Stratospheric Intrusions Research Articles

Deep stratospheric intrusion events in China revealed on the ground by cosmogenic 10Be/7Be

Given the impact of deep stratospheric intrusion on air quality, the development of more extensive trace substances to quantify stratospheric intrusion intensity can better distinguish between stratospheric ozone pollution and anthropogenic factors. The ratio of cosmogenic beryllium-10 to beryllium-7 (10Be/7Be), primarily generated in the stratosphere, has the potential to identify stratospheric air masses on the ground. Here we constructed a 10Be/7Be time-series (July 2020 to September 2021) in rainwater and aerosols from Xi’an, China. Combining in-situ pollutants, reanalysis data, and model calculations support a stratospheric origin for increased 10Be/7Be and identify it as a means of quantifying intrusion intensity. It was found that anticyclones formed by the Asian summer monsoon drive a sudden increase in deep stratospheric intrusion in spring, exacerbating ozone pollution beyond China’s air quality standards. Based on the sufficiently sensitive 10Be/7Be, it further indicates the process of six weak upper atmosphere intrusions in Xi’an during winter.

Unravelling the impacts of stratospheric intrusions on near-surface ozone during the springtime ozone pollution episodes in Lhasa, China

While air pollution due to fine particulate matter (PM2.5) has been effectively controlled in China; the photochemical pollution characterized by elevated ozone (O3) has emerged as a major concern for air quality improvement. Except for ozone, Lhasa is one of the cleanest cities in China with the lowest annual PM2.5 concentration in 2017. The levels of major air pollutants in Lhasa are much lower than those of other cities in the eastern region of China, especially in May, when the O3 concentration peaks. This study was based on multi-source observations in combination with the Goddard Earth Observing System coupled with chemistry (GEOS-Chem) and the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) models to explore the causes of O3 pollution in Lhasa in May 2023. The results indicated that during the high O3 episodes, the concentrations of O3 precursors were low in Lhasa. Surrounding cities in other parts of the Tibetan Plateau also experienced high ozone concentrations despite being in different airsheds, suggesting that the O3 pollution in Lhasa was caused by regional transport rather than purely local emissions. Stratospheric intrusion events modulated by the westerly jet led to elevated ozone in the troposphere above Lhasa City. The results of the GEOS-Chem model indicated that horizontal advection, turbulence, diffusion, and other effects led to high concentrations of ozone in the near-surface above Lhasa. Vertical transport was the dominant factor leading to ozone concentration increases during high ozone days, with a contribution of 6.33 Gg/day. In addition, high-altitude air masses with a maximum altitude of over 8000 m, were observed arriving in Lhasa during the high ozone days. This study revealed that stratospheric intrusions have a greater contribution to the high O3 concentration in Lhasa in spring and provided a scientific basis for mitigating O3 pollution in the plateau cities.

Influence of deep stratosphere-to-troposphere transport on atmospheric carbon dioxide and methane at the Mt. Cimone WMO/GAW global station (2165 m a.s.l., Italy): A multi-year (2015–2022) investigation

This work was dedicated to provide a first systematic assessment of the role played by deep stratosphere-to-troposphere transport events (denoted as “stratospheric intrusions–SIs”) in influencing the CO2 and CH4 variability at a high-elevation site in Italy (Monte Cimone, CMN, 2165 m a.s.l.). To select SI events, we used a methodology based on the analysis of the temporal variability of in-situ observed stratospheric tracers (O3, CO and RH), modeled PV along 5-day back-trajectories starting at the measurement site, and satellite measurements of total column ozone (TCO). The analysis of two case studies revealed clear fingerprints of the stratospheric air masses on the atmospheric composition at CMN: CO2 and CH4 decreases were observed in winter, while CO2CH4 increased (decreased) in summer. For 2015–2022, 9.8% of the hourly data were selected as influenced by SI events. The systematic analysis of the SI effect on CO2 confirmed differences between vegetative and non-vegetative seasons: during May–September, air masses from the stratosphere were richer in CO2 (+1.2 ± 0.1 ppm, average ±σ/N). On the other hand, during winter, air masses from higher altitudes were characterised by a lower amount of CO2 (−1.5 ± 0.1 ppm). Furthermore, SI events were usually characterised by an overall decrease in CH4 compared to the remaining data throughout the solar year (with average values ranging from −7.6 to −16.1 ppb as a function of the seasonal period). However, based on the analysis of seasonal anomalies of residuals, it looks that SI events did not play a major role in determining the interannual variability of CO2 and CH4 at CMN.

