Temperature inversions (TIs) strongly regulate the accumulation and dispersion of air pollutants, yet their nationwide impacts on surface PM2.5 remain poorly quantified. Here we integrate high-resolution Lband radiosonde profiles with PM2.5 monitoring data from 2016-2021 to characterize the frequency, strength, thickness, and diurnal variability of TIs-including surface-based inversions (SBIs) and elevated inversions (EIs)-across mainland China. We show that TIs are pervasive, occurring on average 52 % of days, with mean strength of 2.1 degrees C and thickness of 214 m, and are more common at 08:00 than 20:00 Beijing Time (BJT). Distinct regional patterns emerge: SBIs dominate in northern China, whereas EIs prevail in eastern China. Overall, SBIs are 1.3 degrees C stronger than EIs. TIs intensify seasonal pollution, with 76 % of PM2.5 episodes coinciding with inversion events. SBI strength correlates positively with PM2.5 concentrations nationwide, while EI parameters show negative associations in eastern and southern regions. These findings reveal the spatiotemporal dynamics of TIs, establish quantitative links to surface pollution, and highlight regionally divergent mechanisms, providing critical insight for air-quality forecasting and targeted emission control.

Daily minimum solar zenith angle(SZA)≥80° during or near the period of polar night(May—July)at Zhongshan station of China(69.37°S,76.38°E)in Antarctic restricts ground-based observations of Total Ozone Column(TOC)by using direct solar light(DS),which limits further accurate understanding of TOC seasonality.To address this issue,the following four types of TOC observations at SZA≥80°:(1)Brewer ozone spectrophotometer DS,TOCDS≥80°,(2)focused sun-disk(FS),TOCFS≥80°,(3)focused moon disk(FM),TOCFM and(4)SAOZ spectrometer zenith twilight observations,TOCSAOZ,made from 2009 to 2023 are analyzed with reference to the Brewer DS observations within SZA of[72°,80°),i.e.,TOCDS[72,80°).The results show that,on average,TOCDS≥80° are lower by(5.5±9.5(1σ))DU or(2.1±4.1(1σ))%when SZA is[81°,81.5°)while TOCFS≥80° are higher by(2.8±4.3(1σ))DU or(1.75±4.3(1σ))%when SZA is[80°,84.5°).The fixed stratospheric ozone layer height(～22 km)in the air mass factor(μ)calculation causes a decreased μ with increasing SZA,which is the main reason for the higher TOCFS≥80°.On average,TOCFM is lower by(6.8±16.7(1σ))DU or(2.2±6.8(1σ))%while TOCSAOZ is lower by(12.1±12.9(1σ))DU or(4.4%±5.0(1σ))%on average.The longer optical path of twilight in the atmosphere together with the nature latitudinal distribution of TOC are the reasons for lower TOCSAOZ values.The significant diurnal variation of TOC during the"ozone hole"(TOC≤220 DU)period increases the differences between those observed TOC at SZA≥80° and the reference.It is confirmed that the annual"ozone hole"at Zhongshan station starts in August.The average TOC is within 270-290 DU while the minimum value can be～230 DU,indicating that ozone depletion has already occurred during the polar night when the polar vortex has been well developed.The secondary seasonal lower TOC in autumn(March),which are due to the influence of the nature stratospheric Brewer-Dobson circulation,is similar to the lowest TOC in autumn in the middle and high latitudes of the northern Hemisphere.FS observations should be preferentially applied when the SZA≥80°,and FM should be implemented because it is the only effective way during the polar night for routine TOC observations.

