The electrochemical concentration cell (ECC) ozonesonde is a small instrument flown on weather balloons coupled to radiosondes at ~60 stations. Ozone concentrations, pressure, temperature and humidity are measured from surface to ~10 hPa. Although designed 50-plus years ago, ozonesondes remain a crucial technology for global ozone-observations. Not limited by cloud interferences, ozonesondes record ozone partial pressures with 100-150 m vertical resolution, supplying profiles for satellite retrievals and validation of other ozone-measuring instruments. Sondes provide the longest record of ozone vertical trends, supporting the Montreal Protocol (UNEP, 2020; WMO/UNEP, 2022) and Tropospheric Ozone Assessment Report (Thompson et al., 2025; Van Malderen et al., 2025a,b). The drawback to sondes is that each launch uses a new instrument, requiring protocols for preparation and data processing to ensure that all measurements are comparable. The ozonesonde community employs a three-pronged strategy to optimize data quality and to correct for inhomogeneities over time. First, JOSIE (Jülich Ozonesonde Intercomparison Experiment; 1996-) laboratory tests intercompare sondes of varying manufacture in a World Calibration Center for Ozonesondes. Second, since 2004 an ASOPOS (Assessment of Standard Operating Procedures of Ozonesondes) panel evaluates JOSIEs and issues standard operating procedures (SOP) with WMO (GAW Report No. 268, 2021). Third, an O3S-DQA (Ozonesondes Data Quality Assessment) prescribes how to homogenize long-term records and continuously evaluate data quality for ongoing operations. We show: (1) how this framework has improved sonde precision from the 15-20% in the 1990s to 5% or better today with each ozonesonde record now traceable to a single standard; (2) examples of significant advances facilitated by increasingly rigorous ozone measurement standards.

Abstract. Stratospheric aerosol plays critical roles in the global radiative balance and stratospheric chemistry. A robust evaluation of stratospheric aerosol abundance and size distributions over time is required to determine and accurately model aerosol impacts on the Earth system. The Balloon Baseline Stratospheric Aerosol Profiles (B2SAP) project uses weather balloons to collect regular vertical profiles of stratospheric aerosol number and size distributions, at eight latitudinally diverse locations across the globe from the ground to ~30 km altitude. These systematic in situ measurements allow the micro-physical properties of the background stratospheric aerosol and their variability to be examined. These on-going measurements provide constraints for aerosol modules of large coupled models and reference for satellite retrievals relying on previous size distribution assumptions. The lightweight, low-powered Portable Optical Particle Spectrometer (POPS) instrument is deployed on balloon platforms, allowing for high vertical resolution and the detection of stratospheric aerosol down to 140 nm diameter. This paper presents the B2SAP aerosol dataset (DOI: https://doi.org/10.25921/mchy-y552, Smith et al., 2026), including a description of the B2SAP network, POPS measurement principles, sensitivity analysis, calibration procedures, file processing workflow and a comparison with aerosol extinction from the GloSSAC product.

This study presents an updated evaluation of stratospheric ozone profile trends at Arosa/Davos/Hohenpeißenberg, Switzerland/Germany; Observatory de Haute-Provence (OHP), France; Boulder, Colorado, Mauna Loa Observatory (MLO) and Hilo, Hawaii; and Lauder, Aotearoa / New Zealand, with a focus on the ozone recovery period post-2000. Trends are derived using vertical ozone profiles from NOAA's Dobson network via the Umkehr method (with a recent new homogenization), ozonesondes, and the NOAA COHesive Solar Backscatter Ultraviolet Instrument (SBUV)/Ozone Mapping and Profiler Suite (OMPS) satellite-based record (COH) sampled to match the geographical coordinates of the ground-based stations used in this study. Analyses of long-term changes in stratospheric ozone time series were performed using the updated version (0.8.0) of the Long-term Ozone Trends and Uncertainties in the Stratosphere (LOTUS) independent linear trend (ILT) regression model. This study finds consistency between the trends derived from the different observational records, which is a key factor to the understanding of the recovery of the ozone layer after the implementation of the Montreal Protocol and its amendments that control ozone-depleting substance production and release into the atmosphere. The northern hemispheric Umkehr records of Arosa/Davos, OHP, and MLO all show positive trends in the mid- to upper stratosphere, with trends peaking at ∼ +2 % per decade. Although the upper-stratospheric ozone trends derived from COH satellite records are more positive than those detected by the Umkehr system, the agreement is within the 2 times the standard error uncertainty. Umkehr trends in the upper stratosphere at Boulder and Lauder are positive but not statistically significant, while COH trends are larger and statistically significant (within 2 times the standard error uncertainty). In the lower stratosphere, trends derived from Umkehr and ozonesonde records are mostly negative (except for positive ozonesonde trends at OHP); however, the uncertainties are quite large. Additional dynamical proxies were investigated in the LOTUS model at five ground-based sites. The use of additional proxies did not significantly change trends, but the equivalent latitude reduced the uncertainty in the Umkehr and COH trends in the upper stratosphere and at higher latitudes. In lower layers, additional predictors (tropopause pressure for all stations; two extra components of Quasi-Biennial Oscillation at MLO; Arctic Oscillation at Arosa/Davos, OHP, and MLO) improve the model fit and reduce trend uncertainties as seen by Umkehr and sonde.

