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The vertical distribution of tropospheric O3 from ozonesondes is compared with that from In-service Aircraft for a Global Observing System (IAGOS) measurements at 23 pairs of sites between about 30°S and 55°N, from 1995 to 2021. Profiles of tropospheric O3 from IAGOS aircraft are in generally good agreement with ozonesonde observations, for Electrochemical concentration cells (ECC), Brewer-Mast, and Carbon-Iodine sensors, with average biases of 7.03 ppb, 6.28 ppb, and -4.48 ppb, and correlation coefficients (R) of 0.72, 0.86, and 0.82, respectively. Agreement between the aircraft and Indian-sonde observations is poor, with an average bias of 24.07 ppb and R of 0.41. The O3 concentration observed by ECC sondes is on average higher by 5-10% than that observed by IAGOS aircraft, and the relative bias increases modestly with altitude. For other sonde types, there are some seasonal and altitude variations in the relative bias with respect to IAGOS measurements, but these appear to be caused by local differences. The distance between station and airport within 4° has little effect on the comparison results. For the ECC ozonesonde, the overall bias with respect to IAGOS measurements varies from 5.7 to 9.8 ppb, when the station pairs are grouped by station-airport distances of <1° (latitude and longitude), 1-2°, and 2-4°. Correlations for these groups are R = 0.8, 0.9 and 0.7. These comparison results provide important information for merging ozonesonde and IAGOS measurement datasets. They can also be used to evaluate the relative biases of the different sonde types in the troposphere, using the aircraft as a transfer standard. [ABSTRACT FROM AUTHOR]

Abundance of tropospheric ozone is determined by the interplay among emissions, chemistry, and climate dependent transport. It is important to identify the essential factors that modulate ozone in China and Korea because of increasing ozone trends in this area. In this study, we investigated the correlations of tropospheric ozone in East Asia with El Niño Southern Oscillations (ENSO). The model simulated ozone from 2000 to 2020 with Community Atmosphere Model version 6 with chemistry (CAM6‐chem), ECMWF Atmospheric Composition Reanalysis data, and ozone profiles measured in South Korea, Japan, and Hong Kong. These data were utilized to understand the ozone–ENSO relationship in this region. We found that tropospheric ozone in East Asia is significantly correlated with the wintertime Niño 3.4 index in El Niño/La Niña developing summer and decaying spring, with correlation coefficient up to 0.9. Particularly tropospheric ozone originated from stratosphere is strongly correlated with the Niño 3.4 index. Ozone concentrations in China and the Korean Peninsula are ∼5 ppb higher during El Niño developing summer compared to La Niña in a broad area, because of anomalous cyclonic/anticyclonic flows in the extratropics. In El Niño/La Niña decaying spring, the opposite occurs in the same region. In the decaying spring, stronger variability was observed in the subtropical regions of the lower to mid troposphere, indicating higher ozone concentrations during El Niño compared to La Niña. This variability is associated with the fluctuations in the Walker circulation, large‐scale sinking/rising motions, and the accompanying biomass burning events. The model reproduced the ozone variability associated with ENSO in the ozonesonde observations. Plain Language Summary: Ozone in the troposphere (surface to ∼16 km) is a greenhouse gas and an air pollutant harmful to public health and agriculture. Asian countries such as China and South Korea have increasing ozone trends in the last decades. Therefore, it is imperative to understand critical factors affecting ozone variability in this region. Ozone abundance is determined by emissions, chemistry, and transport related to weather and climate. El Niño–Southern Oscillation (ENSO), an air–sea coupled phenomenon in the tropical ocean can have a global impact on the atmospheric environment. In this study, we found that tropospheric ozone in Asia varies significantly with ENSO. Ozone concentrations in China and the Korean Peninsula in El Niño developing summer are generally higher than its La Niña counterpart throughout the troposphere. In contrast, ozone concentrations in El Niño decaying spring are lower than that in La Niña in the mid to upper troposphere extratropical region, indicating important roles of stratospheric ozone intrusion. The opposite occurs in the subtropical region: higher ozone concentrations during El Niño decaying spring than during La Niña are found in association with large‐scale longitudinal circulation varying with ENSO. Long‐term ozone profile observations support findings in the global chemistry climate model results. Key Points: A new connection between El Niño Southern Oscillations (ENSO) and tropospheric ozone in East Asia is introduced using CAM‐chem, reanalysis data and ozonesonde observationsENSO‐driven ozone variations are affected by stratospheric intrusion, continental and maritime transport, and biomass burning emissionsThe impacts of ENSO on tropospheric ozone in East Asia vary depending on location, altitude, and season (developing summer, decaying spring) [ABSTRACT FROM AUTHOR]

Tropospheric ozone pollution poses a major environmental challenge in China. As its primary natural source, Stratosphere-to-Troposphere Transport (STT) has been recognized as a significant contributor to tropospheric ozone in western, northeastern, and eastern China. However, the extent of STT’s influence on southeastern China has been less studied due to data limitations. Using a recently available one-year dataset of ozonesonde observations from a regional background station, we find that STT contributes significantly to tropospheric and surface ozone elevation in southeastern China. Our results show that STT plays a more substantial role in shaping tropospheric ozone during spring than previously believed, accounting for over 30% of ozone concentrations above 4 km. Without the stratospheric contribution, the spring seasonal peak almost disappears. STT can also significantly influence ozone concentrations at the surface. For example, a distinct ozone profile was observed on 4 May 2022, with a notable increase in tropospheric ozone. This tropospheric ozone increase was caused by a STT event triggered by a robust horizontal trough and subsequent southward movement of subtropical jets in the upper troposphere. According to a stratospheric tracer derived from an atmospheric chemistry model, this STT event contributed to 25%–30% of the surface ozone increase. Overall, this study highlights the important role of STT in driving tropospheric ozone variations, even in regions with comparatively lower ozone levels in southeastern China.

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. Plain Language Summary: Ozonesondes provide accurate ozone measurements from the surface to ∼30‐km altitude and are used as a reference for studies of satellite data, trends, pollution, and climate. Updated guidelines for sonde preparation and adoption of sonde total column ozone (TCO) comparisons with satellite TCO as a "data quality" reference were published in 2021 by the Assessment of Standard Operating Procedures for Ozonesondes (ASOPOS) 2.0 panel in WMO/GAW Report 268. We report the first application of the ASOPOS 2.0 protocol to TCO evaluation from the 60‐station global ozonesonde network (42,042 profiles total). With Aura OMI TCO as the satellite reference (October 2004 to mid‐2021), we find that TCO readings from the global ozonesonde network are remarkably stable, consistently within ±2% of the satellite. An exception occurs at only a small subset of tropical and subtropical locations that use one type of ozonesonde instrument. The latter result confirms our earlier report that a sudden TCO drop occurs at selected sites after 2013. The timing and magnitude of the dropoff are revisited. The hypothesis that ozonesonde production changes are a contributor remains, with station‐specific factors possibly affecting the magnitude of the bias. Overall, global ozonesonde network data are of high quality and stability. Key Points: Global ozonesonde total column ozone stability averages ∼±2% relative to measurements from multiple satellite instruments since 2004A sudden ozonesonde low bias affects a subset of stations using one manufacturer, and is mostly confined to the tropicsContinuous evaluation of ozonesonde data against independent measurements will facilitate ongoing monitoring of the stability of the data [ABSTRACT FROM AUTHOR]