AIRS and MODIS Satellite-Based Assessment of Air Pollution in Southwestern China: Impact of Stratospheric Intrusions and Cross-Border Transport of Biomass Burning

The exacerbation of air pollution during spring in Yunnan province, China, has attracted widespread attention. However, many studies have focused solely on the impacts of anthropogenic emissions while ignoring the role of natural processes. This study used satellite data spanning 21 years from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Atmospheric Infrared Sounder (AIRS) to reveal two natural processes closely related to springtime ozone (O3) and PM2.5 pollution: stratospheric intrusions (SIs) and cross-border transport of biomass burning (BB). We aimed to assess the mechanisms through which SIs and cross-border BB transport influence O3 and PM2.5 pollution in Southwestern China during the spring. The unique geographical conditions and prevalent southwest winds are considered the key driving factors for SIs and cross-border BB transport. Frequent tropopause folding provides favorable dynamic conditions for SIs in the upper troposphere. In the lower troposphere, the distribution patterns of O3 and stratospheric O3 tracer (O3S) are similar to the terrain, indicating that O3 is more likely to reach the surface with increasing altitude. Using stratospheric tracer tagging methods, we quantified the contributions of SIs to surface O3, ranging from 6 to 31 ppbv and accounting for 10–38% of surface O3 levels. Additionally, as Yunnan is located downwind of Myanmar and has complex terrain, it provides favorable conditions for PM2.5 and O3 generation from cross-border BB transport. The decreasing terrain distribution from north to south in Yunnan facilitates PM2.5 transport to lower-elevation border cities, whereas higher-elevation cities hinder PM2.5 transport, leading to spatial heterogeneity in PM2.5. This study provides scientific support for elucidating the two key processes governing springtime PM2.5 and O3 pollution in Yunnan, SIs and cross-border BB transport, and can assist policymakers in formulating optimal emission reduction strategies.

Impact of ENSO on the UTLS chemical composition in the Asian Summer Monsoon Anticyclone

The changes in the convection and background dynamics over the Asian Summer Monsoon (ASM) are mainly responsible for the changes in the concentrations of the tropospheric pollutants within the Asian Summer Monsoon Anticyclone (ASMA). In this study, we have investigated the structural and chemical composition changes of the ASMA during different El Niño–Southern Oscillation (ENSO) conditions by making use of reanalysis products and Microwave Limb Sounder (MLS) satellite observations. Our analysis mainly concentrates on the July and August months of 2015 (El Niño) and 2022 (La Niña), which are the cold (Nino Index 3.4 exceeding −0.5 in magnitude throughout the year) and warm (Nino Index exceeding +0.5 in magnitude throughout the year) phase of the ENSO. The ASMA structure during different phases of the ENSO is found to be different when compared with the long-term (2005–2023) mean. The ASMA is found to have a larger zonal extent during La Niña (2022), extending from 20° to 140° E compared to El Niño (2015), which is from 20° to 100–105°E. A significant decrease (increase) in the concentration of the tropospheric species (CO and WV) is noticed within the ASMA in El Niño (La Niña). The decrease in the tropospheric species is clearly evident throughout the upper troposphere. The northeastern edge of the ASMA at 100 hPa and 150 hPa showed quite distinct behaviour with an enhancement (reduction) in the tropospheric species during La Niña (El Niño). Interestingly, there is an increase in O3 concentrations over the northeastern edge of ASMA during 2015, which is attributed to the stratospheric intrusion. Our results also revealed the enhancement of the tropospheric species over the western Pacific region during 2015 but not in 2022. Current findings highlight dynamic shifts linked to varying ENSO phases, leading to notable alterations in both the ASMA structure and Upper Troposphere and Lower Stratosphere (UTLS) chemical composition.