Tropospheric ozone trends are important indicators of climate forcing and surface pollution, yet relevant satellite observations are too uncertain for assessments. The assessment project TOAR-II has used multi-instrument, ground-based data for global trends over 2000–2022 (Van Malderen et al., 2025a, b). For the tropics, trends are derived from SHADOZ ozonesonde profiles (Thompson et al., 2021, “T21”; Stauffer et al., 2024) or combinations of satellite, SHADOZ and IAGOS aircraft measurements (Gaudel et al., 2024). We extend T21 that covered 1998–2019, analyzing SHADOZ data at five sites with a Multiple Linear Regression (MLR) model for 1998–2023 and reporting trends for two free-tropospheric (FT) segments, the lowermost stratosphere and the total tropospheric column (TrCOsonde). Trends for the Aura period, 2005–2023, are computed from OMI/MLS TrCOsatellite. We find the following: Extending SHADOZ analyses 4 years shows little change from T21; TrCOsonde trends are small (0.5–1 DU/decade) except over SE Asia. Annual trends for TrCOsonde and OMI/MLS TrCOsatellite agree within uncertainties at four of five sites, with the largest differences at Samoa. Sensitivity tests show the following: (a) Adding thousands of FT IAGOS profiles to SHADOZ yields little change in trends; SHADOZ sampling is sufficient. (b) Quantile Regression (QR) and MLR median trends are both near zero, but QR captures extremes (5th percentile, 95th percentile) with changes up to ±1 DU/decade (p< 0.10). (c) Twelve-year analyses for trends lead to uncertainty changes too large for an assessment. This study and Van Malderen et al. (2025a, b) provide the most reliable TOAR-II trends to date: over the past ∼ 25 years, tropical FT ozone changes have been modest, ∼ (−3–+3) %/decade, except over SE Asia.

Surface ozone (O 3 ) mixing ratios exceeding the National Ambient Air Quality Standard were measured at rural monitors along the Colorado Front Range on 17 April 2020 during the COVID‐19 lockdown. This unusual episode followed back‐to‐back upslope snowstorms and coincided with the presence of a deep stratospheric intrusion, but ground‐based lidar and ozonesonde measurements show that little, if any, of the O 3 ‐rich lower stratospheric air reached the surface. Instead, the statically stable lower stratospheric air suppressed the growth of the daytime boundary layer and trapped nitrogen oxides (NO x = NO + NO 2 ) and volatile organic compounds (VOCs) emitted by motor vehicles and oil and natural gas (O&NG) operations near the ground where the clear skies and extensive snow cover triggered a short‐lived photochemical episode similar to those observed in the O&NG producing basins of northeastern Utah and southwestern Wyoming. In this study, we use a combination of lidar, ozonesonde, and surface measurements, together with the WRF‐Chem and Goddard Earth Observing System composition forecast models, to describe the stratospheric intrusion and characterize the boundary layer structure, HYSPLIT back trajectories to show the low‐level transport of O 3 and its precursors to the exceedance sites, and surface measurements of NO x and VOCs together with a 0‐D box model to investigate the roles of urban and O&NG emissions and the COVID‐19 quarantine in the O 3 production. The box model showed the O 3 production to be NO x saturated, such that the NO x reductions associated with COVID‐19 exacerbated the event rather than mitigating it.

