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The 2022 eruption of the Hunga Tonga‐Hunga Ha'apai volcano caused substantial impacts on the atmosphere, including a massive injection of water vapor, and the largest increase in stratospheric aerosol for 30 years. The Ozone Mapping and Profiler Suite (OMPS) Limb Profiler instrument has been critical in monitoring the amount and spread of the volcanic aerosol in the stratosphere. We show that the rapid imagery from the OMPS instrument enables a tomographic retrieval of the aerosol extinction that reduces a critical bias of up to a factor of two, and improves vertical structure and agreement with coincident lidar and occultation observations. Due to the vertically thin and heterogeneous nature of the volcanic aerosol, this impacts integrated values of aerosol across latitude, altitude, and time for several months. We also investigate the systematic impact of uncertainty in assumed particle size that result in an underestimation of the aerosol extinction at the peak of the volcanic aerosol layer. Plain Language Summary: The Hunga Tonga‐Hunga Ha'apai volcano erupted in 2022. The eruption plume went higher into the atmosphere than ever observed before in the modern age. It also carried large amounts of water vapor and other gases and particles, called aerosols, into the stratosphere. The NASA satellite instrument, called the Ozone Mapping and Profiler Suite (OMPS) Limb Profiler, has given us valuable measurements of these aerosols, which are helpful in understanding the impact the volcanic eruption might have on climate. We use an advanced technique to analyze the OMPS measurements that provides a clearer view of the plume. This analysis gives somewhat different results about the thickness of the volcanic plume than the standard method. Key Points: Tomographic retrievals reduce a critical bias in Ozone Mapping and Profiler Suite Limb Profiler volcanic aerosol extinction, improving agreement with Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation and Stratospheric Aerosol and Gas Experiment III/International Space StationBiases of up to a factor of two extend beyond the early plume, with zonal, temporal, and altitude integrated values affected for monthsUncertainty in particle size distribution also has an impact that should be considered when analyzing aerosol loading [ABSTRACT FROM AUTHOR]

The Stratospheric Aerosol and Gas Experiment (SAGE) III has been operating on the International Space Station (ISS) since mid-2017. Nitrogen dioxide (NO2) number density profiles are routinely retrieved from SAGE III/ISS solar occultation measurements in the middle atmosphere. Although NO2 density varies throughout the day due to photochemistry, the standard SAGE NO2 retrieval algorithm neglects these variations along the instrument's line of sight by assuming that the number density has a constant gradient within a given vertical layer of the atmosphere. This assumption will result in a retrieval bias for a species like NO2 that changes rapidly across the terminator. In this work we account for diurnal variations in retrievals of NO2 from the SAGE III/ISS measurements, and we determine the impact of this algorithm improvement on the resulting NO2 number densities. The first step in applying the diurnal correction is to use publicly available SAGE III/ISS products to convert the retrieved number density profiles to optical depth profiles. The retrieval is then re-performed with a new matrix that applies photochemical scale factors for each point along the line of sight according to the changing solar zenith angle. In general NO2 that is retrieved by accounting for these diurnal variations is more than 10 % lower than the standard algorithm below 30 km. This effect is greatest in winter at high latitudes and generally greater for sunrise occultations than sunset. Comparisons with coincident profiles from the Optical Spectrograph and InfraRed Imager System (OSIRIS) show that NO2 from SAGE III/ISS is generally biased high; however the agreement improves by up to 20 % in the mid-stratosphere when diurnal variations are accounted for in the retrieval. We conclude that diurnal variations along the SAGE III/ISS line of sight are an important term to consider for NO2 analyses at altitudes below 30 km. [ABSTRACT FROM AUTHOR]

The Stratospheric Aerosol and Gas Experiment (SAGE) III has been operating on the International Space Station (ISS) since mid 2017. Nitrogen dioxide (NO2) number density profiles are routinely retrieved from SAGE III/ISS solar occultation measurements in the middle atmosphere. Although NO2 density varies throughout the day due to photochemistry, the standard SAGE NO2 retrieval algorithm neglects these variations along the instrument's line of sight by assuming that the number density has a constant gradient within a given vertical layer of the atmosphere. This assumption will result in a retrieval bias for a species like NO2 that changes rapidly across the terminator. In this work we account for diurnal variations in retrievals of NO2 from the SAGE III/ISS measurements, and determine the impact of this algorithm improvement on the resulting NO2 number densities. The diurnal correction is applied by first undoing the SAGE III/ISS retrieval using publicly available SAGE III/ISS products to obtain an optical depth profile. The retrieval is then performed with a new matrix that applies photochemical scale factors for each point along the line of sight according to the changing solar zenith angle. In general NO2 that is retrieved by accounting for these diurnal variations is more than 10 % lower than the standard algorithm below 30 km. This effect is greatest in winter at high latitudes, and generally greater for sunrise occultations than sunset. Comparisons with coincident profiles from the Optical Spectrograph and InfraRed Imager System (OSIRIS) show that NO2 from SAGE III/ISS is generally biased high, however the agreement improves by up to 20 % in the mid stratosphere when diurnal variations are accounted for in the retrieval. We conclude that diurnal variations along the SAGE III/ISS line of sight are an important term to consider for NO2 analyses at altitudes below 30 km. [ABSTRACT FROM AUTHOR]

A small long-term drift in the Optical Spectrograph and Infrared Imager System (OSIRIS) stratospheric ozone product, manifested mostly since 2012, is quantified and attributed to a changing bias in the limb pointing knowledge of the instrument. A correction to this pointing drift using a predictable shape in the measured limb radiance profile is implemented and applied within the OSIRIS retrieval algorithm. This new data product, version 5.10, displays substantially better both long- and short-term agreement with Microwave Limb Sounder (MLS) ozone throughout the stratosphere due to the pointing correction. Previously reported stratospheric ozone trends over the time period 1984–2013, which were derived by merging the altitude–number density ozone profile measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) II satellite instrument (1984–2005) and from OSIRIS (2002–2013), are recalculated using the new OSIRIS version 5.10 product and extended to 2017. These results still show statistically significant positive trends throughout the upper stratosphere since 1997, but at weaker levels that are more closely in line with estimates from other data records. [ABSTRACT FROM AUTHOR]

Techniques for remote sensing of atmospheric composition often involve the minimization of the residual between spectrally-resolved atmospheric radiometry and the output of a radiative transfer model which solves the equations of radiative transfer in a candidate atmosphere. Here, SASKTRAN, a framework of climatologies, optical property calculators, and scalar radiative transfer models specifically designed for modeling limb scattering observations, is upgraded to solve the vector radiative transfer equations. In particular, the Monte Carlo and High Resolution Successive Orders modules of the SASKTRAN framework are extended to handle polarized scattering of light in a fully spherical atmosphere, with the assumption of a Lambertian earth underneath. The fast, polarized, High Resolution module is validated against the statistically exact polarized Monte Carlo module for limb-scatter observations in the UV–visible–NIR spectral range and is found to be accurate to within 0.2% in its fully polarized mode. An approximate method to solve the vector radiative transfer equations is introduced; in this mode, the high resolution engine is accurate for limb-scatter geometries to within about 0.2% in almost all cases with minimal degradation in performance as compared to the scalar mode. The models are used to show that scalar simulations of limb-observed UV–vis radiances may contain absolute radiometric errors of up to 5% for typical stratospheric aerosol loads. Furthermore, limb-observed radiances may have nearly any linear polarization, and this is in general altitude and wavelength dependent. [ABSTRACT FROM AUTHOR]

A small, long-term drift in the Optical Spectrograph and Infrared Imager System (OSIRIS) stratospheric ozone product, manifested mostly since 2012, is quantified and attributed to a changing bias in the limb pointing knowledge of the instrument. A correction to this pointing drift using a predictable shape in the measured limb radiance profile is implemented and applied within the OSIRIS retrieval algorithm. This new data product, version 5.10, displays substantially better both long- and short-term agreement with MLS ozone throughout the stratosphere due to the pointing correction. Previously reported stratospheric ozone trends over the time period 1984-2013, which were derived by merging the altitude/number density ozone profile measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) II satellite instrument (1984-2005) and from OSIRIS (2002-2013) are recalculated using the new OSIRIS version 5.10 product, and extended to 2017. These results still show statistically significant positive trends throughout the upper stratosphere since 1997, but at weaker levels that are more closely in line with estimates from other data records. [ABSTRACT FROM AUTHOR]