A mechanism of stratospheric O3 intrusion into the atmospheric environment: a case study of the North China Plain

Abstract. Stratosphere-to-troposphere transport results in the stratospheric intrusion (SI) of O3 into the free troposphere through the folding of the tropopause. However, the mechanism of SI that influences the atmospheric environment through the cross-layer transport of O3 from the stratosphere and free troposphere to the atmospheric boundary layer has not been elucidated thoroughly. In this study, an SI event over the North China Plain (NCP; 33–40° N, 114–121° E) during 19–20 May 2019 was chosen to investigate the mechanism of the cross-layer transport of stratospheric O3 and its impact on the near-surface O3 based on multi-source reanalysis, observation data, and air quality modeling. The results revealed a mechanism of stratospheric O3 intrusion into the atmospheric environment induced by an extratropical cyclone system. The SI with downward transport of stratospheric O3 to the near-surface layer was driven by the extratropical cyclone system, with vertical coupling of the upper westerly trough, the middle of the northeast cold vortex (NECV), and the lower extratropical cyclone, in the troposphere. The deep trough in the westerly jet aroused the tropopause folding, and the lower-stratospheric O3 penetrated the folded tropopause into the upper and middle troposphere; the westerly trough was cut off to form a typical cold vortex in the upper and middle troposphere. The compensating downdrafts of the NECV further pushed the downward transport of stratospheric O3 in the free troposphere; the NECV activated an extratropical cyclone in the lower troposphere; and the vertical cyclonic circulation governed the stratospheric O3 from the free troposphere across the boundary layer top, invading the near-surface atmosphere. In this SI event, the average contribution of stratospheric O3 to near-surface O3 was accounted for at 26.77 %. The proposed meteorological mechanism of the vertical transport of stratospheric O3 into the near-surface atmosphere, driven by an extratropical cyclone system, could improve the understanding of the influence of stratospheric O3 on the atmospheric environment, with implications for the coordinated control of atmospheric pollution.

Stratospheric influence on surface ozone pollution in China

Events of stratospheric intrusions to the surface (SITS) can lead to severe ozone (O3) pollution. Still, to what extent SITS events impact surface O3 on a national scale over years remains a long-lasting question, mainly due to difficulty of resolving three key SITS metrics: frequency, duration and intensity. Here, we identify 27,616 SITS events over China during 2015-2022 based on spatiotemporally dense surface measurements of O3 and carbon monoxide, two effective indicators of SITS. An overview of the three metrics is presented, illustrating large influences of SITS on surface O3 in China. We find that SITS events occur preferentially in high-elevation regions, while those in plain regions are more intense. SITS enhances surface O3 by 20 ppbv on average, contributing to 30-45% of O3 during SITS periods. Nationally, SITS-induced O3 peaks in spring and autumn, while over 70% of SITS events during the warm months exacerbate O3 pollution. Over 2015-2022, SITS-induced O3 shows a declining trend. Our observation-based results can have implications for O3 mitigation policies in short and long terms.