The ozonesonde observations in Hanoi, Vietnam, over 14 years since 2004, have confirmed an enhancement in lower tropospheric ozone concentration at about 3 km altitude in the spring season. We investigated the evolution of the ozone enhancement from analysis of meteorological data, backward trajectories, and model sensitivity experiments. In spring, air masses over Hanoi exhibit strong height dependence. At 3 km, the high‐ozone air masses originate from the land area to the west of Hanoi, while low‐ozone air masses below about 1.5 km are from the oceanic area to the east. Above 4 km, the air masses are mostly traced back to the farther west area. The chemical transport model simulations revealed that precursor emissions from biomass burning in the inland Indochina Peninsula have the largest contribution to the lower tropospheric ozone enhancement, which is transported upward and eastward and overhangs the clean air intrusion from the ocean to the east of Hanoi. At this height level, the polluted air has the horizontal extent of about 20° in longitude and latitude. The polluted air observed in Hanoi is transported further east and widely spread over the northern Pacific Ocean. Plain Language Summary: Ozone in the lower atmosphere acts as an air pollutant and is one of the strong greenhouse gases. Understanding chemical and transport processes that control ozone variations provide important implications for the origin of air pollution and for climate change. In the past 20 years, the global tropospheric ozone amount has increased in the tropics and subtropics, with particularly large increases in Southeast Asia. We analyze a seasonal enhancement in ozone over Hanoi, Vietnam, using the 14‐year record of upper air ozone measurements that began in 2004. The measurements reveal high levels of ozone particularly during the spring that peak at around 3 km. Numerical model simulations show that agricultural burning in the inland Indochina Peninsula is the major contributor to these high ozone amounts that are transported upward and eastward and overhang the clean air from the ocean to the east of Hanoi. Over time, the high‐ozone air observed in Hanoi is transported further east and widely spread over the northern Pacific Ocean. The identified source of the ozone enhancements and its three‐dimensional distribution provide new insights into the impact of agricultural burning over Southeast Asia on large‐scale ozone distributions and are relevant to global air quality and climate studies. Key Points: Ozonesonde observations in Hanoi, Vietnam, for about 14 years since 2004 display an ozone enhancement at ∼3 km altitude in March–AprilEnhanced ozone air at ∼3 km originates from burning areas west of Hanoi whereas low‐ozone near‐surface air is marine from the eastModel experiments show biomass fires in the Indochina Peninsula are the cause of Hanoi's enhanced ozone [ABSTRACT FROM AUTHOR]

Abstract The modern reanalysis datasets provide not only meteorological variables, but also atmospheric chemical compositions such as tropospheric ozone and aerosol concentration. However, the quality of chemical compositions has been rarely assessed especially over East Asia. To better understand the characteristics of reanalysis datasets on regional scale, the present study evaluates tropospheric ozone derived from seven reanalyses against five independent ozonesonde observations in East Asia. The reanalysis datasets are the ECMWF Reanalysis 5th (ERA5), Monitoring Atmospheric Composition and Climate reanalysis (MACCRA), Copernicus Atmosphere Monitoring Service reanalysis (CAMSRA), as well as the NCEP Climate Forecast System Reanalysis (CFSR), NASA Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA2), Japanese 55-year Reanalysis (JRA-55), and updated Tropospheric Chemistry Reanalysis (TCR-2). It turns out that MACCRA, CAMSRA, and TCR-2, which incorporate chemical transport model, depict most reasonable spatio-temporal variability of tropospheric ozone in East Asia. The MACC exhibits a better quality with relatively small mean biases of 6.4 ± 1.3% in tropospheric column ozone than biases of 7.8 ± 2.7% and 7.8 ± 2.8% for CAMSRA and TCR-2. The CAMSRA further shows a significant monthly correlation with the observation of up to 0.7 at 850 hPa. Among the seven reanalyses, MACC, CAMSRA, and TCR-2 are suitable for local tropospheric ozone study on seasonal to inter-annual time scales. However, none of the seven reanalysis datasets reproduce the observed trend of tropospheric ozone. This result suggests that even the latest datasets are inadequate for the long-term ozone change study.

While surface concentrations of ozone are routinely monitored, ozone aloft is infrequently measured, but critical for a full understanding of ozone production and transport. In this study, twenty‐six balloon‐borne ozonesondes were launched near the north shore of central Long Island in the summers of 2018 and 2019 as part of the Long Island Sound Tropospheric Ozone Study (LISTOS). The observed vertical ozone profiles are presented and analyzed with additional data sources and modeling tools, including lidar wind profiles from the New York State Mesonet, back trajectories based on 3 km resolution high‐resolution rapid refresh model data, and surface data, aircraft observations, sonde, and ozone lidar measurements from other LISTOS participants. Special attention is given to the region of interest for ozone pollution in the lower troposphere, from the surface to 2 km altitude. Cases analyzed in detail illustrate events with high ozone levels observed in the lower troposphere, often with pronounced vertical structure in the profile. Specifically, easily discernible layers are identified with ozone excursions of up to 40 ppb over short vertical distances. Analysis indicates that synoptic and local meteorological processes can combine to generate the observed vertical profiles. Hot, sunny days with high‐pressure systems are accompanied by high precursor emissions due to increased power demands, plentiful radiation for photochemistry, and stagnation of synoptic winds. Under these conditions, meso‐ and smaller‐scale flows like low‐level jets and sea/bay/land breeze circulations may dominate synoptic flow to produce shearing and the complex vertical layered structure observed. Plain Language Summary: Ground‐level ozone pollution is a persistent threat to public health. Ozone is not directly emitted, but forms when emitted pollutants are exposed to sunlight. Winds that would normally dissipate ozone may be disrupted by local weather features produced in coastal regions where land and water meet. Understanding the meteorology behind high ozone events is important to forecasting unhealthy air quality days accurately. Twenty‐six ozone sensors were launched via balloon from Long Island (LI) in 2018 and 2019 as part of the Long Island Sound Tropospheric Ozone Study. These sensors provide detailed vertical measurements of ozone and weather useful for understanding how ozone is formed and transported. Through five case studies, we described the impacts of small‐scale, local circulations on ground‐level ozone. Ozone buildup comes from large‐scale transport, low‐level jets, stagnation events when the winds were very light for a sustained period of time, and sea breeze circulations. These situations occur on both the Sound and Atlantic sides of LI, bringing in pollutants from the water onto land. This article illustrates the complex meteorology behind high ozone days in coastal areas near cities. Key Points: Vertical ozone profiles below 2 km can exhibit layering with little connection to surface concentrationsSea breeze circulation during stagnation near the land‐water interface often induces layeringVertical shearing as a result of low‐level jets or sea breeze circulation provides mechanisms for the formation of layers [ABSTRACT FROM AUTHOR]

Abstract We review the current state of knowledge of ozonesonde uncertainty and bias, with reference to recent developments in laboratory and field experiments. In the past 20 years ozonesonde precision has improved by a factor of 2, primarily through the adoption of strict standard operating procedures. The uncertainty budget for the ozone partial pressure reading has contributions from stoichiometry, cell background current, pump efficiency and temperature, sensing solution type, and volume. Corrections to historical data for known issues may reduce biases but simultaneously introduce additional uncertainties. This paper describes a systematic approach to quantifying these uncertainties by considering the physical and chemical processes involved and attempts to place our estimates on a firm theoretical or empirical footing. New equations or tables for ozone/iodine conversion efficiency, humidity and temperature corrections to pump flow rate, and altitude‐dependent pump flow corrections are presented, as well as detailed discussion of stoichiometry and conversion efficiencies. The nature of the so‐called “background current” is considered in detail. Two other factors particularly affecting past measurements, uncertainties and biases in the pressure measurement, and the comparison of sonde profiles to spectrophotometric measurements of total column ozone, are also discussed. Several quality assurance issues remain, but are tractable problems that can be addressed with further research. This will be required if the present goal of better than 5% overall uncertainty throughout the global ozonesonde network is to be achieved.