Positive trends in tropical free-tropospheric (FT) ozone are frequently ascribed to emissions growth, but less is known about the effects of changing dynamics. Extending a prior study (Thompson et al., 2021; https://doi.org/10.1029/2021JD034691; “T21”), we re-examine Southern Hemisphere Additional Ozonesondes (SHADOZ) ozone trends over equatorial Southeast Asia (ESEA), one of Earth's most convectively active regions, using 25 years (1998–2022) of ozone soundings. T21 posited that early-year positive FT ozone trends at equatorial SHADOZ stations are related to decreasing convection. The 25-year analysis of Kuala Lumpur and Watukosek SHADOZ records finds that FT ozone trends of +5 % to +15 % (+2 to +6 nmol mol−1) per decade from ∼ February–April coincide with large increases in satellite infrared brightness temperatures and outgoing longwave radiation, indicators of declining convective activity. MERRA-2 reanalyses exhibit increases in upper-tropospheric velocity potential and decreases in precipitable water, also indicating diminished convection. In contrast, trends in ozone and convective indicators are weak the rest of the year. These results suggest that decreases in convective intensity and frequency are primary drivers of FT ozone build-up over ESEA early in the year; i.e., waning convection suppresses lofting and dilution of ozone. Decreasing convection promotes accumulation of biomass burning emissions typical of boreal spring even though satellite FT carbon monoxide trends (2002–2022) over ESEA follow a global decrease pattern. Finally, our results demonstrate the advantages of monthly or seasonally resolved analyses over annual means for robust attribution of observed ozone trends, challenging models to reproduce these detailed features in simulations of the past 25 years.

Volcanic and pyrocumulonimbus (pyroCB) injections into the stratosphere perturb the aerosol layer and can have important radiative and chemical impacts on timescales spanning from months to several years. Repeated in situ balloon-borne measurements of aerosol size and number concentration (>140 nm in diameter), ozone, water vapor, and atmospheric state variables made at midlatitudes in the southern hemisphere (SH) since 2019 enable us to better characterize such events. We use this record and coincident lidar extinction profiles to study several moderate to large stratospheric perturbations in the SH between 2019 and 2022 in detail, including the Australian New Year Super Outbreak (ANYSO) pyroCB in 2020. Median vertical profiles of aerosol number concentration, effective radius, and surface area in SH midlatitudes are also compared with those recorded in Northern Hemisphere midlatitudes under baseline conditions using an identical payload. These data depict the variability in stratospheric aerosol properties in the SH midlatitudes during this period and provide a benchmark for global sectional aerosol models. They reveal that sulfate particle size distributions under baseline conditions and in volcanic plumes are relatively well represented in the Community Earth System Model-Community Aerosol Radiation Model for Atmospheres (CESM-CARMA), but more observations of biomass burning plumes are needed to improve model skill in simulating pyroCB. Comparisons between in situ and lidar observations also highlight a need for more observations of aerosol composition and refractive index in both fresh and aging biomass burning plumes.

Abstract. Balloon-borne ozonesondes launched weekly from South Pole Station (1986–2021) measure high-vertical-resolution profiles of ozone and temperature from the surface to 30–35 km altitude. The launch frequency is increased in late winter before the onset of rapid stratospheric ozone loss in September. Ozone hole metrics show that the yearly total column ozone and 14–21 km partial column ozone minimum values and September loss rate trends have been improving (less severe) since 2001. The 36-year record also shows interannual variability, especially in recent years (2019–2021). Here we show additional details of these 3 years by comparing annual minimum profiles observed on the date when the lowest integrated total column ozone occurs. We also compare the July–December time series of the 14–21 km partial column ozone values to the 36-year median with percentile intervals. The 2019 anomalous vortex breakdown showed stratospheric temperatures began warming in early September followed by reduced ozone loss. The minimum total column ozone of 180 Dobson units (DU) was observed on 24 September. This was followed by two stable and cold polar vortex years during 2020 and 2021 with total column ozone minimums at 104 DU (1 October) and 102 DU (7 October), respectively. These years also showed broad near-zero-ozone (loss saturation) regions within the 14–21 km layer by the end of September which persisted into October. Validation of the ozonesonde observations is conducted through the ongoing comparison of total column ozone measurements with the South Pole ground-based Dobson spectrophotometer. The ozonesondes show a more positive bias of 2 ± 3 % (higher) than the Dobson following a thorough evaluation and homogenization of the long-term ozonesonde record completed in 2018.