Abstract Antarctic springtime ozone reduction is spatially heterogeneous, making it essential to examine local and vertical ozone changes. However, the long‐term total column ozone (TCO) and vertical ozone structure changes in the coastal region of East Antarctica remain uncertain. This study investigates the TCO variability and vertical distribution of ozone, based on Zhongshan TCO observations (1993–2024), Davis ozonesondes (2003–2024), complemented by satellite and reanalysis data sets (SBUV, ERA5, MERRA2, and MSR2) from 1980 to 2024. TCO at stations is significantly correlated with the Antarctic regional (60°–90°S) mean ( R > 0.8) and most effectively reflects the TCO variations over the coastal sector of East Antarctica (about 61°–79°S, 34°–120°E; R > 0.9) from August to December. A positive TCO trend from September to December has emerged since 2003 at Zhongshan, although persistent ozone deficits since 2020 have partially offset this upward tendency. At Davis, springtime ozone deficit occurs mainly within 165–30 hPa. In 2019, unusually high column ozone (187.81 Dobson units (DU)) in this layer coincided with sudden stratospheric warming. Since 2003, September column ozone (165–30 hPa) has exhibited an upward inclination of 0.80 ± 0.75 DU/yr, and September ozone mixing ratio deficit rates from 2018 to 2022 were lower than in 2013–2017. However, ozone deficit after mid‐September state has intensified (0.86 ± 0.62 DU/yr), highlighting the role of late‐spring reduction. This study offers a complete view of the long‐term temporal and vertical variations in ozone over the coastal region of East Antarctica and provides additional insights and evidence for springtime Antarctic ozone layer recovery.

The Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument, launched in April 2023, is North America's first geostationary air pollution monitoring satellite mission. Together with Asia's Geostationary Environment Monitoring Spectrometer (GEMS) launched in 2020 and Europe's upcoming Sentinel-4, TEMPO contributes to nearly global coverage provided by geostationary satellite constellation. TEMPO and GEMS offer hourly, high-resolution data of ozone surpassing the once-daily observations of instruments like the TROPOspheric Monitoring Instrument (TROPOMI) in temporal resolution. This study presents TEMPO's total ozone data, demonstrating TEMPO's ability to observe sudden changes in ozone and UV index. Furthermore, TEMPO and GEMS measurements are validated using ground-based monitoring networks (Brewer, Dobson, and Pandora). Results show good agreement but also highlight latitude-dependent discrepancies between the satellite and ground-based data sets (-2% to 2% for TEMPO, -1% to -3% for GEMS). Findings are further validated using TROPOMI data and reanalysis models.

The downward shortwave radiation(DSR)is an important part of the Earth's energy balance,driving Earth's system's energy,water,and carbon cycles.Due to the harsh Antarctic environment,the accuracy of DSR derived from satellite and reanalysis has not been systematically evaluated over the transect of Zhongshan station to Dome A,East Antarctica.Therefore,this study aims to evaluate DSR reanalysis products(ERA5-Land,ERA5,MERRA-2)and satellite products(CERES and ICDR)in this area.The results indicate that DSR exhibits obvious monthly and seasonal variations,with higher values in summer than in winter.The ERA5-Land(ICDR)DSR product demonstrated the highest(lowest)accuracy,as evidenced by a correlation coefficient of 0.988(0.918),a root-mean-square error of 23.919(69.383)W m-2,a mean bias of-1.667(-28.223)W m-2 and a mean absolute error of 13.37(58.99)W m-2.The RMSE values for the ERA5-Land reanalysis product at seven stations,namely Zhongshan,Panda 100,Panda 300,Panda 400,Taishan,Panda 1100,and Kunlun,were 30.938,29.447,34.507,29.110,20.339,17.267,and 14.700 W m-2,respectively;with corresponding bias values of 9.887,-12.159,-19.181,-15.519,-8.118,6.297,and 3.482 W m-2.Regarding seasonality,ERA5-Land,ERA5,and MERRA-2 reanalysis products demonstrate higher accuracies during spring and summer,while ICDR products are least accurate in autumn.Cloud cover,water vapor,total ozone,and severe weather are the main factors affecting DSR.The error of DSR products is greatest in coastal areas(particularly at the Zhongshan station)and decreases towards the inland areas of Antarctica.

Chen et al. (2022) analyzed the event of rapid nocturnal O-3 enhancement (NOE) observed on 31 July 2021 at the surface level in the North China Plain and proposed transport of substantial stratosphere ozone to the surface by Typhoon In-fa followed by downdraft of shallow convection as the mechanism of the NOE event. The analysis seems to be valid from the viewpoint of atmospheric physics. This comment revisits the NOE phenomenon on the basis of the China National Environmental Monitoring Center (CNEMC) network data used in Chen et al. (2022), together with the CNEMC data from Zibo (ZB) and O-3 , NOx , PAN (peroxyacetic nitric anhydride), and VOC (volatile organic compound) data from the Zibo supersite operated by the China Research Academy of Environmental Sciences (CRAES). We found (a) O-x (O-3 + NO2 ) levels during the NOE period approaching those of O-3 during 14:00-17:00 LT, (b) levels of PAN and the relationship between O-3 and PAN consistent with dominance of chemical and physical processes within the boundary layer, and (c) estimated photochemical ages of air mass shorter than 1 d and showing no drastic increases during the NOE. We argue that the NOE was not caused by typhoon-induced stratospheric intrusion but originated from fresh photochemical production in the lower troposphere. Our argument is well supported by the analysis of atmospheric transport as well as ground-based remote sensing data.