Abstract Typhoons play a key role in causing low ozone concentrations in the upper troposphere and lower stratosphere (UTLS) over the western Pacific. In this study, 18 years records (2000–2017) of balloon‐borne ozone measurements in Hong Kong (22.3°N, 114.2°E) and Naha (26.2°N, 127.7°E) are combined with trajectory calculations to quantitatively assess the impact of western Pacific typhoons on ozone in the UTLS. The results show that 46.3% (44.9%) of the ozone profiles in Hong Kong (Naha) are impacted by western Pacific typhoons from July to October, with negative ozone anomalies exceeding −20%. Vertical transport by intense typhoons affects 16.8% (18.8%) of the ozone profiles in Hong Kong (Naha) with even larger negative anomalies (

We review the current state of knowledge of ozonesonde uncertainty and bias, with reference to recent developments in laboratory and field experiments. In the past 20 years ozonesonde precision has improved by a factor of 2, primarily through the adoption of strict standard operating procedures. The uncertainty budget for the ozone partial pressure reading has contributions from stoichiometry, cell background current, pump efficiency and temperature, sensing solution type, and volume. Corrections to historical data for known issues may reduce biases but simultaneously introduce additional uncertainties. This paper describes a systematic approach to quantifying these uncertainties by considering the physical and chemical processes involved and attempts to place our estimates on a firm theoretical or empirical footing. New equations or tables for ozone/iodine conversion efficiency, humidity and temperature corrections to pump flow rate, and altitude‐dependent pump flow corrections are presented, as well as detailed discussion of stoichiometry and conversion efficiencies. The nature of the so‐called "background current" is considered in detail. Two other factors particularly affecting past measurements, uncertainties and biases in the pressure measurement, and the comparison of sonde profiles to spectrophotometric measurements of total column ozone, are also discussed. Several quality assurance issues remain, but are tractable problems that can be addressed with further research. This will be required if the present goal of better than 5% overall uncertainty throughout the global ozonesonde network is to be achieved. Plain Language Summary: Ozonesondes are a stable reference for the global ozone observing network, making relatively inexpensive, accurate measurements of ozone from the ground to 30 km, with high vertical resolution, for more than 50 years. Ozonesonde data are used extensively for validation of satellite ozone measurements, models, and for trend analyses. The current state of knowledge of ozonesonde uncertainty and bias is reviewed, and a systematic approach to quantifying these uncertainties by considering the physical and chemical processes involved is presented. While ozonesonde precision has improved through the adoption of strict standard operating procedures, further improvement is possible with further research, toward a goal of less than 5% overall uncertainty throughout the global network. Key Points: Review of the current state of knowledge of ozonesonde uncertainty and biasA systematic approach to quantifying these uncertainties by considering the physical and chemical processes involvedUncertainties related to stoichiometry and sensor response may be reduced with further research, toward a goal of less than 5% uncertainty in the global network [ABSTRACT FROM AUTHOR]

Typhoons play a key role in causing low ozone concentrations in the upper troposphere and lower stratosphere (UTLS) over the western Pacific. In this study, 18 years records (2000–2017) of balloon‐borne ozone measurements in Hong Kong (22.3°N, 114.2°E) and Naha (26.2°N, 127.7°E) are combined with trajectory calculations to quantitatively assess the impact of western Pacific typhoons on ozone in the UTLS. The results show that 46.3% (44.9%) of the ozone profiles in Hong Kong (Naha) are impacted by western Pacific typhoons from July to October, with negative ozone anomalies exceeding −20%. Vertical transport by intense typhoons affects 16.8% (18.8%) of the ozone profiles in Hong Kong (Naha) with even larger negative anomalies (<−40%). Vertical transport of tropical cyclones reduces ozone by about 20–60 ppbv compared to the mean ozone profile near the tropopause. Ozone values below 60 ppbv in the upper troposphere in Hong Kong and Naha are mainly caused by the uplift in the western Pacific typhoons and by horizontal transport within the Asian summer monsoon (ASM) anticyclone. The horizontal transport in the ASM anticyclone has larger contributions to the low ozone values measured over Hong Kong than over Naha. Plain Language Summary: Ozone profiles from July to October during 2000–2017 in Hong Kong and Naha are used to analyze the impact of western Pacific cyclones on ozone in the upper troposphere and lower stratosphere. During the summer season, typhoons over the western Pacific influence 46.3% (44.9%) of the ozone profiles in Hong Kong (Naha) based on the trajectory calculations of air masses. We also find that in Hong Kong (Naha) intense typhoons contribute to large negative ozone anomalies (<−40%) for 16.8% (18.8%) of the profiles. Vertical transport associated with typhoons and the horizontal transport of the Asian summer monsoon anticyclone reduces ozone by about 20–60 ppbv near the tropopause in Hong Kong and Naha. Key Points: Western Pacific typhoons impact 46.3% (44.9%) of ozone profiles in Hong Kong (Naha) from July to October of 2000–2017Ozone values below 60 ppbv near the tropopause in Hong Kong and Naha are caused by vertical transport of ozone‐poor air by typhoon convectionTyphoons reduce ozone by 20–60 ppbv compared to 18 years mean ozone profile in the upper troposphere and lower stratosphere [ABSTRACT FROM AUTHOR]

An objective method, employing HYSPLIT back-trajectories and Moderate Resolution Imaging Spectroradiometer (MODIS) fire observations, is developed to estimate ozone enhancement in air transported from regions of active forest fires at 18 ozone sounding sites located across North America. The Differential Back Trajectory (DBT) method compares mean differences between ozone concentrations associated with fire-affected and fire-unaffected parcels. It is applied to more than 1100 ozonesonde profiles collected from these sites during the summer months June to August 2006, 2008, 2010 and 2011. Layers of high ozone associated with low humidity were first removed from the ozonesonde profiles to minimize the potential effects of stratospheric intrusions on the calculations. No significant influence on average ozone levels by North American fires was found for stations located at Arctic latitudes. The ozone enhancement for stations nearer large fires, such as Trinidad Head and Bratt's Lake, was up to 4.8% of the TTOC (Total Tropospheric Ozone Column). Fire ozone accounted for up to 8.3% of TTOC at downwind sites such as Yarmouth, Sable Island, Narragansett, and Walsingham. The results are consistent with other studies that have reported an increase in ozone production with the age of the smoke plume.

Vertical distribution of ozone has been obtained for the first time using INSAT-3D for the period 2013- 2017 over the central Himalaya and validated utilizing balloon-borne observations from a high-altitude site in Nainital (29.4?N, 79.5?E, 1793 m amsl). The INSAT- 3D retrieved ozone profiles captured ozone gradient and ozone peak altitude successfully, despite only one IR channel for ozone. This demonstrates the capability of the INSAT-3D Sounder in capturing the observed features, with a smaller bias in the stratosphere and somewhat larger bias in the troposphere. Total ozone column from INSAT-3D showed maximum difference of 8% with ozonesonde-derived total ozone column. Larger ozone bias in the lower troposphere could be attributed to lower reliability of regression coefficient and INSAT-3D channel constraints itself, whereas high variability near the tropopause is possibly due to low ozone, poor temperature retrieval near the tropopause and stratosphere-troposphere transport process in the Himalayan region. [ABSTRACT FROM AUTHOR]

The modern reanalysis datasets provide not only meteorological variables, but also atmospheric chemical compositions such as tropospheric ozone and aerosol concentration. However, the quality of chemical compositions has been rarely assessed especially over East Asia. To better understand the characteristics of reanalysis datasets on regional scale, the present study evaluates tropospheric ozone derived from seven reanalyses against five independent ozonesonde observations in East Asia. The reanalysis datasets are the ECMWF Reanalysis 5th (ERA5), Monitoring Atmospheric Composition and Climate reanalysis (MACCRA), Copernicus Atmosphere Monitoring Service reanalysis (CAMSRA), as well as the NCEP Climate Forecast System Reanalysis (CFSR), NASA Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA2), Japanese 55-year Reanalysis (JRA-55), and updated Tropospheric Chemistry Reanalysis (TCR-2). It turns out that MACCRA, CAMSRA, and TCR-2, which incorporate chemical transport model, depict most reasonable spatio-temporal variability of tropospheric ozone in East Asia. The MACC exhibits a better quality with relatively small mean biases of 6.4 ± 1.3% in tropospheric column ozone than biases of 7.8 ± 2.7% and 7.8 ± 2.8% for CAMSRA and TCR-2. The CAMSRA further shows a significant monthly correlation with the observation of up to 0.7 at 850 hPa. Among the seven reanalyses, MACC, CAMSRA, and TCR-2 are suitable for local tropospheric ozone study on seasonal to inter-annual time scales. However, none of the seven reanalysis datasets reproduce the observed trend of tropospheric ozone. This result suggests that even the latest datasets are inadequate for the long-term ozone change study. [ABSTRACT FROM AUTHOR]

