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We analyze tropical ozone (O3) and carbon monoxide (CO) distributions in the upper troposphere (UT) and their temporal changes for 2005–2020 using Aura Microwave Limb Sounder (MLS) observations and chemistry climate models. The models are the Whole Atmosphere Community Climate Model (WACCM6) and two variants of the Community Atmosphere Model with Chemistry (CAM-chem), each variant using different anthropogenic emissions. Upper tropospheric trends and variability diagnostics are obtained from multiple linear regression analyses. We compare the model and MLS annual climatologies, focusing on 147 and 215 hPa pressure levels; climatological values generally fall within 10–20 % of each other, with both positive and negative differences for O3, and with models generally underestimating observed CO. In the northern hemisphere tropics, we find significantly poorer model fits to the observed phasing of CO seasonal changes at 215 hPa than at 147 hPa. This discrepancy is much smaller for the comparison of modeled and Measurements of Pollution in the Troposphere (MOPITT) V9J CO columns. We also find that the sensitivity of UT CO to El Niño / Southern Oscillation (ENSO) is positive at all tropical longitudes, in contrast to the dipolar longitudinal structure that exists for UT O3 ENSO sensitivity. MLS O3 has a zonal mean trend at 20° S–20° N of +0.39 ± 0.28 % yr-1; CAM-chem and WACCM have similar trends, though the WACCM trend is somewhat smaller. Our analyses for specific latitude/longitude bins yield positive trends up to 1.4 % yr-1 over Indonesia and East of that region, as well as over tropical Africa and the tropical Atlantic. We find broad similarities between the mapped MLS-derived UT O3 trends and corresponding mapped trends of tropospheric column ozone. Positive tropical UT mapped O3 trends are generally captured by the models, although in a more muted way. There is room for improvements in modeled tropical UT CO trends. Indeed, the MLS zonal mean CO trend is -0.25 ± 0.30 % yr-1, whereas the corresponding modeled CO trends are near zero (0.0 ± 0.14 % yr-1) when the anthropogenic emissions used in CAM-chem and WACCM are taken from Community Emissions Data System (CEDS) version 2. The non-CEDS version of CAM-chem yields CO UT trends of 0.22 ± 0.19 % yr-1, in contrast to the negative MLS CO trends throughout the tropics. The negative MLS tropical UT CO trends for 2005–2020 agree with (but tend to be smaller in magnitude than) previously published total column CO trends. Decreasing CO emissions from anthropogenic and biomass burning sources have previously been suggested as the main causes of tropospheric CO decreases, although significant regional emission trend variations exist. [ABSTRACT FROM AUTHOR]

We use Aura Microwave Limb Sounder (MLS) trace gas measurements to investigate whether water vapor (H2O) injected into the stratosphere by the Hunga Tonga‐Hunga Ha'apai (HTHH) eruption affected the 2022 Antarctic stratospheric vortex. Other MLS‐measured long‐lived species are used to distinguish high HTHH H2O from that descending in the vortex from the upper‐stratospheric H2O peak. HTHH H2O reached high southern latitudes in June–July but was effectively excluded from the vortex by the strong transport barrier at its edge. MLS H2O, nitric acid, chlorine species, and ozone within the 2022 Antarctic polar vortex were near average; the vortex was large, strong, and long‐lived, but not exceptionally so. There is thus no clear evidence of HTHH influence on the 2022 Antarctic vortex or its composition. Substantial impacts on the stratospheric polar vortices are expected in succeeding years since the H2O injected by HTHH has spread globally. Plain Language Summary: The 2022 Hunga Tonga‐Hunga Ha'apai eruption injected vast amounts of water vapor into the stratosphere. Concern arose that this excess water vapor could affect the 2022 Antarctic stratospheric polar vortex and ozone hole: Water vapor plays a crucial role in forming polar stratospheric clouds, which provide surfaces upon which chemical reactions that destroy ozone take place. Enhanced water vapor also affects temperatures, which in turn affect the powerful winds defining the polar vortex boundary. Antarctic polar vortex development began in April–May; by June the intense vortex‐edge winds presented a formidable obstacle to transport. Satellite trace‐gas measurements show that when water vapor from the Hunga Tonga eruption reached the vortex edge in June, it faced an impenetrable barrier and "besieged" the vortex, building up exceptionally strong water vapor gradients across the vortex edge. Water vapor, ozone, and chemicals involved in ozone destruction remained near historical average levels within the vortex through spring 2022. Because excess water vapor spread throughout the south polar regions after vortex breakup, much larger effects on the Antarctic vortex and chemical processing within it are expected in 2023 and beyond, when high water vapor will be entrained into the vortex as it develops. Key Points: Microwave Limb Sounder (MLS) trace gas data show that the Hunga Tonga‐Hunga Ha'apai H2O plume was effectively excluded from the 2022 Antarctic polar vortexAntarctic lower stratospheric vortex strength, size, and longevity were among the largest on record, but within the range of previous yearsAntarctic chemical ozone loss in 2022 was unexceptional, with MLS ozone and related trace gases observed to be near average [ABSTRACT FROM AUTHOR]