The Nabro stratovolcano in Eritrea, northeastern Africa, erupted on 13 June 2011, injecting approximately 1.3 teragrams of sulfur dioxide (SO2) to altitudes of 9 to 14 kilometers in the upper troposphere, which resulted in a large aerosol enhancement in the stratosphere. The SO2 was lofted into the lower stratosphere by deep convection and the circulation associated with the Asian summer monsoon while gradually converting to sulfate aerosol. This demonstrates that to affect climate, volcanic eruptions need not be strong enough to inject sulfur directly to the stratosphere. [ABSTRACT FROM AUTHOR]

Abstract: The scattering of visible light by ice crystals and dust in radiative transfer models is challenging in part due to the large amount of scattering in the forward direction. We introduce a technique that ensures numerical conservation of photons in any radiative transfer model and that quantifies the integration error associated with highly asymmetric phase functions. When applied to a successive-orders of scatter model, the technique illustrates the high accuracy obtained in numerical integration of molecular and aerosol scattering. As well, a phase function truncation and renormalization technique is applied to scattering by ice crystals with very large size parameters, between 100 and 1000, and the scaled radiative transfer equation is solved with the spherical successive-orders model, SASKTRAN. Since computations shown this work are performed in a fully spherical model atmosphere, the computed radiances are not subject to the discontinuity at the horizon that is inherent in models using a plane–parallel assumption. The methods introduced in this work are of particular interest in modeling limb radiances in the presence of thin cirrus clouds. [Copyright &y& Elsevier]

Systematic errors in plane-parallel radiative transfer calculations are examined in the context of limb radiance. Calculation of the multiple-scattering component of limb radiance using plane-parallel geometry can lead to a systematic overestimation that increases with surface albedo and tangent height. It is demonstrated that the cause of this is the overestimate of the surface-reflected radiance component. A simple yet effective correction is introduced that reduces these errors from 4%–8% to <2%. This error source may manifest itself in any application of radiative transfer calculations employing plane-parallel geometry. [ABSTRACT FROM AUTHOR]

The v7.2 NO2 retrieval for the Optical Spectrograph and InfraRed Imager System (OSIRIS) was designed to improve sensitivity in the upper troposphere–lower stratosphere (UTLS) and to reduce an observed low bias in the previous version, v6.0. The details of this retrieval are described and then the data are compared to coincident NO2 profiles from the Atmospheric Chemistry Experiment–Fourier Transform Spectrometer (ACE-FTS) and the Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGE III/ISS). The PRATMO photochemical box model was used to account for differences in the measurement times of the instruments: all datasets were scaled to the same local solar time of 12:00 LST. Coincident ACE-FTS and OSIRIS NO2 measurements agree within 20 % throughout much of the stratosphere. Coincident SAGE III/ISS and OSIRIS NO2 measurements also agree within 20 %, with OSIRIS biased low at all altitudes and latitudes. The ACE-FTS, OSIRIS, and SAGE III-ISS NO2 monthly zonal mean data show very similar variability in time at most altitude and latitudes. [ABSTRACT FROM AUTHOR]

Deep convection within the Asian summer monsoon (ASM) transports surface level air into the upper troposphere‐lower stratosphere (UTLS). This work aims to understand the distribution of NO2, NO, and NOx in the UTLS ASM anticyclone from satellite measurements. Observations of NO2 from the Optical Spectrograph and InfraRed Imager System, the Atmospheric Chemistry Experiment ‐ Fourier Transform Spectrometer (ACE‐FTS), and the Stratospheric Aerosol and Gas Experiment III on the International Space Station are considered. The PRATMO photochemical box model is used to quantify the NOx photochemistry, and to derive the NOx concentration using OSIRIS NO2 and O3 observations. The satellite data show a relative minimum in NO2 over the ASM in the summer months, while the corresponding NO and NOx anomalies are elevated, mainly due to low O3 and cold temperatures within the ASM. The observations within the ASM show reasonable agreement to simulations from the Whole Atmosphere Community Climate Model. Plain Language Summary: Motion within the Asian summer monsoon (ASM) transports air from the Earth's surface to the upper troposphere‐lower stratosphere (UTLS), an altitude of around 10–15 km. This provides a pathway for surface level pollutants to influence the chemistry of the UTLS. One such pollutant is NOx, which is the sum of NO2 and NO. Here we consider observations of NO2 from the Optical Spectrograph and InfraRed Imager System, the Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer (ACE‐FTS), and the Stratospheric Aerosol and Gas Experiment III on the International Space Station, along with NOx derived from OSIRIS NO2 and O3 observations. The satellite data show that a relative maximum in NOx within the UTLS ASM corresponds to a relative minimum in NO2. This is mainly due to low O3 and cold temperatures within the ASM that alter the NOx chemistry. The observations show good agreement with climate model simulations. These findings provide new information on UTLS NOx behavior and the chemistry that occurs within the ASM. Key Points: Satellite data show a relative minimum in the NO2 concentration within the Asian summer monsoon (ASM) in the upper troposphere‐lower stratosphere (UTLS)Photochemical box model calculations show corresponding maxima in UTLS NO and NOx, due to low O3 and temperature in the monsoonNO2 and NOx from satellite data and a climate model show good agreement within the ASM [ABSTRACT FROM AUTHOR]

The understanding of the excitation mechanism of the sodium D-line nightglow emission is important for many investigations of atomic sodium in the upper mesosphere region. In 1939, Sidney Chapman proposed a first scheme of the chemical reactions involving sodium and ozone to explain the yellow sodium line doublet in the terrestrial nightglow. Later, it became clear that this mechanism could not explain the observed variability in the Na D-line ratio. Hence, Slanger et al. (2005) suggested a modified Chapman mechanism. However, it is well accepted that there is still a parameter that is not well quantified, i.e., the branching ratio f A that defines how many sodium atoms are produced (via the reaction Na + O 3) in the excited Na( P 2 J) state. Here, we retrieve sodium profiles in the mesosphere region from limb measurements carried out with the OSIRIS instrument on the Odin satellite between 2002 and until recently. These profiles are compared to profiles obtained from ground-based Lidar measurements. Until 2010, the Lidar was located in Fort Collins, Colorado at 40.5° N and 105.0° W and was then moved to Logan, Utah at 41.8° N and 111.2° W. In this study, we found 20 nights with measurements suitable for comparison. By adjusting the branching ratio in the retrieval algorithm until the Lidar Na-profile and the satellite Na-profile agree best and combining the three different approaches for the comparison we propose a value of the branching ratio of: 0.064 ± 0.028. • Retrieval of mesospheric sodium from OSIRIS nightglow measurements. • Validation of retrieved profiles by comparison to Lidar measurements. • Constraining of branching ratio f A of the reaction NaO + O → Na (P 2 / S 2) + O 2. [ABSTRACT FROM AUTHOR]

The v7.2 NO2 retrieval for the Optical Spectrograph and InfraRed Imager System (OSIRIS) was designed to improve sensitivity in the Upper Troposphere-Lower Stratosphere (UTLS) and to reduce an observed low bias in the previous version, v6.0. The details of this retrieval are described, and then the data are compared to coincident NO2 profiles from the Atmospheric Chemistry Experiment - Fourier Transform Spectrometer (ACE-FTS) and the Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGE III/ISS). The the PRATMO photochemical box model was used to account for differences in the measurement times of the instruments: all datasets were scaled to the same local solar time of 12:00 pm. Coincident ACE-FTS and OSIRIS NO2 measurements agree within 20% throughout much of the stratosphere. Coincident SAGE III/ISS and OSIRIS NO2 measurements also agree within 20%, with OSIRIS biased low at all altitudes and latitudes. The ACE-FTS, OSIRIS, and SAGE III-ISS NO2 monthly zonal mean data show very similar variability in time at most altitude and latitudes. [ABSTRACT FROM AUTHOR]

Stratospheric aerosols play a key role in atmospheric chemistry and climate. Their particle size is a crucial factor controlling the microphysical, radiative, and chemical aerosol processes in the stratosphere. Despite its importance, available observations on aerosol particle size are rather sparse. This limits our understanding and knowledge about the mechanisms and importance of chemical and climate aerosol feedbacks. The retrieval described by provides the stratospheric particle size distribution (PSD) from SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) limb observations in the tropics. This algorithm has now been improved and extended to work on the entire globe. Two PSD parameters of a unimodal lognormal PSD, the median radius and the geometric standard deviation, are retrieved between 18 and 35 km altitude from SCIAMACHY limb observations by a multiwavelength nonlinear regularized inversion. The approach assumes an aerosol particle number density profile that does not change during the retrieval. The effective Lambertian surface albedo pre-retrieved from coinciding SCIAMACHY nadir observations is integrated into the retrieval algorithm to mitigate the influence of the surface albedo on the retrieval results. The extinction coefficient and the effective radius are calculated from the PSD parameters. The aerosol characteristics from SCIAMACHY are compared with in situ balloon-borne measurements from Laramie, Wyoming, and retrievals from the satellite instruments of the Stratospheric Aerosol and Gas Experiment series (SAGE II and SAGE III) and Optical Spectrograph and InfraRed Imager System (OSIRIS). In the Northern Hemisphere, the median radius differs by less than 27 % and the geometric standard deviation by less than 11 % from both balloon-borne and SAGE III data. Differences are mainly attributed to errors in the assumed a priori number density profile. Globally, the SCIAMACHY extinction coefficient at 750 nm deviates by less than 35 % from SAGE II, SAGE III, and OSIRIS data. The effective radii from SCIAMACHY, balloon-borne measurements, and SAGE III agree within about 18 %, while the effective radius based on SAGE II measurements is systematically larger. The novel data set containing the PSD parameters, the effective radius, and the aerosol extinction coefficients at 525, 750, and 1020 nm from SCIAMACHY observations is publicly available. [ABSTRACT FROM AUTHOR]