Impact of stratospheric intrusions on surface ozone enhancement in Hong Kong in the lower troposphere: Implications for ozone control strategy

Understanding the impact of stratospheric intrusion (SI) is crucial for elucidating atmospheric complexities and implications for ozone (O3) control. However, current studies have not focused on the influence of SI on surface O3, thus limiting the effectiveness of control strategies. This study delves into an SI event that occurred from March 5 to 12, 2022, employing a comprehensive integration of the Weather Research and Forecasting model coupled with Chemistry and multi-reanalysis data. The lower tropospheric O3 enhancement in Hong Kong has been noticed, despite the absence of SIs in the upper troposphere. This study employed a comprehensive methodology, integrating the use of a box model to estimate the stratosphere–troposphere exchange (STE) O3 flux, integrated process rate (IPR) analysis to quantify the contributions from individual physical and chemical processes, and tracer methods to detect stratospheric O3. During the deep SI episode, the STE flux peaked at −81.33 × 10−8 kg m−2·s−1, surpassing the monthly average by 15.2-fold. The IPR results indicate that vertical transport within the 0–16 km range contributes between 18.4 and 39.0 ppbv h−1 during the SI event, whereas horizontal advection shows a negative contribution. Stratospheric O3 tagging revealed that SI contributes 15.5–31.3 ppb (29.6–50.2%, respectively, of surface O3) when stratospheric O3 mixes down. Five emission reduction paths were designed in response to the negative impacts of SI. The “anthropogenic VOC (AVOC) only” path was the most efficient; however, when SI contributed 31 ppb on March 9, the “NOx only” path required a 69% reduction, while the “AVOC only” path required an 85% reduction for efficiency. This research not only elucidates the complex interplay between SI and surface O3 concentrations but also emphasizes the significance of refining existing emission reduction paths.

Spatiotemporal characterization of PM2.5, O3, and trace gases associated with East Asian continental outflows via drone sounding

East Asian continental outflows with PM2.5, O3, and other species may determine the baseline conditions and affect the air quality in downwind areas via long-range transport (LRT). To gain insight into the impact and spatiotemporal characteristics of airborne pollutants in East Asian continental outflows, a versatile multicopter drone sounding platform was used to simultaneously observe PM2.5, O3, CO2, and meteorological variables (temperature, specific humidity, pressure, and wind vector) above the northern tip of Taiwan, Cape Fuiguei, which often encounters continental outflows during winter monsoon periods. By coordinating hourly high-spatial-resolution profiles provided by drone soundings, WRF/CMAQ model air quality predictions, HYSPLIT-simulated backward trajectories, and MERRA-2 reanalysis data, we analyzed two prominent phenomena of airborne pollutants in continental outflows to better understand their physical/chemical characteristics. First, we found that pollutants were well mixed within a sounding height of 500 m when continental outflows passed through and completely enveloped Cape Fuiguei. Eddies induced by significant fluctuations in wind speeds coupled with minimal temperature inversion and LRT facilitated vertical mixing, possibly resulting in high homogeneity of pollutants within the outflow layer. Second, the drone soundings indicated exceptionally high O3 concentrations (70–100 ppbv) but relatively low concentrations of PM2.5 (10–20 μg/m3), CO2 (420–425 ppmv), and VOCs in some air masses. The low levels of PM2.5, CO2, and VOCs ruled out photochemistry as the cause of the formation of high-level O3. Further coordination of spatiotemporal data with air mass trajectories and O3 cross sections provided by MERRA-2 suggested that the high O3 concentrations could be attributed to stratospheric intrusion and advection via continental outflows. High-level O3 concentrations persisted in the lower troposphere, even reaching the surface, suggesting that stratospheric intrusion O3 may be involved in the rising trend in O3 concentrations in parts of East Asia in recent years in addition to surface photochemical factors.

3-D Changes of Tropospheric O3 in Central and Eastern China Induced by Tropical Cyclones over the Northwest Pacific: Recent-Year Characterization with Multi-Source Observations

