Stratosphere-to-troposphere Transport Research Articles

Impacts of stratosphere-to-troposphere transport on tropospheric ozone in southeastern China: insights from ozonesonde observations

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.

Impacts of the SSTs over different regions on the troposphere-to-stratosphere transport over the tropical western Pacific in the boreal winter

The strong upward motion in the lower tropical tropopause layer (TTL) over the tropical western Pacific (TWP) and the large-scale tropical upwelling of the Brewer-Dobson circulation (BDC) in boreal winter form a critical pathway for tropospheric trace gases entering the stratosphere. An index, called the TSTI, is defined using the combination of the upward motion in the lower TTL and the tropical upwelling in the lower stratosphere to reflect the troposphere-to-stratosphere transport (TST) over the TWP in boreal winter (NDJFM). The TSTIs derived from JRA55 and CMIP6-AMIP historical simulations show significantly increasing trends in the past decades. The impacts of the changes in the SSTs over 9 different regions on the TSTI in NDJFM during 1958–2017 are further investigated with WACCM4 numeric simulations. The results show that the warmed sea surface temperatures (SSTs) over the TWP make a significant contribution to the increasing trends of the upward motion over the TWP and the tropical upwelling in the lower stratosphere, resulting in an enhanced TSTI over the TWP. The changes in SSTs over other regions may singly impact the upward motion in the lower TTL over the TWP or the tropical upwelling in the lower stratosphere, but their overall impacts on the TSTI over the TWP are weak. The WACCM4 simulations further suggest that the increased TST over the TWP could not lead to a wetter lower stratosphere since the enhanced deep convection accompanied by the increased TST over the TWP induces a decreased tropopause temperature and hence a drier lower stratosphere.

The Stratosphere-to-Troposphere Transport Related to Rossby Wave Breaking and Its Impact on Summertime Ground-Level Ozone in Eastern China

In summertime, eastern China experiences severe ozone pollution. Stratosphere-to-troposphere transport (STT), as the primary natural source of tropospheric ozone, may have a non-negligible contribution to ground-level ozone. Rossby wave breaking (RWB) is a leading mechanism that triggers STT, which can be categorized as anticyclonic wave breakings (AWBs) and cyclonic wave breakings (CWBs). This study uses an objective method to diagnose AWBs and CWBs and to investigate their influence on the surface ozone in eastern China using ground-based ozone observations, satellite ozone data from AIRS, a stratospheric ozone tracer simulated by CAM-chem, and meteorological fields from MERRA-2. The results indicate that AWBs occur mainly and frequently over northeast China, while CWBs occur mostly over the northern Sea of Japan. STTs triggered by AWBs mainly have sinking areas over the North China Plain, increasing the ground-level ozone concentrations by 5–10 ppbv in eastern China. The downwelling zones in the CWBs extend from Mongolia to the East China Sea, potentially causing an elevation of 5–10 ppbv of ozone in both central and eastern China. This study gives an overview of the impacts of AWBs and CWBs on surface ozone in eastern China and helps to improve our understanding of summertime ozone pollution in eastern China.

The response of the North Pacific jet and stratosphere-to-troposphere transport of ozone over western North America to RCP8.5 climate forcing

