Attribution of Antarctic ozone recovery to the Montreal protocol requires evidence that (1)Antarctic chlorine levels are declining and (2) there is a reduction in ozone depletion in response to ...achlorine decline. We use Aura Microwave Limb Sounder measurements of O3, HCl, and N2O to demonstratethat inorganic chlorine (Cly) from 2013 to 2016 was 223 ± 93 parts per trillion lower in the Antarctic lowerstratosphere than from 2004 to 2007 and that column ozone depletion declined in response. The mean Clydecline rate, ~0.8%/yr, agrees with the expected rate based on chlorofluorocarbon lifetimes. N2Omeasurements are crucial for identifying changes in stratospheric Cly loading independent of dynamicalvariability. From 2005 to 2016, the ozone depletion and Cly time series show matching periods of decline,stability, and increase. The observed sensitivity of O3 depletion to changing Cly agrees with the sensitivitysimulated by the Global Modeling Initiative chemistry transport model integrated
The Network for the Detection of Atmospheric Composition Change (NDACC) is an international global network of more than 90 stations making high-quality measurements of atmospheric composition that ...began official operations in 1991 after 5 years of planning. Apart from sonde measurements, all measurements in the network are performed by ground-based remote-sensing techniques. Originally named the Network for the Detection of Stratospheric Change (NDSC), the name of the network was changed to NDACC in 2005 to better reflect the expanded scope of its measurements. The primary goal of NDACC is to establish long-term databases for detecting changes and trends in the chemical and physical state of the atmosphere (mesosphere, stratosphere, and troposphere) and to assess the coupling of such changes with climate and air quality. NDACC's origins, station locations, organizational structure, and data archiving are described. NDACC is structured around categories of ground-based observational techniques (sonde, lidar, microwave radiometers, Fourier-transform infrared, UV-visible DOAS (differential optical absorption spectroscopy)-type, and Dobson-Brewer spectrometers, as well as spectral UV radiometers), timely cross-cutting themes (ozone, water vapour, measurement strategies, cross-network data integration), satellite measurement systems, and theory and analyses. Participation in NDACC requires compliance with strict measurement and data protocols to ensure that the network data are of high and consistent quality. To widen its scope, NDACC has established formal collaborative agreements with eight other cooperating networks and Global Atmosphere Watch (GAW). A brief history is provided, major accomplishments of NDACC during its first 25 years of operation are reviewed, and a forward-looking perspective is presented.
SignificanceLarge wildfires have been observed to inject smoke into the stratosphere, raising questions about their potential to affect the stratospheric ozone layer that protects life on Earth from ...biologically damaging ultraviolet radiation. Multiple observations of aerosol and NO
concentrations from three independent satellite instruments are used here together with model calculations to identify decreases in stratospheric NO
concentrations following major Australian 2019 through 2020 wildfires. The data confirm that important chemistry did occur on the smoke particle surfaces. The observed behavior in NO
with increasing particle concentrations is a marker for surface chemistry that contributes to midlatitude ozone depletion. The results indicate that increasing wildfire activity in a warming world may slow the recovery of the ozone layer.
The lifetime of nitrous oxide, the third‐most‐important human‐emitted greenhouse gas, is based to date primarily on model studies or scaling to other gases. This work calculates a semiempirical ...lifetime based on Microwave Limb Sounder satellite measurements of stratospheric profiles of nitrous oxide, ozone, and temperature; laboratory cross‐section data for ozone and molecular oxygen plus kinetics for O(1D); the observed solar spectrum; and a simple radiative transfer model. The result is 116 ± 9 years. The observed monthly‐to‐biennial variations in lifetime and tropical abundance are well matched by four independent chemistry‐transport models driven by reanalysis meteorological fields for the period of observation (2005–2010), but all these models overestimate the lifetime due to lower abundances in the critical loss region near 32 km in the tropics. These models plus a chemistry‐climate model agree on the nitrous oxide feedback factor on its own lifetime of 0.94 ± 0.01, giving N2O perturbations an effective residence time of 109 years. Combining this new empirical lifetime with model estimates of residence time and preindustrial lifetime (123 years) adjusts our best estimates of the human‐natural balance of emissions today and improves the accuracy of projected nitrous oxide increases over this century.