In order to investigate the variations of cloud and aerosol vertical profiles over the Tibetan Plateau (TP) in winter, we performed ground-based lidar observations in Lhasa, a city on the TP, from November 2021 to January 2022. The profiles of extinction coefficient, depolarization ratio, and signal-to-noise ratio (SNR) were retrieved using the atmospheric echo signals collected by the lidar. Clouds were identified by the range-correction echo signals and classified into water clouds, mixed clouds, horizontally oriented ice crystal clouds (HOICC), and ice clouds by the depolarization ratio and the hourly temperature from the European Centre for Medium-Range Weather Forecasts Reanalysis version 5 (ERA5). The clouds mainly appeared at a height of 3~5 km from 14:00–22:00 Beijing Time throughout the field campaign. The height and frequency (~30%) for cloud appearance were significantly lower than that reported in previous studies in summer. The cloud categories were dominated by mixed clouds and ice clouds during the observation period. The proportions of ice clouds gradually increased with increasing heights. After eliminating profiles influenced by clouds, the aerosol extinction coefficient and depolarization ratio were obtained, and the atmospheric boundary layer height (ABLH) was calculated. The aerosol extinction coefficient decreased with increasing height in the ABLH, and there were no obvious changes for the aerosol extinction coefficient above the ABL. The aerosol extinction coefficients near the Earth’s surface presented two peaks, appearing in the morning and evening, respectively. The high aerosols at the surface in the morning continually spread upward for 4–5 h and finally reached an altitude of 1 km with the development of ABLH. In addition, the depolarization ratio of aerosols decreased slowly with increasing altitudes. There was no obvious diurnal variation for depolarization ratios, indicating partly that the source of aerosols did not change significantly. These results are beneficial in understanding the evolution of cloud and aerosol vertical profiles over the TP.

The New Baseline Surface Radiation (NBSR) system was established at the Shangdianzi (SDZ) regional Global Atmosphere Watch (GAW) station in 2013 to observe nine broadband radiation components, i.e. the global, direct, diffuse, and upwelling shortwave irradiance (GSWI, DSWI, DifSWI, and UpSWI); the photosynthetically active radiation (PAR); the ultraviolet irradiance (UVAI and UVBI); and the down- and upwelling longwave irradiance (DnLWI and UpLWI). To test the 1 min raw radiometric data, a Hybrid Algorithm for Radiation Data Quality Control (HARDQC) is presented in this study based on well-established methods, together with the solar irradiance dataset and the spectral features of the instrument bands. Subsequently, a NBSR dataset, which consists of radiation data at multiple timescales (i.e. 1 min, hourly, daily, monthly, monthly average hourly, and monthly average daily) over 2013–2022, is established and evaluated. Results show that more than 98.7 % of all radiation components passed the physical possibility test. The percentages of those that passed the extremely rare test are greater than 98.6 % for all radiation components except for the DnLWI (97.1 %). The percentages of those that passed the comparison test are greater than 83.3 % (GSWI), 78.3 % (DSWI), 81.7 % (DifSWI), 93.1 % (UpSWI), 88.9 % (PAR), 95.6 % (UVAI), 96.3 % (UVBI), 99.8 % (DnLWI), and 99.7 % (UpLWI), respectively. Due to data logger faults, removal of the instruments for calibration, and lightning strikes, some apparent data gaps in the upwelling radiation components (January 2015–August 2017) and all radiation components (December 2018; July to September 2021) were detected. Despite the existence of a few imperfections in the NBSR dataset, it is still reliable to apply it in many fields such as the validation of satellite products and numerical models, the investigation of relationships between radiation and atmospheric composition, and the detection of changes in the surface fluxes. The dataset described in this paper is available at https://doi.org/10.1594/PANGAEA.963330 (Quan et al., 2023b).