Abstract. Stratosphere-to-troposphere transport (STT) is an important source of ozone for the troposphere, particularly over western North America. STT in this region is predominantly controlled by a combination of the variability and location of the Pacific jet stream and the amount of ozone in the lower stratosphere, two factors which are likely to change if greenhouse gas concentrations continue to increase. Here we use Whole Atmosphere Community Climate Model experiments with a tracer of stratospheric ozone (O3S) to study how end-of-the-century Representative Concentration Pathway (RCP) 8.5 sea surface temperatures (SSTs) and greenhouse gases (GHGs), in isolation and in combination, influence STT of ozone over western North America relative to a preindustrial control background state. We find that O3S increases by up to 37 % during late winter at 700 hPa over western North America in response to RCP8.5 forcing, with the increases tapering off somewhat during spring and summer. When this response to RCP8.5 greenhouse gas forcing is decomposed into the contributions made by future SSTs alone versus future GHGs alone, the latter are found to be primarily responsible for these O3S changes. Both the future SSTs alone and the future GHGs alone accelerate the Brewer–Dobson circulation, which modifies extratropical lower-stratospheric ozone mixing ratios. While the future GHGs alone promote a more zonally symmetric lower-stratospheric ozone change due to enhanced ozone production and some transport, the future SSTs alone increase lower-stratospheric ozone predominantly over the North Pacific via transport associated with a stationary planetary-scale wave. Ozone accumulates in the trough of this anomalous wave and is reduced over the wave's ridges, illustrating that the composition of the lower-stratospheric ozone reservoir in the future is dependent on the phase and position of the stationary planetary-scale wave response to future SSTs alone, in addition to the poleward mass transport provided by the accelerated Brewer–Dobson circulation. Further, the future SSTs alone account for most changes to the large-scale circulation in the troposphere and stratosphere compared to the effect of future GHGs alone. These changes include modifying the position and speed of the future North Pacific jet, lifting the tropopause, accelerating both the Brewer–Dobson circulation's shallow and deep branches, and enhancing two-way isentropic mixing in the stratosphere.

The Troposphere-to-Stratosphere Transport Caused by a Rossby Wave Breaking Event over the Tibetan Plateau in Mid-March 2006

Based on reanalysis data, satellite ozone concentration observations, and a Lagrangian trajectory simulation, a Rossby wave breaking (RWB) event and its effect on stratosphere–troposphere exchange (STE) over the Tibetan Plateau in mid-March 2006 were investigated. Results showed that the increased eddy heat flux from the subtropical westerly jet magnified the amplitude of the Rossby wave, which contributed to the occurrence of the cyclonic RWB event. The quasi-horizontal cyclonic motion of the isentropic potential vorticity in the RWB cut the tropical tropospheric air mass into the extratropical stratosphere, completing the stratosphere–troposphere mass exchange. Meanwhile, the tropopause folding zone extended polewards by 10° of latitude and the tropospheric air mass escaped from the tropical tropopause layer into the extratropical stratosphere through the tropopause folding zone. The particles in the troposphere-to-stratosphere transport (TST) pathway migrated both eastwards and polewards in the horizontal direction, and shifted upwards in the vertical direction. Eventually, the mass of the TST particles reached about 3.8 × 1014 kg, accounting for 42.2% of the particles near the tropopause in the RWB event. The rest of the particles remained in the troposphere, where they moved eastwards rapidly along the westerly jet and slid down in the downstream upper frontal zone.

Dynamics of ENSO-driven stratosphere-to-troposphere transport of ozone over North America

Abstract. The El Niño–Southern Oscillation (ENSO) is known to modulate the strength and frequency of stratosphere-to-troposphere transport (STT) of ozone over the Pacific–North American region during late winter to early summer. Dynamical processes that have been proposed to account for this variability include variations in the amount of ozone in the lowermost stratosphere that is available for STT and tropospheric circulation-related variations in the frequency and geographic distribution of individual STT events. Here we use a large ensemble of Whole Atmosphere Community Climate Model (WACCM) simulations (forced by sea-surface temperature (SST) boundary conditions consistent with each phase of ENSO) to show that variability in lower-stratospheric ozone and shifts in the Pacific tropospheric jet constructively contribute to the amount of STT of ozone in the North American region during both ENSO phases. In terms of stratospheric variability, ENSO drives ozone anomalies resembling the Pacific–North American teleconnection pattern that span much of the lower stratosphere below 50 hPa. These ozone anomalies, which dominate over other ENSO-driven changes in the Brewer–Dobson circulation (including changes due to both the stratospheric residual circulation and quasi-isentropic mixing), strongly modulate the amount of ozone available for STT transport. As a result, during late winter (February–March), the stratospheric ozone response to the teleconnections constructively reinforces anomalous ENSO-jet-driven STT of ozone. However, as ENSO forcing weakens as spring progresses into summer (April–June), the direct effects of the ENSO-jet-driven STT transport weaken. Nevertheless, the residual impacts of the teleconnections on the amount of ozone in the lower stratosphere persist, and these anomalies in turn continue to cause anomalous STT of ozone. These results should prove helpful for interpreting the utility of ENSO as a subseasonal predictor of both free-tropospheric ozone and the probability of stratospheric ozone intrusion events that may cause exceedances in surface air quality standards.

