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  • 1
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    Tropospheric SF6: Age of Air from the Northern Hemisphere Mid-latitude Surface (2013)
    Wiley
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    Publication Date: 2013-09-21
    Description: [1]  Observations of SF 6 are used to quantify the mean time since air was in (“mean age” from) the northern hemisphere (NH) mid-latitude surface layer. The mean age is a fundamental property of tropospheric transport that can be used in theoretical studies and used to evaluate transport in comprehensive models. Comparisons of simulated SF 6 and an idealized clock tracer confirm that the time lag between the SF 6 mixing ratio at a given location and the NH mid-latitude surface provides an accurate estimate of the mean age. The ages calculated from surface SF 6 measurements show large meridional gradients in the tropics but weak gradients in the extratropics, with near zero ages at the surface north of 30 o N and ages around 1.4 yr south of 30 o S. Aircraft measurements show weak vertical age gradients in the lower-middle troposphere, with only slight increases of age with height in the NH and slight decreases with height in the SH. There are large seasonal variations in the age at tropical stations (annual amplitudes around 0.5-1.0 yr), with younger ages during northern winter, but only weak seasonal variations at higher latitudes. The seasonality and interannual variations in the tropics and southern hemisphere are related to changes in locations of tropical convection. There is qualitative agreement, in both spatial and temporal variations, between the simulated ages and observations. The model ages tend to be older than observed, with differences of ~0.2 yr in the northern hemisphere upper troposphere and throughout the southern hemisphere troposphere.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Modulation of Antarctic vortex composition by the Quasi-Biennial Oscillation (2015)
    Wiley
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    Publication Date: 2015-05-07
    Description: Using a decade of Aura Microwave Limb Sounder (MLS) observations we show distinctly different N 2 O distributions in southern hemisphere winter that depend on the phase of the quasi-biennial oscillation (QBO). Composites of the nitrous oxide (N 2 O) anomalies calculated for westerly and easterly phases show that QBO-generated variability originating in the subtropical middle stratosphere fills the midlatitude surf zone by late winter. After the spring vortex breakup, the anomaly is transported to the Antarctic where it remains until the next vortex forms in fall. Trapped in the newly formed vortex, the anomaly descends in isolation through fall and winter, arriving in the Antarctic lower stratosphere in September – about 1 year after it formed. This transport pathway explains previously reported variability of N 2 O and inorganic chlorine (Cl y ) inside the Antarctic vortex, and demonstrates that the middle stratosphere QBO affects ozone depletion by modulating Antarctic Cl y .
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Chemical and Dynamical Impacts of Stratospheric Sudden Warmings on Arctic Ozone Variability (2016)
    Wiley
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    Publication Date: 2016-09-16
    Description: We use the Global Modeling Initiative (GMI) chemistry and transport model with Modern-Era Retrospective Analysis for Research and Applications (MERRA) meteorological fields to quantify heterogeneous chemical ozone loss in Arctic winters 2005-2015. Comparisons to Aura Microwave Limb Sounder N 2 O and O 3 observations show the GMI simulation credibly represents the transport processes and net heterogeneous chemical loss necessary to simulate Arctic ozone. We find that the maximum seasonal ozone depletion varies linearly with the number of cold days and with wave driving (eddy heat flux) calculated from MERRA fields. We use this relationship and MERRA temperatures to estimate seasonal ozone loss from 1993-2004 when inorganic chlorine levels were in the same range as during the Aura period. Using these loss estimates and the observed March mean 63-90 o N column O 3 , we quantify the sensitivity of the ozone dynamical resupply to wave driving, separating it from the sensitivity of ozone depletion to wave driving. The results show that about 2/3 of the deviation of the observed March Arctic O 3 from an assumed climatological mean is due to variations in O 3 resupply and 1/3 is due to depletion. Winters with a stratospheric sudden warming (SSW) before mid-February have about 1/3 the depletion of winters without one and export less depletion to the midlatitudes. However, a larger effect on the spring midlatitude ozone comes from dynamical differences between warm and cold Arctic winters, which can mask or add to the impact of exported depletion.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    The contributions of chemistry and transport to low Arctic ozone in March 2011 derived from Aura MLS observations (2013)
    Wiley
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    Publication Date: 2013-01-19
    Description: [1]  Stratospheric and total columns of Arctic O 3 (63–90 o N) in late March 2011 averaged 320 and 349 DU, respectively, 50–100 DU lower than any of the previous 6 years. We use Aura MLS O 3 observations to quantify the roles of chemistry and transport and find there are two major reasons for low O 3 in March 2011: heterogeneous chemical loss and a late final warming that delayed the resupply of O 3 until April. Daily vortex-averaged partial columns in the lowermost stratosphere (p 〉 133 hPa) and middle stratosphere (p 〈 29 hPa) are largely unaffected by local heterogeneous chemistry, according to model calculations. Very weak transport into the vortex between late January and late March contributes to the observed low ozone. The lower stratospheric (LS) column (133–29 hPa, ~370–550 K) is affected by both heterogeneous chemistry and transport. Because MLS N 2 O data show strong isolation of the vortex, we estimate the contribution of vertical transport to LS O 3 using the descent of vortex N 2 O profiles. Simulations with the Global Modeling Initiative (GMI) chemistry and transport model (CTM) with and without heterogeneous chemical reactions show 73 DU vortex averaged O 3 loss; the loss derived from MLS O 3 is 84 ± 12 DU. The GMI simulation reproduces the observed O 3 and N 2 O with little error and demonstrates credible transport and chemistry. Without heterogeneous chemical loss, March 2011 vortex O 3 would have been at least 40 DU lower than climatology due to the late final warming that did not resupply O 3 until mid-April.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 5