Balloon sounding with the Compact Optical Backscatter Aerosol Detector (COBALD) and Frost Point hygrometers (FPs) provides in situ data for a better understanding of the vertical distribution of cirrus clouds. In this study, eight summer balloon-borne measurements in Kunming (2012, 2014, 2015, and 2017) and Lhasa (2013, 2016, 2018, and 2020) over the Tibetan Plateau were used to show the distribution characteristics of cirrus clouds. Differences of cirrus occurrence were compared by different indices: the backscatter ratio (BSR) at a 455 nm/940 nm wavelength (BSR455 > 1.2/BSR940 > 2), the color index (CI > 7), and the relative humidity with respect to ice (RHice > 70%). Analysis of the profiles indicated that BSR455 > 1.2 was the optimal criterion to identify the cirrus layer and depict the distribution of the CI and RHice within cirrus clouds. The results showed that the median CI (RHice) within the cirrus clouds at both sites was mostly in the 18-20 (90%-110%) range at pressures below 120 hPa. Furthermore, the balloon-borne measurements combined with Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) measurements indicated a high frequency of cirrus occurrence near the tropopause in Kunming and Lhasa. The top height of cirrus occurrence at both sites was above the cold point tropopause and the lapse rate tropopause. Both Kunming and Lhasa had the highest frequency of thin cirrus clouds in the 0-0.4 km vertical cirrus thickness range.

Solar irradiance is one important element in conventional meteorology observations. Long-term observations of solar radiation by using China-made wide-band pyranometers have been carried out. However, the performance of instrumental systems is not sufficiently evaluated or analyzed, especially in the polar regions where harsh condition and large seasonal variations of solar elevation causes dramatic variation of surface solar irradiance. To fill this gap, the performance of the domestically made solar radiation observation systems at Zhongshan Station, Antarctica in 2017, including global solar radiation (GSR), direct solar radiation (DIR) and the diffuse solar radiation (DIF) measurements, is evaluated. The averaged nighttime thermal offsets of two domestic FS-6A pyranometers, respectively for GSR and DIF observations, are both less than 3 W·m-2, and their temporal variations are highly consistent. Compared with CM21 or CM22 pyranometer that reach the requirement of the second-class standard and are globally deployed, the additional heating effect of the auxiliary ventilation heater of FS-6A pyranometers significantly reduces the inherently physically-based correlation coefficient between the night thermal offset and the net longwave radiation, and the absolute values of FS-6A thermal offset significantly increase but are within 5 W·m-2 under higher wind speeds (noless than 15 m·s-1). The temporal variations of solar DIF irradiances from two FS-6A pyranometers are highly consistent under cloudy overcast condition, and their solar irradiance values are systemically lower (about -6 W·m-2 or -1%) than that of CM22 as the solar DIF irradiance is about 500 W·m-2 from CM22. However, the absolute (relative) difference is respectively lower than 2.6 W·m-2 (4.0%) as the solar zenith angle (θ) is less than 86 °. The GSR close examination suggests that the ratio of FS-6A GSR absolute difference from the sum of horizontally projected DIR and DIF meeting the requirement of threshold value (less than 2% or 15 W·m-2 with θ≤ 80°) proposed by the baseline surface radiation network (BSRN) is more than 80%. But only 44% samples meet the requirement of the BSRN threshold value (less than 3.5% or 20 W·m-2 with θ> 80°) when the four-quadrant tracking solar disk model is applied in operation. Under cloud-free condition, the measurements of GSR, DIR and DIF from the domestically made instruments are well comparable with the simulations from the parameterized solar radiation model that has been extensively applied in middle-low latitudes, and the correlation coefficients between the simulations and observations are more than 0.95. However, the observations are significantly higher than the simulations as the solar irradiance increases. The results suggest that China domestically made solar radiation observation system is fully qualified for the routine observation in polar regions.