The OH airglow has been used to investigate the chemistry and dynamics of the mesosphere and the lower thermosphere (MLT) for a long time. The infrared imager (IRI) aboard the Odin satellite has been recording the night-time 1.53 µ m OH (3-1) emission for more than 15 years (2001–2015), and we have recently processed the complete data set. The newly derived data products contain the volume emission rate profiles and the Gaussian-approximated layer height, thickness, peak intensity and zenith intensity, and their corresponding error estimates. In this study, we describe the retrieval steps for these data products. We also provide data screening recommendations. The monthly zonal averages depict the well-known annual oscillation and semi-annual oscillation signatures, which demonstrate the fidelity of the data set (10.5281/zenodo.4746506,). The uniqueness of this Odin IRI OH long-term data set makes it valuable for studying various topics, for instance, the sudden stratospheric warming events in the polar regions and solar cycle influences on the MLT. [ABSTRACT FROM AUTHOR]

The Optical Spectrograph and InfraRed Imaging System (OSIRIS) was launched on boardthe Odin satellite in 2001, and has since provided vertical profiles of limb scattered radiancebetween 280 and 810 nm. These measurements have been used to provide reliable aerosolextinction measurements at 750 nm from altitudes above the tropopause to near 35 km with avertical resolution of approximately 2 km. The relatively high sampling rate produceshundreds of measurements per day over the sunlit portion of the globe, enabling excellentspatial and temporal sampling of short-lived events. This work compares the newmulti-wavelength OSIRIS version 7 aerosol retrieval with data sets from multipleinstruments. Occultation measurements from SAGE III/ISS are used as a baseline toinvestigate general accuracy of the OSIRIS retrieval, while comparisons with CALIPSO andOMPS-LP are used to investigate differences at higher spatial and temporal resolution. Focusis given to measurements during volcanic periods to explore differences arisingfrom spatial inhomogeneities, retrieval assumptions and measurement techniques. [ABSTRACT FROM AUTHOR]

The OH airglow has been used to investigate the chemistry and dynamics of the mesosphere and the lower thermo-sphere (MLT) for a long time. The infrared imager (IRI) aboard the Odin satellite has been recording the nighttime 1.53 μm OH (3-1) emission for more than 15 years (2001-2015) and we have recently processed the complete data set. The newly derived data products contain the volume emission rate profiles and the Gaussian approximated layer height, thickness, peak intensity and zenith intensity, and their corresponding error estimates. In this study, we describe the retrieval steps of these data products. We also provide data screening recommendations. The monthly zonal averages depict the well known annual oscillation and semi-annual oscillation signatures, which demonstrate the fidelity of the data set (https://doi.org/10.5281/zenodo.4746506, Li et al. (2021)). The uniqueness of this Odin-IRI OH long-term data set makes it valuable for studying various topics, for instance, the sudden stratospheric warming events in the polar regions and solar cycle influences on the MLT. [ABSTRACT FROM AUTHOR]

A new upper stratospheric (35–60 km) temperature data product has been produced using Optical Spectrograph and InfraRed Imager System (OSIRIS) limb-scattered spectra that now spans over 22 years. Temperature is calculated by first estimating the Rayleigh scattering signal and then integrating hydrostatic balance combined with the ideal gas law. Uncertainties are estimated to be 1–5 K, with a vertical resolution of 3–4 km. Correlative comparisons with the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) and the Microwave Limb Sounder on Aura (MLS) are consistent with these uncertainty estimates and generally have no regions of statistically significant drift. The data product has been publicly released as part of the nominal OSIRIS v7.3 processing. [ABSTRACT FROM AUTHOR]

Climate-related studies need information about the distribution of stratospheric aerosols, which influence the energy balance of the Earth's atmosphere. In this work, we present a merged dataset of vertically resolved stratospheric aerosol extinction coefficients, which is derived from data by six limb and occultation satellite instruments: SAGE II on ERBS, GOMOS and SCIAMACHY on Envisat, OSIRIS on Odin, OMPS on Suomi-NPP, and SAGE III on the International Space Station. The merging of aerosol profiles is performed by transformation of the aerosol datasets from individual satellite instruments to the same wavelength 750 nm and their de-biasing and homogenization by adjusting the seasonal cycles. After such homogenization, the data from individual satellite instruments are in good agreement. The merged aerosol extinction coefficient is computed as the median of the adjusted data from the individual instruments. The merged time series of vertically resolved monthly mean aerosol extinction coefficients at 750 nm is provided in 10° latitudinal bins from 90° S to 90° N, in the altitude range from 8.5 km to 39.5 km. The time series of the stratospheric aerosol optical depth (SAOD) is created by integration of aerosol extinction profiles from the tropopause to 39.5 km; it is also provided as monthly mean data in 10° latitudinal bins. The created aerosol climate record covers the period from October 1984 until May 2022, and it is intended to be extended in the future. It can be used in various climate-related studies. [ABSTRACT FROM AUTHOR]

A new retrieval algorithm to obtain vertical profiles of the aerosol extinction coefficient from the measurements of the scattered solar light in the limb viewing geometry made by the OMPS-LP instrument is presented. The method employs the normalization of the limb radiances to the solar irradiance in contrast to the normalization by a limb measurement at an upper tangent height, which is used by most of the other published limb-scatter retrievals. The main advantage of this approach is a nearly complete elimination of the dependence of the retrieval results on the prior aerosol extinction profile used in the retrieval. This makes the retrieval well suitable to analyze the observation scenes with highly elevated aerosol plumes as occurred after the Hunga Tonga–Hunga Ha'apai volcanic eruption in January 2022. The results from the new approach were compared to independent data from SAGE III/ISS and OSIRIS. In general, an agreement within 25% between the different data products was observed although larger differences were seen after very strong volcanic eruptions and wildfires. The new data product was used to investigate the evolution of the aerosol plume after the Hunga Tonga–Hunga Ha'apai volcanic eruption. [ABSTRACT FROM AUTHOR]

Air quality measurements from geostationary orbit by the instrument TEMPO (Tropospheric Emissions: Monitoring of Pollution) will offer an unprecedented view of atmospheric composition over North America. Measurements over Canadian latitudes, however, offer unique challenges: TEMPO's lines of sight are shallower, the sun is lower, and snow cover is more common. All of these factors increase the impact of the sphericity and the horizontal inhomogeneity of the atmosphere on the accuracy of the air quality measurements. Air mass factors encapsulate the complex paths of the measured sunlight, but traditionally they ignore horizontal variability. For the high spatial resolution of modern instruments such as TEMPO, the error due to neglecting horizontal variability is magnified and needs to be characterized. Here we present developments to SASKTRAN, the radiative transfer framework developed at the University of Saskatchewan, to calculate air mass factors in a spherical atmosphere, with and without consideration of horizontal inhomogeneity. Recent upgrades to SASKTRAN include first-order spherical corrections for the discrete ordinates method and the capacity to compute air mass factors with the Monte Carlo method. Together with finite-difference air mass factors via the successive orders method, this creates a robust framework for computing air mass factors. One-dimensional air mass factors from all three methods are compared in detail and are found to be in good agreement. Two-dimensional air mass factors are computed with the deterministic successive orders method, demonstrating an alternative for a calculation which would typically be done only with a nondeterministic Monte Carlo method. The two-dimensional air mass factors are used to analyze a simulated TEMPO-like measurement over Canadian latitudes. The effect of a sharp horizontal feature in surface albedo and NO 2 was quantified while varying the distance of the feature from the intended measurement location. Such a feature in the surface albedo or NO2 could induce errors on the order of 5 % to 10 % at a distance of 50 km, and their combination could induce errors on the order of 10 % as far as 100 km away. [ABSTRACT FROM AUTHOR]