A new algorithm to derive near-real-time (NRT) data products for the Aura Microwave Limb Sounder (MLS) is presented. The old approach was based on a simplified optimal estimation retrieval algorithm (OE-NRT) to reduce computational demands and latency. This paper describes the setup, training, and evaluation of a redesigned approach based on artificial neural networks (ANN-NRT), which is trained on >17 years of MLS radiance observations and composition profile retrievals. Comparisons of joint histograms and performance metrics derived between the two NRT results and the operational MLS products demonstrate a noticeable statistical improvement from ANN-NRT. This new approach results in higher correlation coefficients, in addition to lower root-mean-square deviations and biases at almost all retrieval levels compared to OE-NRT. The exceptions are pressure levels with concentrations close to 0 ppbv (parts per billion by volume), where the ANN models fail to establish a functional relationship and tend to predict 0. Depending on the application, this behavior might be advantageous. While the developed models can take advantage of the extended MLS data record, this study demonstrates that training ANN-NRT on just a single year of MLS observations is sufficient to improve upon OE-NRT. This confirms the potential of applying machine learning to the NRT efforts of other current and future mission concepts. [ABSTRACT FROM AUTHOR]

Nineteen limb-viewing data sets (occultation, passive thermal, and UV scattering) and two nadir upper tropospheric humidity (UTH) data sets are intercompared and also compared to frost-point hygrometer balloon sondes. The upper troposphere considered here covers the pressure range from 300–100 hPa. UTH is a challenging measurement, because concentrations vary between 2–1000 ppmv (parts per million by volume), with sharp changes in vertical gradients near the tropopause. Cloudiness in this region also makes the measurement challenging. The atmospheric temperature is also highly variable ranging from 180–250 K. The assessment of satellite-measured UTH is based on coincident comparisons with balloon frost-point hygrometer sondes , multi-month mapped comparisons, zonal mean time series comparisons, and coincident satellite-to-satellite comparisons. While the satellite fields show similar features in maps and time series, quantitatively they can differ by a factor of 2 in concentration, with strong dependencies on the amount of UTH. Additionally, time-lag response-corrected Vaisala RS92 radiosondes are compared to satellites and the frost-point hygrometer measurements. In summary, most satellite data sets reviewed here show on average ∼30 % agreement amongst themselves and frost-point data but with an additional ∼30 % variability about the mean bias. The Vaisala RS92 sonde, even with a time-lag correction, shows poor behavior for pressures less than 200 hPa. [ABSTRACT FROM AUTHOR]

This paper describes the Earth Observing System (EOS) Aura Microwave Limb Sounder (MLS) forward model for a clear-sky atmosphere emitting unpolarized radiation. This model is used for the majority of the EOS MLS radiance calculations. The orbital and viewing geometry of MLS on Aura is such that a two-dimensional (vertical and line-of-sight, horizontal) forward model provides the most accurate basis for the inversion performed in retrievals. The model consists of atmospheric radiative transfer convolved with the instrumental field of view and the spectral response. An additional model relating the pointing heights supplied by the Aura satellite operations and MLS scan pointing encoders to the limb tangent pressure and atmospheric temperature is described. The analytic computation of Jacobians needed for retrievals is given. Index Terms--Aura, Earth Observing System (EOS), Microwave Limb Sounder (MLS), radiative transfer, spectroscopy.