Tropospheric ozone (O 3) plays predominant role in atmospheric chemistry, exacerbates the air quality, and contributes to the climate change. Near the Earth's surface, O 3 levels exhibit large heterogeneity due to diverse emissions of precursors and effects of meteorological conditions. Nevertheless, the understanding of key dynamical and photochemical processes governing O 3 remains limited in the Western Ghats, a tropical region of immense significance in context of the geosphere-biosphere processes. In this regard, we combined surface O 3 measurements at Ponmudi (8.758° N, 77.114° E, ∼1 km above mean sea level–amsl), a high-altitude site in the Western Ghats, with ground- and balloon-borne O 3 measurements at western coast of India (Thumba: 8.542° N, 76.858° E, ∼3 m amsl) and photochemical box model simulations. O 3 diurnal variation exhibits a small amplitude (3–9 ppbv) with lower values during the daytime, in contrast to substantial urban O 3 build-up (25–37 ppbv) at Thumba. The influence of regional pollution from the urban boundary layer, downdraft of O 3 -rich air during night, and dry deposition processes are suggested to govern the O 3 diurnal pattern over Ponmudi. Lower O 3 /CO and O 3 /NO 2 ratios indicating lower O 3 production efficiency, and dominant role of dynamics are consistently seen over the study region. O 3 variability between Ponmudi and Thumba are found to be well correlated (r = 0.7–0.8) during pre-monsoon and post-monsoon seasons as the sea breeze circulation transport the airmass from coast to the Western Ghats during the daytime. Mean O 3 levels at Ponmudi are typically lower than those above Thumba showing an effective O 3 loss towards Ponmudi. Chemistry involving strong natural emissions (e.g., isoprene) as simulated by photochemical box model, and greater dry deposition tend to supress O 3 build up over this densely vegetated tropical region. Unlike other high-altitude sites, O 3 seasonal cycle at Ponmudi is associated closely with synoptic wind changes, and peaks during the winter owing to the transport from Indo Gangetic Plain/northern India. Our study highlights the roles of regional pollution as well as natural processes including biogenic emissions in governing the surface O 3 variability over the Western Ghats. [Display omitted] • O 3 over the Western Ghats (Ponmudi) is well correlated (r = 0.7–0.8) with O 3 at western coastal station (Thumba). • Lower levels of O 3 is attributed to the higher dry deposition loss and biogenic emission of isoprene over the Western Ghats. • Unlike other high-altitude sites, Ponmudi-O 3 peaks during winter due to transport from Indo Gangetic Plain/northern India. [ABSTRACT FROM AUTHOR]

Model simulations of ozone (O3) driven by meteorological reanalyses are useful for filling observational gaps and interpreting observed O3 variability and trends. However, the transport circulation of reanalysis products is impacted by changes to the observing system (the data assimilated into the reanalyses). We examine the impacts of these changes on simulated O3 from two models, Global Modeling Initiative (GMI) Chemistry Transport Model (GMI CTM) and Modern‐Era Retrospective Analysis for Research Applications version 2 (MERRA‐2) GMI Replay (M2 GMI Replay) simulation, using observations from global ozonesondes (>50,000 profiles) and satellites from 1980 to 2016. Both models are constrained by meteorology from the NASA MERRA‐2 reanalysis, and both use versions of NASA's GMI chemical mechanism. We focus on an observing system change affecting simulated O3 after 1998, associated with the assimilation of temperature and humidity data from new microwave profiling satellites. A large post‐1998 O3 increase, mainly confined to 15–20‐km altitude, of ~10 Dobson units (DU) in midlatitudes occurs in the GMI CTM, worsening the bias compared to observations. In contrast, an increase in M2 GMI Replay simulation O3 of ~10 DU is observed only near −60° latitude, reducing the bias compared to observations. The GMI CTM O3 high biases display a Quasi‐Biennial Oscillation (QBO)‐like periodicity that result from excessive transport from the tropical stratosphere to the midlatitude lower stratosphere during the QBO westerly phase. We quantify O3 discontinuities caused by MERRA‐2 observing system changes and demonstrate how the MERRA‐2 Global Modeling Initiative Replay simulation dampens the effects of these changes and QBO‐driven artifacts on simulated lower stratospheric and total O3. We caution against using simulations driven by a reanalysis to derive multidecadal O3 trends, especially prior to 1998. Key Points: Ozonesonde and satellite observations are used to evaluate two 1980–2016 model simulations of O3 driven by MERRA‐2 meteorologyAn observing system change in 1998 that is known to affect MERRA‐2 meteorology causes major O3 discontinuities in both simulationsThe 1998 change impacts the stratosphere‐to‐troposphere O3 transport differently in the simulations, improving one and worsening the other [ABSTRACT FROM AUTHOR]

In the Mexico City Metropolitan Area (MCMA), ozone (O3) concentration is still higher than in other urban areas in developed countries. In order to reveal the current state of photochemical air pollution and to provide data for validation of chemical transport models, vertical profiles of meteorological parameters and ozone concentrations were measured by ozonesonde in two field campaigns: the first one, during the change of season from wet to dry-cold (November 2011) and the second during the dry-warm season (March 2012). Unlike previous similar field campaigns, ozonesonde was launched twice daily. The observation data were used to analyze the production and distribution of ozone in the convective boundary layer. The observation days covered a wide range of meteorological conditions, and various profiles were obtained. The evolution of the mixing layer (ML) height was analyzed, revealing that ML evolution was faster during daytime in March 2012 than in November 2011. On a day in November 2011, the early-morning strong wind and the resulting vertical mixing was observed to have brought the high-ozone-concentration air-mass to the ground and caused relatively high surface ozone concentration in the morning. The amount of produced ozone in the MCMA was estimated by taking the difference between the two profiles on each day. In addition to the well-known positive correlation between daily maximum temperature and ozone production, effect of the ML height and wind stagnation was identified for a day in March 2012 when the maximum ground-level ozone concentration was observed during the two field campaigns. The relatively low ventilation coefficient in the morning and the relatively high value in the afternoon on this day implied efficient accumulation of the O3 precursors and rapid production of O3 in the ML.

This study analyzes tropospheric column ozone variability in the southern hemisphere as a function of ozone transport from the stratosphere to the troposphere and photochemical formation. Geographically, the study area was located in the mid-latitudes in South America (33° S), to the west of the Andes mountain range, in an area highly susceptible to stratospheric intrusions. Monthly ozonesonde measurements were recorded in Colina to ascertain seasonal vertical ozone distribution from the surface to the stratosphere between September 2010 and May 2012. Vertical distribution of the tropospheric ozone was measured in Talagante for fronts crossing from west to east in central Chile, during two periods in September 2014 and March 2015. These periods were significantly different in terms of the stratospheric ozone annual cycle and height of the tropopause. Our results showed rapid increases of approximately 50% in the tropospheric column ozone at time intervals shorter than 1 week. At the surface level, unusually enhanced ozone levels up to 10 parts per billion volume (ppbv) were observed during nighttime. Additionally, stratosphere-troposphere exchange (STE) preferentially occurred in spring and winter, with higher contribution during spring when the tropospheric column ozone attained its maximum concentration. These results provide valuable information regarding tropospheric ozone, a major local and global climate pollutant, to decision makers. In addition, they provide the research community with experimental data from the southern hemisphere, which helps bridge knowledge gaps in a region that has been rarely studied by national and international scientific communities. [ABSTRACT FROM AUTHOR]

Abstract: Reprocessed ozonesonde data from eight SHADOZ (Southern Hemisphere ADditional OZonesondes) sites have been used to derive the first analysis of uncertainty estimates for both profile and total column ozone (TCO). The ozone uncertainty is a composite of the uncertainties of the individual terms in the ozone partial pressure (PO3) equation, those being the ozone sensor current, background current, internal pump temperature, pump efficiency factors, conversion efficiency, and flow rate. Overall, PO3 uncertainties (ΔPO3) are within 15% and peak around the tropopause (15 ± 3 km) where ozone is a minimum and ΔPO3 approaches the measured signal. The uncertainty in the background and sensor currents dominates the overall ΔPO3 in the troposphere including the tropopause region, while the uncertainties in the conversion efficiency and flow rate dominate in the stratosphere. Seasonally, ΔPO3 is generally a maximum in the March–May, with the exception of SHADOZ sites in Asia, for which the highest ΔPO3 occurs in September–February. As a first approach, we calculate sonde TCO uncertainty (ΔTCO) by integrating the profile ΔPO3 and adding the ozone residual uncertainty, derived from the McPeters and Labow (2012, doi:10.1029/2011JD017006) 1σ ozone mixing ratios. Overall, ΔTCO are within ±15 Dobson units (DU), representing ~5–6% of the TCO. Total Ozone Mapping Spectrometer and Ozone Monitoring Instrument (TOMS and OMI) satellite overpasses are generally within the sonde ΔTCO. However, there is a discontinuity between TOMS v8.6 (1998 to September 2004) and OMI (October 2004–2016) TCO on the order of 10 DU that accounts for the significant 16 DU overall difference observed between sonde and TOMS. By comparison, the sonde‐OMI absolute difference for the eight stations is only ~4 DU. [ABSTRACT FROM AUTHOR]

