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 assess the interactions between stratospheric water vapor (SWV) and surface temperature during the past two decades using satellite observations and the Community Earth System Model (CESM). From ...1992 to 2013, to first order, the observed SWV exhibited three distinct piece-wise trends: a steady increase from 1992 to 2000, an abrupt drop from 2000 to 2004, and a gradual recovery after 2004, while the global-mean surface temperature experienced a strong increase until 2000 and a warming hiatus after 2000. The atmosphere-only CESM shows that the seasonal variation of tropical-mean (30°S–30°N) SWV is anticorrelated with that of the tropical-mean sea surface temperature (SST), while the correlation between the tropical SWV and SST anomalies on the interannual time scale is rather weak. By nudging the modeled SWV to prescribed profiles in coupled atmosphere-slab ocean experiments, we investigate the impact of SWV variations on surface temperature change. We find that a uniform 1 ppmv (0.5 ppmv) SWV increase (decrease) leads to an equilibrium global mean surface warming (cooling) of 0.12 ± 0.05 °C (−0.07 ± 0.05 °C). Sensitivity experiments show that the equilibrium response of global mean surface temperature to SWV perturbations over the extratropics is larger than that over the tropics. The observed sudden drop of SWV from 2000 to 2004 produces a global mean surface cooling of about −0.048 ± 0.041 °C, which suggests that a persistent change in SWV would make an imprint on long-term variations of global-mean surface temperature. A constant linear increase in SWV based on the satellite-observed rate of SWV change yields a global mean surface warming of 0.03 ± 0.01 °C/decade over a 50-year period, which accounts for about 19 % of the observed surface temperature increase prior to the warming hiatus. In the same experiment, trend analyses during different periods reveal a multi-year adjustment of surface temperature before the response to SWV forcing becomes strong relative to the internal variability in the model.
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.
Key Points
This article is the first validation study of the SMILES Level 2 product
Comparison results of the SMILES ozone data with other data sources are shown
Performance and characteristics of the SMILES ozone product are presented
Derived Meteorological Products (DMPs, including potential temperature, potential vorticity (PV), equivalent latitude (EqL), horizontal winds and tropopause locations) from several meteorological ...analyses have been produced for the locations and times of measurements taken by several solar occultation instruments and the Aura Microwave Limb Sounder (MLS). MLS and solar occultation data are analyzed using DMPs to illustrate sampling issues that may affect interpretation and comparison of data sets with diverse sampling patterns and to provide guidance regarding the kinds of studies that benefit most from analyzing satellite data in relation to meteorological conditions using the DMPs. Using EqL or PV as a vortex‐centered coordinate does not alleviate all sampling problems, including those in studies using “vortex averages” of solar occultation data and in analyses of localized features (such as polar stratospheric clouds) and other fields that do not correlate well with PV. Using DMPs to view measurements with respect to their air mass characteristics is particularly valuable in studies of transport of long‐lived trace gases, polar processing in the winter lower stratosphere, and distributions and transport of O3 and other trace gases from the upper troposphere through the lower stratosphere. The comparisons shown here demonstrate good agreement between MLS and solar occultation data for O3, N2O, H2O, HNO3, and HCl; small biases are attributable to sampling effects or are consistent with detailed validation results presented elsewhere in this special section. The DMPs are valuable for many scientific studies and to facilitate validation of noncoincident measurements.
The primary ozone loss process in the cold polar lower stratosphere hinges on chlorine monoxide (ClO) and one of its dimers, chlorine peroxide (ClOOCl). Recently, analyses of atmospheric observations ...have suggested that the equilibrium constant, Keq, governing the balance between ClOOCl formation and thermal decomposition in darkness is lower than that in the current evaluation of kinetics data. Measurements of ClO at night, when ClOOCl is unaffected by photolysis, provide a useful means of testing quantitative understanding of the ClO/ClOOCl relationship. Here we analyze nighttime ClO measurements from the National Aeronautics and Space Administration Aura Microwave Limb Sounder (MLS) to infer an expression for Keq. Although the observed temperature dependence of the nighttime ClO is in line with the theoretical ClO/ClOOCl equilibrium relationship, none of the previously published expressions for Keq consistently produces ClO abundances that match the MLS observations well under all conditions. Employing a standard expression for Keq, A x exp(B/T), we constrain the parameter A to currently recommended values and estimate B using a nonlinear weighted least squares analysis of nighttime MLS ClO data. ClO measurements at multiple pressure levels throughout the periods of peak chlorine activation in three Arctic and four Antarctic winters are used to estimate B. Our derived B leads to values of Keq that are ~1.4 times smaller at stratospherically relevant temperatures than currently recommended, consistent with earlier studies. Our results are in better agreement with the newly updated (2009) kinetics evaluation than with the previous (2006) recommendation.