In this study, the multi-year data of meteorology and O3 from remote sensing and ground observations are applied to characterize the 3-D changes of O3 in the troposphere over central and eastern China (CEC) induced by the tropical cyclones (TCs) in the tropical and subtropical ocean regions over Northwest Pacific. The CEC-regional average of near-surface O3 levels is significantly elevated with 6.0 ppb in the large coverage by the TCs in the subtropical ocean, while the TCs in the tropical ocean alter near-surface O3 weakly, indicating the latitudinal-located TCs in the subtropical offshore ocean could largely influence the O3 variations over CEC. The sub-seasonal change with the positive and negative anomalies of near-surface O3 is induced by the tropical TCs from June to July and from August to October. The peripheral circulation of TCs in the subtropical offshore ocean persistently enhances the O3 concentrations over CEC during the season of East Asian summer monsoons. The positive O3 anomalies maintain from the entire troposphere to the lower stratosphere over CEC in the peripheries of subtropical TCs, while the tropical TCs cause the positive O3 anomalies merely in the lower troposphere. The O3 transport and accumulation, photochemical production and stratospheric intrusion are climatologically confirmed as the major meteorological mechanisms of TCs affecting the O3 variations. This study reveals that the downward transport of stratospheric O3 of TCs in the subtropical ocean exerts a large impact on the atmospheric environment over CEC, while the regional O3 transport and photochemical productions dominate the lower troposphere over CEC with less impact of stratospheric intrusion from the TCs in the tropical ocean region. These results present the climatology of tropospheric O3 anomalies in China induced by the TCs over the Northwest Pacific with enhancing our comprehension of the meteorological impact on O3 variations over the East Asian monsoon region.

Comment on “Transport of substantial stratospheric ozone to the surface by a dying typhoon and shallow convection” by Chen et al. (2022)

Abstract. Chen et al. (2022) analyzed the event of rapid nocturnal O3 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 O3, 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) Ox (O3 + NO2) levels during the NOE period approaching those of O3 during 14:00–17:00 LT, (b) levels of PAN and the relationship between O3 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.

The Evolutions and Large‐Scale Mechanisms of Summer Stratospheric Ozone Intrusion Across Global Hotspots

AbstractStratospheric ozone intrusions can have a significant impact on regional near‐surface ozone levels. Especially in summer, intrusions can contribute to extreme ozone events because of preexisting high ozone levels near the surface and cause serious health issues. Considering the increasing trend of surface ozone level, an understanding of stratospheric ozone intrusion is necessary. From a 19‐year Whole Atmosphere Community Climate Model, version 6 simulation and a stratospheric origin ozone tracer, we identify the global hotspots of stratospheric intrusions based on extreme tracer concentrations near the surface: North America, Africa, the Mediterranean, and the Middle East. We investigate the common underlying large‐scale mechanisms of the stratospheric intrusions over the identified hotspots from the lower stratosphere to the lower troposphere. From the trajectory analysis, we find that the upper‐level jet drives isentropic mixing near the jet axis and initiates stratospheric ozone intrusion. Subsequently, climatological descent at the lower troposphere brings the ozone down to the surface, which explains the spatial preference of summertime stratospheric intrusion events.

Himalayas as a global hot spot of springtime stratospheric intrusions: Insight from isotopic signatures in sulfate aerosols

Downward transport of stratospheric air into the troposphere (identified as stratospheric intrusions) could potentially modify the radiation budget and chemical of the Earth's surface atmosphere. As the highest and largest plateau on earth, the Tibetan Plateau including the Himalayas couples to global climate, and has attracted widespread attention due to rapid warming and cryospheric shrinking. Previous studies recognized strong stratospheric intrusions in the Himalayas but are poorly understood due to limited direct evidences and the complexity of the meteorological dynamics of the third pole. Cosmogenic 35S is a radioactive isotope predominately produced in the lower stratosphere and has been demonstrated as a sensitive chemical tracer to detect stratospherically sourced air mass in the planetary boundary layer. Here, we report 6-month (April–September 2018) observation of 35S in atmospheric sulfate aerosols (35SO42−) collected from a remote site in the Himalayas to reveal the stratospheric intrusion phenomenon as well as its potential impacts in this region. Throughout the sampling campaign, the 35SO42− concentrations show an average of 1,070 ± 980 atoms/m3. In springtime, the average is 1,620 ± 730 atoms/m3, significantly higher than the global existing data measured so far. The significant enrichments of 35SO42− measured in this study verified the hypothesis that the Himalayas is a global hot spot of stratospheric intrusions, especially during the springtime as a consequence of its unique geology and atmospheric couplings. In combined with the ancillary evidences, e.g., oxygen-17 anomaly in sulfate and modeling results, we found that the stratospheric intrusions have a profound impact on the surface ozone concentrations over the study region, and potentially have the ability to constrain how the mechanisms of sulfate oxidation are affected by a change in plateau atmospheric properties and conditions. This study provides new observational constraints on stratospheric intrusions in the Himalayas, which would further provide additional information for a deeper understanding on the environment and climatic changes over the Tibetan Plateau.