The Spatial Heterodyne Observations of Water instrument (SHOW) is a limb-sounding satellite prototype that utilizes the Spatial Heterodyne Spectroscopy (SHS) technique, operating in a limb-viewing configuration, to observe limb-scattered sunlight in a vibrational band of water vapour within a spectral window from 1363 to 1366 nm. The goal is to retrieve high vertical and horizontal resolution measurements of water vapour in the upper troposphere and lower stratosphere. The prototype instrument has been configured for observations from NASA's ER-2 high-altitude airborne remote science airplane. Flying at a maximum altitude of ∼21.34 km with a maximum speed of ∼760 km h -1 , the ER-2 provides a stable platform to simulate observations from a low-earth orbit satellite. Demonstration flights were performed from the ER-2 during an observation campaign from 15 to 22 July 2017. In this paper, we present the laboratory characterization work and the level 0 to level 1 processing of flight data that were obtained during an engineering flight performed on 18 July 2017. Water vapour profile retrievals are presented and compared to in situ radiosonde measurements made of the same approximate column of air. These measurements are used to validate the SHOW measurement concept and examine the sensitivity of the technique. [ABSTRACT FROM AUTHOR]

The Spatial Heterodyne Observations of Water instrument (SHOW) is a limb-sounding satellite prototype that utilizes the Spatial Heterodyne Spectroscopy Technique (SHS), operating in a limb-viewing configuration, to observe limb-scattered sunlight in a vibrational band of water vapour within a spectral window from 1363 nm to 1366 nm. The goal is to retrieve high vertical and horizontal resolution measurements of water vapour in the upper troposphere and lower stratosphere. The prototype instrument has been configured for observations from NASA's ER-2 high altitude airborne remote science airplane. Flying at a maximum altitude of ~ 21.34 km with a maximum speed of ~ 760 km/hr, the ER-2 provides a stable platform to simulate observations from a low-earth orbit satellite. Demonstration flights were performed from the ER-2 during an observation campaign from July 15-July 22, 2017. In this paper, we present the laboratory characterization work and the Level 0 to Level 1 processing of flight data that was obtained during an engineering flight performed on July 18th, 2017. Water vapour profile retrievals are presented and compared to in situ radiosonde measurements made of the same approximate column of air. These measurements are used to validate the SHOW measurement concept and examine the sensitivity of the technique. [ABSTRACT FROM AUTHOR]

Measurements of limb-scattered sunlight from the Ozone Mapping and Profiler Suite Limb Profiler (OMPSLP) can be used to obtain vertical profiles of ozone in the stratosphere. In this paper we describe a two-dimensional, or tomographic, retrieval algorithm for OMPS-LP where variations are retrieved simultaneously in altitude and the along-orbital-track dimension. The algorithm has been applied to measurements from the center slit for the full OMPSLP mission to create the publicly available University of Saskatchewan (USask) OMPS-LP 2D v1.0.2 dataset. Tropical ozone anomalies are compared with measurements from the Microwave Limb Sounder (MLS), where differences are less than 5% of the mean ozone value for the majority of the stratosphere. Examples of near-coincident measurements with MLS are also shown, and agreement at the 5% level is observed for the majority of the stratosphere. Both simulated retrievals and coincident comparisons with MLS are shown at the edge of the polar vortex, comparing the results to a traditional one-dimensional retrieval. The one-dimensional retrieval is shown to consistently overestimate the amount of ozone in areas of large horizontal gradients relative to both MLS and the two-dimensional retrieval. [ABSTRACT FROM AUTHOR]

Stratospheric aerosols play a key role in atmospheric chemistry and climate. Their particle size is a crucial factor controlling the microphysical, radiative, and chemical aerosol processes in the stratosphere. Despite its importance, available observations on aerosol particle size are rather sparse. This limits our understanding and knowledge about the mechanisms and importance of chemical and climate aerosol feedbacks. The retrieval described by Malinina et al. (2018) provides the stratospheric particle size distribution (PSD) from SCIAMACHY limb observations in the tropics. This algorithm has now been improved and extended to work on the entire globe. Two PSD parameters of a unimodal lognormal PSD, the median radius and the geometric standard deviation, are retrieved between 18 and 35 km altitude from SCIAMACHY limb observations by a multi-wavelength non-linear regularized inversion. This assumes a fixed number density profile. The extinction coefficient and the effective radius are calculated. The effective Lambertian surface albedo pre-retrieved from coinciding SCIAMACHY nadir observations is integrated into the retrieval algorithm to mitigate the influence of the surface albedo on the retrieval results. The aerosol characteristics from SCIAMACHY are compared with in-situ balloon-borne measurements from Laramie, Wyoming, and retrievals from the satellite instruments SAGE II, SAGE III, and OSIRIS. In the northern hemisphere, the median radius differs by less than 27% and the geometric standard deviation by less than 11% from both balloon-borne and SAGE III data. Differences are mainly attributed to errors in the assumed a priori number density profile. Globally, the SCIAMACHY extinction coefficients at 750 nm deviate by less than 35% from SAGE II, SAGE III, and OSIRIS data. The effective radius from SCIAMACHY, balloon-borne measurements, and SAGE III agree within about 18% while the effective radius based on SAGE II measurements is systematically larger. The novel data set containing the effective radius and the aerosol extinction coefficient at 525, 750, and 1020 nm from SCIAMACHY observations is publicly available. [ABSTRACT FROM AUTHOR]

The Limb Imaging Fourier Transform Spectrometer Experiment (LIFE) instrument is a balloon-borne prototype of a satellite instrument designed to take vertical images of atmospheric limb emission spectra in the 700–1400 cm -1 wavenumber range from the upper-troposphere–lower-stratosphere (UTLS) altitude region of the atmosphere. The prototype builds on the success of past and existing instruments while reducing the complexity of the imaging design. This paper details the results of a demonstration flight on a stabilized stratospheric balloon gondola from Timmins, Canada, in August 2019. Retrievals of vertical trace gas profiles for the important greenhouse gases H2O , O3 , CH4 , and N2O , as well as HNO3 , are performed using an optimal estimation approach and the SASKTRAN radiative transfer model. The retrieved profiles are compared to approximately coincident observations made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) solar occultation and Microwave Limb Sounder (MLS) instruments. An evaluation of the LIFE measurements is performed, and areas of improvement are identified. This work increases the overall technical readiness of the approach for future balloon, aircraft, and space applications. [ABSTRACT FROM AUTHOR]

In this paper, we present the updated SAGE-CCI-OMPS + climate data record of monthly zonal mean ozone profiles. This dataset covers the stratosphere and combines measurements by nine limb and occultation satellite instruments – SAGE II (Stratospheric Aerosol and Gases Experiment II), OSIRIS (Optical Spectrograph and InfraRed Imaging System), MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), SCIAMACHY (SCanning Imaging Spectrometer for Atmospheric CHartographY), GOMOS (Global Ozone Monitoring by Occultation of Stars), ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer), OMPS-LP (Ozone Monitor Profiling Suite Limb Profiler), POAM (Polar Ozone and Aerosol Measurement) III, and SAGE III/ISS (Stratospheric Aerosol and Gases Experiment III on the International Space Station). Compared to the original version of the SAGE-CCI-OMPS dataset (Sofieva et al., 2017b), the update includes new versions of MIPAS, ACE-FTS, and OSIRIS datasets and introduces data from additional sensors (POAM III and SAGE III/ISS) and retrieval processors (OMPS-LP). In this paper, we show detailed intercomparisons of ozone profiles from different instruments and data versions, with a focus on the detection of possible drifts in the datasets. The SAGE-CCI-OMPS + dataset has a better coverage of polar regions and of the upper troposphere and the lower stratosphere (UTLS) than the previous dataset. We also studied the influence of including new datasets on ozone trends, which are estimated using multiple linear regression. The changes in the merged dataset do not change the overall morphology of post-1997 ozone trends; statistically significant trends are observed in the upper stratosphere. The largest changes in ozone trends are observed in polar regions, especially in the Southern Hemisphere. The updated SAGE-CCI-OMPS + dataset contains profiles of deseasonalized anomalies and ozone concentrations from 1984 to 2021, in 10 ∘ latitude bins from 90 ∘ S to 90 ∘ N and in the altitude range from 10 to 50 km. The dataset is open access and available at https://climate.esa.int/en/projects/ozone/data/ (last access: 9 March 2023) and at ftp://cci_web@ftp-ae.oma.be/esacci (ESA Climate Office; last access: 9 March 2023). [ABSTRACT FROM AUTHOR]

