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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; 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. [ABSTRACT FROM AUTHOR]

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 +5 to +15% (+2 to +6 nmol mol-1) per decade FT ozone trends 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 decreases in upper tropospheric velocity potential and precipitable water, also indicating diminished convection. In contrast, trends in ozone and convective indicators are generally 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 trends over annual means for robust attribution of observed ozone trends, challenging models to reproduce these detailed features in simulations of the past 25 years. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Balloon-borne ozonesondes launched weekly from South Pole station (1986-2021) measure high vertical resolution profiles of ozone and temperature from 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 the yearly total column ozone and 14-21 km column ozone minimum values and September loss rates remain on an upward (less severe) trend since 2001. However, the data series also illustrate interannual variability, especially in the last three years (2019-2021). Here we show additional details of these three years by comparing minimum ozone profiles and the July-December 14-21 km column ozone time series. The 2019 anomalous vortex breakdown showed stratospheric temperatures began warming in early September leading to 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 in 2020 and 2021 with total column ozone minimums at 104 DU (01 October) and 102 DU (07 October), respectively. These years also showed broad near-zero ozone (saturation loss) 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 (TCO) measurements with the South Pole ground-based Dobson spectrophotometer. The ozonesondes show a constant positive offset of 2 ± 3% (higher) than the Dobson following a thorough evaluation/homogenization of the ozonesonde record in 2018. [ABSTRACT FROM AUTHOR]

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 (O 3) 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 O 3 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 O 3 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 O 3 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 O 3 on surface air quality across the greater southwestern US and Intermountain West. The FAST-LVOS measurements found elevated O 3 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 O 3 concentrations of 50–55 parts per billion by volume (ppbv), or about 85–95 µ g m -3. These high background concentrations constitute 70 %–80 % of the current US National Ambient Air Quality Standard (NAAQS) of 70 ppbv (≈ 120 µ g 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. [ABSTRACT FROM AUTHOR]

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 U.S. (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 neighbouring California, and to probe the influence of stratospheric intrusions, wildfires, and local, regional, and Asian pollution on surface O3 concentrations in Las Vegas and the surrounding area. In this paper, we describe the FAST-LVOS campaign and present case studies illustrating the influence of different transport processes on background O3 and air quality attainment in the SWUS. The 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 background O3 concentrations of 50-55 parts-per-billion by volume (ppbv) across southern Nevada. These background concentrations constitute 70-80% of the current U.S. National Ambient Air Quality Standard (NAAQS) of 70 ppbv, and illustrate some of the challenges facing air quality managers tasked with O3 attainment in the SWUS during late spring and early summer. The companion paper by Zhang et al. (2020) describes the use of the AM4 and GEOS-Chem global models to estimate the impacts of transported O3 on surface air quality in the Southwestern U.S. and Intermountain West during the FAST-LVOS campaign. [ABSTRACT FROM AUTHOR]

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. On board of the ESA/EU Copernicus Sentinel-5 Precursor (S5P) satellite launched in October 2017, 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 data (TrOC). Derived with the Convective Cloud Differential (CCD) method, TROPOMI daily TrOC data represent the three-day moving mean ozone column between surface and 270 hpa under clear sky conditions gridded at 0.5° latitude by 1° longitude resolution. Comparisons to almost two 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 four times better spatial resolution and almost twice 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 two- 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. A quasi-periodic signal of 1-2 weeks and 3-5 DU amplitude in TrOC time series, especially at low latitudes, is reminiscent of Kelvin wave activity. TROPOMI's combination of higher precision and higher resolution reveal 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. [ABSTRACT FROM AUTHOR]