Stratospheric aerosol plays an important role in Earth's radiative balance. Systematic measurements of stratospheric aerosol number concentration and size are needed to characterize their baseline values and variability, improve understanding of aerosol microphysics, and assess the representation of stratospheric aerosol processes in global models. In situ measurements of particle size distribution provide insight into the responses of stratospheric aerosol processes to perturbations from volcanoes and wildfires, anthropogenic emissions from rockets and aircraft, or intentional injections related to climate intervention. The Baseline Balloon Stratospheric Aerosol Profiles (B 2 SAP) project uses compact, lightweight payloads carried by meteorological balloons to measure aerosol number density and size distributions, water vapor, ozone, and meteorological data from the surface to the middle stratosphere. The long‐term goal for the B 2 SAP project is to produce climatologies of aerosol number and size distributions into the middle stratosphere at latitudinally distributed measurement sites. Since March 2019, B 2 SAP payloads have been launched from Boulder, CO, USA once to twice per month and four to six times per year from Lauder, NZ. These measurements provide a new record of in situ stratospheric aerosol observations and are being used to help validate satellite‐based estimates of stratospheric aerosol size distributions. Here we present 50 vertical profiles from Boulder, CO, from March 2019 to March 2022, to build a NH mid‐latitude climatology of stratospheric aerosol and its variability and investigate the impact of natural perturbations on stratospheric aerosol microphysics.

The Antarctic polar vortex creates unique chemical and dynamical conditions when the stratospheric air over Antarctica is isolated from the rest of the stratosphere. As a result, stratospheric ozone within the vortex remains largely unchanged for a 5-month period from April until late August when the sunrise and extremely cold temperatures create favorable conditions for rapid ozone loss. Such prolonged stable conditions within the vortex make it possible to estimate the total ozone levels there from sparse wintertime ozone observations at the South Pole. The available records of focused Moon (FM) observations by Dobson and Brewer spectrophotometers at the Amundsen–Scott South Pole Station (for the periods 1964–2022 and 2008–2022, respectively) as well as integrated ozonesonde profiles (1986–2022) and MERRA-2 reanalysis data (1980–2022) were used to estimate the total ozone variability and long-term changes over the South Pole. Comparisons with MERRA-2 reanalysis data for the period 1980–2022 demonstrated that the uncertainties of Dobson and Brewer daily mean FM values are about 2.5 %–4 %. Wintertime (April–August) MERRA-2 data have a bias with Dobson data of −8.5 % in 1980–2004 and 1.5 % in 2005–2022. The mean difference between wintertime Dobson and Brewer data in 2008–2022 was about 1.6 %; however, this difference can be largely explained by various systematic errors in Brewer data. The wintertime ozone values over the South Pole during the last 20 years were about 12 % below the pre-1980s level; i.e., the decline there was nearly twice as large as that over southern midlatitudes. It is probably the largest long-term ozone decline aside from the springtime Antarctic ozone depletion. While wintertime ozone decline over the pole has hardly any impact on the environment, it can be used as an indicator to diagnose the state of the ozone layer, particularly because it requires data from only one station. Dobson and ozonesonde data after 2001 show a small positive, but not statistically significant, trend in ozone values of about 1.5 % per decade that is in line with the trend expected from the concentration of the ozone-depleting substances in the stratosphere.