An abnormal decrease in ozonesonde sensor signal occurred during air-pollution study campaigns in November 2011 and March 2012 in Mexico City Metropolitan Area (MCMA). Sharp drops in sensor signal around 5 km above sea level and above were observed in November 2011, and a reduction of signal over a broad range of altitude was observed in the convective boundary layer in March 2012. Circumstantial evidence indicated that SO2 gas interfered with the electrochemical concentration cell (ECC) ozone sensors in the ozonesonde and that this interference was the cause of the reduced sensor signal output. The sharp drops in November 2011 were attributed to the SO2 plume from Popocatépetl volcano southeast of MCMA. Experiments on the response of the ECC sensor to representative atmospheric trace gases showed that only SO2 could cause the observed abrupt drops in sensor signal. The vertical profile of the plume reproduced by a Lagrangian particle diffusion simulation supported this finding. A nearground reduction in the sensor signal in March 2012 was attributed to an SO2 plume from the Tula industrial complex north-west of MCMA. Before and at the time of ozonesonde launch, intermittent high SO2 concentrations were recorded at ground-level monitoring stations north of MCMA. The difference between the O3 concentration measured by the ozonesonde and that recorded by a UV-based O3 monitor was consistent with the SO2 concentration recorded by a UV-based monitor on the ground. The vertical profiles of the plumes estimated by Lagrangian particle diffusion simulation agreed fairly well with the observed profile. Statistical analysis of the wind field in MCMA revealed that the effect Popocatépetl was most likely to have occurred from June to October, whereas the effect of the industries north of MCMA, including the Tula complex, was predicted to occur throughout the year.

Tropospheric ozone variability occurs because of multiple forcing factors including surface emission of ozone precursors, stratosphere-to-troposphere transport (STT), and meteorological conditions. Analyses of ozonesonde observations made in Huntsville, AL, during the peak ozone season (May to September) in 2013 indicate that ozone in the planetary boundary layer was significantly lower than the climatological average, especially in July and August when the Southeastern United States (SEUS) experienced unusually cool and wet weather. Because of a large influence of the lower stratosphere, however, upper tropospheric ozone was mostly higher than climatology, especially from May to July. Tropospheric ozone anomalies were strongly anticorrelated (or correlated) with water vapor (or temperature) anomalies with a correlation coefficient mostly about 0.6 throughout the entire troposphere. The regression slopes between ozone and temperature anomalies for surface up to midtroposphere are within 3.0-4.1 ppbv K−1. The occurrence rates of tropospheric ozone laminae due to STT are ≥50% in May and June and about 30% in July, August, and September suggesting that the stratospheric influence on free-tropospheric ozone could be significant during early summer. These STT laminae have a mean maximum ozone enhancement over the climatology of 52 ± 33% (35 ± 24 ppbv) with a mean minimum relative humidity of 2.3 ± 1.7%. [ABSTRACT FROM AUTHOR]

The global budget of tropospheric ozone is governed by a complicated ensemble of coupled chemical and dynamical processes. Simulation of tropospheric ozone has been a major focus of the GEOS-Chem chemical transport model (CTM) over the past 20 years, and many developments over the years have affected the model representation of the ozone budget. Here we conduct a comprehensive evaluation of the standard version of GEOS-Chem (v10-01) with ozone observations from ozonesondes, the OMI satellite instrument, and MOZAIC-IAGOS commercial aircraft for 2012–2013. Global validation of the OMI 700-400 hPa data with ozonesondes shows that OMI maintained persistent high quality and no significant drift over the 2006–2013 period. GEOS-Chem shows no significant seasonal or latitudinal bias relative to OMI and strong correlations in all seasons on the 2° × 2.5° horizontal scale ( r = 0.88–0.95), improving on previous model versions. The most pronounced model bias revealed by ozonesondes and MOZAIC-IAGOS is at high northern latitudes in winter-spring where the model is 10–20 ppbv too low. This appears to be due to insufficient stratosphere-troposphere exchange (STE). Model updates to lightning NO x , Asian anthropogenic emissions, bromine chemistry, isoprene chemistry, and meteorological fields over the past decade have overall led to gradual increase in the simulated global tropospheric ozone burden and more active ozone production and loss. From simulations with different versions of GEOS meteorological fields we find that tropospheric ozone in GEOS-Chem v10-01 has a global production rate of 4960–5530 Tg a −1 , lifetime of 20.9–24.2 days, burden of 345–357 Tg, and STE of 325–492 Tg a −1 . Change in the intensity of tropical deep convection between these different meteorological fields is a major factor driving differences in the ozone budget. [ABSTRACT FROM AUTHOR]

To further verify the ozone profile reliability of satellite remote sensing for the ozone valley over the Tibetan Plateau in the core area of the South Asian high in summer, we validate the ozone products from the microwave limb sounder (MLS) onboard the Aura satellite over the Tibetan plateau using electrochemical concentration cell (ECC) ozonesonde data of 2016 for Ngari, Tibet. The MLS version four ozone profiles have lower standard deviation in the middle stratosphere (38–10 hPa), whereas the ozonesonde profiles have lower standard deviation in the upper troposphere and lower stratosphere region (200–83 hPa). There are statistically significant differences between these two datasets in most of the stratosphere (10–83 hPa). The mean value of MLS ozone is about 0.8–1.5 mPa greater than that of ECC ozone, which corresponds to a relative deviation of 59 ± 24% at 83 hPa, 24 ± 13% at 68 hPa, 20 ± 5% at 56 hPa, 14 ± 4% at 46 hPa and 38 hPa, and 9 ± 4% in the layers between 32 and 10 hPa. However, there is no statistically significant difference between the two datasets in the upper troposphere (100–200 hPa). [ABSTRACT FROM AUTHOR]

An ozonesonde launched near electrically active convection in Houston, TX on 5 September 2013 during the NASA DISCOVER-AQ project measured a large enhancement of ozone throughout the troposphere. A separate ozonesonde was launched from Smith Point, TX (∼58 km southeast of the Houston site) at approximately the same time as the launch from Houston and did not measure that enhancement. Furthermore, ozone profiles for the descent of both sondes agreed well with the ascending Smith Point profile, suggesting a highly localized event in both space and time in which an anomalously large enhancement of 70–100 ppbv appeared in the ascending Houston ozonesonde data. Compared to literature values, such an enhancement appears to be the largest observed to date. Potential sources of the localized ozone enhancement such as entrainment of urban or biomass burning emissions, downward transport from the stratosphere, photochemical production from lightning NO x , and direct ozone production from corona discharges were investigated using model simulations. We conclude that the most likely explanation for the large ozone enhancement is direct ozone production by corona discharges. Integrating the enhancement seen in the Houston ozone profile and using the number of electrical discharges detected by the NLDN (or HLMA), we estimate a production of 2.48 × 10 28 molecules of ozone per flash which falls within the range of previously recorded values (9.89 × 10 26 –9.82 × 10 28 molecules of ozone per flash). Since there is currently no parameterization for the direct production of ozone from corona discharges we propose the implementation of an equation into a chemical transport model. Ultimately, additional work is needed to further understand the occurrence and impact of corona discharges on tropospheric chemistry on short and long timescales. [ABSTRACT FROM AUTHOR]