Connecting an Offshore Dry Air Stratospheric Intrusion with the Outbreak of Soberanes Fire 2016

Abstract This study investigates the connection between the arrival of dry stratospheric air and the Soberanes Fire (2016). The Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) and Goddard Earth Observing System Forward Processing model (GEOS-FP) are used for back-trajectories and offshore deep stratospheric intrusion in conjunction with the ignition and outbreak of the fire. The back-trajectory analysis indicates that most air reaching the vertical column was critically dry, exhibiting relative humidity values below 10%. As the fire ignited, dry air arrived from due west at heights of 1–3 km about 24 h prior. During the overnight fire growth, dry air arrived from the northwest to north-northwest at heights of 3.5–5.5 km 48–72 h prior. The synoptic and GEOS-FP analyses demonstrate offshore mid-to-low stratospheric intrusion. On 21 July 2016, an enclosed upper-level low approached the California–Oregon border along the northwesterly subtropical jet stream hours before the fire outbreak. The GEOS-FP results of potential vorticity, specific humidity, and ozone along the back-trajectories to the west and northwest of the fire suggest a stratospheric intrusion event into the mid-to-low troposphere at the back-trajectory start points, and vertical velocity indicates sinking motion. The specific humidity analyzed at the arrival time shows the transport of the abnormally dry air to the Soberanes Fire. Results suggest a connection between dry stratospheric air transported to the Soberanes Fire at ignition and overnight accelerated growth, supported by a dark bank in satellite water vapor imagery. The prediction of low-level transport of dry stratospheric air to the coastal communities could help to predict the occurrence of wildfire outbreaks, or periods of accelerated fire growth.

A specific stratospheric precursory signal of overshooting convective systems over the southern slope of the Tibetan Plateau

Using the TRMM PR observations, ERA5 reanalysis and Outgoing Longwave Radiation (OLR) datasets, a specific stratospheric precursory signal to the occurrence of overshooting convective systems (OCSs) over the southern slope of the Tibetan Plateau (SSTP) is investigated. We compared two types of OCS events: OCSs associated with stratospheric intrusions and other OCSs. The results indicate that there is a notable decline in the tropopause height starting two days before to on the day of the OCS onset for OCSs associated with stratospheric intrusions, but the tropopause height for other OCSs increases with the occurrence of OCS. It is found that the descending of the tropopause height is accompanied by downward intrusion of stratospheric high potential vorticity (PV) air into the troposphere for the OCSs associated with stratospheric intrusions. The increase in PV near the tropopause can cause an increase in temperature and relative vorticity near the tropopause. The OCSs associated with stratospheric intrusions are closely related to strong synoptic variability. As the stratospheric intrusion deepens on the day of the OCS onset, increased relative vorticity near the tropopause, stronger upward motion in the troposphere associated with divergence and convergence, more specific humidity in the troposphere, and weakened static stability in the middle troposphere are found within the OCS region, which are conducive to the development of deep convection that penetrates into the stratospheric intrusion. Compared with OCSs associated with stratospheric intrusions, other OCS events are characterized by stronger high cloud cover (HCC) and negative Outgoing Longwave Radiation (OLR) anomaly, as well as higher upward movement in the troposphere. This is conducive to other OCSs occurring in a more barotropic convective environment characteristic of the Asian monsoon, the vertical development height of weakened static stability and upward motion are higher, promoting the development of convective activity to higher altitudes and penetrating into the tropopause, thereby causing the uplift of the tropopause.