Atmospheric chemistry observations aboard ships are vulnerable to pollution from ship fuel engine exhaust. This local pollution leads to an increase in CO and CO2 and a decrease in O3. Concentrations (presented as volume mixing ratio) of CO, CO2, O3, CH4 and N2O in the marine-atmosphere boundary layer were measured along the R/V Beijing cruise track between Bohai bay of China and the Ross sea in Antarctica from January to April 2020. Based on the successive observations of 1-minute resolution, the impact of local pollution emission on the measured values of each trace gas is respectively implied by the persistence of variations in CO, CO2, and combinations of CO and CO2 and CO/CO2 for the study of data quality assessment quality controls (QA/QC). The results show that CO, CO2 and O3 (CH4 and N2O) data are significantly (slightly) contaminated by local pollution. A method of trace concentration difference threshold between any immediate adjacent 1-minute is proposed, which can effectively remove the outliers of CH4 and N2O from their time series datasets. The pollution period implied by CO or CO2 can be partially but not completely used to identify the contaminations in the dataset. The ratio of CO/CO2 implication can effectively determine the global baseline concentration of CO or CO2 in highly polluted region but at a high cost of discarding many data samples. The combination of CO2 and CO (CO2+CO) is the best method to imply the pollution signals in the CO2, CO and O3 datasets, and this method decreases the measured values of CO and CO2 concentrations in the southern hemisphere and the pristine region by (5—11)×10−9 (10%—18%) and (3—7)×10−6 (1%—2%) respectively, while O3 increases by (3—5)×10−9 (20%—25%). The final version concentration data of each trace gas are reasonably comparable with those measured at continental sites and the concentration differences of CO, CO2, CH4 and N2O in the southern ocean and Antarctica are respectively within 2×10−9, 0.7 ×10−6, 1.4×10−9 and 0.5×10−9. All the atmospheric trace gases data after QA/QC display their concentration features of high (low) in the northern (southern) hemisphere, and they remain stable in the south of the Southern Ocean and Antarctica region. Additionally, regional distribution characteristics of atmospheric O3 and their mechanisms are quite reasonable. All these reflect the rationality and accuracy of the proposed QA/QC method applied for the trace gases observations along the R/V Beijing cruise.

The extensive global climate observing system (GCOS) reference upper-air network (GRUAN) datasets provide a chance to validate newly released Atmospheric Infrared Sounder (AIRS) version 7 (v7) products over the Arctic. This manuscript reports on the analysis performed to evaluate errors from AIRS version 6 (v6) and v7 temperature profiles and to characterize the derived low-level temperature inversion (LLI) representativeness in the Arctic region. The AIRS averaging kernel, representing the AIRS measurement sensitivity, is applied to reduce the vertical resolution of the radiosonde profiles for comparison. Due to improved retrieval algorithms, v7 produces smaller biases in the troposphere and suppresses the cold bias in v6. Nevertheless, the profile-averaged root mean square error (RMSE) increased by over 30% in v7, particularly in the winter half-year when v7 showed a larger RMSE below 800 hPa. The AIRS temperature retrieval accuracy is primarily sensitive to surface type and cloud fraction. Compared to v6, v7 has less bias over frozen land and sea ice in different cloud fraction conditions. However, the RMSEs of v7 are more sensitive to the effective cloud fraction (ECF) and are highly influenced by a more significant contribution from nonfrozen land samples. Compared to the kernel-averaged radiosonde profiles, more than 80% of the temperature profiles from v6 and v7 accurately detect LLIs. The discreteness of the AIRS's predefined pressure level results is consistent with the radiosondes only 65% of the time for LLI depth calculation. In contrast, the AIRS can obtain LLI intensity with a relatively high correlation (>0.9). With the AIRS temperature retrieval in the boundary layer further improved, it has the potential to be used as an independent LLI detector in the Arctic region.

Tropospheric ozone（O 3 ） has significance impacts on climate and air quality, yet observations of tropospheric O 3 is not sufficient. In this study, vertical distribution of tropospheric O 3 and associated meteorological processes from 25 July to 22 August2016 observed at Golmud, a city in Qinghai province on the Northern Tibet Plateau（TP）, is analyzed to better understand characteristics and mechanism of tropospheric O 3 variation there. One general feature is that high（low） O 3 corresponds to low（high）water vapor and high（low） potential vorticity. Except for the tropospheric O 3 enhancement associated with the passage of a lowpressure trough from 25 to 27 July, the impacts of a warm blocking high and the passage of a deep convective system from the main area of the TP on the distribution of tropospheric O 3 are observed and analyzed for the first time in the northern TP. After the formation of the blocking warm high to the northeast of the observation site, easterly winds maintain in the whole troposphere at Golmud from 31 July to 8 August, but tropospheric O 3 concentration is only slightly higher on 2 August due to the anticyclonic shear along the northeast-southwest direction. Pollutant air mass with high O 3 and specific humidity（q） below 6 km appear due to westward transport. The lowest O 3 and the highest q during the entire observational period appear at the height above 10 km from 12 to 14August with the passage of a deep convective system. Compared with the historical tropospheric O 3 observed in Xining（from July to early August 1996）, O 3 at Golmud exhibits a seasonal low value in August, which is consistent with seasonal influences of the summer monsoon. Compared with historical tropospheric O 3 observations in Linzhi（July 2014）, Naqu（from late July to mid-August2011） and Lhasa（August 1998）, it is found that latitude has certain influences on O 3 over the Tibetan Plateau.