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    Understanding differences in upper stratospheric ozone response to changes in chlorine and temperature as computed using CCMVal-2 models (2012)
    Wiley
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    Publication Date: 2012-08-18
    Description: Projections of future ozone levels are made using models that couple a general circulation model with a representation of atmospheric photochemical processes, allowing interactions among photochemical processes, radiation, and dynamics. Such models are known as coupled chemistry-climate models (CCMs). Although developed from common principles and subject to the same boundary conditions, simulated ozone time series vary among models for scenarios for ozone depleting substances (ODSs) and greenhouse gases. Photochemical processes control the upper stratospheric ozone level, and there is broad agreement among CCMs in that ozone increases as ODSs decrease and temperature decreases due to greenhouse gas increase. There are quantitative differences in the ozone sensitivity to chlorine and temperature. We obtain insight into differences in sensitivity by examining the relationship between the upper stratospheric seasonal cycles of ozone and temperature as produced by fourteen CCMs. All simulations conform to expectation in that ozone is less sensitive to temperature when chlorine levels are highest because chlorine catalyzed loss is nearly independent of temperature. Analysis reveals differences in simulated temperature, ozone and reactive nitrogen that lead to differences in the relative importance of ozone loss processes and are most obvious when chlorine levels are close to background. Differences in the relative importance of loss processes underlie differences in simulated sensitivity of ozone to composition change. This suggests 1) that the multimodel mean is not a best estimate of the sensitivity of upper stratospheric ozone to changes in ODSs and temperature; and 2) that the spread of values is not an appropriate measure of uncertainty.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 6
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    Using transport diagnostics to understand chemistry climate model ozone simulations (2011)
    Wiley
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    Publication Date: 2011-09-09
    Description: We use observations of N2O and mean age to identify realistic transport in models in order to explain their ozone predictions. The results are applied to 15 chemistry climate models (CCMs) participating in the 2010 World Meteorological Organization ozone assessment. Comparison of the observed and simulated N2O, mean age and their compact correlation identifies models with fast or slow circulations and reveals details of model ascent and tropical isolation. This process-oriented diagnostic is more useful than mean age alone because it identifies models with compensating transport deficiencies that produce fortuitous agreement with mean age. The diagnosed model transport behavior is related to a model's ability to produce realistic lower stratosphere (LS) O3 profiles. Models with the greatest tropical transport problems compare poorly with O3 observations. Models with the most realistic LS transport agree more closely with LS observations and each other. We incorporate the results of the chemistry evaluations in the Stratospheric Processes and their Role in Climate (SPARC) CCMVal Report to explain the range of CCM predictions for the return-to-1980 dates for global (60°S–60°N) and Antarctic column ozone. Antarctic O3 return dates are generally correlated with vortex Cly levels, and vortex Cly is generally correlated with the model's circulation, although model Cl chemistry and conservation problems also have a significant effect on return date. In both regions, models with good LS transport and chemistry produce a smaller range of predictions for the return-to-1980 ozone values. This study suggests that the current range of predicted return dates is unnecessarily broad due to identifiable model deficiencies.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    Multi-model estimates of atmospheric lifetimes of long-lived Ozone-Depleting Substances: Present and future (2014)
    Wiley
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    Publication Date: 2014-01-24
    Description: [1]  We have diagnosed the lifetimes of long-lived source gases emitted at the surface and removed in the stratosphere using six three-dimensional chemistry-climate models (CCMs) and a two-dimensional model. The models all used the same standard photochemical data. We investigate the effect of different definitions of lifetimes, including running the models with both mixing ratio (MBC) and flux (FBC) boundary conditions. Within the same model, the lifetimes diagnosed by different methods agree very well. Using FBCs versus MBCs leads to a different tracer burden as the implied lifetime contained in the MBC value does not necessarily match a model's own calculated lifetime. In general, there are much larger differences in the lifetimes calculated by different models, the main causes of which are variations in the modelled rates of ascent and horizontal mixing in the tropical mid-lower stratosphere. The model runs have been used to compute instantaneous and steady-state lifetimes. For chlorofluorocarbons (CFCs) their atmospheric distribution was far from steady state in their growth phase through to the 1980s and the diagnosed instantaneous lifetime is accordingly much longer. Following the cessation of emissions, the resulting decay of CFCs is much closer to steady-state. For 2100 conditions the model circulation speeds generally increase, but a thicker ozone layer due to recovery and climate change reduces photolysis rates. These effects compensate so the net impact on modelled lifetimes is small. For future assessments of stratospheric ozone use of FBCs would allow a consistent balance between rate of CFC removal and model circulation rate.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 8
