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We present an analysis of Antarctic polar winters from 2005 to 2023 as observed by the Atmospheric Chemistry Experiment (ACE). The unique broad band infrared spectral features in ACE "residual" spectra are used to classify the spectra of polar aerosols by composition into polar stratospheric clouds (PSCs) and sulfate aerosols. The spectra of PSCs are further classified into nitric acid trihydrate, supercooled ternary solutions, supercooled nitric acid, ice‐mix, and mixtures of PSCs. A breakdown of PSC composition is presented for each year. Antarctic winter seasons with unusual compositions are: 2011, in which volcanic ash mixed with PSCs was observed from July to August; 2019, which experienced a stratospheric warming event; 2020, the PSC season following the Australian Black Summer pyrocumulonimbus event; and 2023, which had unusually large sulfate aerosols following the Honga‐Tonga Honga Ha'apai eruption of 2022. Plain Language Summary: Polar Stratospheric Clouds (PSCs) are clouds that form during polar night when temperatures in the stratosphere drop to extreme lows. These clouds play a crucial role in the annual depletion of ozone (the Antarctic "ozone hole"). We present a summary of PSC observations made from the Atmospheric Chemistry Experiment (ACE), which is a satellite‐based mission that has been operational since 2004. Key Points: Antarctic polar stratospheric cloud composition distribution during August and September observed by the Atmospheric Chemistry Experiment (ACE)Polar stratospheric cloud (PSC) composition impacted by large scale stratospheric events such as Australian Black Summer pyrocumulonimbus event and volcanic eruptionsPSC composition distribution is mostly governed by temperature [ABSTRACT FROM AUTHOR]

We have developed an empirical model of nitric oxide (NO) number density at altitudes from ∼73 km to the exobase, as a function of altitude, latitude, day of year, solar zenith angle, solar activity, and geomagnetic activity. The model is part of the NRLMSIS® 2.1 empirical model of atmospheric temperature and species densities; this upgrade to NRLMSIS 2.0 consists solely of the addition of NO. MSIS 2.1 assimilates observations from six space-based instruments: UARS/HALOE, SNOE, Envisat/MIPAS, ACE/FTS, Odin/SMR, and AIM/SOFIE. We additionally evaluated the new model against independent extant NO data sets. In this paper, we describe the formulation and fitting of the model, examine biases between the data sets and model and among the data sets, compare with another empirical NO model (NOEM), and discuss scientific aspects of our analysis. [ABSTRACT FROM AUTHOR]

Using multidecadal time series of ground‐based and satellite Fourier transform infrared measurements of inorganic fluorine (i.e., total fluorine resident in stratospheric fluorine reservoirs), we investigate stratospheric circulation changes over the past 20 years. The representation of these changes in five modern reanalyses is further analyzed through chemical‐transport model (CTM) simulations. From the observations but also from all reanalyses, we show that the inorganic fluorine is accumulating less rapidly in the Southern Hemisphere than in the Northern Hemisphere during the 21st century. Comparisons with a study evaluating the age‐of‐air of these reanalyses using the same CTM allow us to link this hemispheric asymmetry to changes in the Brewer‐Dobson circulation (BDC), with the age‐of‐air of the Southern Hemisphere getting younger relative to that of the Northern Hemisphere. Large differences in simulated total columns and absolute trend values are, nevertheless, depicted between our simulations driven by the five reanalyses. Superimposed on this multidecadal change, we, furthermore, confirm a 5–7‐year variability of the BDC that was first described in a recent study analyzing long‐term time series of hydrogen chloride (HCl) and nitric acid (HNO3). It is important to stress that our results, based on observations and meteorological reanalyses, are in contrast with the projections of chemistry‐climate models in response to the coupled increase of greenhouse gases and decrease of ozone‐depleting substances, calling for further investigations and the continuation of long‐term observations. Plain Language Summary: The overturning circulation of the stratosphere is projected to change in response to increases in greenhouse gases and decreases in ozone‐depleting substances, with the Southern Hemisphere branch expected to get weaker relative to the Northern Hemisphere. Here, we use 30‐year time series of observations and simulations of a long‐lived tracer to investigate stratospheric circulation changes. The observations analyzed are from ground‐based and satellite instruments. We compare them with simulations that use a chemical‐transport model driven by winds from meteorological reanalyses of 1990–2019. A reanalysis assimilates meteorological measurements into a forecast model to simulate the best possible representation of the atmospheric state. All five simulations, which use different reanalyses, agree with the observational analysis showing that the analyzed tracer is accumulating less rapidly in the Southern Hemisphere than in the Northern Hemisphere through 2019. This hemispheric asymmetry is attributed to changes in the stratospheric transport circulation, with the Southern Hemisphere branch getting stronger relative to the Northern Hemisphere. Our results support the conclusions of a recent observational study but do not support circulation changes projected by chemistry‐climate models. This study highlights the crucial importance of long‐term observation time series. Key Points: Fluorine in the stratosphere is accumulating less rapidly in the Southern Hemisphere than in the Northern HemisphereThe observed fluorine trend asymmetry, attributed to transport circulation changes, opposes trends predicted by chemistry‐climate modelsObservations and five modern reanalyses are qualitatively in agreement with these changes, calling for maintaining long‐term observations [ABSTRACT FROM AUTHOR]