We develop an algorithm that uses the aerosol extinction at two wavelengths (color ratio) to derive the size and number density for stratospheric aerosols. We apply our algorithm to Ozone Mapping Profiler Suite Limb Profiler (OMPS-10 LP) L2 and Stratospheric Aerosol and Gas Experiment (SAGE) data. We show that the color ratio between two wavelengths (e.g. 510 nm/869 nm) is insensitive to aerosol concentration and thus can be used to derive aerosol size assuming a log-normal size distribution. With the size and the extinction, we can compute a number density consistent with both wavelengths. Our results compare favorably to balloon borne particle size and concentration measurements. Our results are also consistent with SAGE solar occultation measurements. Finally, we show the background distribution of stratospheric aerosols and the changes 15 in those distributions during the Reikoke and Hunga Tonga-Hunga Ha'apai volcanic eruptions. We also show the evolution of the size and number density of aerosols following both of those eruptions. [ABSTRACT FROM AUTHOR]

The Limb Imaging Fourier-Transform Spectrometer Experiment (LIFE) instrument is a balloon-borne prototype of a satellite instrument designed to take vertical images of atmospheric limb emission spectra in the 700 to 1400 cm-1 wavenumber range from the upper troposphere/lower stratosphere (UTLS) altitude region of the atmosphere. The prototype builds on the success of past and existing instruments while reducing the complexity of the imaging design. This paper details the results of a demonstration flight on a stabilized stratospheric balloon gondola from Timmins, Canada in August 2019. Retrievals 5 of vertical trace gas profiles for the important greenhouse gases H2O, O3, CH4 and N2O, as well as HNO3, are performed using an Optimal Estimation Approach and the SASKTRAN radiative transfer model. The retrieved profiles are compared to approximately coincident observations made by the ACE-FTS solar occultation satellite instrument. An evaluation of the LIFE measurements is performed and areas of improvement are identified. This work increases the overall technical readiness of the approach for future balloon, aircraft and space applications. [ABSTRACT FROM AUTHOR]

In this paper, we present the updated SAGE-CCI-OMPS+ climate data record of monthly zonal mean ozone profiles. This dataset covers the stratosphere and combines measurements by 9 limb and occultation satellite instruments - SAGE II, OSIRIS, MIPAS, SCIAMACHY, GOMOS, ACE-FTS, OMPS-LP, POAM III and SAGE III/ISS. Compared to the original version of the SAGE-CCI-OMPS dataset (Sofieva et al., 2017), the update includes new versions of MIPAS, ACE-FTS, and OSIRIS datasets, and introduces data from additional sensors (POAM III, SAGE III/ISS) and retrieval processors (OMPSLP). In the paper, we show detailed intercomparisons of ozone profiles from different instruments and data versions, with a focus on the detection of possible drifts in the datasets. The SAGE-CCI-OMPS+ dataset has a better coverage of polar regions and of the upper troposphere and the lower stratosphere (UTLS) than the SAGE-CCI-OMPS dataset. We also studied the influence of including new datasets on ozone trends, which are estimated using multiple linear regression. The changes in the merged dataset do not change the overall morphology of post-1997 ozone trends: statistically significant trends are observed in the upper stratosphere. The largest changes in ozone trends are observed in polar regions, especially in the Southern Hemisphere. The updated SAGE-CCI-OMPS+ dataset contains profiles of deseasonalized anomalies and ozone concentrations from 1984 to 2021, in 10° latitude bins from 90°S to 90°N, and in the altitude range from 10 km to 50 km. The dataset is in open access at https://climate.esa.int/en/projects/ozone/data/and at ftp://cci_web@ftp-ae.oma.be/esacci (ESA Climate Office, last access: 10 August 2022). [ABSTRACT FROM AUTHOR]

In the mesosphere, the vibrationally excited hydroxyl layer is sensitive to changes in incoming solar flux. An enhanced photodissociation of molecular oxygen will lead to more atomic oxygen production, thus we expect the OH layer emission rate to be positively with the Lyman- α flux and the emission height to be negatively correlated. The Optical Spectrograph and InfraRed Imager System (OSIRIS) has recorded the Meinel band centred at 1.53 μ m from 2001 to 2015. In this study, we show how the 11-year solar cycle signature manifests itself in this data set, in terms of OH zenith emission rate and emission height. As expected, the emission height is negatively correlated with the Lyman- α flux at all latitudes. The zenith emission rate is positively correlated with the Lyman- α flux at most latitudes except near the equator. By the means of a time dependent photochemical model, we show that the changing local time sampling of the Odin satellite was the cause of the observed distortion of the solar cycle signature near the equator. • The responses of the OH emission to the solar radiation are studied based on 15-year observation by OSIRIS. • The 11-year solar cycle signature is found in both the airglow intensity and the layer height. • The local solar time sampling effect is investigated by means of a photochemical model. [ABSTRACT FROM AUTHOR]

For the past 17 years, the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument on the Canadian SCISAT satellite has been measuring profiles of atmospheric ozone. The latest operational versions of the level 2 ozone data are versions 3.6 and 4.1. This study characterizes how both products compare with correlative data from other limb-sounding satellite instruments, namely MAESTRO, MLS, OSIRIS, SABER, and SMR. In general, v3.6, with respect to the other instruments, exhibits a smaller bias (which is on the order of ∼ 3 %) in the middle stratosphere than v4.1 (∼ 2 %–9 %); however, the bias exhibited in the v4.1 data tends to be more stable, i.e. not changing significantly over time in any altitude region. In the lower stratosphere, v3.6 has a positive bias of about 3 %–5 % that is stable to within ± 1 % per decade, and v4.1 has a bias on the order of - 1 % to + 5 % and is also stable to within ± 1 % per decade. In the middle stratosphere, v3.6 has a positive bias of ∼ 3 % with a significant negative drift on the order of 0.5 %–2.5 % per decade, and v4.1 has a positive bias of 2 %–9 % that is stable to within ± 0.5 % per decade. In the upper stratosphere, v3.6 has a positive bias that increases with altitude up to ∼ 16 % and a significant negative drift on the order of 2 %–3 % per decade, and v4.1 has a positive bias that increases with altitude up to ∼ 15 % and is stable to within ± 1 % per decade. Estimates indicate that both versions 3.6 and 4.1 have precision values on the order of 0.1–0.2 ppmv below 20 km and above 45 km (∼ 5 %–10 %, depending on altitude). Between 20 and 45 km, the estimated v3.6 precision of ∼ 4 %–6 % is better than the estimated v4.1 precision of ∼ 6 %–10 %. [ABSTRACT FROM AUTHOR]

Massive Australian wildfires lofted smoke directly into the stratosphere in the austral summer of 2019/20. The smoke led to increases in optical extinction throughout the midlatitudes of the southern hemisphere that rivalled substantial volcanic perturbations. Previous studies have assumed that the smoke became coated with sulfuric acid and water and would deplete the ozone layer through heterogeneous chemistry on those surfaces, as is routinely observed following volcanic enhancements of the stratospheric sulfate layer. Here, observations of extinction and reactive nitrogen species from multiple independent satellites that sampled the smoke region are compared to one another and to model calculations. The data display a strong decrease in reactive nitrogen concentrations with increased aerosol extinction in the stratosphere, which is a known fingerprint for key heterogeneous chemistry on sulfate/H2O particles (specifically the hydrolysis of N2O5 to form HNO3). This chemical shift affects not only reactive nitrogen but also chlorine and reactive hydrogen species and is expected to cause midlatitude ozone layer depletion. Comparison of the model ozone to observations suggests that N2O5 hydrolysis contributed to reduced ozone, but additional chemical and/or dynamical processes are also important. These findings suggest that if wildfire smoke injection into the stratosphere increases sufficiently in frequency and magnitude as the world warms due to climate change, ozone recovery under the Montreal Protocol could be impeded, at least sporadically. Modeled austral midlatitude total ozone loss was about 1% in March 2020, which is significant comparedtoexpectedozone recovery ofabout 1%per decade. [ABSTRACT FROM AUTHOR]

