L. Froidevaux

California Institute of Technology, Pasadena, California, United States

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Publications (252)763.33 Total impact

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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; 7:3971-3987. · 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. · 42.35 Impact Factor
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    ABSTRACT: Variability in the chemistry of the upper stratosphere / lower mesosphere (USLM) region has been analyzed focusing on high latitudes during the boreal winter 2009 characterized by the strong sudden stratospheric warming (SSW) on 24 January. Data from Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) aboard ENVISAT and the Microwave Limb Sounder (MLS) on Aura have been used to exemplify these changes. Record high (low) values of O3 and ClO (temperature and HCl) for the winters of 2005-2012, coupled with a simultaneous enhancement of ClONO2, have been observed in February 2009. This suggests that the very low temperatures favor a more effective ozone production and a greater O3/O ratio. The latter is the main factor controlling active chlorine partitioning. Increases of ClO lead to high ClONO2 concentrations in the upper stratosphere at high latitudes, where its photodissociation rate is smaller. Since this increase of ClONO2 happens at the expense of HCl, the region of high ClONO2 coincides roughly with the region of low HCl. Although this period was characterized by an elevated stratopause event, the investigated region was not influenced by the descent of mesospheric air rich in NOx. Some limited enhancements in NOx at ~1 hPa occurred at latitudes greater than 80° N after about 20 February but they became consistent only in March. Intrusion of mid-latitude air mostly occurred between the SSW and early February. Then, the sum of VMRs of ClONO2 + ClO + HCl remained approximately constant and close to the values of the other years. In contrast, it was up to 0.2 ppbv lower during the SSW period. These atypical chemical conditions occurred also in February 2006, but 2009 stands out for its long-lasting effects, which persisted until late March.
    Journal of Geophysical Research Atmospheres 09/2014; · 3.44 Impact Factor
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    ABSTRACT: Satellite measurements sample continuous fields of atmospheric constituents at discrete locations and times. However, insufficient or inhomogeneous sampling, if not taken into account, can result in inaccurate average estimates and even induce spurious features. We propose to characterize the spatio-temporal inhomogeneity of atmospheric measurements by a measure, which is a linear combination of the asymmetry and entropy of a sampling distribution. It is shown that this measure is related to the so-called sampling uncertainty, which occurs due to non-uniform sampling patterns. We have estimated the sampling uncertainty of zonal mean ozone profiles for six limb-viewing satellite instruments participating in the European Space Agency Ozone Climate Change Initiative project using the high-resolution ozone field simulated with the FinROSE chemistry-transport model. It is shown that the sampling uncertainty for the instruments with coarse sampling is not negligible and can be as large as a few percent. It is found that the standard deviation of the sampling uncertainty in the monthly zonal mean data allows for a simple parameterization in terms of the product of the standard deviation of natural variations and the proposed inhomogeneity measure. The focus of this work is the vertical ozone distributions measured by limb-viewing satellite instruments, but the developed methods can also be applied to different satellite, ground-based and in-situ measurements.
    Atmospheric Measurement Techniques 06/2014; 7:1891-1900. · 3.21 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.
    Journal of Geophysical Research: Atmospheres. 05/2014; 119(11).