NRLMSIS® 2.0 is an empirical atmospheric model that extends from the ground to the exobase and describes the average observed behavior of temperature, eight species densities, and mass density via a parametric analytic formulation. The model inputs are location, day of year, time of day, solar activity, and geomagnetic activity. NRLMSIS 2.0 is a major, reformulated upgrade of the previous version, NRLMSISE‐00. The model now couples thermospheric species densities to the entire column, via an effective mass profile that transitions each species from the fully mixed region below ~70 km altitude to the diffusively separated region above ~200 km. Other changes include the extension of atomic oxygen down to 50 km and the use of geopotential height as the internal vertical coordinate. We assimilated extensive new lower and middle atmosphere temperature, O, and H data, along with global average thermospheric mass density derived from satellite orbits, and we validated the model against independent samples of these data. In the mesosphere and below, residual biases and standard deviations are considerably lower than NRLMSISE‐00. The new model is warmer in the upper troposphere and cooler in the stratosphere and mesosphere. In the thermosphere, N2 and O densities are lower in NRLMSIS 2.0; otherwise, the NRLMSISE‐00 thermosphere is largely retained. Future advances in thermospheric specification will likely require new in situ mass spectrometer measurements, new techniques for species density measurement between 100 and 200 km, and the reconciliation of systematic biases among thermospheric temperature and composition data sets, including biases attributable to long‐term changes. Key Points: A major, reformulated upgrade to NRLMSISE‐00 is presented using extensive new data sets from the ground to ~100 km altitudeVertical structure of the atmosphere is now self‐consistently coupled; O density now extends down to 50 kmNew model is warmer in upper troposphere, cooler in stratosphere and mesosphere; thermospheric N2 and O densities are lower [ABSTRACT FROM AUTHOR]

The Atmospheric Chemistry Experiment (ACE) is a satellite‐based mission that probes Earth's atmosphere via solar occultation. The primary instrument on board is a high‐resolution infrared Fourier transform spectrometer (Atmospheric Chemistry Experiment Fourier Transform Spectrometer, ACE‐FTS), providing altitude‐resolved volume mixing ratio measurements for numerous atmospheric constituents, including many biomass burning products. The ACE mission has observed the aftermath of three major pyrocumulonimbus events, in which extreme heat from intense fires created a pathway for directly injecting into the stratosphere plumes of gaseous and aerosol pollutants. These three events were associated with severe Australian bushfires from 2009 and 2019/2020, along with intense North American wildfires from summer 2017. The ACE‐FTS measured stratospheric plumes containing aerosols, enhanced levels of gaseous fire products, and tropospheric air transported into the stratosphere. Infrared spectral features indicate strikingly similar aerosol composition for all three events, characteristic of oxygenated organic matter. Plain Language Summary: The Atmospheric Chemistry Experiment (ACE) is a satellite‐based mission for studying the Earth's atmosphere. During the 16+ years of operation for the mission, three extreme fire events were observed that injected gases and smoke particles very high into the atmosphere (near 20 km in altitude). The amount of gas and the nature of the smoke particles were studied, in an effort to provide insight into the effect of such fires on climate and atmospheric chemistry. Key Points: Plumes from three pyrocumulonimbus eruptions were measured with the Atmospheric Chemistry Experiment Fourier transform spectrometerEnhanced stratospheric levels of atmospheric constituents from fire emission and transport were measured for many speciesInfrared spectra measured for the associated stratospheric aerosols were strikingly similar for all three events [ABSTRACT FROM AUTHOR]