The Stratosphere-troposphere Processes and their Role in Climate (SPARC) Data Initiative (SPARC, 2017) performed the first comprehensive assessment of currently available stratospheric composition measurements obtained from an international suite of space-based limb sounders. The initiative's main objectives were (1) to assess the state of data availability, (2) to compile time series of vertically resolved, zonal monthly mean trace gas and aerosol fields, and (3) to perform a detailed intercomparison of these time series, summarizing useful information and highlighting differences among datasets. The datasets extend over the region from the upper troposphere to the lower mesosphere (300–0.1 hPa) and are provided on a common latitude–pressure grid. They cover 26 different atmospheric constituents including the stratospheric trace gases of primary interest, ozone (O3) and water vapor (H2O), major long-lived trace gases (SF6 , N2O , HF , CCl3F , CCl2F2 , NOy), trace gases with intermediate lifetimes (HCl , CH4 , CO , HNO3), and shorter-lived trace gases important to stratospheric chemistry including nitrogen-containing species (NO , NO2 , NOx , N2O5 , HNO4), halogens (BrO , ClO , ClONO2 , HOCl), and other minor species (OH , HO2 , CH2O , CH3CN), and aerosol. This overview of the SPARC Data Initiative introduces the updated versions of the SPARC Data Initiative time series for the extended time period 1979–2018 and provides information on the satellite instruments included in the assessment: LIMS, SAGE I/II/III, HALOE, UARS-MLS, POAM II/III, OSIRIS, SMR, MIPAS, GOMOS, SCIAMACHY, ACE-FTS, ACE-MAESTRO, Aura-MLS, HIRDLS, SMILES, and OMPS-LP. It describes the Data Initiative's top-down climatological validation approach to compare stratospheric composition measurements based on zonal monthly mean fields, which provides upper bounds to relative inter-instrument biases and an assessment of how well the instruments are able to capture geophysical features of the stratosphere. An update to previously published evaluations of O3 and H2O monthly mean time series is provided. In addition, example trace gas evaluations of methane (CH4), carbon monoxide (CO), a set of nitrogen species (NO , NO2 , and HNO3), the reactive nitrogen family (NOy), and hydroperoxyl (HO2) are presented. The results highlight the quality, strengths and weaknesses, and representativeness of the different datasets. As a summary, the current state of our knowledge of stratospheric composition and variability is provided based on the overall consistency between the datasets. As such, the SPARC Data Initiative datasets and evaluations can serve as an atlas or reference of stratospheric composition and variability during the "golden age" of atmospheric limb sounding. The updated SPARC Data Initiative zonal monthly mean time series for each instrument are publicly available and accessible via the Zenodo data archive (Hegglin et al., 2020). [ABSTRACT FROM AUTHOR]

The upper troposphere and lower stratosphere (UTLS) region is a highly variable region of the atmosphere and critical for understanding climate. Yet, it remains undersampled in the observational satellite record. Due to recent advances in interferometer and infrared detection technologies, imaging Fourier transform spectrometer (FTS) technology has been identified as a feasible remote sensing approach to obtain the required precision and spatial resolution of atmospheric trace gas composition in the UTLS. Building on the success of instruments such as the Michelson Interferometer for Passive Atmospheric Sounding and gimbaled limb observer for radiance imaging of the atmosphere, the limb imaging Fourier transform spectrometer experiment (LIFE) instrument, of which this paper details the design and performance, is a balloon-borne infrared imaging FTS developed as an early prototype of a low earth orbit satellite instrument. LIFE is constructed primarily with commercially available off-the-shelf components, with a design emphasis on greatly reducing the complexity of the instrument, particularly the cooling requirements, with a minimal reduction in information gain on the target atmospheric greenhouse gases of water vapor, methane, ozone, and nitrous oxide. The developed instrument was characterized through a series of thermal and vacuum tests and validated through a successful demonstration balloon flight during the 2019 Strato-Science campaign in Canada. In the calibration of the data from the balloon flight, an issue was identified regarding a lack of knowledge in the emissivity of the on-board blackbody calibration sources. These systematic effects were minimized through the application of an emissivity ratio determined from the characterization tests where a wider range of known blackbody temperatures were available. Despite this identified calibration issue, the results demonstrate that the instrument is capable of meeting primary performance requirements for trace gas retrievals of the target atmospheric species. [ABSTRACT FROM AUTHOR]

For the past 17 years, the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument on the Canadian SCISAT satellite has been measuring profiles of atmospheric ozone. The latest two operational versions of the level 2 ozone data are versions 3.6 and 4.1. This technical note characterizes how both products compare with correlative data from other limb-sounding satellite instruments, namely MAESTRO, MLS, OSIRIS, SABER, and SMR. In general, v3.6, with respect to the other instruments, exhibits a smaller bias (which is on the order of ~3 %) in the middle stratosphere than v4.1 (~2-9 %), however the bias exhibited in the v4.1 data tends to be more stable, i.e. not changing significantly over time in any altitude region. In the lower stratosphere, v3.6 has a positive bias of about 3-5 % that is stable to within ±1 % dec-1, and v4.1 has a bias on the order of -1 to +5 % and is also stable to within ±1 % dec-1. In the middle stratosphere, v3.6 has a positive bias of ~3 % with a significant negative drift on the order of 0.5-2.5 % dec-1, and v4.1 has a positive bias of 2-9 % that is stable to within ±0.5 % dec-1. However, the v4.1 bias in the middle stratosphere is reduced to 0-5 % after being corrected for field-of-view modelling errors. In the upper stratosphere, v3.6 has a positive bias that increases with altitude up to ~16 % and a significant negative drift on the order of 2-3 % dec-1, and v4.1 has a positive bias that increases with altitude up to ~15 % and is stable to within ±1 % dec-1. [ABSTRACT FROM AUTHOR]

A comprehensive inter-comparison of seven radiative transfer models in the limb scattering geometry has been performed. Every model is capable of accounting for polarization within a spherical atmosphere. Three models (GSLS, SASKTRAN-HR, and SCIATRAN) are deterministic, and four models (MYSTIC, SASKTRAN-MC, Siro, and SMART-G) are statistical using the Monte Carlo technique. A wide variety of test cases encompassing different atmospheric conditions, solar geometries, wavelengths, tangent altitudes, and Lambertian surface reflectances have been defined and executed for every model. For the majority of conditions it was found that the models agree to better than 0.2 % in the single-scatter test cases and better than 1 % in the scalar and vectorial test cases with multiple scattering included, with some larger differences noted at high values of surface reflectance. For the first time in limb geometry, the effect of atmospheric refraction was compared among four models that support it (GSLS, SASKTRAN-HR, SCIATRAN, and SMART-G). Differences among most models with multiple scattering and refraction enabled were less than 1 % , with larger differences observed for some models. Overall the agreement among the models with and without refraction is better than has been previously reported in both scalar and vectorial modes. [ABSTRACT FROM AUTHOR]

The OMPS Limb Profiler (LP) instrument is designed to provide high-vertical-resolution ozone and aerosol profiles from measurements of the scattered solar radiation in the 290–1000 nm spectral range. It collected its first Earth limb measurement on 10 January 2012 and continues to provide daily global measurements of ozone and aerosol profiles from the cloud top up to 60 and 40 km , respectively. The relatively high vertical and spatial sampling allow detection and tracking of sporadic events when aerosol particles are injected into the stratosphere, such as volcanic eruptions or pyrocumulonimbus (PyroCb) events. In this paper we discuss the newly released Version 2.0 OMPS multi-wavelength aerosol extinction coefficient retrieval algorithm. The algorithm now produces aerosol extinction profiles at 510, 600, 674, 745, 869 and 997 nm wavelengths. The OMPS LP Version 2.0 data products are compared to the SAGE III/ISS, OSIRIS and CALIPSO missions and shown to be of good quality and suitable for scientific studies. The comparison shows significant improvements in the OMPS LP retrieval performance in the Southern Hemisphere (SH) and at lower altitudes. These improvements arise from use of the longer wavelengths, in contrast with the V1.0 and V1.5 OMPS aerosol retrieval algorithms, which used radiances only at 675 nm and therefore had limited sensitivity in those regions. In particular, the extinction coefficients at 745, 869 and 997 nm are shown to be the most accurate, with relative accuracies and precisions close to 10 % and 15 %, respectively, while the 675 nm relative accuracy and precision are on the order of 20 %. The 510 nm extinction coefficient is shown to have limited accuracy in the SH and is only recommended for use between 20–24 km and only in the Northern Hemisphere. The V2.0 retrieval algorithm has been applied to the complete set of OMPS LP measurements, and the new dataset is publicly available. [ABSTRACT FROM AUTHOR]

A comprehensive inter-comparison of seven radiative transfer models in the limb scattering geometry has been performed. Every model is capable of accounting for polarisation within a fully spherical atmosphere. Three models (GSLS, SASKTRAN-HR, and SCIATRAN) are deterministic, and four models (MYSTIC, SASKTRAN-MC, Siro, and SMART-G) are statistical using the Monte Carlo technique. A wide variety of test cases encompassing different atmospheric conditions, solar geometries, wavelengths, tangent altitudes, and Lambertian surface reflectances have been defined and executed for every model. For the majority of conditions it was found that the models agree to better than 0.2 % in the single scatter test cases and better than 1 % in the multiple scatter scalar and vector test cases, with some larger differences noted at high values of surface reflectance in multiple scatter. For the first time in limb geometry, the effect of atmospheric refraction was compared among four models that support it (GSLS, SASKTRAN-HR, SCIATRAN, and SMART-G). Differences among most models in multiple scatter with refraction enabled was less than 1 %, with larger differences observed for some models. Overall the agreement among the models with and without refraction is better than has been previously reported in both scalar and vector modes. [ABSTRACT FROM AUTHOR]