Sensitivities of Cross‐Tropopause Transport in Midlatitude Overshooting Convection to the Lower Stratosphere Environment

AbstractTropopause‐overshooting convection in the midlatitudes rapidly transports lower troposphere air and cloud material to the upper troposphere and lower stratosphere (UTLS), with notable events resulting in the formation of above‐anvil cirrus plumes (AACPs). However, there is limited understanding of how transport driven by overshooting convection and AACP properties are modified by variations in UTLS environments, especially the extent of stratosphere‐to‐troposphere (i.e., downward) transport. Here, AACP development and UTLS transport sensitivities to lower stratosphere stability and the UTLS wind environment are evaluated. The Bryan Cloud Model is used to simulate midlatitude supercell convection and evaluate resulting UTLS composition changes within two common midlatitude thermodynamic environments: a single tropopause and a double tropopause, and two UTLS wind environments: one with a stronger stratospheric wind profile supportive of frequent gravity wave breaking and AACP development and the other with a weaker stratospheric wind profile to suppress gravity wave breaking and discourage AACP formation. Multiple passive tracers and water vapor concentrations are used to evaluate the exchange of air between the troposphere and stratosphere and transport confined to each layer. It is found that greater troposphere‐to‐stratosphere transport (TST) and stratospheric hydration occurs in storms with AACPs, while stratosphere‐to‐troposphere transport (STT) is greater in storms without AACPs. This STT is facilitated by a mechanical oscillation induced by the overshoot. AACP‐producing storms have increased downward transport of stratospheric overworld air to the lowermost stratosphere, which is enhanced within a double tropopause environment. More expansive AACPs, deeper overshoots, and greater TST also occurs in double tropopause environments.

Substantial Near-Surface Spring Ozone Enhancement due to Stratospheric Intrusion in the Northeastern Qinghai–Tibet Plateau, China

Stratospheric ozone intrusion plays an important role in affecting the tropospheric ozone concentrations in the troposphere through stratosphere to troposphere transport (STT). Nevertheless, there are very limited studies on the quantification of the effect. Focusing on a typical event on 9–12 May 2015, when STT occurs over the northeastern Qinghai–Tibet Plateau of China, the observations and reanalysis data indicate that STT is accompanied by high potential vorticity and low specific humidity. In order to quantify the extent to which the STT may elevate the ozone concentrations, an inert tracer is added to the boundary conditions of the Community Multiscale Air Quality (CMAQ), which is driven by the Weather Research and Forecasting (WRF) model. The meteorological conditions simulated by WRF nicely reproduce the distributions of potential vorticity (PV) and water vapor in the upper troposphere. Through the physical processes of diffusion, advection, and dry and wet deposition, the ozone tracer concentrations simulated from CMAQ well capture the spatial propagation and evolution of stratospheric ozone intrusion over Qinghai–Tibet Plateau, warranting the confidence in interpreting the simulated results in quantifying the STT. The STT event indicates the near-surface ozone enhancement of approximately 10–20 ppbv covering half of Qinghai province, even spreading to a broader area of eastern China. For the typical remote mountain such as Waliguan, clear ozone enhancement is obtained over the lower level of the troposphere. The method used in this study is applicable to other regions as well, which can be applied in the future to detect the STT at a wider spatiotemporal scale and help the policymakers identify the ozone sources and make efficient strategies for the ozone pollution control.

Influence of stratosphere-to-troposphere transport on summertime surface O3 changes in North China Plain in 2019