Ozone measurements made by the Microwave Limb Sounder (MLS) on board the Earth Observing System (EOS) Aura Satellite are compared with measurements made by ground‐based microwave radiometers (MWR) in ...the Network for Detection of Atmospheric Composition Change (NDACC) stations at Lauder, New Zealand (45°S, 169°E) and Mauna Loa, Hawaii (20°N, 204°E). The latter instruments measure ozone over the pressure range 56 to 0.03 hPa (about 20 to 72 km), allowing validation of ozone to the upper range of the MLS profiles. In addition, because they operate continuously, separate daytime and nighttime comparisons with MLS can be made to account for the large diurnal variations of ozone in the upper stratosphere and mesosphere. MLS‐MWR comparisons show agreement generally within 5% between 24 and 0.04 hPa (about 26 to 70 km) and 5 to 13% elsewhere. To more thoroughly investigate ozone in the stratosphere and mesosphere and establish a consensus between different sets of measurements, comparisons, and analyses with other satellite‐borne instruments, including the Stratospheric Aerosol and Gas Experiment II (SAGE‐II), Halogen Occultation Experiment (HALOE), Global Ozone Monitoring by Occultation of Stars (GOMOS), and Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), are also made, using the ground‐based microwave measurements as a reference. The resulting MLS‐consensus difference profiles remove some of the features present in the MLS‐MWR comparisons and indicate that the overall agreement between MLS and the correlative data, between 56 and 0.04 hPa, is mostly within 5% at both sites.
Aura Microwave Limb Sounder (MLS) measurements show that chemical processing was critical to the observed record‐low Arctic stratospheric ozone in spring 2020. The 16‐year MLS record indicates more ...polar denitrification and dehydration in 2019/2020 than in any Arctic winter except 2015/2016. Chlorine activation and ozone depletion began earlier than in any previously observed winter, with evidence of chemical ozone loss starting in November. Active chlorine then persisted as late into spring as it did in 2011. Empirical estimates suggest maximum chemical ozone losses near 2.8 ppmv by late March in both 2011 and 2020. However, peak chlorine activation, and thus peak ozone loss, occurred at lower altitudes in 2020 than in 2011, leading to the lowest Arctic ozone values ever observed at potential temperature levels from ∼400–480 K, with similar ozone values to those in 2011 at higher levels.
Plain Language Summary
Unlike the Antarctic, the Arctic does not usually experience an ozone hole because temperatures are often too high for the chemistry that destroys ozone. In 2019/2020, satellite measurements show record‐low stratospheric wintertime temperatures and record‐low springtime ozone concentrations in the Arctic lower stratosphere (about 12‐ to 20‐km altitude). Only one other winter/spring season, 2010/2011, in this 16‐year satellite data record comes close. Low temperatures, which result in chlorine being converted from nonreactive forms into forms that destroy ozone, started earlier than in any previous Arctic winter in the record and lingered later than in any year except 2011. The ozone‐destroying chemistry in 2019/2020 occurred at lower altitudes (where more of the ozone that filters out harmful ultraviolet radiation resides) than in 2010/2011. Such extensive ozone loss can have important health and biological impacts because it leads to more ultraviolet radiation reaching the Earths surface. While the success of the Montreal Protocol in limiting human emissions that increase ozone‐destroying gases in the stratosphere has resulted in much less Arctic ozone destruction than we would have otherwise had, future temperature changes could lead to other winters with even more chemical ozone depletion than in 2019/2020.
Key Points
MLS trace gas data show that exceptional polar vortex conditions led to record‐low ozone in the Arctic lower stratosphere in 2019/2020
Early and persistent cold conditions led to the longest period with chlorine in ozone‐destroying forms in the 16‐year MLS data record
Chemical ozone destruction began earlier than in any Arctic winter in the MLS record and ended later than in any year except 2010/2011
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