Mobile Observations of Ozone and Aerosols in Alabama: Southeastern US Summer Pollution and Coastal Variability

AbstractWe present a detailed analysis of measurements of ozone (O3) and aerosols collected using multiple platforms and instruments during a 5‐day mini‐campaign in Alabama, for improving our understanding of the chemistry and air quality in the Southeastern United States. Our study features the first on‐road O3 lidar profiling on highways and the coordinated airborne observations that captured O3 and aerosol structures associated with synoptic weather conditions, stratospheric intrusions, wildfire smoke transport, and enhancements in urban areas. The Doppler wind lidar data suggest that the vertical structures and variability of O3 and aerosols at Dauphin Island, a coastal site in the Gulf of Mexico, were related to both the synoptic flow and local‐scale land‐sea breeze circulation. The onshore gulf breeze, which typically started in the afternoon, helped to quickly dilute air pollutants near the surface, resulting in large gradients toward inland areas. In contrast, gradients were much smaller on days without a gulf breeze. O3 and humidity were highly anti‐correlated in the lower free troposphere, but weakly anti‐correlated in the planetary boundary layer (PBL) suggesting more complicated emissions and sink of O3 in the PBL. Our on‐road and airborne measurement strategies to quantify the vertical and horizontal chemical gradients will be useful for future geostationary satellite validation.

Surface ozone over the Tibetan Plateau controlled by stratospheric intrusion

Abstract. The Tibetan Plateau is a global hotspot of stratospheric intrusion, and elevated surface ozone was observed at ground monitoring sites. Still, links between the variability of surface ozone and stratospheric intrusion at the regional scale remain unclear. This study synthesized ground measurements of surface ozone over the Tibetan Plateau and analyzed their seasonal variations. The monthly mean surface ozone concentrations over the Tibetan Plateau peaked earlier in the south in April and May and later in the north in June and July. The migration of monthly surface ozone peaks was coupled with the synchronous movement of tropopause folds and the westerly jet that created conditions conducive to stratospheric ozone intrusion. Stratospheric ozone intrusion significantly contributed to surface ozone across the Tibetan Plateau, especially in the areas with high surface ozone concentrations during their peak-value month. We demonstrated that monthly variation of surface ozone over the Tibetan Plateau is mainly controlled by stratospheric intrusion, which warrants proper consideration in understanding the atmospheric chemistry and the impacts of ozone over this highland region and beyond.

Transport mechanisms of nocturnal surface ozone over Riyadh, Kingdom of Saudi Arabia

This study investigated ozone (O3) transport mechanisms and their impact on observed nocturnal surface O3 (NSO) enhancements over Riyadh, Kingdom of Saudi Arabia (KSA). We used O3 measurements for the summer of 2012 to examine the NSO enhancements across the city. We found that out of the 88 days for which observations were available, NSO enhancements could be observed on 38 days. The average difference in the NSO concentration between NSO-enhanced days and nonenhanced days was ∼15 ppb, which was statistically significant (p < 0.05). Our analysis of the regional surface emission sources, potential vorticity (PV), and back trajectories using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model indicated that out of 38 days, the NSO enhancements on 18 days (47%), 14 days (37%), 4 days (11%), and 2 days (5%) are due to local transport, long-range transport from highly polluted regions and from stratospheric intrusion regions located at a considerable distance from Riyadh, and a combination of long-range transport from stratospheric intrusion and highly polluted regions, respectively. We then investigated the relevance of each transport mechanism for the increased NSO concentrations during five protracted episodes, each marked by three consecutive days of NSO enhancement. Our results demonstrated that the local transport, long-range transport of O3-rich air masses from highly polluted regions to the east and northeast of the KSA, and local descent over Riyadh increased the average NSO concentration by up to 43%. Moreover, we found that long-range transport from stratospheric intrusion regions north of the KSA increased the average NSO concentration by up to 46%. The combination of long-range transport and stratospheric intrusion increased the average NSO concentration by ∼180%. These increases were usually associated with strong northwest regional winds flowing over highly polluted and major stratospheric intrusion regions.