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    Understanding Differences in Chemistry Climate Model Projections of Stratospheric Ozone (2014)
    Wiley
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    Publication Date: 2014-04-02
    Description: Chemistry climate models (CCMs) are used to project future evolution of stratospheric ozone as concentrations of ozone-depleting substances (ODSs) decrease and GHGs increase, cooling the stratosphere. CCM projections exhibit many common features, but also a broad range of values for quantities such as year of ozone-return-to-1980 and global ozone level at the end of the 21 st century. Multiple linear regression is applied to each of fourteen CCMs to separate ozone response to ODS concentration change from that due to climate change. We show that the sensitivity of lower stratospheric ozone to chlorine change ΔO 3 /ΔCl y is a near linear function of partitioning of total inorganic chlorine (Cl y ) into its reservoirs; both Cl y and its partitioning are largely controlled by lower stratospheric transport. CCMs with best performance on transport diagnostics agree with observations for chlorine reservoirs and produce similar ozone responses to chlorine change. After 2035 differences in ΔO 3 /ΔCl y contribute little to the spread in CCM projections as the anthropogenic contribution to Cl y becomes unimportant. Differences among upper stratospheric ozone increases due to temperature decreases are explained by differences in ozone sensitivity to temperature change ΔO 3 /ΔT due to different contributions from various ozone loss processes, each with its own temperature dependence. Ozone decrease in the tropical lower stratosphere caused by a projected speed-up in the Brewer-Dobson circulation may or may not be balanced by ozone increases in the middle and high latitude lower stratosphere and upper troposphere. This balance, or lack thereof, contributes most to the spread in late 21 st century projections.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
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    Assessment and Applications of NASA Ozone Data Products Derived from Aura OMI/MLS Satellite Measurements in Context of the GMI Chemical Transport Model (2014)
    Wiley
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    Publication Date: 2014-04-15
    Description: Measurements from the Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS), both onboard the Aura spacecraft, have been used to produce daily global maps of column and profile ozone since August 2004. Here we compare and evaluate three strategies to obtain daily maps of tropospheric and stratospheric ozone from OMI and MLS measurements: trajectory mapping, direct profile retrieval, and data assimilation. Evaluation is based on an assessment that includes validation using ozonesondes and comparisons with the Global Modeling Initiative (GMI) chemical transport model (CTM). We investigate applications of the three ozone data products from near-decadal and inter-annual timescales to day-to-day case studies. Inter-annual changes in zonal mean tropospheric ozone from all of the products in any latitude range are of the order 1-2 Dobson Units while changes (increases) over the 8-year Aura record investigated vary by 2-4 Dobson Units. It is demonstrated that all of the ozone products can measure and monitor exceptional tropospheric ozone events including major forest fire and pollution transport events. Stratospheric ozone during the Aura record has several anomalous inter-annual events including split stratospheric warmings in the Northern Hemisphere extra-tropics that are well captured using the data assimilation ozone profile product. Data assimilation with continuous daily global coverage and vertical ozone profile information is the best of the three strategies at generating a global tropospheric and stratospheric ozone product for science applications.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    Chemical loss of ozone in the arctic polar vortex in the winter of 1991-1992 (1993)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 1993-08-27
    Description: In situ measurements of chlorine monoxide, bromine monoxide, and ozone are extrapolated globally, with the use of meteorological tracers, to infer the loss rates for ozone in the Arctic lower stratosphere during the Airborne Arctic Stratospheric Expedition II (AASE II) in the winter of 1991-1992. The analysis indicates removal of 15 to 20 percent of ambient ozone because of elevated concentrations of chlorine monoxide and bromine monoxide. Observations during AASE II define rates of removal of chlorine monoxide attributable to reaction with nitrogen dioxide (produced by photolysis of nitric acid) and to production of hydrochloric acid. Ozone loss ceased in March as concentrations of chlorine monoxide declined. Ozone losses could approach 50 percent if regeneration of nitrogen dioxide were inhibited by irreversible removal of nitrogen oxides (denitrification), as presently observed in the Antarctic, or without denitrification if inorganic chlorine concentrations were to double.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Salawitch, R J -- Wofsy, S C -- Gottlieb, E W -- Lait, L R -- Newman, P A -- Schoeberl, M R -- Loewenstein, M -- Podolske, J R -- Strahan, S E -- Proffitt, M H -- Webster, C R -- May, R D -- Fahey, D W -- Baumgardner, D -- Dye, J E -- Wilson, J C -- Kelly, K K -- Elkins, J W -- Chan, K R -- Anderson, J G -- New York, N.Y. -- Science. 1993 Aug 27;261(5125):1146-9.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17790349" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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