利用Aura卫星臭氧观测仪(OMI)反演的臭氧总量产品OMTO3对FY-3B TOU(2010年11月至2016年12月)臭氧总量产品进行验证分析,通过TOU与OMTO3相对偏差(RD)在空间和时间尺度上的比较分析来评估FY-3B TOU臭氧总量产品的可靠性.研究结果显示,赤道、南北半球中纬度的大部分区域RD值分布在2％～4％之间,南北极区扩到4％～6％和8％～10％之间,在赤道到两极地不同空间上RD随纬度升高而增大.除南北极地区域外,TOU臭氧总量RD月平均值均呈现出明显的季节变化特征,尤其南半球中纬度区域的季节变化较其他区域更为显著.南半球中纬度区域,波谷值出现在6月～7月,北半球中纬度区域,波谷出现在2月～3月.赤道区域RD值随卫星臭氧总量的变化波动较小,基本稳定在0.1％～2.9％之间,其余区域仅卫星臭氧总量在230 DU～500 DU之间时,RD才出现相对稳定的波动,TOU臭氧总量RD在不同区域对卫星臭氧总量值均出现了一定程度的依赖性.南北极区域SZA在45°～65°期间,TOU臭氧总量RD随SZA增大均呈正的下降趋势,在70°～80°之间均呈现小幅度回升趋势.从总体上来看,TOU臭氧总量RD值受SZA变化情况并不明显.

Environment context One-year-long on-line measurements of surface O3 and CO mixing ratios were performed on the southeast Tibetan Plateau to examine O3 behaviour. During the daytime, the O3 mixing ratio was strongly affected by vertical air exchange. The O3 mixing ratio was high in the afternoon and decreased at night, indicating a sink of tropospheric O3. The upper limit of the tropospheric O3 sink averaged from 4.5 to 5.5 ppb h−1. Rationale Ozone (O3) behaviour over the Tibetan Plateau has attracted attention in recent decades. However, few long-term measurements have been performed in the region. Methodology Field observations were conducted at a mountain site on the southeastern Tibetan Plateau from June 2014 to July 2015 in order to understand the behaviour of surface O3 and its influencing factors. Backward trajectory cluster analysis was applied to understand long-range transport sources and their relative contributions. Results The monthly average O3 ranged from 22.1 to 48.6 ppb with a common high spring ozone concentration phenomenon. The O3 diurnal variation exhibited a similar pattern to those in polluted areas but the cause was different. The O3 mixing ratio was significantly positively correlated with mixed-layer depth and wind speed, and negatively with temperature and relative humidity, indicating strong vertical air exchange. Approximately 50% of air mass trajectories originated from the northeastern Bengal Bay region, with fairly low O3 (CO) mixing ratios and high humidity. Others originated from the north Indian subcontinent (28%) and the Middle East (18%), with fairly high O3 (and CO) and low humidity. Discussion The average relative contributions of different air masses to surface O3 and CO were small and scattered but large for trajectories arriving at 14:00 hours when vertical air exchange was close to its strongest for the day. The tropospheric O3 sink may be common in the highlands, indicating a negative greenhouse effect there. The O3 sink at Linzhi was estimated in the range of 4.5–5.5 ppb h−1 at maximum.