The North China Plain (NCP) is a major region of summer O3 pollution in China. However, the contribution of stratosphere-to-troposphere transport (STT) to summertime surface O3 in the NCP and the meteorological mechanisms are poorly understood. In this study, meteorological reanalysis data, O3 observations, and Lagrange modeling were used to analyze the contribution of the STT to the change in surface O3 in the NCP, the vertical transport pathways, and the associated meteorological mechanism. In summer 2019, the STT contributed an estimated 5.7%–18.8% to the surface O3 concentrations in the NCP. The influence of the STT in the NCP is closely related to the changes in westerly circulation in the upper troposphere and lower stratosphere (UTLS) over the Eurasian region. The troughs and vortices in the westerlies led to the convergences of O3 in the UTLS and tropopause folding, forming downward intrusions of O3-rich air from the UTLS to the lower troposphere. Controlled by the westerly circulation, the O3 source regions of STT in the stratosphere are found between 40° and 70°N, oscillating zonally with changing transport periods, where a key source region of STT is situated steadily over 60°-100°E northwest of the NCP. In association with the cut-off low of westerly circulation in the UTLS over Siberia evolved from the splitting of the polar vortex, a slanted channel of O3 downward intrusion was set up from the UTLS to the lower troposphere over the NCP in 7 days. As the cut-off low in westerly circulation moves southeastward close to the NCP, the channel of the O3 downward intrusion is set upright with a shorter period of vertical transport to the NCP ground. Our results provide insights into the the O3 source–receptor relationship in the STT with vertical O3 transport structures in changes in the atmospheric environment.

A process-oriented evaluation of CAMS reanalysis ozone during tropopause folds over Europe for the period 2003–2018

Abstract. Tropopause folds are the key process underlying stratosphere-to-troposphere transport (STT) of ozone, and thus they affect tropospheric ozone levels and variability. In the present study we perform a process-oriented evaluation of Copernicus Atmosphere Monitoring Service (CAMS) reanalysis (CAMSRA) O3 during folding events over Europe and for the time period from 2003 to 2018. A 3-D labeling algorithm is applied to detect tropopause folds in CAMSRA, while ozonesonde data from WOUDC (World Ozone and Ultraviolet Radiation Data Centre) and aircraft measurements from IAGOS (In-service Aircraft for a Global Observing System) are used for CAMSRA O3 evaluation. The profiles of observed and CAMSRA O3 concentrations indicate that CAMSRA reproduces the observed O3 increases in the troposphere during the examined folding events. Nevertheless, at most of the examined sites, CAMSRA overestimates the observed O3 concentrations, mostly at the upper portion of the observed increases, with a median fractional gross error (FGE) among the examined sites >0.2 above 400 hPa. The use of a control run without data assimilation reveals that the aforementioned overestimation of CAMSRA O3 arises from the data assimilation implementation. Overall, although data assimilation assists CAMSRA O3 to follow the observed O3 enhancements in the troposphere during the STT events, it introduces biases in the upper troposphere resulting in no clear quantitative improvement compared to the control run without data assimilation. Less biased assimilated O3 products, with finer vertical resolution in the troposphere, in addition to higher IFS (Integrated Forecasting System) vertical resolution, are expected to provide a better representation of O3 variability during tropopause folds.

Anomalous surface O3 changes in North China Plain during the northwestward movement of a landing typhoon

As high impact weather in a large scale, typhoon movement from the northwest Pacific into inland regions influencing ambient O3 changes is unclear, especially in North China Plain (NCP). A landing Typhoon Ampil during July 17–24, 2018 was studied herein to characterize the surface O3 anomalies during its movement over NCP. Landing typhoons present large negative O3 anomalies at the center of the typhoon and positive O3 anomalies 600–1700 km away from the center. During the northwest movement of Typhoon Ampil to the NCP, the area and magnitude of both positive and negative O3 anomalies shrank, particularly in the western and northern periphery, where the typical diurnal change of O3 dissipated with nocturnal O3 enhancement in the NCP. The spatiotemporal patterns of surface O3 anomalies in the NCP were induced significantly during various stages of typhoon movement with a stable structure in the atmospheric boundary layer, strong solar radiation on sunny days, and stratosphere-to-troposphere transport (STT) in the typhoon periphery, depending on the changing intensity, distance, and orientation of the typhoon center. Among them, the STT played a considerable role and contributed 32% to the positive anomalies of surface O3 in the NCP. Under the influence of westerly jets and high pressure at mid-latitudes on the typhoon movement, strong wind convergences in the upper troposphere were formed intensifying the downdraft of O3-rich stratospheric air to the boundary layer in the NCP with an asymmetrical distribution of surface positive O3 anomalies over the periphery of typhoon. This study could improve our understanding of regional ozone changes with meteorological influences.