Key Points
Nitrous oxide lifetime is computed empirically from MLS satellite data
Empirical N2O lifetimes compared with models including interannual variability
Results improve values for present anthropogenic and preindustrial emissions
We investigate the observed trends and interannual variability in surface ozone over the United States using the Global Modeling Initiative chemical transport model. We discuss the roles of ...meteorology, emissions, and transport from the stratosphere in driving the interannual variability in different regions and seasons. We demonstrate that a hindcast simulation for 1991–2010 can reproduce much of the observed variability and the trends in summertime ozone, with correlation coefficients for seasonally and regionally averaged median ozone ranging from 0.46 to 0.89. Reproducing the interannual variability in winter and spring in the western United States may require higher‐resolution models to adequately represent stratosphere‐troposphere exchange. Hindcast simulations with fixed versus variable emissions show that changes in anthropogenic emissions drive the observed negative trends in monthly median ozone concentrations in the eastern United States during summer, as well as the observed reduction in the amplitude of the seasonal cycle. The simulation underestimates positive trends in the western United States during spring, but excluding the first 4 years of data removes many of the statistically significant trends in this region. The reduction in the slope of the ozone versus temperature relationship before and after major emission reductions is also well represented by the model. Our results indicate that a global model can reproduce many of the important features of the meteorologically induced ozone variability as well as the emission‐driven trends, lending confidence to model projections of future changes in regional surface ozone.
Key Points
We examine trends and variability in U.S. surface ozone for each season
Emission changes decrease East U.S. summer O3 and amplitude of the annual cycle
Strong correlation of observed and modeled surface O3 IAV in summer
Tropospheric trends in long-lived source gases N2O and the chlorofluorocarbons (CFCs) cause trends in O3 through changes in their reactive product gases. Transport affects the product gases because ...it controls the distribution of the long-lived source gases. We find that large changes in tropical upwelling 10-5 hPa since 2012 have strengthened the northern branch of the upper stratospheric (UpS) transport circulation, dramatically altering the abundances of N2O and its odd nitrogen product gases, NOx and HNO3. Increased upwelling is connected to stronger and more frequent Quasi-Biennial Oscillation easterly winds at 10 hPa and above. We use simulations with and without time varying MERRA2 meteorology to quantify the impact of dynamical changes on O3 loss via the NOx and ClOx cycles. We find that dynamical impacts on these cycles explain the mid-stratospheric tropical O3 increase and Arctic UpS O3 decrease since 2005.
The inorganic chlorine (Cly) and odd nitrogen (NOy) chemical families influence stratospheric O3. In January 2020 Australian wildfires injected record-breaking amounts of smoke into the southern ...stratosphere. Within 1-2 months ground-based and satellite observations showed Cly and NOy were repartitioned. Compared to decadal means, lower stratospheric (LS) HCl columns declined by ~30% between March and May while ClONO2 columns increased by 40-50%. The Cly perturbations recovered from June to October. The LS NO2 decreased from February to April, consistent with sulfate aerosol reactions, but returned to typical values by June - months earlier than the Cly recovery. Transport tracers show an air mass change explains the observed O3 decrease after April. Simulations assuming wildfire smoke behaves identically to sulfate aerosols couldn’t reproduce observed Cly changes, suggesting that their composition and chemistry differs from sulfate aerosols. This undermines our ability to predict ozone in a changing climate.