This study comprehensively analyzed nearly 20 years of ozonesonde data from Beijing on the North China Plain collected between 2001 and 2019 to determine the vertical structure of tropospheric ozone as well as its sources of precursors through a combination of radiosonde data, satellite products, and ERA5 reanalysis data. Quantitative analysis involved the self-organizing map (SOM) algorithm, calculating backward trajectories, and applying the Potential Source Contribution Function (PSCF) algorithm. The SOM clustering results (3 × 3 classes) showed that the ozone profiles over Beijing were primarily affected by seasonal variability, and the vertical ozone structure and its precursors exhibited obvious seasonal patterns (high VOCs in warm seasons and high NO x in cold seasons). The tropospheric ozone concentrations in Beijing, which were influenced by both ozone precursors and seasonal meteorological factors, were high in warm seasons and low in cold seasons. Further detailed investigations of SOM clustering results found that biomass burning, pollutant transport and stratospheric intrusion contributed significantly to tropospheric ozone over Beijing. Ozone in spring and summer was influenced by biomass burning activities south of Beijing, resulting in an increase in tropospheric ozone concentrations (at 0–10 km) of 60.6 ppbv. Biomass burning activities on the North China Plain have been adequately controlled since the implementation of the Clean Air Action Plan in 2013, which greatly reduced the frequency of ultrahigh tropospheric ozone concentrations in Beijing during the spring and summer. Stratospheric intrusions, which are closely associated with ozone transport from high latitudes (including the influence of Brewer-Dobson circulation subsidence), occur more frequently during spring and winter and increase ozone concentrations in the middle and upper troposphere in Beijing. • Ozonesonde data from Beijing were used to analyze the vertical ozone structure based on the SOM algorithm. • Ozone in spring and summer was influenced by biomass burning activities south of Beijing. • The small probability of stratospheric intrusion affected the tropospheric ozone structure. [ABSTRACT FROM AUTHOR]

This paper examines the influence of meteorological conditions in ozone mixing ratio measured at the Thai Meteorological Department (TMD) in Bangkok, Thailand. In addition to surface wind speed and direction, surface ozone concentrations, ozonesondes and CALIPSO Lidar images were collected during the study period extending from 01/01/2014 to 30/04/2015. Surface ozone concentrations show a strong seasonality, with maximum in the dry months of December to April and minimum during the wet southwest (SW) monsoon period extending from May to October. High ozone concentrations are related to biomass burning in the northeast highland regions of the country and neighboring Myanmar and southern China. These precursors travel in a southerly direction towards Bangkok in a well-defined aerosol layer which may be at ground level or at elevated heights. The growth of the daytime mixed layer scavenges some of the upper level aerosols, although local maxima in ozone concentrations at 1–2 km are a frequent feature at Bangkok. There is an evidence of fumigation in the Gulf of Thailand and a return flow via the southerly sea breezes. [ABSTRACT FROM AUTHOR]

This paper assesses the quality of ozone data of modified Brewer Mast (MBM) balloonborne ozonesonde and its comparison with MOZAIC Data (version 4), TES data (version 6), UARS MLS (version 5), EOS Aura MLS Data (version 5) and SBUV (version 8.1). The stations that have been used for this analysis: Delhi (28.58N, 77.20E) (Ozonesonde, MOZAIC, TES, UARS MLS, AURA MLS and SBUV), Pune (18.53N, 73.85E) Ozonesonde, UARS MLS, AURA MLS, SBUV and MOZAIC over Bombay presently called Mumbai (20.19N, 72.34E)), Madras presently called Chennai (13N, 80.18E) (MOZAIC and MLS) and Trivandrum (8.48N, 76.95E) (Ozonesonde, UARS MLS, AURA MLS and SBUV). Analysis shows that reasonable amounts of ozonesonde data are of good quality according to WMO criteria (1982) as 70-80 % of data over all the three stations are within the normalization factor of 1.3-0.8 ± (0.05-0.1) although some major changes in instrumentation e.g., new fast running nonreactive Teflon pump, modernized electronics, and smaller case since 1971. Several international intercomparisons carried out in 1970, 1982, 1991 and 1996 respectively, has also been confirmed the same, in spite of fundamental differences among the methods of these intercomparisons as well as ozonesonde types on procedure for sonde preparation, data processing and analysis. The intercomparison of Indian ozonesonde data (1995-1999) is made at troposphere with MOZAIC data (1995-2000) over Delhi and Pune/Mumbai, ozonesonde data (2013-2014) also compare with TES (2013-2014) special observation data over particular station Delhi at troposphere and at stratosphere with UARS MLS (1995-1999), EOS Aura MLS (2005-2014) and SBUV (2005-2013) data at Delhi, Pune and Trivandrum. Tropospheric value of Ozonesonde shows on average 10-20 % higher value than MOZAIC value over Delhi and Pune except in the lower height (<800 hPa) but ozonesonde show 10-15 % higher value than TES over Delhi in the lower height (<800 hPa). The percentage difference between ozonesonde data and UARS MLS data at Delhi, Pune and Trivandrum show variation of ±15 % and ozonesonde data, EOS Aura MLS and SBUV data at Delhi, Pune and Trivandrum show variation of ±45 % in the vertical range of 46 to 10 hPa, where, MLS data show highest accuracy. [ABSTRACT FROM AUTHOR]

Ozone concentrations have been increasing in the Guadalajara Metropolitan Area (GMA) in Mexico. To help devise efficient mitigation measures, we investigated the ozone formation regime by a chemical transport model (CTM) system WRF-CMAQ. The CTM system was validated by field measurement data of ground-level volatile organic compounds (VOC) and vertical profiles of ozone in GMA as well as in the Mexico City Metropolitan Area (MCMA). By conducting CTM simulations with modified emission rates of VOC and nitrogen oxides (NOx), the ozone formation regime in GMA was found to lie between VOC-sensitive and NOx-sensitive regimes. The result is consistent with the relatively large VOC/NOx emission ratio in GMA compared to that in MCMA where the ozone formation regime is in the VOC-sensitive regime. [ABSTRACT FROM AUTHOR]

In situ measurements of the vertical structure of ozone were made in Changchun (43.53°N, 125.13°E), China, by the Institute of Atmosphere Physics, in the summers of 2010-13. Analysis of the 89 validated ozone profiles shows the variation of ozone concentration in the upper troposphere and lower stratosphere (UTLS) caused by cut-off lows (COLs) over Changchun. During the COL events, an increase of the ozone concentration and a lower height of the tropopause are observed. Backward simulations with a trajectory model show that the ozone-rich airmass brought by the COL is from Siberia. A case study proves that stratosphere-troposphere exchange (STE) occurs in the COL. The ozone-rich air mass transported from the stratosphere to the troposphere first becomes unstable, then loses its high ozone concentration. This process usually happens during the decay stage of COLs. In order to understand the influence of COLs on the ozone in the UTLS, statistical analysis of the ozone profiles within COLs, and other profiles, are employed. The results indicate that the ozone concentrations of the in-COL profiles are significantly higher than those of the other profiles between ±4 km around the tropopause. The COLs induce an increase in UTLS column ozone by 32% on average. Meanwhile, the COLs depress the lapse-rate tropopause (LRT)/dynamical tropopause height by 1.4/1.7 km and cause the atmosphere above the tropopause to be less stable. The influence of COLs is durable because the increased ozone concentration lasts at least one day after the COL has passed over Changchun. Furthermore, the relative coefficient between LRT height and lower stratosphere (LS) column ozone is -0.62, which implies a positive correlation between COL strength and LS ozone concentration. [ABSTRACT FROM AUTHOR]

Using ozonesonde measurements from 2015 to 2018 at the Jang Bogo station located in the southeastern Antarctic region, we evaluate ozone profiles retrieved from the three satellite measurements that are widely used: Ozone Monitoring Instrument (OMI), Microwave Limb Sounder (MLS), and Ozone Mapping Profiler Suite (OMPS) data. For the fair validation, ozonesonde profiles are smoothed using the weighting function of each satellite retrieval algorithm (i.e., convolution process). Compared with limb-viewing MLS and OMPS ozone profiles, the OMI ozone profiles are relatively less qualified: coarser vertical resolution and larger inter-annual variation. Nevertheless, our validation reveals that the quality of all three satellite ozone profiles looks comparable; In general, difference from ozonesonde profile is ∼1 ppm absolutely, and −20 to 30% relatively at maximum. This quantitative range well corresponds to previous work, meaning that our new validation confirms the reliability of satellite ozone profiles in the southeastern Antarctic region where the measurement data for the validation were not enough. Another interesting feature is the role of a priori ozone profile; Nadir-viewing OMI satellite can have qualified ozone profiles by a proper assumption of a priori ozone profile. Since the performance of limb-viewing ozone profiles is better, however, the careful usage of nadir-viewing ozone profile is still required. We think that the simultaneous usage of multiple satellite ozone profiles can contribute to better understanding of Antarctic ozone characteristics. • Ozonesonde data in Jang Bogo site are newly used for the satellite ozone validation. • Limb-viewing MLS and OMPS ozone profiles show similar accuracy. • The quality of nadir-viewing OMI PROFOZ ozone profile is also reliable. • The importance of a priori data is confirmed for the ozone profile retrieval. [ABSTRACT FROM AUTHOR]