We present results of early validation studies using retrieved atmospheric profiles from the Earth Observing System Microwave Limb Sounder (MLS) instrument on the Aura satellite. 'Global' results are presented for MLS measurements of atmospheric temperature, ozone, water vapor, hydrogen chloride, nitrous oxide, nitric acid, and carbon monoxide, with a focus on the January-March 2005 time period. These global comparisons are made using long-standing global satellites and meteorological datasets, as well as some measurements from more recently launched satellites. Comparisons of MLS data with measurements from the Ft. Sumner, NM, September 2004 balloon flights are also presented. Overall, good agreeement is obtained, often within 5% to 10%, but we point out certain issues to resolve and some larger systematic differences; some artifacts in the first publicly released MLS (version 1.5) dataset are noted. We comment briefly on future plans for validation and software improvements. Index Terms--Atmospheric retrievals, data validation.

The retrieval algorithms for the Earth Observing System Microwave Limb Sounder (MLS) on the Aura spacecraft, launched on July 15, 2004, are described. These algorithms are used to produce estimates of geophysical parameters such as vertical profiles of atmospheric temperature and composition ('Level 2' data) from the calibrated MLS observations of microwave limb radiance ('Level 1' data). The MLS algorithms are based on the standard optimal estimation approach, a weighted nonlinear least squares optimization with a priori constraints. New aspects include adaptation to a two-dimensional system, and an approach to the issues of retrieval 'phasing' and error propagation that differs from that taken for previous similar instruments. Important new aspects of the software that implements these algorithms are also described, along with the algorithm configuration for the 'version 1.5' dataset. Some examples are shown from MLS in-orbit observations. Index Terms--Atmospheric measurements, inverse problems, millimeterwave measurements, remote sensing, terrestrial atmosphere.

The Earth Observing System Microwave Limb Sounder measures several atmospheric chemical species (OH, H[O.sub.2], [H.sub.2]O, [O.sub.3], HCl, ClO, HOCl, BrO, HN[O.sub.3], [N.sub.2]O, CO, HCN, C[H.sub.3] CN, volcanic S[O.sub.2]), cloud ice, temperature, and geopotential height to improve our understanding of stratospheric ozone chemistry, the interaction of composition and climate, and pollution in the upper troposphere. All measurements are made simultaneously and continuously, during both day and night. The instrument uses heterodyne radiometers that observe thermal emission from the atmospheric limb in broad spectral regions centered near 118, 190, 240, and 640 GHz, and 2.5 THz. It was launched July 15, 2004 on the National Aeronautics and Space Administration's Aura satellite and started full-up science operations on August 13, 2004. An atmospheric limb scan and radiometric calibration for all bands are performed routinely every 25 s. Vertical profiles are retrieved every 165 km along the suborbital track, covering 82[degrees]S to 82[degrees]N latitudes on each orbit. Instrument performance to date has been excellent; data have been made publicly available; and initial science results have been obtained. Index Terms--Microwave, remote sensing, stratosphere, submillimeter wave.

This work supplements the Earth Observing System (EOS) Microwave Limb Sounder (MLS) clear-sky unpolarized forward model with algorithms for modeling polarized emission from the Zeeman-split 118.75-GHz [O.sub.2] spectral line. The model accounts for polarization-dependent emission and for correlation between polarizations with complex, 2 x 2 intensity and absorption matrices. The oxygen line is split into three Zeeman components by the interaction of oxygen's electronic spin with an external magnetic field, and the splitting is of order [+ or -] 0.5 MHz in a typical geomagnetic field. Zeeman splitting is only significant at pressures low enough that coilisional broadening (~1.6 MHz/hPa) is not very large by comparison. The polarized forward model becomes significant for MLS temperature retrievals at pressure below 1.0 hPa and is crucial at pressures below ~0.03 hPa. Interaction of the [O.sub.2] molecule with the radiation field depends upon the relative orientation of the radiation polarization mode and the geomagnetic field direction. The model provides both limb radiances and the derivatives of these radiances with respect to atmospheric temperature and composition, as required by MLS temperature retrievals. EOS MLS views the atmospheric limb at 118.75 GHz with a pair of linear-cross-polarized, 100-kHz-resolution, 10-MHz-wide spectrometers. The antennas of the associated receivers are scanned to view rays with tangent heights from the Earth's surface to 0.001 hPa. Comparisons of the modeled MLS radiances with measurements show generally good agreement in line positions and strengths, however residuals in the line centers at the highest tangent heights are larger than desired and still under investigation. Index Terms--Earth Observing System (EOS) Aura, forward model, Microwave Limb Sounder (MLS), Zeeman.