Improving knowledge of the ozone global distributions in the mesosphere–lower thermosphere (MLT) is a crucial step in understanding the behaviour of the middle atmosphere. However, the concentration of ozone under sunlit conditions in the MLT is often so low that its measurement requires instruments with very high sensitivity. Fortunately, the bright oxygen airglow can serve as a proxy to retrieve the daytime ozone density indirectly, due to the strong connection to ozone photolysis in the Hartley band. The OSIRIS IR imager (hereafter, IRI), one of the instruments on the Odin satellite, routinely measures the oxygen infrared atmospheric band (IRA band) at 1.27 µ m. In this paper, we will primarily focus on the detailed description of the steps done for retrieving the calibrated IRA band limb radiance (with <10 % random error), the volume emission rate of O2 (a1Δg) (with <25 % random error) and finally the ozone number density (with <20 % random error). This retrieval technique is applied to a 1-year sample from the IRI dataset. The resulting product is a new ozone dataset with very tight along-track sampling distance (<20 km). The feasibility of the retrieval technique is demonstrated by a comparison of coincident ozone measurements from other instruments aboard the same spacecraft, as well as zonal mean and monthly average comparisons between Odin-OSIRIS (both spectrograph and IRI), Odin-SMR and Envisat-MIPAS. We find that IRI appears to have a positive bias of up to 25 % below 75 km , and up to 50 % in some regions above. We attribute these differences to uncertainty in the IRI calibration as well as uncertainties in the photochemical constants. However, the IRI ozone dataset is consistent with the compared dataset in terms of the overall atmospheric distribution of ozone between 50 and 100 km. If the origin of the bias can be identified before processing the entire dataset, this will be corrected and noted in the dataset description. The retrieval technique described in this paper can be further applied to all the measurements made throughout the 19 year mission, leading to a new, long-term high-resolution ozone dataset in the middle atmosphere. [ABSTRACT FROM AUTHOR]

A robust stratospheric aerosol climate data record enables the depiction of the radiative forcing of this highly variable component of climate. In addition to the radiative forcing, stratospheric aerosol also plays a key role in the chemical processes leading to ozone depletion. Therefore, stratospheric aerosol is one of the crucial parameters in understanding climate change in the past and potential changes in the future. As a part of Stratospheric-tropospheric Processes and their Role in Climate (SPARC) Stratospheric Sulfur and its Role in Climate (SSiRC) activity, the Global Space-based Stratospheric Aerosol Climatology (GloSSAC) was created to support the World Climate Research Programme's (WCRP) Coupled Model Intercomparison Project Phase 6 (CMIP6). This data set is a follow-on to one created as a part of SPARC's Assessment of Stratospheric Aerosol Properties (ASAP) activity and a data created for the Chemistry-Climate Model Initiative (CCMI) in 2012. Herein, we discuss changes to the original release version including those as a part of v1.1 that was released in September 2018 that primarily corrects an error in the conversion of Cryogenic Limb Array Etalon Spectrometer (CLAES) data to Stratospheric Aerosol and Gas Experiment (SAGE) II wavelengths, as well as the new release, v2.0. Version 2.0 is focused on improving the post-SAGE II era (after 2005) with the goal of mitigating elevated aerosol extinction in the lower stratosphere at mid- and high latitudes noted in v1.0 as noted in. Changes include the use of version 7.0 of the Optical Spectrograph and InfraRed Imaging System (OSIRIS), the recently released Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar Level 3 stratospheric aerosol profile monthly product and the new addition of SAGE III/ISS. Here, we use an observed relationship between (i) OSIRIS extinction at 750 nm and (ii) SAGE II and SAGE III/ISS extinction at 525 nm to derive an altitude–latitude-based monthly climatology of Ångström exponent to compute OSIRIS extinction at 525 nm, resulting in a better agreement between OSIRIS and SAGE measurements. We employ a similar approach to convert OSIRIS 750 nm extinction to 1020 nm extinction for the post-SAGE II period. Additionally, we incorporate the recently released standard CALIPSO stratospheric aerosol profile monthly product into GloSSAC with an improved conversion technique of the 532 nm backscatter coefficient to extinction using an observed relationship between OSIRIS 525 nm extinction and CALIPSO 532 nm backscatter. SAGE III/ISS data are also incorporated in GloSSAC to extend the climatology to the present and to test the approach used to correct OSIRIS/CALIPSO data. The GloSSAC v2.0 netCDF file is accessible at 10.5067/glossac-l3-v2.0. [ABSTRACT FROM AUTHOR]

The OMPS Limb Profiler (LP) instrument is designed to provide high vertical resolution ozone and aerosol profiles from measurements of the scattered solar radiation in the 290-1000 nm spectral range. It collected its first Earth limb measurement on January 10, 2012 and continues to provide daily global measurements of ozone and aerosol profiles from the cloud top up to 60 km and 40 km respectively. The relatively high vertical and spatial sampling allow detection and tracking of sporadic events when aerosol particles are injected into the stratosphere, such as volcanic eruptions or PyroCb events. In this paper we discuss the newly released Version 2.0 OMPS multi-wavelength aerosol extinction coefficient retrieval algorithm. The algorithm now produces aerosol extinction profiles at 510, 600, 674, 745, 869, and 997 nm wavelengths. The OMPS LP Version 2.0 data products are compared to the SAGE III/ISS, OSIRIS and CALIPSO missions and shown to be of good quality and suitable for scientific studies. The comparison shows significant improvements in the OMPS LP retrieval performance in the Southern Hemisphere and at lower altitudes. These improvements arise from use of the longer wavelengths, in contrast with the V1.0 and V1.5 OMPS aerosol retrieval algorithms, which used radiances only at 675 nm and therefore had limited sensitivity in those regions. In particular, the extinction coefficients at 745, 869 and 997 nm are shown to be the most accurate, with relative accuracies and precisions close to 10 % and 15 % respectively, while the 675 nm relative accuracy and precision are on the order of 20 %. The 510 nm extinction coefficient is shown to have limited accuracy in SH and is only recommended for use between 20-24 km, and only in the Northern Hemisphere. The V2.0 retrieval algorithm has been applied to the complete set of OMPS LP measurements and the new data set is publicly available. [ABSTRACT FROM AUTHOR]

A robust stratospheric aerosol climate data record enables the depiction of the radiative forcing of this highly variable component of climate. Since stratospheric aerosol also plays a key role in the chemical processes leading to ozone depletion, stratosphere is one of the crucial parameters in understanding climate change in the past and potential changes in the future. As a part of Stratospheric-tropospheric Processes and their Role in Climate (SPARC) Stratospheric Sulfur and its Role in Climate (SSiRC) activity, the Global Space-based Stratospheric Aerosol Climatology (GloSSAC) was created (Thomason et al., 2018) to support the World Climate Research Programme (WCRP)'s Coupled Model Intercomparison Project Phase 6 (CMIP6) (Zanchettin et al., 2016). This data set is a follow-on to one created as a part of Stratosphere-Troposphere Process and their Role in Climate Project (SPARC)'s Assessment of Stratospheric Aerosol Properties (ASAP) activity(SPARC, 2006) and a data created for Chemistry-Climate Model Initiative (CCMI) in 2012 (Eyring and Lamarque, 2012). Herein, we discuss changes to the original release version including those as a part of v1.1 that was released in September 2018 that primarily corrects an error in the conversion of Cryogenic Limb Array Etalon Spectrometer (CLAES) data to Stratospheric Aerosol and Gas Experiment (SAGE) II wavelengths, and the new release, v2.0. Version 2.0 is focused on improving the post-SAGE II era (after 2005) with the goal to mitigate elevated aerosol extinction in the lower stratosphere at mid and high latitudes noted in v1.0 as noted in Thomason et al. (2018). Changes include the use of version 7.0 of Optical Spectrograph and InfraRed Imaging System(OSIRIS), the recently released Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Lidar Level 3 Stratospheric Aerosol profile monthly product, and the new addition of SAGE III/ISS. Although, the version 7.0 OSIRIS data is substantially improved at its native wavelength of 750 nm, conversion to 525 nm using a constant Angstrom exponent often results in disagreements with SAGEII/ SAGE III/ISS overlap measurements. We, therefore use an observed relationship between OSIRIS extinction at 750 nm and SAGEII/SAGE III/ISS extinction at 525 nm to derive Altitude-Latitude based monthly climatology of Angstrom exponent to compute extinction at 525 nm, resulting in a better agreement between OSIRIS and SAGE measurements. We employ a similar approach to convert OSIRIS 750 nm extinction to 1020 nm extinction for the post-SAGEII period. Additionally, we incorporate the recently released standard CALIPSO stratospheric aerosol profile monthly product into GloSSAC with an improved conversion technique of 532 nm backscatter coefficient to extinction using an observed relationship between OSIRIS 525 nm extinction and CALIPSO 532 nm backscatter. We also investigate for any cloud contamination in OSIRIS/standard CALIPSO stratospheric aerosol product, which may have caused apparent enhancement in the aerosol extinction particularly in the lower stratosphere. SAGE III/ISS data is also incorporated in GloSSAC to extend the climatology to the present and to test the approach used to correct OSIRIS/CALIPSO data. The GloSSAC v2.0 netcdf file is accessible at https://doi.org/10.5067/glossac-l3-v2.0 (Thomason, 2020). [ABSTRACT FROM AUTHOR]