Tropopause folds are one of the key mechanisms of stratosphere-troposphere exchange (STE) in extratropical regions, transporting ozone-rich stratospheric air into the middle and lower troposphere. Although there have been many studies of tropopause folds that have occurred over Europe and North America, a very limited amount of work has been carried out over northeastern Asia. Ozonesondes produced by the Institute of Atmospheric Physics were launched in Changchun (43.9°N, 125.2°E), Northeast China, in June 2013, and observed an ozone-enriched layer with thickness of 3 km and an ozone peak of 180 ppbv at 6 km in the troposphere. The circulation field from the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim) dataset shows that this ozone peak was caused by a tropopause fold associated with a jet stream at the eastern flank of the East Asian trough. By analyzing the ozone data from the ozone monitoring instrument and Weather Research and Forecasting model with Chemistry (WRF-Chem) simulations, it was found that a high ozone concentration tongue originating from the lower stratosphere at high latitude (near central Siberia) intruded into the middle troposphere over Changchun between 5 and 8 km on 12 June 2013. The high-resolution WRF-Chem simulation was capable of describing events such as the tropopause fold that occurred on the cyclonic shear side of the jet stream. In addition, the TRAJ3D trajectory model was used to trace the origin of measured secondary ozone peaks in the middle troposphere back, for example, to stratospheric intrusion through the tropopause fold. [ABSTRACT FROM AUTHOR]

In the present study, vertical distribution of ozone observed using ozonesonde measurements obtained during 2010–2014 from a tropical station Gadanki (13.48°E, 79.18°E) is presented with special emphasis on the variability within the atmospheric boundary layer (ABL), in the free troposphere and in the lower stratosphere. In general, a clear semi-annual oscillation in ozone is seen within ABL and free-troposphere with peak during pre- and post-monsoon seasons. Whereas annual oscillation with peak during monsoon season dominates in the lower stratosphere. Interestingly, there is a delay of one month in the peak in the ozone mixing ratio (OMR) in the free troposphere when compared to the ABL during pre-monsoon and these features are quite different compared to other stations in the Indian region. One of the main objectives of the present study is to examine how well the present satellite (MLS and SABER) and re-analysis (ERA-Interim) ozone data sets represent the variability of ozone over the tropical station. In general, MLS and ERA-Interim overestimate the OMR by 97% and 80%, respectively, near the tropical tropopause, in comparison with ozonesonde data. Thus, caution is to be exercised in using these data sets for investigating the stratosphere–troposphere exchange (STE) process. However, all the three data sets show very good agreement with ozonesonde measurements above 22.5 km. ERA-Interim data set is found to agree well in the free troposphere but not in the ABL. Interestingly, the ozone tropopause is found to be more associated with the convective outflow level than cold point tropopause height. Back trajectory analysis reveals that the observed ozone in the troposphere over Gadanki region is mostly transported from the western region and from the higher altitudes. These features are discussed in the light of current understanding of the generation and transport of the ozone in the troposphere. [ABSTRACT FROM AUTHOR]

ECC (Electrochemical Concentration Cell) ozonesondes and UV DIAL (DIfferential Absorption Lidar) lidar measurements have been carried out simultaneously at OHP (Observatoire de Haute Provence, 44°N, 6.7°E, 690 m) since 1991 and provide: (i) a good basis for long-term trend assessment by two different instruments at the same location and (ii) a 20-year climatology of tropospheric ozone in the Western Mediterranean basin. Air mass trajectories and mixing layer height (MLH) have been calculated for all the ozone observations available at OHP. The bias between the seasonal mean calculated with lidar and ECC ozone vertical profiles for 4 time-periods of 5 years is ∼0.6 ppbv in the free troposphere (4–8 km). Larger differences (≥10 ppbv) are explained by the need for clear sky conditions during lidar observations. Measurements of both instruments have been combined to decrease the impact of short-term atmospheric variability on the trend estimate. In the upper and the mid-troposphere, the spring maximum decreases by 5 ppbv during the 20-year period. In the mid-troposphere, the summer average decreases by 3 ppbv during the 20-year period. At altitude above 4 km, the variability of ozone attributed to different source regions can be in the range of 5–10 ppbv over 20 years and is mitigated by transport variability. In the lower troposphere and after year 2000, there is a sharp 5 ppbv decrease of the spring and summer averages of the surface measurements and of the ECC observations made in the 0.6–4 km altitude range. There is also a decrease of the lowermost troposphere ozone vertical gradient which is explained by the variability of the MLH (mixing layer height). The trends of ozone annual mean show significant decrease of 3 and 4 ppbv/decade from 1998 to 2010 at the surface and at the 2–4 km altitude range, while no significant trends are observed at higher altitude. Positive North Atlantic Oscillation (NAO) periods correspond to positive yearly mean ozone anomalies in the upper troposphere in relation with stratosphere–troposphere exchange. A negative NAO period in 2010 corresponds to an increase of the yearly mean ozone anomaly in the lower troposphere due to a more suitable transport of ozone precursors from Northern America and Western Europe. [ABSTRACT FROM AUTHOR]

Ozonesonde measurements over Nairobi, Kenya are presented for the period 2000 - 2014. Ozone concentration is influenced by anthropogenic activities, calling for its continuous monitoring since it affects the climate system and human health. The study utilizes weekly ozonesonde flights, winds and RH from ERA-interim gridded data. The results indicate that the tropopause over Nairobi is approximately 1.3 km in depth. Ozone exhibits a negative trend upwards within the troposphere, up until the tropopause. There is a high increase in the lower stratosphere, peaking in the mid stratosphere. The maximum ozone value of 13.04 ppb is found at a pressure of 20 hpa and approximately, 80% of ozone is found in the stratosphere. The June-August season experiences the highest ozone levels in the low levels and December - February the highest concentration in upper levels as compared to the other seasons. Easterlies are predominant in the lower troposphere, up to about 500mb, westerlies in the mid troposphere and again, easterlies in upper troposphere, extending into the lower stratosphere, commonly known 'steering winds' in this region. [ABSTRACT FROM AUTHOR]

In the Mexico City Metropolitan Area (MCMA), ozone (O3) concentration is still higher than in other urban areas in developed countries. In order to reveal the current state of photochemical air pollution and to provide data for validation of chemical transport models, vertical profiles of meteorological parameters and ozone concentrations were measured by ozonesonde in two field campaigns: the first one, during the change of season from wet to dry-cold (November 2011) and the second during the drywarm season (March 2012). Unlike previous similar field campaigns, ozonesonde was launched twice daily. The observation data were used to analyze the production and distribution of ozone in the convective boundary layer. The observation days covered a wide range of meteorological conditions, and various profiles were obtained. The evolution of the mixing layer (ML) height was analyzed, revealing that ML evolution was faster during daytime in March 2012 than in November 2011. On a day in November 2011, the early-morning strong wind and the resulting vertical mixing was observed to have brought the high-ozone-concentration air-mass to the ground and caused relatively high surface ozone concentration in the morning. The amount of produced ozone in the MCMA was estimated by taking the difference between the two profiles on each day. In addition to the well-known positive correlation between daily maximum temperature and ozone production, effect of the ML height and wind stagnation was identified for a day in March 2012 when the maximum ground-level ozone concentration was observed during the two field campaigns. The relatively low ventilation coefficient in the morning and the relatively high value in the afternoon on this day implied efficient accumulation of the O3 precursors and rapid production of O3 in the ML. [ABSTRACT FROM AUTHOR]