The detection and attribution of high background ozone (O3) events in the southwestern U.S. is challenging but relevant to the effective implementation of the lowered National Ambient Air Quality Standard (NAAQS; 70 ppbv). Here we leverage intensive field measurements from the Fires, Asian, and Stratospheric TransportLas Vegas Ozone Study (FAST-LVOS) in MayJune 2017, alongside high-resolution simulations with two global models (GFDL-AM4 and GEOS-Chem), to pinpoint the sources of O3 during high-O3 events. We show stratospheric influence on four out of the ten events with daily maximum 8-hour average (MDA8) surface O3 above 65 ppbv in the greater Las Vegas region. While O3 produced from regional anthropogenic emissions dominates pollution in the Las Vegas Valley, stratospheric intrusions can mix with regional pollution to push surface O3 above 70 ppbv. GFDL-AM4 captures the key characteristics of deep stratospheric intrusions consistent with ozonesondes, lidar profiles, and co-located measurements of O3, CO, and water vapor at Angel Peak, whereas GEOS-Chem has difficulty simulating the observed features and underestimates observed O3 by ~ 20 ppbv at the surface. The two models also differ substantially during a wildfire event, with GEOS-Chem estimating ~ 15 ppbv greater O3, in better agreement with lidar observations. At the surface, the two models bracket the observed MDA8 O3 values during the wildfire event. Both models capture the large-scale transport of Asian pollution, but neither resolves some fine-scale pollution plumes, as evidenced from aerosol backscatter, aircraft, and satellite measurements. U.S. background O3 estimates from the two models differ by 5 ppbv on average and up to 15 ppbv episodically. Our multi-model approach tied closely to observational analysis yields process insights, suggesting that elevated background O3 may pose challenges to achieving a potentially lower NAAQS level (e.g., 65 ppbv) in the southwestern U.S. [ABSTRACT FROM AUTHOR]

Understanding the drivers of atmospheric ozone variations in the Arctic is difficult because there are few long-term records of vertical ozone profiles in this region. We present 12 years of ozone profiles over Summit Station, Greenland (72.6 N, 38.4 W; 3200 meters) that were measured from 2005 to 2016. These profiles are subjected to data screening and are extended to 60 km using a robust extrapolation method. The total column ozone and the partial column ozone in four atmospheric layers (troposphere to upper stratosphere) are analyzed. The monthly mean total column ozone reaches a maximum of about 400 DU in April, then decreases to minimum values between 275 and 300 DU in the late summer and early fall. The partial column ozone values peak at different times between late winter and early summer. There is a positive trend in the total column that is likely due to increases in springtime ozone, however, these trends are not robust given the short period of record. A stepwise multiple regression analysis is performed to determine the primary drivers of ozone variations over Summit Station. This analysis shows that the variations in total column ozone are due primarily to changes in the tropopause pressure, the quasi-biennial oscillation (QBO), and the volume of polar stratospheric clouds. The eddy heat flux is also important for variations in the partial column ozone in the different altitude regions. The importance of the QBO appears to be a unique characteristic for ozone variations over the Greenland Ice Sheet (when compared to other nearby Arctic Stations) and may be related to the fact that Greenland is particularly sensitive to the phase of the QBO. [ABSTRACT FROM AUTHOR]

NOAA's program of long term monitoring of the vertical distribution of ozone with Electrochemical Concentration Cell (ECC) ozonesondes has undergone a number of changes over the 50 year record. In order to produce a homogenous data set, these changes must be documented and where necessary, appropriate corrections applied. This is the first comprehensive and consistent reprocessing of NOAA's ozonesonde data records that corrects for these changes using the rawest form of the data (cell current and pump temperature) in native resolution as well as a point by point uncertainty calculation that is unique to each sounding. The reprocessing is carried out uniformly at all eight ozonesonde sites in NOAA's network with differences in sensing solution and ozonesonde types accounted for in the same way at all sites. The corrections used to homogenize the NOAA ozonesonde data records greatly improve the ozonesonde measurements with an average one sigma uncertainty of ±4-6 % in the stratosphere and ±5-20 % in the troposphere. A comparison of the integrated column ozone from the ozonesonde profile with co-located Dobson spectrophotometers total column ozone measurements shows agreement within ±5 % for > 70 % of the profiles. Very good agreement is also found in the stratosphere between ozonesonde profiles and profiles retrieved from the Solar Backscatter Ultraviolet Instruments (SBUV). [ABSTRACT FROM AUTHOR]