performed using a multilinear parametric trend model which includes a linear term, several harmonics with period lengths from 3 to 24 months and the quasi-biennial oscillation (QBO). Drifts at 2-sigma significance level were mainly negative for ozone relative to Aura MLS and Odin OSIRIS and negative or near zero for most of the comparisons to lidar measurements. Lidar stations used here include those at Hohenpeissenberg (47.8° N, 11.0° E), Lauder (45.0° S, 169.7° E), Mauna Loa (19.5° N, 155.6° W), Observatoire Haute Provence (43.9° N, 5.7° E) and Table Mountain (34.4° N, 117.7° W). Drifts against the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) were found to be mostly insignificant. The assessed MIPAS ozone trends cover the time period of July 2002 to April 2012 and range from -0.56 ppmv decade-1 to +0.48 ppmv decade-1 (-0.52 ppmv decade-1 to +0.47 ppmv decade-1 when displayed on pressure coordinates) depending on altitude/pressure and latitude. From the empirical drift analyses we conclude that the real ozone trends might be slightly more positive/less negative than those calculated from the MIPAS data, by conceding the possibility of MIPAS having a very small (approximately within -0.3 ppmv decade-1) negative drift for ozone. This leads to drift-corrected trends of -0.41 ppmv decade-1 to +0.55 ppmv decade-1 (-0.38 ppmv decade-1 to +0.53 ppmv decade-1 when displayed on pressure coordinates) for the time period covered by MIPAS Envisat measurements, with very few negative and large areas of positive trends at mid-latitudes for both hemispheres around and above 30 km (~10 hPa). Negative trends are found in the tropics around 25 and 35 km (~25 and 5 hPa), while an area of positive trends is located right above the tropical tropopause. These findings are in good agreement with the recent literature. Differences of the trends compared with the recent literature could be explained by a possible shift of the subtropical mixing barriers. Results for the altitude-latitude distribution of amplitudes of the quasi-biennial, annual and the semi-annual oscillation are overall in very good agreement with recent findings.
    Atmospheric Chemistry and Physics 02/2014; 14(5). DOI:10.5194/acp-14-2571-2014 · 4.88 Impact Factor
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    ABSTRACT: We present measurements of a long range smoke transport event recorded on 20–21 July 2011 over Halifax, Nova Scotia, Canada, during the Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites (BORTAS-B) campaign. Ground-based Fourier transform spectrometers and photometers detected air masses associated with large wildland fires burning in eastern Manitoba and western Ontario. We investigate a plume with high trace gas amounts but low amounts of particles that preceded and overlapped at the Halifax site with a second plume with high trace gas loadings and significant amounts of particulate material. We show that the first plume experienced a meteorological scavenging event but the second plume had not been similarly scavenged. This points to the necessity to account carefully for the plume history when considering long range transport since simultaneous or near-simultaneous times of arrival are not necessarily indicative of either similar trajectories or meteorological history. We investigate the origin of the scavenged plume, and the possibility of an aerosol wet deposition event occurring in the plume ~24 h prior to the measurements over Halifax. The region of lofting and scavenging is only monitored on an intermittent basis by the present observing network, and thus we must consider many different pieces of evidence in an effort to understand the early dynamics of the plume. Through this discussion we also demonstrate the value of having many simultaneous remote-sensing measurements in order to understand the physical and chemical behaviour of biomass burning plumes.
    Atmospheric Chemistry and Physics 02/2014; 14:3395-3426. DOI:10.5194/acpd-14-3395-2014 · 4.88 Impact Factor
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    ABSTRACT: An algorithm is developed to retrieve the vertical profile of carbon dioxide in the 5 to 25 km altitude range using mid-infrared solar occultation spectra from the main instrument of the ACE (Atmospheric Chemistry Experiment) mission, namely the Fourier Transform Spectrometer (FTS). The main challenge is to find an atmospheric phenomenon which can be used for accurate tangent height determination in the lower atmosphere, where the tangent heights (THs) calculated from geometric and timing information is not of sufficient accuracy. Error budgets for the retrieval of CO2 from ACE-FTS and the FTS on a potential follow-on mission named CASS (Chemical and Aerosol Sounding Satellite) are calculated and contrasted. Retrieved THs are typically within 60 m of those retrieved using the ACE version 3.x software after revisiting the temperature dependence of the N2 CIA (Collision-Induced Absorption) laboratory measurements and accounting for sulfate aerosol extinction. After correcting for the known residual high bias of ACE version 3.x THs expected from CO2 spectroscopic/isotopic inconsistencies, the remaining bias for tangent heights determined with the N2 CIA is -20m. CO2 in the 5-13 km range in the 2009-2011 time frame is validated against aircraft measurements from CARIBIC, CONTRAIL and HIPPO, yielding typical biases of -1.7 ppm in the 5-13 km range. The standard error of these biases in this vertical range is 0.4 ppm. The multi-year ACE-FTS dataset is valuable in determining the seasonal variation of the latitudinal gradient which arises from the strong seasonal cycle in the Northern Hemisphere troposphere. The annual growth of CO2 in this time frame is determined to be 2.5 ± 0.7 ppm yr-1, in agreement with the currently accepted global growth rate based on ground-based measurements.