Eight years of ozone measurements retrieved from the Ozone Monitoring Instrument and the Microwave Limb Sounder, both on the EOS Aura satellite, have been assimilated into the Goddard Earth Observing ...System Version 5 (GEOS‐5) data assimilation system. This study evaluates this assimilated product, highlighting its potential for science. The impact of observations on the GEOS‐5 system is explored by examining the spatial distribution of the observation‐minus‐forecast statistics. Independent data are used for product validation. The correlation of the lower stratospheric (the tropopause to 50 hPa) ozone column with ozonesondes is 0.99 and the (high) bias is 0.5%, indicating the success of the assimilation in reproducing the ozone variability in that layer. The upper tropospheric (500 hPa to the tropopause) assimilated ozone column is about 10% lower than the ozonesonde column, but the correlation is still high (0.87). The assimilation is shown to realistically capture the sharp cross‐tropopause gradient in ozone mixing ratio. Occurrence of transport‐driven low ozone laminae in the assimilation system is similar to that obtained from the High Resolution Dynamics Limb Sounder (HIRDLS) above the 400 K potential temperature surface, but the assimilation produces fewer laminae than seen by HIRDLS below that surface. Although the assimilation produces about 25% fewer occurrences per day during the 3 years of HIRDLS data, the interannual variability is captured correctly. This data‐driven assimilated product is complementary to ozone fields generated from chemistry and transport models. Applications include study of the radiative forcing by ozone and tracer transport near the tropopause.
Key Points
Ozone observations from OMI and MLS are assimilated into GEOS‐5
Very good agreement with ozonesondes in the lower stratosphere
Representation of transport‐driven ozone structures in the UTLS
Concerns for ozone recovery Liang, Qing; Strahan, Susan E.; Fleming, Eric L.
Science (American Association for the Advancement of Science),
12/2017, Letnik:
358, Številka:
6368
Journal Article
Recenzirano
Climate change mitigation and compliance with the Montreal Protocol are crucial for ozone layer recovery
Reactive halogen gases containing chlorine (Cl) or bromine (Br) can destroy stratospheric ...ozone via catalytic cycles. The main sources of atmospheric reactive halogen are the long-lived synthetic chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), carbon tetrachloride (CCl
4
), methyl chloroform (CH
3
CCl
3
), and bromine-containing halons, all of which persist in the atmosphere for years. These ozone-depleting substances are now controlled under the Montreal Protocol and its amendments. Natural methyl bromide (CH
3
Br) and methyl chloride (CH
3
Cl) emissions are also important long-lived sources of atmospheric reactive halogen. Rising concentrations of very-short-lived substances (VSLSs) with atmospheric lifetimes of less than half a year may also contribute to future stratospheric ozone depletion. A greater concern for ozone layer recovery is incomplete compliance with the Montreal Protocol, which will impact stratospheric ozone for many decades, as well as rising natural emissions as a result of climate change.
Offline chemical transport models (CTMs) have traditionally been used to perform studies of atmospheric chemistry in a fixed dynamical environment. An alternative to using CTMs is to constrain the ...flow in a general circulation model using winds from meteorological analyses. The Goddard Earth Observing System (GEOS) “replay” approach involves reading in analyzed fields every 6 h and recomputing the analysis increments, which are applied as a forcing to the meteorology at every model time step. Unlike in CTM, all of the subgrid‐scale processes are recalculated online so that they are consistent with the large‐scale analysis fields, similar in spirit to “nudged” simulations, in which the online meteorology is relaxed to the analysis. Here we compare the transport of idealized tracers in different replay simulations constrained with meteorological fields taken from The Modern‐Era Retrospective Analysis for Research and Applications, Version 2 (MERRA‐2). We show that there are substantial differences in their large‐scale stratospheric transport, depending on whether analysis fields or assimilated fields are used. Replay simulations constrained with the instantaneous analysis fields produce stratospheric mean age values that are up to 30% too young relative to observations; by comparison, simulations constrained with the time‐averaged assimilated fields produce more credible stratospheric transport. Our study indicates that care should be taken to correctly configure the model when the replay technique is used to simulate stratospheric composition.
Key Points
GEOS replay simulations produce credible large‐scale stratospheric and tropospheric transport for use in studies of atmospheric composition
Simulations constrained with analysis fields produce stratospheric mean ages that are too young, versus when assimilated fields are used
By comparison, large‐scale tropospheric transport properties are relatively insensitive to whether analysis or assimilated fields are used
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