The recent Assessment of Standard Operating Procedures for Ozonesondes 2.0 (WMO/GAW Report #268) addressed questions of homogeneity and long‐term stability in global electrochemical concentration cell (ECC) ozone sounding network time series. Among its recommendations was adoption of a standard for evaluating data quality in ozonesonde time series. Total column ozone (TCO) derived from the sondes compared to TCO from Aura's Ozone Monitoring Instrument (OMI) is a primary quality indicator. Comparisons of sonde ozone with Aura's Microwave Limb Sounder (MLS) are used to assess the stability of stratospheric ozone. This paper provides a comprehensive examination of global ozonesonde network data stability and accuracy since 2004 in light of the sudden post‐2013 TCO “dropoff” of ∼3%–4% that was reported previously at select stations (Stauffer et al., 2020, https://doi.org/10.1029/2019GL086791 ). Comparisons with Aura OMI TCO averaged across the network of 60 stations are stable within about ±2% over the past 18 years. Sonde TCO has similar stability compared to three other TCO satellite instruments, and the stratospheric ozone measurements average to within ±5% of MLS from 50 to 10 hPa. Thus, sonde data are reliable for trends, but with a caveat applied for a subset of dropoff stations in the tropics and subtropics. The dropoff is associated with only one of two major ECC instrument types. A detailed examination of ECC serial numbers pinpoints the timing of the dropoff. However, we find that overall, ozonesonde data are stable and accurate compared to independent measurements over the past two decades.

The Stratospheric Aerosol and Gas Experiment III (SAGE III) instrument installed on the International Space Station (ISS) has completed almost half of a decade of data collection and production of science data products. The SAGE III/ISS is a solar and lunar occultation instrument that scans the light from the Sun and Moon through the limb of the Earth’s atmosphere to produce vertical profiles of aerosol, ozone, water vapor, and other trace gases. It continues the legacy of previous SAGE instruments dating back to the 1970s to provide data continuity of stratospheric constituents critical for assessing trends in the ozone layer. This presentation shows how SAGE III/ISS aerosol and gas vertical profiles continue to benefit a worldwide database of in situ and satellite data for climate observation.

&lt;p&gt;The Stratospheric Aerosol and Gas Experiment III (SAGE III) instrument installed on the International Space Station (ISS) has completed almost half of a decade of data collection and production of science data products. The SAGE III/ISS is a solar and lunar occultation instrument that scans the light from the Sun and Moon through the limb of the Earth&amp;#8217;s atmosphere to produce vertical profiles of aerosol, ozone, water vapor, and other trace gases. It continues the legacy of previous SAGE instruments dating back to the 1970s to provide data continuity of stratospheric constituents critical for assessing trends in the ozone layer. This presentation shows how SAGE III/ISS aerosol and gas vertical profiles continue to benefit a worldwide database of in situ and satellite data for climate observation.&lt;/p&gt;

The Fires, Asian, and Stratospheric Transport–Las Vegas Ozone Study (FAST-LVOS) was conducted in May and June of 2017 to study the transport of ozone (O3) to Clark County, Nevada, a marginal non-attainment area in the southwestern United States (SWUS). This 6-week (20 May–30 June 2017) field campaign used lidar, ozonesonde, aircraft, and in situ measurements in conjunction with a variety of models to characterize the distribution of O3 and related species above southern Nevada and neighboring California and to probe the influence of stratospheric intrusions and wildfires as well as local, regional, and Asian pollution on surface O3 concentrations in the Las Vegas Valley (≈ 900 m above sea level, a.s.l.). In this paper, we describe the FAST-LVOS campaign and present case studies illustrating the influence of different transport processes on background O3 in Clark County and southern Nevada. The companion paper by Zhang et al. (2020) describes the use of the AM4 and GEOS-Chem global models to simulate the measurements and estimate the impacts of transported O3 on surface air quality across the greater southwestern US and Intermountain West. The FAST-LVOS measurements found elevated O3 layers above Las Vegas on more than 75 % (35 of 45) of the sample days and show that entrainment of these layers contributed to mean 8 h average regional background O3 concentrations of 50–55 parts per billion by volume (ppbv), or about 85–95 µg m−3. These high background concentrations constitute 70 %–80 % of the current US National Ambient Air Quality Standard (NAAQS) of 70 ppbv (≈ 120 µg m−3 at 900 m a.s.l.) for the daily maximum 8 h average (MDA8) and will make attainment of the more stringent standards of 60 or 65 ppbv currently being considered extremely difficult in the interior SWUS.

The Stratospheric Aerosol and Gas Experiment III (SAGE III) instrument installed on the International Space Station (ISS) has completed over three and a half years of data collection and production of science data products. The SAGE III/ISS is a solar and lunar occultation instrument that scans the light from the Sun and Moon through the limb of the Earth’s atmosphere to produce vertical profiles of aerosol, ozone, water vapor, and other trace gases. It continues the legacy of previous SAGE instruments dating back to the 1970s to provide data continuity of stratospheric constituents critical for assessing trends in the ozone layer. This presentation shows the validation results of comparing SAGE III/ISS ozone and water vapor vertical profiles from the newly released v5.2 science product with those of in situ and satellite data .