High Spatial Resolution Ozone Profiles Retrieved from the First Chinese Ultraviolet–Visible Hyperspectral Satellite Instrument

Understanding the vertical distribution of ozone is crucial when assessing both its horizontal and vertical transport, as well as when analyzing the physical and chemical properties of the atmosphere. One of the most effective ways to obtain high spatial resolution ozone profiles is through satellite observations. The Environmental Trace Gases Monitoring Instrument (EMI) deployed on the Gaofen-5 satellite is the first Chinese ultraviolet–visible hyperspectral spectrometer. However, retrieving ozone profiles using backscattered radiance values measured by the EMI is challenging due to unavailable measurement errors and a low signal-to-noise ratio. The algorithm developed for the Tropospheric Monitoring Instrument did not allow us to retrieve 87% of the EMI pixels. Therefore, we developed an algorithm specific to the characteristics of the EMI. The fitting residuals are smaller than 0.3% in most regions. The retrieved ozone profiles were in good agreement with ozonesonde data, with maximum mean biases of 20% at five latitude bands. By applying EMI averaging kernels to the ozonesonde profiles, the integrated stratospheric column ozone and tropospheric column ozone also showed excellent agreement with ozonesonde data. The lower layers (0–7.5 km) of the EMI ozone profiles reflected the seasonal variation in surface ozone derived from the China National Environmental Monitoring Center (CNEMC). However, the upper layers (9.7–16.7 km) of the ozone profiles show different trends, with the ozone peak occurring at an altitude of 9.7–16.7 km in March, 2019. A stratospheric intrusion event in central China from August 11 to 15, 2019, is captured using the EMI ozone profiles, potential vorticity data, and relative humidity data. The increase in the CNEMC ozone concentration showed that downward transport enhanced surface ozone pollution.

Zugspitze ozone 1970–2020: the role of stratosphere–troposphere transport

Abstract. The pronounced increase in ozone observed at the Alpine station Zugspitze (2962 ma.s.l.) since the 1970s has been ascribed to an increase in stratospheric air descending to the Alps. In this paper, we present a reanalysis of the data from for both ozone (1978 to 2011) and carbon monoxide (1990–2011), which has been extended until 2020 by the data from the Global Atmosphere Watch site at the Umweltforschungsstation Schneefernerhaus (UFS; 2671 ma.s.l. – above sea level), which is located just below the Zugspitze summit. For ozone between 1970 and 1977, a constant annual average of 36.25 ppb (parts per billion) was assumed to have been obtained by extrapolation. The analysis is based on data filtering, utilizing the isotope 7Be (measured between 1970 and 2006) and relative humidity (1970 to 2011; UFS from 2002 to 2020). We estimate both the influence of stratospheric intrusions directly descending to the northern rim of the Alps from the full data filtering and the aged (“indirect”) intrusions from applying a relationship between ozone and the 7Be data. The evaluated total stratospheric contribution to the annual average ozone rises roughly from 12 ppb in 1970 to 24 ppb in 2003. It turns out that the increase in the stratospheric influence is particularly strong in winter. A lowering in positive trend is seen afterwards, with a delay of roughly 1 decade after the beginning of the decrease in the solar irradiation. The air masses hitting the Zugspitze summit became drier until 2003, and we see the growing stratospheric contribution as being an important factor for this drying. Both an increase in the lower-stratospheric ozone and the growing thickness of the intruding layers departing downward from just above the tropopause must be taken into consideration. Carbon monoxide in the intrusions did not change much during the full measurement period from 1990 to 2020, with a slight increase until 2005. This is remarkable since, for air outside intrusions, a decrease by approximately 44 % was found, indicating a substantial improvement in the tropospheric air quality.