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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). · 4.88 Impact Factor
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    ABSTRACT: Vertical profiles of the rate of linear change (trend) in the altitude range 15–50 km are determined from decadal O3 time series obtained from SCIAMACHY/ENVISAT measurements in limb-viewing geometry. The trends are cal-culated by using a multivariate linear regression. Seasonal variations, the quasi-biennial oscillation, signatures of the solar cycle and the El Niño–Southern Oscillation are ac-counted for in the regression. The time range of trend cal-culation is August 2002–April 2012. A focus for analy-sis are the zonal bands of 20 N–20S (tropics), 60–50N, and 50–60S (midlatitudes). In the tropics, positive trends of up to 5 % per decade between 20 and 30 km and nega-tive trends of up to 10 % per decade between 30 and 38 km are identified. Positive O3 trends of around 5 % per decade are found in the upper stratosphere in the tropics and at midlatitudes. Comparisons between SCIAMACHY and EOS MLS show reasonable agreement both in the tropics and at midlatitudes for most altitudes. In the tropics, measure-ments from OSIRIS/Odin and SHADOZ are also analysed. These yield rates of linear change of O3 similar to those from SCIAMACHY. However, the trends from SCIAMACHY near 34 km in the tropics are larger than MLS and OSIRIS by a factor of around two.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2014; 14:831-846. · 5.30 Impact Factor
  • EGU General Assembly Conference Abstracts; 01/2014
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    ABSTRACT: The Optical Spectrograph and InfraRed Imaging System (OSIRIS) was launched aboard the Odin satellite in 2001 and is continuing to take limb-scattered sunlight measurements of the atmosphere. This work aims to characterize and assess the stability of the OSIRIS 11 yr v5.0x ozone data set. Three validation data sets were used: the v2.2 Microwave Limb Sounder (MLS) and v6 Global Ozone Monitoring by Occultation of Stars (GOMOS) satellite data records, and ozonesonde measurements. Global mean percent differences between coincident OSIRIS and validation measurements are within 5% at all altitudes above 18.5 km for MLS, above 21.5 km for GOMOS, and above 17.5 km for ozonesondes. Below 17.5 km, OSIRIS measurements agree with ozonesondes within 5% and are well-correlated (R > 0.75) with them. For low OSIRIS optics temperatures (< 16 °C), OSIRIS ozone measurements have a negative bias of 1-6% compared with the validation data sets for 25.5-40.5 km. Biases between OSIRIS ascending and descending node measurements were investigated and found to be related to aerosol retrievals below 27.5 km. Above 30 km, agreement between OSIRIS and the validation data sets was related to the OSIRIS retrieved albedo, which measures apparent upwelling, with a positive bias in OSIRIS data with large albedos. In order to assess the long-term stability of OSIRIS measurements, global average drifts relative to the validation data sets were calculated and were found to be < 3% per decade for comparisons with MLS for 19.5-36.5 km, GOMOS for 18.5-54.5 km, and ozonesondes for 12.5-22.5 km. Above 36.5 km, the relative drift for OSIRIS versus MLS ranged from ~ 0 to 6% per decade, depending on the data set used to convert MLS data to the OSIRIS altitude versus number density grid. Overall, this work demonstrates that the OSIRIS 11 yr ozone data set from 2001 to the present is suitable for trend studies.
    Atmospheric Measurement Techniques 12/2013; 7(1). · 3.21 Impact Factor
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    ABSTRACT: [1] Volcanoes release large amounts of halogen species such as HCl and HBr, which can be converted into reactive halogens by heterogeneous photochemical reactions that are currently not fully characterized. Here we report on the first satellite detection of volcanic chlorine dioxide (OClO). Measurements were performed using the SCanning Imaging Absorption spectroMeter for Atmospheric CHartograpHY (SCIAMACHY) instrument for the ash-laden plume emitted after the 2011 eruption of Puyehue-Cordón Caulle in Chile. We also identified volcanic BrO using the Ozone Monitoring Instrument (OMI) instrument, as well as enhanced HCl in data of the Microwave Limb Sounder (MLS) instrument. These observations suggest that OClO was formed in the plume by the ClO + BrO reaction in presence of a large excess of ClO. The present satellite data set could help better understand reactive halogen chemistry in volcanic plumes, and its impact on atmospheric composition.