The structure of the tropopause transition layer (TTL) over the Asian summer monsoon (ASM) anticyclone is analyzed on the basis of high-resolution balloon soundings of temperature, ozone, and water vapor at Kunming and Lhasa, China during 11 summer monsoon seasons between 2009 and 2019. Two definitions of the TTL, one based on thermal structure and one based on tracer correlations of O-3 and H2O, are analyzed and compared. In tracer-tracer space, the air masses of mixed stratospheric and tropospheric characteristics are identified by using the O-3-H2O correlations. The mixed air formed a transitional layer and shows strong spatial variation in the altitude space. Statistical result shows that the altitude range of mixed air parcels spans a layer of approximately 5.5 km depth between 12.5 km and 18 km. This vertical distribution of mixed air masses is basically consistent with thermally-based TTL, which takes the level of minimum stability (LMS) and the cold point tropopause (CPT) as its lower and upper boundaries, respectively. The thermal definition regards the TTL as a thermal transition zone, but does not consider the tracer mixing features between stratosphere and troposphere. Tracer correlations can identify the mixed air mass, which is a basic feature of the TTL, whether they are from the range of the thermally-based TTL, or below the LMS, or above the CPT. Based on the O-3-H2O correlations, statistical analysis shows that about 30%-50% of air masses between the LMS and the CPT are identified as mixed. And up to 1-1.5 km above the CPT, a small fraction of air masses is also mixed, which means that air masses from the troposphere can be lifted and mixed into the free stratosphere within the ASM anticyclone region.

With the support of the Chinese National Antarctic Research Expedition, near-surface ozone (O 3 ) was continuously monitored at Zhongshan Station (ZOS) (69°22′12″ S, 76°21′49″ E, 18.5 m above sea level) in East Antarctica from 2008 to 2020. The seasonal and diurnal variability of near-surface O 3 at ZOS were investigated. O 3 enhancement events (OEEs) were frequently observed in the warm season (OEEs in January accounted for 23.0% of all OEEs). The OEEs at ZOS were related to the photochemical reaction processes under the influences of O 3 and solar radiation in the stratosphere and synoptic-scale air mass transport from coastal areas (Princess Elizabeth Land, Wilkes Land, and Queen Mary Land), as evidenced by the recorded wind speed, solar shortwave irradiance, and total column ozone data and the computed potential source contribution function and concentration-weighted trajectory models. The results computed by the tool Stratosphere-to-Troposphere Exchange Flux indicated that stratosphere-to-troposphere transport had no direct impact on OEEs at ZOS. Therefore, synoptic-scale air mass transport is the main cause of OEEs in Antarctica, which is consistent with previous studies. Unlike OEEs at inland Antarctic stations, which are mainly affected by air mass transport from inland plateaus, OEEs at ZOS, a coastal station, are mainly affected by air mass transport from coastal land in East Antarctica.

Ozonesonde launches were routinely performed in Beijing from March 2001 to February 2019 to generate a unique long-term (18 years) vertical ozone profile dataset over mainland China. This study elucidates the vertical ozone structure on various temporal scales during this 18 years period by using the entire ozonesonde data product for the first time. Moreover, the long-term variability in the integrated ozone column over the North China Plain (NCP) is also explored by comparing the retrievals from ozonesonde at the Beijing urban site and a Dobson ozone spectrometer at the Xianghe suburban site. Our results indicate that vertical ozone exhibited clear monthly variability characterized by high values of tropospheric ozone during warm seasons and high values of stratospheric ozone during cold seasons. Stratospheric intrusions frequently occurred during spring and effectively transported cold air masses with high ozone from the lower stratosphere downward into the upper troposphere. Evident interannual variability in the lower troposphere and in ozone-rich areas of the stratosphere was revealed by vertical ozone distributions. The integrated total ozone columns retrieved from ozonesonde and Dobson bear close resemblance and exhibit strong sinusoidal monthly variations. In the troposphere and boundary layer, the integrated ozone column presented a significant positive trend during 2001–2012 in Beijing; a sudden decline occurred between 2011 and 2013, which was followed by a slow and insignificant increase after the implementation of the Clean Air Action plan in 2013 on the NCP.