Nineteen limb viewing (occultation and passive thermal) and two nadir humidity data sets are intercompared and also compared to frostpoint hygrometer balloon sondes. The upper troposphere considered here covers the pressure range from 300-100 hPa. Water vapor in this region is a challenging measurement because concentrations vary between 2-1000 parts per million volume with sharp changes in vertical gradients near the tropopause. The atmospheric temperature is also highly variable ranging from 180-250 K. The assessment of satellite measured humidity is based on coincident comparisons with frostpoint hygrometer sondes, multi month mapped comparisons, zonal mean time series comparisons and coincident satellite to satellite comparisons. While the satellite fields show similar features in maps and time series, quantitatively, they can differ by a factor of two in concentration, with strong dependencies on the amount of H2O. Additionally, time-lag response corrected Vaisala-RS92 radiosondes are compared to satellites and the frostpoint hygrometer measurements. In summary, most satellite data sets reviewed here show on average ~30 % agreement amongst themselves and frostpoint data but with an additional ~30 % variability about the mean. The Vaisala-RS92 sonde even with a time-lag correction shows poor behavior for pressure less than 200 hPa. [ABSTRACT FROM AUTHOR]

We analyze Aura Microwave Limb Sounder (MLS) monthly zonal mean time series of ClO and HOCl between 50°S and 50°N to estimate upper stratospheric trends in these chlorine species from 2005 through 2020. We compare these observations to those from the Whole Atmosphere Community Climate Model version 6 (WACCM6), run under the specified dynamics configuration. The model sampling follows the MLS coverage in space and local time. We use version 5 MLS ClO zonal mean daytime profiles and similarly binned daytime ClO model profiles from 32 to 1.5 hPa. For MLS HOCl, we use the version 5 offline product derived from daily zonal mean radiances rather than averaged Level 2 profiles; MLS HOCl is scientifically useful between 10 and 2 hPa, and the HOCl monthly zonal means are separated into day and night for comparison to WACCM6. We find good agreement (mostly within ~10%) between the climatological MLS ClO daytime distributions and the model ClO climatology for 2005-2020. The model HOCl climatology, however, underestimates the MLS HOCl climatology by about 30%. This could well be caused by a combination of fairly large systematic uncertainties in both the model-assumed rate constant for the formation of HOCl and the MLS HOCl retrievals themselves. The model daytime ClO trends versus latitude and pressure agree quite well with those from MLS. MLS22 derived near-global upper stratospheric daytime trends between 7 and 2 hPa are -0.73 ± 0.40 %yr-1 for ClO and 0.39 ± 0.35 %yr-1 for HOCl, with 2σ uncertainty estimates used here. The corresponding model decreases are somewhat faster than observed (although the difference is not statistically significant), with trend values of -0.85 ± 0.45 %yr-1 for ClO and -0.64 ± 0.37 %yr-1 for HOCl. Both data and model results point to a faster trend in ClO than in HOCl. The MLS ClO trends are consistent with past estimates of upper stratospheric ClO trends from satellite and ground-based microwave data. As discussed in the past, trends in other species (in particular, positive trends in CH4 and H2O) can lead to a ClO decrease that is faster than the decrease in total inorganic chlorine. Regarding trends in HOCl, positive trends in HO2 can lead to a faster rate of formation for HOCl as a function of time, which partially offsets the decreasing trend in active chlorine. The decreasing trends in upper stratospheric ClO and HOCl provide additional confirmation of the effectiveness of the Montreal Protocol and its amendments, which have led to the early stages of an expected long-term ozone recovery from the effects of ozone-depleting substances. [ABSTRACT FROM AUTHOR]