    Geophysical Research Letters. 12/2013;
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    ABSTRACT: There is presently renewed interest in diurnal variations of stratospheric and mesospheric ozone for the purpose of supporting homogenization of records of various ozone measurements that are limited by the technique employed to being made at certain times of day. We have made such measurements for 18 yr using a passive microwave remote sensing technique at the Mauna Loa Observatory in Hawaii, which is a primary station in the Network for Detection of Atmospheric Composition Change (NDACC). We have recently reprocessed these data with hourly time resolution to study diurnal variations. We inspected differences between pairs of the ozone spectra (e.g. day and night) from which the ozone profiles are derived to determine the extent to which they may be contaminated by diurnally varying systematic instrumental or measurement effects. These are small, and we have reduced them further by selecting data that meet certain criteria that we established. We have calculated differences between profiles measured at different times: morning-night, afternoon-night, and morning-afternoon and have intercompared these with like profiles derived from Aura-MLS, UARS-MLS, SMILES, and SBUV/2 measurements. Differences between averages of coincident profiles are typically <1.5% of typical nightime values over most of the covered altitude range with some exceptions. We calculated averages of ozone values for each hour from the Mauna Loa microwave data, and normalized these to the average for the first hour after midnight for comparison with corresponding values calculated with the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM). We found that the measurements and model output mostly agree to better than 1.5% of the midnight value, with one noteworthy exception: the measured morning-night values are significantly (2-3%) higher than the modeled ones from 3.2 to 1.8 hPa (~39-43 km), and there is evidence that the measured values are increasing compared to the modeled values before sunrise in this region.
    Atmospheric Chemistry and Physics 12/2013; 13(12):31855-31890. · 4.88 Impact Factor
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    ABSTRACT: AbstractA comprehensive quality assessment of the ozone products from 18 limb‐viewing satellite instruments is provided by means of a detailed intercomparison. The ozone climatologies in form of monthly zonal mean time series covering the upper troposphere to lower mesosphere are obtained from LIMS, SAGE I/II/III, UARS‐MLS, HALOE, POAM II/III, SMR, OSIRIS, MIPAS, GOMOS, SCIAMACHY, ACE‐FTS, ACE‐MAESTRO, Aura‐MLS, HIRDLS, and SMILES within 1978–2010. The intercomparisons focus on mean biases of annual zonal mean fields, interannual variability, and seasonal cycles. Additionally, the physical consistency of the data is tested through diagnostics of the quasi‐biennial oscillation and Antarctic ozone hole. The comprehensive evaluations reveal that the uncertainty in our knowledge of the atmospheric ozone mean state is smallest in the tropical and midlatitude middle stratosphere with a 1σ multi‐instrument spread of less than ±5%. While the overall agreement among the climatological data sets is very good for large parts of the stratosphere, individual discrepancies have been identified, including unrealistic month‐to‐month fluctuations, large biases in particular atmospheric regions, or inconsistencies in the seasonal cycle. Notable differences between the data sets exist in the tropical lower stratosphere (with a spread of ±30%) and at high latitudes (±15%). In particular, large relative differences are identified in the Antarctic during the time of the ozone hole, with a spread between the monthly zonal mean fields of ±50%. The evaluations provide guidance on what data sets are the most reliable for applications such as studies of ozone variability, model‐measurement comparisons, detection of long‐term trends, and data‐merging activities.
    Journal of Geophysical Research Atmospheres 11/2013; 118(21). · 3.44 Impact Factor
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    ABSTRACT: [1] Monthly zonal mean climatologies of atmospheric measurements from satellite instruments can have biases due to the nonuniform sampling of the atmosphere by the instruments. We characterize potential sampling biases in stratospheric trace gas climatologies of the Stratospheric Processes and Their Role in Climate (SPARC) Data Initiative using chemical fields from a chemistry climate model simulation and sampling patterns from 16 satellite-borne instruments. The exercise is performed for the long-lived stratospheric trace gases O3 and H2O. Monthly sampling biases for O3 exceed 10% for many instruments in the high-latitude stratosphere and in the upper troposphere/lower stratosphere, while annual mean sampling biases reach values of up to 20% in the same regions for some instruments. Sampling biases for H2O are generally smaller than for O3, although still notable in the upper troposphere/lower stratosphere and Southern Hemisphere high latitudes. The most important mechanism leading to monthly sampling bias is nonuniform temporal sampling, i.e., the fact that for many instruments, monthly means are produced from measurements which span less than the full month in question. Similarly, annual mean sampling biases are well explained by nonuniformity in the month-to-month sampling by different instruments. Nonuniform sampling in latitude and longitude are shown to also lead to nonnegligible sampling biases, which are most relevant for climatologies which are otherwise free of biases due to nonuniform temporal sampling.