Ozone has become the main factor affecting the air quality in the Pearl River Delta (PRD) in recent years, and a clear understanding source of ozone pollution is a top priority of ozone pollution prevention. However, the vertical distribution of tropospheric ozone in the PRD is poorly understood. Based on daily ozone sounding data obtained in Yangjiang (111 degrees 58'00" E, 21 degrees 50'00" N), Guangdong Province, from 18 November tjo 3 December 2013, the detailed vertical distribution of tropospheric ozone and its influencing factors were determined and compared with weekly autumn ozone profile data obtained at the Hong Kong Observatory (HKO) and corresponding tropospheric column ozone (TCO) datasets from the Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS) onboard satellite AURA. Overall, the ozone profiles observed in Yangjiang are in accordance with those of the HKO in terms of the vertical structure. The TCO values are 34.2 +/- 4.0 and 34.3 +/- 4.6 DU, respectively, that are, 4 DU higher than those of satellite data, indicating that the OMI/MLS product underestimates the TCO level in the PRD region. The Yangjiang ozone profiles indicate significant variations in the ozone vertical structure and concentration during the observation period. A significant ozone peak perma-nently occurs in the lower troposphere at 1.1 +/- 0.4 km above the surface. The vertical structural difference is mainly reflected in the presence of low-ozone strata (below 30 ppbv) near the tropopause and ozone peaks and high-ozone stratification in the middle and upper troposphere. The average ozone peak concentrations in the lower, middle, and upper troposphere are 65.2 +/- 9.9, 57.8 +/- 7.7, 71.8 +/- 13.6 ppbv, respectively, which are higher than the average ground-level ozone concentration (48.9 +/- 9.4 ppbv). Based on the analysis of the profile characteristics, weather conditions, and backward trajectories, it can be concluded that the low-ozone strata near the tropopause are caused by the regional transport of the tropical air mass from the tropical cyclone storm area. Potential vorticity analysis shows that stratosphere-troposphere exchange (STE) is the main reason for the for-mation of the ozone peaks and its varying concentrations in the middle and upper troposphere. Photochemical reactions represent the main factor affecting the ozone concentration in the lower troposphere. The STE is also an important factor affecting the ozone in the lower troposphere, leading to an increase in the surface ozone concentration by-10.9 ppbv.

Ground-based zenith scattered light differential optical absorption spectroscopy (DOAS) measurements were performed in summer and autumn (27 May–30 November) 2020 at Golmud (94°54′ E, 36°25′ N; 2807.6 m altitude) to investigate the abundances and temporal variations of ozone (O3) and its depleting substances over the northern Tibetan Plateau (TP). The differential slant column densities (dSCDs) of O3, nitrogen dioxide (NO2), bromine monoxide (BrO), and chlorine dioxide (OClO) were simultaneously retrieved from scattered solar spectra in the zenith direction during the twilight period. The O3 vertical column densities (VCDs) were derived by applying the Langley plot method, for which we investigated the sensitivities to the chosen wavelength, the a-priori O3 profile and the aerosol extinction profile used in O3 air mass factor (AMF) simulation as well as the selected solar zenith angle (SZA) range. The mean O3 VCDs from June to November 2020 are 7.21 × 1018 molec·cm−2 and 7.18 × 1018 molec·cm−2 at sunrise and sunset, respectively. The derived monthly variations of the O3 VCDs, ranging from a minimum of 6.9 × 1018 molec·cm−2 in October to 7.5 × 1018 molec·cm−2 in November, well matched the OMI satellite product, with a correlation coefficient R = 0.98. The NO2 VCDs at SZA = 90°, calculated by a modified Langley plot method, were systematically larger at sunset than at sunrise as expected with a pm/am ratio of ~1.56. The maximum of the monthly NO2 VCDs, averaged between sunrise and sunset, was 3.40 × 1015 molec·cm−2 in July. The overall trends of the NO2 VCDs were gradually decreasing with the time and similarly observed by the ground-based zenith DOAS and OMI. The average level of the BrO dSCD90°–80° (i.e., dSCD between 90° and 80° SZA) was 2.06 × 1014 molec·cm−2 during the period of June–November 2020. The monthly BrO dSCD90°–80° presented peaks in August and July for sunrise and sunset, respectively, and slowly increased after October. During the whole campaign period, the OClO abundance was lower than the detection limit of the instrument. This was to be expected because during that season the stratospheric temperatures were above the formation temperature of polar stratospheric clouds. Nevertheless, this finding is still of importance, because it indicates that the OClO analysis works well and is ready to be used during periods when enhanced OClO abundances can be expected. As a whole, ground-based zenith DOAS observations can serve as an effective way to measure the columns of O3 and its depleting substances over the TP. The aforementioned results are helpful in investigating stratospheric O3 chemistry over the third pole of the world.