Improving knowledge of the ozone global distributions in the mesosphere-lower thermosphere (MLT) is a crucial step in understanding the behaviour of the middle atmosphere. However, the ozone concentration under sunlit conditions in the MLT is often so low that its measurement requires instruments with very high sensitivity. Fortunately, the bright oxygen airglow can serve as a proxy to retrieve the daytime ozone density indirectly, due to the strong connection to ozone photolysis in the Hartley band. The OSIRIS IR imager (hereafter IRI), one of the instruments on the Odin satellite, routinely measures the oxygen infrared atmospheric band (IRA band) at 1.27 µm. In this paper, we will describe the detailed steps of retrieving the calibrated IRA band limb radiance, the volume emission rate of O2(a¹Δg) and, finally, the ozone number density. This retrieval technique is applied to a one-year-sample IRI dataset. The resulting product is a completely new ozone dataset with very high along-track resolution. The performance of the retrieval technique is demonstrated by a comparison of the coincident ozone measurements from the same spacecraft, as well as zonal mean and monthly average comparisons between OS, SMR, MIPAS and ACE-FTS. The consistency of this IRI ozone dataset implies that such a retrieval technique can be further applied to all the measurements made throughout the 19 years-long mission, leading to a long-term, high resolution dataset in the middle atmosphere. [ABSTRACT FROM AUTHOR]

We report on the retrieval of stratospheric aerosol particle size and extinction coefficient profiles from multi-color backscatter measurements with the Rayleigh-Mie-Raman lidar operated at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) in northern Norway. The retrievals are based on a two-step approach. In a first step the median radius of an assumed log-normal particle size distribution with fixed width is retrieved based on the color ratio formed from the measured backscatter ratios at wavelenghts of 1064 nm and 532 nm. An intrinsic ambiguity of the retrieved aerosol size information is discussed. In a second step, this particle size information is used to convert the measured lidar backscatter ratio to aerosol extinction coefficients. The retrieval is currently based on monthly-averaged lidar measurements covering the period from the year 2000 to present. A sensitivity study is presented that allows establishing an error budged for the aerosol retrievals. Assuming a log-normal aerosol particle size distribution with a geometric width of S = 1.3, median radii on the order of 100 nm are retrieved. The median radii are found to generally decrease with increasing altitude. The retrieved aerosol extinction profiles are compared to observations with various current and past satellite instruments. [ABSTRACT FROM AUTHOR]

Stratospheric aerosols are of a great importance to the scientific community, predominantly because of their role in climate, but also because accurate knowledge of aerosol characteristics is relevant for trace gas retrievals from remote-sensing instruments. There are several data sets published which provide aerosol extinction coefficients in the stratosphere. However, for the instruments measuring in the limb-viewing geometry, the use of this parameter is associated with uncertainties resulting from the need to assume an aerosol particle size distribution (PSD) within the retrieval process. These uncertainties can be mitigated if PSD information is retrieved. While occultation instruments provide more accurate information on the aerosol extinction coefficient, in this study, it was shown that limb instruments are more sensitive to the smaller particles in the visible–near-infrared spectral range. However, the sensitivity of occultation instruments improves if the UV part of the wavelength spectrum is considered. A data set containing PSD information was recently retrieved from SCIAMACHY limb measurements and provides two parameters of the unimodal lognormal PSD for the SCIAMACHY operational period (2002–2012). In this study, the data set is expanded by aerosol extinction coefficients and Ångström exponents calculated from the retrieved PSD parameters. Parameter errors for the recalculated Ångström exponents and aerosol extinction coefficients are assessed using synthetic retrievals. For the extinction coefficient the resulting parameter error is within ±25 %, and for the Ångström exponent, it is better than 10 %. The SCIAMACHY aerosol extinction coefficients recalculated from PSD parameters are compared to those from SAGE II. The differences between the instruments vary from 0 % to 25 % depending on the wavelength. Ångström exponent comparison with SAGE II shows differences between 10 % at 31 km and 40 % at 18 km. Comparisons with SAGE II, however, suffer from the low number of collocated profiles. Furthermore, the Ångström exponents obtained from the limb-viewing instrument OSIRIS are used for the comparison. This comparison shows an average difference within 7 %. The time series of these differences do not show signatures of any remarkable events (e.g., volcanic eruptions or biomass burning events). In addition, the temporal behaviour of the Ångström exponent in the tropics is analyzed using the SCIAMACHY data set. It is shown that there is no trivial relation between the Ångström exponent value at a single wavelength pair and the PSD because the same value of Ångström exponent can be obtained from an infinite number of combinations of the PSD parameters. [ABSTRACT FROM AUTHOR]

AIM-North is a proposed satellite mission that would provide observations of unprecedented frequency and density for monitoring northern greenhouse gases (GHGs), air quality (AQ) and vegetation. AIM-North would consist of two satellites in a highly elliptical orbit formation, observing over land from ∼40°N to 80°N multiple times per day. Each satellite would carry a near-infrared to shortwave infrared imaging spectrometer for CO2, CH4, and CO, and an ultraviolet-visible imaging spectrometer for air quality. Both instruments would measure solar-induced fluorescence from vegetation. A cloud imager would make near-real-time observations, which could inform the pointing of the other instruments to focus only on the clearest regions. Multiple geostationary (GEO) AQ and GHG satellites are planned for the 2020s, but they will lack coverage of northern regions like the Arctic. AIM-North would address this gap with quasi-geostationary observations of the North and overlap with GEO coverage to facilitate intercomparison and fusion of these datasets. The resulting data would improve our ability to forecast northern air quality and quantify fluxes of GHG and AQ species from forests, permafrost, biomass burning and anthropogenic activity, furthering our scientific understanding of these processes and supporting environmental policy. [ABSTRACT FROM AUTHOR]

TROPOspheric Monitoring Instrument (TROPOMI), on‐board the Sentinel‐5 Precurser satellite, is a nadir‐viewing spectrometer measuring reflected sunlight in the ultraviolet, visible, near‐infrared, and shortwave infrared. From these spectra several important air quality and climate‐related atmospheric constituents are retrieved, including nitrogen dioxide (NO2) at unprecedented spatial resolution from a satellite platform. We present the first retrievals of TROPOMI NO2 over the Canadian Oil Sands, contrasting them with observations from the Ozone Monitoring Instrument satellite instrument, and demonstrate TROPOMI's ability to resolve individual plumes and highlight its potential for deriving emissions from individual mining facilities. Further, the first TROPOMI NO2 validation is presented, consisting of aircraft and surface in situ NO2 observations, and ground‐based remote‐sensing measurements between March and May 2018. Our comparisons show that the TROPOMI NO2 vertical column densities are highly correlated with the aircraft and surface in situ NO2 observations, and the ground‐based remote‐sensing measurements with a low bias (15–30 %); this bias can be reduced by improved air mass factors. Plain Language Summary: Nitrogen dioxide (NO2) is a pollutant that is linked to respiratory health issues and has negative environmental impacts such as soil and water acidification. Near the surface the most significant sources of NO2 are fossil fuel combustion and biomass burning. With a recently launched satellite instrument (TROPOspheric Monitoring Instrument [TROPOMI]), NO2 can be measured with an unprecedented combination of accuracy, spatial coverage, and resolution. This work presents the first TROPOMI NO2 measurements near the Canadian Oil Sands and shows that these measurements have an outstanding ability to detect NO2 on a very high horizontal resolution that is unprecedented for satellite NO2 observations. Further, these satellite measurements are in excellent agreement with aircraft and ground‐based measurements. Key Points: First evaluation of the TROPOMI NO2 retrieval productThe quality of the TROPOMI NO2 data is excellent and captures variation on a very high spatial resolutionTROPOMI tropospheric NO2 retrievals can be corrected with higher‐resolution input data [ABSTRACT FROM AUTHOR]