    Journal of Geophysical Research: Atmospheres. 10/2013; 118(20).
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    ABSTRACT: The diurnal variation of HOCl and the related species ClO, HO 2 and HCl measured by satellites has been compared with the results of a one-dimensional photochem-ical model. The study compares the data from various limb-viewing instruments with model simulations from the middle stratosphere to the lower mesosphere. Data from three sub-millimetre instruments and two infrared spectrometers are used, namely from the Sub-Millimetre Radiometer (SMR) on board Odin, the Microwave Limb Sounder (MLS) on board Aura, the Superconducting Submillimeter-wave Limb-Emission Sounder (SMILES) on the International Space Sta-tion, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board ENVISAT, and the Atmo-spheric Chemistry Experiment Fourier Transform Spectrom-eter (ACE-FTS) on board SCISAT. Inter-comparison of the measurements from instruments on sun-synchronous satel-lites (SMR, MLS, MIPAS) and measurements from solar occultation instruments (ACE-FTS) is challenging since the measurements correspond to different solar zenith angles (or local times). However, using a model which covers all solar zenith angles and data from the SMILES instrument which measured at all local times over a period of several months provides the possibility to verify the model and to indirectly compare the diurnally variable species. The satellite data were averaged for latitudes of 20 • S to 20 • N for the SMILES observation period from November 2009 to April 2010 and were compared at three altitudes: 35, 45 and 55 km. Besides presenting the SMILES data, the study also shows a first comparison of the latest MLS data (version 3.3) of HOCl, ClO, and HO 2 with other satellite observations, as well as a first evaluation of HO 2 observations made by Odin/SMR. The MISU-1D model has been carefully initialised and run for conditions and locations of the observations. The diurnal cycle features for the species investigated here are generally well reproduced by the model. The satellite observations and the model agree well in terms of absolute mixing ratios. The differences between the day and night values of the model are in good agreement with the observations although the ampli-tude of the HO 2 diurnal variation is 10–20 % lower in the model than in the observations. In particular, the data offered the opportunity to study the reaction ClO+HO 2 → HOCl+O 2 in the lower mesosphere at 55 km. At this altitude the HOCl Published by Copernicus Publications on behalf of the European Geosciences Union. 7588 M. Khosravi et al.: HOCl, ClO and HO 2 diurnal variation in the tropics night-time variation depends only on this reaction. The result of this analysis points towards a value of the rate constant within the range of the JPL 2006 recommendation and the upper uncertainty limit of the JPL 2011 recommendation at 55 km.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 08/2013; 13(15):7587-7606. · 5.30 Impact Factor
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    ABSTRACT: The diurnal variation of HOCl and the related species ClO, HO2 and HCl measured by satellites has been compared with the results of a one-dimensional photochemical model. The study compares the data from various limb-viewing instruments with model simulations from the middle stratosphere to the lower mesosphere. Data from three sub-millimeter instruments and two infrared spectrometers are used, namely from the Sub-Millimeter Radiometer (SMR) on board Odin, the Microwave Limb Sounder (MLS) on board Aura, the Superconducting Submillimeter-wave Limb-Emission Sounder (SMILES) on the International Space Station, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board ENVISAT, and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) on board SCISAT. Inter-comparison of the measurements from instruments on sun-synchronous satellites (SMR, MLS, MIPAS) and measurements from solar occultation instruments (ACE-FTS) is challenging since the measurements correspond to different solar zenith angles (or local times). However, using a model which covers all solar zenith angles and the new SMILES instrument which measures at all local times over a period of several months provides the possibility to indirectly compare the diurnally variable species. The satellite data were averaged for latitudes of 20° S to 20° N for the SMILES observation period from November 2009 to April 2010 and were compared at three altitudes: 35, 45 and 55 km. This study presents the first evaluation of HO2 Odin/SMR data and also the first comparison of the new SMILES data and the latest version of MLS (version 3.3) with other satellite observations. The MISU-1D model has been run for conditions and locations of the observations. The diurnal cycle features for the species investigated here are generally well reproduced by the model. The satellite observations and the model generally agree well in terms of absolute mixing ratios as well as differences between the day and night values. This confirms that gas phase chemistry of these species based on latest recommendations of reaction rate constants is fairly well understood.