    Atmospheric Measurement Techniques 01/2014; 7(2). DOI:10.5194/amtd-7-1691-2014 · 3.21 Impact Factor
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    ABSTRACT: We present a five-year time series of seven tropospheric species measured using a ground-based Fourier transform infrared (FTIR) spectrometer at the Polar Environment Atmospheric Research Laboratory (PEARL; Eureka, Nunavut, Canada; 80 degrees 05' N, 86 degrees 42' W) from 2007 to 2011. Total columns and temporal variabilities of carbon monoxide (CO), hydrogen cyanide (HCN) and ethane (C2H6) as well as the first derived total columns at Eureka of acetylene (C2H2), methanol (CH3OH), formic acid (HCOOH) and formaldehyde (H2CO) are investigated, providing a new data set in the sparsely sampled high latitudes. Total columns are obtained using the SFIT2 retrieval algorithm based on the optimal estimation method. The microwindows as well as the a priori profiles and variabilities are selected to optimize the information content of the retrievals, and error analyses are performed for all seven species. Our retrievals show good sensitivities in the troposphere. The seasonal amplitudes of the time series, ranging from 34 to 104%, are captured while using a single a priori profile for each species. The time series of the CO, C2H6 and C2H2 total columns at PEARL exhibit strong seasonal cycles with maxima in winter and minima in summer, in opposite phase to the HCN, CH3OH, HCOOH and H2CO time series. These cycles result from the relative contributions of the photochemistry, oxidation and transport as well as biogenic and biomass burning emissions. Comparisons of the FTIR partial columns with coincident satellite measurements by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) show good agreement. The correlation coefficients and the slopes range from 0.56 to 0.97 and 0.50 to 3.35, respectively, for the seven target species. Our new data set is compared to previous measurements found in the literature to assess atmospheric budgets of these tropospheric species in the high Arctic. The CO and C2H6 concentrations are consistent with negative trends observed over the Northern Hemisphere, attributed to fossil fuel emission decrease. The importance of poleward transport for the atmospheric budgets of HCN and C2H2 is highlighted. Columns and variabilities of CH3OH and HCOOH at PEARL are comparable to previous measurements performed at other remote sites. However, the small columns of H2CO in early May might reflect its large atmospheric variability and/or the effect of the updated spectroscopic parameters used in our retrievals. Overall, emissions from biomass burning contribute to the day-to-day variabilities of the seven tropospheric species observed at Eureka.
    Atmospheric Measurement Techniques 01/2014; 7(6):1547-1570. DOI:10.5194/amt-7-1547-2014 · 3.21 Impact Factor
  • Atmospheric Chemistry and Physics 01/2014; 14(11):16865-16906. DOI:10.5194/acpd-14-16865-2014 · 4.88 Impact Factor
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    ABSTRACT: The long-term evolution of the vertical column abundance of carbon tetrafluoride (CF4) above the high altitude Jungfraujoch station (Swiss Alps, 46.5 °N, 8.0 °E, 3580 m a.s.l.) has been derived from the spectrometric analysis of Fourier transform infrared solar spectra recorded at that site between 1989 and 2012. The investigation is based on a multi-microwindow approach, two encompassing pairs of absorption lines belonging to the strong ν3 band of CF4 centered at 1283 cm-1, and two additional ones to optimally account for weak but overlapping HNO3 interferences. The analysis reveals a steady accumulation of the very long-lived CF4 above the Jungfraujoch at mean rates of (1.38 ± 0.11) × 1013 molec cm-2 yr-1 from 1989 to 1997, and (0.97 ± 0.02) × 1013 molec cm-2 yr-1 from 1998 to 2012, which correspond to linear growth rates of 1.71 ± 0.14 and 1.04 ± 0.02% yr-1, respectively referenced to 1989 and 1998. Related global CF4 emissions required to sustain these mean increases correspond to 15.8 ± 1.3 and 11.1 ± 0.2 Gg yr-1 over the above specified time intervals. Findings reported here are compared and discussed with respect to relevant results obtained remotely from space and balloons as well as in situ on the ground, including new gas chromatography mass spectrometry measurements performed at the Jungfraujoch since 2010.
    01/2014; 7(1-1):333-344. DOI:10.5194/amt-7-333-2014

Publication Stats

3k Citations
679.02 Total Impact Points

Institutions

  • 2005–2015
    • University of Toronto
      • Department of Physics
      Toronto, Ontario, Canada
  • 1991–2011
    • University of Waterloo
      • Department of Chemistry
      Waterloo, Ontario, Canada
  • 2004
    • Trent University
      • Environmental and Resource Studies
      Питерборо, Ontario, Canada