Evaluation of Stratospheric Intrusions and Biomass Burning Plumes on the Vertical Distribution of Tropospheric Ozone Over the Midwestern United States

AbstractNaturally occurring ozone‐rich stratosphere‐to‐troposphere transport (STT) intrusions and biomass burning (BB) plumes reaching the surface can contribute to exceedances of the U.S. National Ambient Air Quality Standards for ground‐level ozone (70 ppbv implemented in 2015). Additionally, fires can inject significant pollution into the free troposphere where it can be transported long distances. The combined air quality impacts from these sources on ozone have only been analyzed in a few case studies for the Midwest United States. Here we study ozone impacts in a Midwestern city, for the first time in St. Louis, Missouri, using a series of ozonesonde profiles taken during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) field campaign in August–September 2013. All ozonesondes showed enhancements above the background profile levels (~55 ppbv) throughout each tropospheric column. Two models were used to estimate ozone origins in columns. A chemical transport model identified STT enhancements equivalent to 10 to 15 ppbv over the background with a 10% to 15% contribution overall to the column. Two FLEXPART‐WRF simulations, one with smoke in the boundary layer and another with smoke above, identified BB enhancements equivalent to 10 to 80 ppbv. Overall, the total BB contribution is 15% to 30% of the total column. Five ozonesondes showed signatures of mixed BB plumes and STT intrusions. During this study period, BB in the western United States contributed 70% to ozone enhancements in the total column compared to 3% from the central United States and 27% from other areas.

Future trends in stratosphere-to-troposphere transport in CCMI models

Abstract. One of the key questions in the air quality and climate sciences is how tropospheric ozone concentrations will change in the future. This will depend on two factors: changes in stratosphere-to-troposphere transport (STT) and changes in tropospheric chemistry. Here we aim to identify robust changes in STT using simulations from the Chemistry Climate Model Initiative (CCMI) under a common climate change scenario (RCP6.0). We use two idealized stratospheric tracers to isolate changes in transport: stratospheric ozone (O3S), which is exactly like ozone but has no chemical sources in the troposphere, and st80, a passive tracer with fixed volume mixing ratio in the stratosphere. We find a robust increase in the tropospheric columns of these two tracers across the models. In particular, stratospheric ozone in the troposphere is projected to increase 10 %–16 % by the end of the 21st century in the RCP6.0 scenario. Future STT is enhanced in the subtropics due to the strengthening of the shallow branch of the Brewer–Dobson circulation (BDC) in the lower stratosphere and of the upper part of the Hadley cell in the upper troposphere. The acceleration of the deep branch of the BDC in the Northern Hemisphere (NH) and changes in eddy transport contribute to increased STT at high latitudes. These STT trends are caused by greenhouse gas (GHG) increases, while phasing out of ozone-depleting substances (ODS) does not lead to robust transport changes. Nevertheless, the decline of ODS increases the reservoir of ozone in the lower stratosphere, which results in enhanced STT of O3S at middle and high latitudes. A higher emission scenario (RCP8.5) produces stronger STT trends, with increases in tropospheric column O3S more than 3 times larger than those in the RCP6.0 scenario by the end of the 21st century.

On the impact of future climate change on tropopause folds and tropospheric ozone

Abstract. Using a transient simulation for the period 1960–2100 with the state-of-the-art ECHAM5/MESSy Atmospheric Chemistry (EMAC) global model and a tropopause fold identification algorithm, we explore the future projected changes in tropopause folds, stratosphere-to-troposphere transport (STT) of ozone, and tropospheric ozone under the RCP6.0 scenario. Statistically significant changes in tropopause fold frequencies from 1970–1999 to 2070–2099 are identified in both hemispheres, regionally exceeding 3 %, and are associated with the projected changes in the position and intensity of the subtropical jet streams. A strengthening of ozone STT is projected for the future in both hemispheres, with an induced increase in transported stratospheric ozone tracer throughout the whole troposphere, reaching up to 10 nmol mol−1 in the upper troposphere, 8 nmol mol−1 in the middle troposphere, and 3 nmol mol−1 near the surface. Notably, the regions exhibiting the largest changes of ozone STT at 400 hPa coincide with those with the highest fold frequency changes, highlighting the role of the tropopause folding mechanism in STT processes under a changing climate. For both the eastern Mediterranean and Middle East (EMME) and Afghanistan (AFG) regions, which are known as hotspots of fold activity and ozone STT during the summer period, the year-to-year variability of middle-tropospheric ozone with stratospheric origin is largely explained by the short-term variations in ozone at 150 hPa and tropopause fold frequency. Finally, ozone in the lower troposphere is projected to decrease under the RCP6.0 scenario during MAM (March, April, and May) and JJA (June, July, and August) in the Northern Hemisphere and during DJF (December, January, and February) in the Southern Hemisphere, due to the decline of ozone precursor emissions and the enhanced ozone loss from higher water vapour abundances, while in the rest of the troposphere ozone shows a remarkable increase owing mainly to the STT strengthening and the stratospheric ozone recovery.