Throughout spring and summer 2020, ozone stations in the northern extratropics recorded unusually low ozone in the free troposphere. From April to August, and from 1 to 8 kilometers altitude, ozone was on average 7% (≈4 nmol/mol) below the 2000–2020 climatological mean. Such low ozone, over several months, and at so many stations, has not been observed in any previous year since at least 2000. Atmospheric composition analyses from the Copernicus Atmosphere Monitoring Service and simulations from the NASA GMI model indicate that the large 2020 springtime ozone depletion in the Arctic stratosphere contributed less than one‐quarter of the observed tropospheric anomaly. The observed anomaly is consistent with recent chemistry‐climate model simulations, which assume emissions reductions similar to those caused by the COVID‐19 crisis. COVID‐19 related emissions reductions appear to be the major cause for the observed reduced free tropospheric ozone in 2020.

Ozone in the troposphere affects humans and ecosystems as a pollutant and as a greenhouse gas. Observing, understanding and modelling this dual role, as well as monitoring effects of international regulations on air quality and climate change, however, challenge measurement systems to operate at opposite ends of the spatio-temporal scale ladder. Aboard the ESA/EU Copernicus Sentinel-5 Precursor (S5P) satellite launched in October 2017, the TROPOspheric Monitoring Instrument (TROPOMI) aspires to take the next leap forward by measuring ozone and its precursors at unprecedented horizontal resolution until at least the mid-2020s. In this work, we assess the quality of TROPOMI's first release (V01.01.05–08) of tropical tropospheric ozone column (TrOC) data. Derived with the convective cloud differential (CCD) method, TROPOMI daily TrOC data represent the 3 d moving mean ozone column between the surface and 270 hPa under clear-sky conditions gridded at 0.5∘ latitude by 1∘ longitude resolution. Comparisons to almost 2 years of co-located SHADOZ ozonesonde and satellite data (Aura OMI and MetOp-B GOME-2) conclude to TROPOMI biases between −0.1 and +2.3 DU (<+13 %) when averaged over the tropical belt. The field of the bias is essentially uniform in space (deviations <1 DU) and stable in time at the 1.5–2.5 DU level. However, the record is still fairly short, and continued monitoring will be key to clarify whether observed patterns and stability persist, alter behaviour or disappear. Biases are partially due to TROPOMI and the reference data records themselves, but they can also be linked to systematic effects of the non-perfect co-locations. Random uncertainty due to co-location mismatch contributes considerably to the 2.6–4.6 DU (∼14 %–23 %) statistical dispersion observed in the difference time series. We circumvent part of this problem by employing the triple co-location analysis technique and infer that TROPOMI single-measurement precision is better than 1.5–2.5 DU (∼8 %–13 %), in line with uncertainty estimates reported in the data files. Hence, the TROPOMI precision is judged to be 20 %–25 % better than for its predecessors OMI and GOME-2B, while sampling at 4 times better spatial resolution and almost 2 times better temporal resolution. Using TROPOMI tropospheric ozone columns at maximal resolution nevertheless requires consideration of correlated errors at small scales of up to 5 DU due to the inevitable interplay of satellite orbit and cloud coverage. Two particular types of sampling error are investigated, and we suggest how these can be identified or remedied. Our study confirms that major known geophysical patterns and signals of the tropical tropospheric ozone field are imprinted in TROPOMI's 2-year data record. These include the permanent zonal wave-one pattern, the pervasive annual and semiannual cycles, the high levels of ozone due to biomass burning around the Atlantic basin, and enhanced convective activity cycles associated with the Madden–Julian Oscillation over the Indo-Pacific warm pool. TROPOMI's combination of higher precision and higher resolution reveals details of these patterns and the processes involved, at considerably smaller spatial and temporal scales and with more complete coverage than contemporary satellite sounders. If the accuracy of future TROPOMI data proves to remain stable with time, these hold great potential to be included in Climate Data Records, as well as serve as a travelling standard to interconnect the upcoming constellation of air quality satellites in geostationary and low Earth orbits.