    Atmospheric Chemistry and Physics 07/2013; 13:7587-7606. · 4.88 Impact Factor
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    ABSTRACT: [1] The Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) onboard the International Space Station provided global measurements of ozone profiles in the middle atmosphere from 12 October 2009 to 21 April 2010. We present validation studies of the SMILES version 2.1 ozone product based on coincidence statistics with satellite observations and outputs of chemistry and transport models (CTMs). Comparisons of the stratospheric ozone with correlative data show agreements that are generally within 10%. In the mesosphere, the agreement is also good and better than 30% even at a high altitude of 73 km, and the SMILES measurements with their local time coverage also capture the diurnal variability very well. The recommended altitude range for scientific use is from 16 to 73 km. We note that the SMILES ozone values for altitude above 26 km are smaller than some of the correlative satellite datasets; conversely the SMILES values in the lower stratosphere tend to be larger than correlative data, particularly in the tropics, with less than 8% difference below ~24 km. The larger values in the lower stratosphere are probably due to departure of retrieval results between two detection bands at altitudes below 28 km; it is ~3% at 24 km and is increasing rapidly down below.
    Journal of Geophysical Research Atmospheres 06/2013; · 3.44 Impact Factor
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    ABSTRACT: Anderson et al. (2012) (A2012) report in situ observations of convectively injected water vapor (H2O) in the North American (NA) summer lowermost stratosphere (LMS), occasionally exceeding 12ppmv. They contend that, in such cold/wet conditions, heterogeneous chemistry on binary water‒sulfate aerosols can activate chlorine, leading to catalytic ozone destruction. Aura Microwave Limb Sounder 100 hPa and 82.5 hPa H2O measurements show that, indeed, the NA LMS is unusually wet, both in mean values and in outliers reaching 18ppmv. Using A2012's threshold, 4% (0.03%) of 100 hPa (82.5 hPa) NA July-August observations are cold/wet enough for activation. Cold parcels, whether wet or dry, typically have much less HCl to activate and O3to destroy than A2012's initial conditions. Slightly lower concentrations of HCl and O3 in cold/wet parcels are attributable, at least in part, to dilution by tropospheric air. Alarming reductions in NA summer column O3suggested by A2012 are not seen in the current climate.
    Geophysical Research Letters 05/2013; 40(10):2316-2321. · 4.46 Impact Factor
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    ABSTRACT: The Optical Spectrograph and InfraRed Imaging System (OSIRIS) was launched aboard the Odin satellite in 2001 and is continuing to take limb-scattered sunlight measurements of the atmosphere. This work aims to characterize and assess the stability of the OSIRIS 11 yr v5.0x ozone data set. Three validation data sets were used: the v2.2 Microwave Limb Sounder (MLS) and v6 Global Ozone Monitoring of Occultation on Stars (GOMOS) satellite data records, and ozone sonde measurements. Global mean percent differences between coincident OSIRIS and validation measurements are within 5% of zero at all altitude layers above 18.5 km for MLS, above 21.5 km for GOMOS, and above 17.5 km for ozone sondes. Below 17.5 km, OSIRIS measurements agree with ozone sondes within 5% and are well-correlated (R > 0.75) with them. For low OSIRIS optics temperatures (< 16 °C), OSIRIS ozone measurements are biased low by up 6% compared with the validation data sets for 25.5-40.5 km. Biases between OSIRIS ascending and descending node measurements were investigated and were found to be related to aerosol retrievals below 27.5 km. Above 30 km, agreement between OSIRIS and the validation data sets was related to the OSIRIS retrieved albedo, which measures apparent upwelling, with a high bias for in OSIRIS data with large albedos. In order to assess the long-term stability of OSIRIS measurements, global average drifts relative to the validation data sets were calculated and were found to be < 3% per decade for comparisons against MLS for 19.5-36.5 km, GOMOS for 18.5-54.5 km, and ozone sondes for 12.5-22.5 km, and within error of 3% per decade at most altitudes. Above 36.5 km, the relative drift for OSIRIS versus MLS ranged from ~ 0-6%, depending on the data set used to convert MLS data to the OSIRIS altitude versus number density grid. Overall, this work demonstrates that the OSIRIS 11 yr ozone data set from 2001 to the present is suitable for trend studies.