Ozone laminae result from complex chemical and dynamical processes in the atmosphere. Unfortunately, there is no accepted definition of ozone laminae due to limited knowledge of the science determining their formation and evolution. Current methods to define ozone laminae are based on a reference profile that results from smoothing or linearly-regressing the original profile. These definitions are likely to underestimate the number of ozone laminae in the troposphere because the production of reference profiles reduces the information content in the original profiles. We present a method using the Continuous Wavelet Transform (CWT) with a two-threshold technique to define ozone laminae number and magnitude without requiring a reference profile. Our method encompasses an efficient and robust capability for identifying ozone laminae in the 15-year ozone profiles observed by ozonesondes in Huntsville, AL. [ABSTRACT FROM AUTHOR]

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Loss of stratospheric ozone has occurred over the last half a century due to catalytic destruction by halogen-containing anthropogenic compounds. Saturated ozone loss events, when the ozone concentrations decreased to less than or equal to 1 mPa (>95% ozone loss), were studied across nine stations in Antarctica using the Microwave Limb Sounder (MLS) instrument onboard the satellite Aura. The satellite observations were corrected using in situ ozonesonde observations to quantify the saturated ozone loss between 2004 and 2020. The analysis shows that at some stations the original MLS observations underestimated the number of loss events by 5–10%, however, at other stations they were overestimated by the same margin. A couple of stations showed a very good match between the original and corrected MLS data. Irrespective of the bias, the number of loss events decreased gradually from 2004 to 2013 suggesting a recovering trend. After 2013, no significant trend is visible, with large variation seen for especially between 2015 and 2019. The interannual variation was strongly coupled to the temperature, highlighting the key role that polar stratospheric clouds play in causing saturated ozone loss. • MLS profiles corrected using ozonesonde profiles show significant differences in the saturated ozone loss at some stations. • A decreasing trend in saturated loss events is observed between 2004 and 13. • The saturated loss events were strongly coupled to temperature, highlighting the role of polar stratospheric clouds. [ABSTRACT FROM AUTHOR]

It is widely accepted that the tropical tropopause is closely linked to climate change. Several campaigns have already been conducted and also are being planned to address various issues related to the tropical tropopause layer (TTL). Despite many campaigns, several scientific issues still remain unexplained including Indian summer monsoon dynamics, cirrus clouds and the trace gas distribution across the tropopause etc. In order to address some of the issues, particularly over the Indian region, intensive observational campaigns called ‘tropical tropopause dynamics (TTD)’ are being conducted since December 2010 at two stations namely Gadanki (13.5°N, 79.2°E) and Trivandrum (8.5°N, 76.9°E) under CAWSES India Phase-II programme. This overview article aims to bring out the current understanding on the tropical tropopause, issues addressed through the TTD campaigns and the details of the data collected in these campaigns using collocated instruments as well as complementary satellite data. So far 32 campaigns have been completed successfully and in this paper main focus is given for describing the systematic data collected using various techniques (MST radar, Mie lidar, Radiosonde, ozonesonde) simultaneously in each month. In general, over the study region affected by the monsoon, a prominent updraft prevails in the middle and upper troposphere regions covering TTL affecting the transport of minor species across the tropopause. The behavior of the cold point tropopause (CPT) at Gadanki and Trivandrum reveals that there are significant differences in the CPT characteristics even within the monsoon region. Cold point tropopause shows stronger sub-daily scale variation over Trivandrum than Gadanki though no indication of deep convection is present at the former location particularly in winter. [ABSTRACT FROM AUTHOR]

The North China Plain (NCP), to the east of the Loess Plateau, experiences severe regional air pollution. During the daytime in the summer, the Loess Plateau acts as an elevated heat source. The impacts of such a thermal effect on meteorological phenomena (e.g., waves, precipitation) in this region have been discussed. However, its impacts on the atmospheric boundary layer structure and air quality have not been reported. It is hypothesized that the thermal effect of the Plateau likely modulates the boundary layer structure and ambient concentrations of pollutants over the NCP under certain meteorological conditions. Thus, this study investigates such effect and its impacts using measurements and three-dimensional model simulations. It is found that in the presence of daytime westerly wind in the lower troposphere (~ 1 km above the NCP), warmer air above the Loess Plateau was transported over the NCP and imposed a thermal inversion above the mixed boundary layer, which acted as a lid and suppressed the mixed layer growth. As a result, pollutants accumulated in the shallow mixed layer and ozone was efficiently produced. The downward branch of the thermally-induced Mountain-Plains Solenoid circulation over the NCP contributed to enhancing the capping inversion and exacerbating air pollution. Previous studies have reported that low mixed layer, a factor for elevated pollution in the NCP, may be caused by aerosol scattering and absorption of solar radiation, frontal inversion, and large scale subsidence. The present study revealed a different mechanism (i.e., westerly warm advection) for the suppression of the mixed layer in summer NCP, which caused severe O 3 pollution. This study has important implications for understanding the essential meteorological factors for pollution episodes in this region and forecasting these severe events. [ABSTRACT FROM AUTHOR]

An abnormal decrease in ozonesonde sensor signal occurred during air-pollution study campaigns in November 2011 and March 2012 in Mexico City Metropolitan Area (MCMA). Sharp drops in sensor signal around 5 km above sea level and above were observed in November 2011, and a reduction of signal over a broad range of altitude was observed in the convective boundary layer in March 2012. Circumstantial evidence indicated that SO2 gas interfered with the electrochemical concentration cell (ECC) ozone sensors in the ozonesonde and that this interference was the cause of the reduced sensor signal output. The sharp drops in November 2011 were attributed to the SO2 plume from Popocatépetl volcano southeast of MCMA. Experiments on the response of the ECC sensor to representative atmospheric trace gases showed that only SO2 could cause the observed abrupt drops in sensor signal. The vertical profile of the plume reproduced by a Lagrangian particle diffusion simulation supported this finding. A near-ground reduction in the sensor signal in March 2012 was attributed to an SO2 plume from the Tula industrial complex north-west of MCMA. Before and at the time of ozonesonde launch, intermittent high SO2 concentrations were recorded at ground-level monitoring stations north of MCMA. The difference between the O3 concentration measured by the ozonesonde and that recorded by a UV-based O3 monitor was consistent with the SO2 concentration recorded by a UV-based monitor on the ground. The vertical profiles of the plumes estimated by Lagrangian particle diffusion simulation agreed fairly well with the observed profile. Statistical analysis of the wind field in MCMA revealed that the effect Popocatépetl was most likely to have occurred from June to October, whereas the effect of the industries north of MCMA, including the Tula complex, was predicted to occur throughout the year. [ABSTRACT FROM AUTHOR]

Ozonesondes are widely used to obtain ozone concentration profiles from the surface to the upper atmosphere. A kind of double-cell ozonesonde has been developed at the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (named the 'IAP ozonesonde') based on previous experience over the past 20 years of developing the singlecell GPSO3 ozonesonde. The IAP ozonesonde is of the Electrochemical Concentration Cell (ECC) type. A detailed description of the IAP ozonesonde is firstly provided in the present paper, followed by a presentation of results from a series of launches carried out to evaluate its performance. The analysis involved comparing its observations with measurements from the GPSO3 and ECC ozonesondes (Model type ENSCI-Z) as well as a Brewer spectrophotometer. The results showed that the IAP ozonesonde is a vast improvement over the GPSO3 ozonesonde, able to capture vertical ozone structures very well and in good agreement with ECC ozonesonde measurements. The average difference in the ozone partial pressure between the IAP and ECC ozonesondes was 0.3 mPa from the surface to 2.5 km, close to zero from 2.5 to 9 km and generally less than 1 mPa for layers higher than 9 km. The apparent deviation is likely caused by a decreasing pump flow rate in the IAP ozonesonde which needs further improvement. The total ozone amounts measured by the IAP ozonesonde profiles were highly comparable with the Brewer data with a relative difference of 6%. The development of the IAP ozonesonde and its strong performance will surely accelerate the process of conventional observations of ozone profiles over China in the near future as well as provide more data for ozone research in general. [ABSTRACT FROM AUTHOR]