    Atmospheric Measurement Techniques Discussions. 04/2013; 6(2):3819-3857.
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    ABSTRACT: The SCIAMACHY/ENVISAT satellite instrument (2002-12) has monitored Earth' atmosphere globally for almost one decade. In its limb viewing geometry, it measured the vertical profiles of the atmospheric limb scatter. O3, NO2, and BRO are among the retrieved species. Their vertical profiles are obtained in 1 km altitude steps. Longterm changes as well as periodically varying features are reflected by the resulting time series. The longterm changes are statistically described by trends. The trend profile of SCIAMACHY limb O3 extends throughout the stratosphere and is a function of the latitude. There are contrasts and parallels between the tropics and the mid-latitudes. With O3 being in the focus of interest for several decades now, there is a range of parallel measurements. Thus, the quality of the SCIAMACHY ozone trends is confirmed by inter-instrumental comparisons. There are contemporary satellite instruments as well as different measurement techniques. In our talk, we further present trend profiles of NO2 and BRO. Being crosslinked through chemical reactions, their trend profiles are potentially related among each other and to O3.
    04/2013;
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    ABSTRACT: The trends and variability of ozone are assessed over a northern mid-latitude station, Haute-Provence Observatory (OHP - 43.93° N, 5.71° E), using total column ozone observations from the Dobson and Système d'Analyse par Observation Zénithale spectrometers, and stratospheric ozone profile measurements from Light detection and ranging, ozonesondes, Stratospheric Aerosol and Gas Experiment II, Halogen Occultation Experiment and Aura Microwave Limb Sounder. A multi-variate regression model with quasi biennial oscillation (QBO), solar flux, aerosol optical thickness, heat flux, North Atlantic oscillation (NAO) and piecewise linear trend (PWLT) or Equivalent Effective Stratospheric Chlorine (EESC) functions is applied to the ozone anomalies. The maximum variability of ozone in winter/spring is explained by QBO and heat flux in 15-45 km and in 15-24 km, respectively. The NAO shows maximum influence in the lower stratosphere during winter while the solar flux influence is largest in the lower and middle stratosphere in summer. The total column ozone trends estimated from the PWLT and EESC functions are of -1.39±0.26 and -1.40±0.25 DU yr-1, respectively over 1984-1996 and about 0.65±0.32 and 0.42±0.08 DU yr-1, respectively over 1997-2010. The ozone profiles yield similar and significant EESC-based and PWLT trends in 1984-1996 and are about -0.5 and -0.8 % yr-1 in the lower and upper stratosphere, respectively. In 1997-2010, the EESC-based and PWLT trends are significant and of order 0.3 and 0.1 % yr-1, respectively in the 18-28 km range, and at 40-45 km, EESC provides significant ozone trends larger than the insignificant PWLT results. Therefore, this analysis unveils ozone recovery signals from total column ozone and profile measurements at OHP, and hence in the mid-latitudes.
    Atmospheric Chemistry and Physics 03/2013; 13(3):7081-7112. · 4.88 Impact Factor