A deep stratosphere-to-troposphere ozone transport event over Europe simulated in CAMS global and regional forecast systems: analysis and evaluation

Abstract. Stratosphere-to-troposphere transport (STT) is an important natural source of tropospheric ozone, which can occasionally influence ground-level ozone concentrations relevant for air quality. Here, we analyse and evaluate the Copernicus Atmosphere Monitoring Service (CAMS) global and regional forecast systems during a deep STT event over Europe for the time period from 4 to 9 January 2017. The predominant synoptic condition is described by a deep upper level trough over eastern and central Europe, favouring the formation of tropopause folding events along the jet stream axis and therefore the intrusion of stratospheric ozone into the troposphere. Both global and regional CAMS forecast products reproduce the “hook-shaped” streamer of ozone-rich and dry air in the middle troposphere depicted from the observed satellite images of water vapour. The CAMS global model successfully reproduces the folding of the tropopause at various European sites, such as Trapani (Italy), where a deep folding down to 550 hPa is seen. The stratospheric ozone intrusions into the troposphere observed by WOUDC ozonesonde and IAGOS aircraft measurements are satisfactorily forecasted up to 3 days in advance by the CAMS global model in terms of both temporal and vertical features of ozone. The fractional gross error (FGE) of CAMS ozone day 1 forecast between 300 and 500 hPa is 0.13 over Prague, while over Frankfurt it is 0.04 and 0.19, highlighting the contribution of data assimilation, which in most cases improves the model performance. Finally, the meteorological and chemical forcing of CAMS global forecast system in the CAMS regional forecast systems is found to be beneficial for predicting the enhanced ozone concentrations in the middle troposphere during a deep STT event.

The development of a digital gamma-gamma coincidence/anticoincidence spectrometer and its applications to monitor low-level atmospheric 22Na/7Be activity ratios in Resolute Bay, Canada

Using a previously developed digital gamma-gamma coincidence/anticoincidence spectrometer, daily aerosol samples collected at Resolute Bay, Canada (74.71°N, 94.97°W) from May 2016 to April 2017 were analysed for activity concentrations of 22Na and 7Be. The spectrometer design allows a more selective measurement of 22Na with a significant background reduction by gamma-gamma coincidence events processing. It has been demonstrated that the improved spectrometer provides a more sensitive and effective way to quantify trace amounts of 22Na and 7Be with a critical limit of 3 mBq and 5 Bq respectively for a 20 h counting. The 7Be/22Na ratio data set obtained in this study showed significant annual variation, which has a distinct spring (typically from February to May) maximum and winter (from September to February of next calendar year) minimum, which suggest that it could be used as a radiochronometer for studying the atmospheric processes. The 7Be/22Na ratios are most likely connected to deep stratosphere-to-troposphere transport (STT) exchange events where air with a higher 7Be/22Na ratio originates from downward flow from stratosphere to the troposphere. The aerosols with lower 7Be/22Na ratios located between two oscillation peaks may have longer residence time. The correlations between 7Be and 22Na activity concentration were the high during these time periods. Compared with other studies based on weekly collected aerosol samples, the techniques greatly improve the temporal resolution of 7Be/22Na data set that will be able to provide more detailed information to study various atmospheric phenomena.

Exploring the relationship between the cloud-top and tropopause height in boreal summer over the Tibetan Plateau and its adjacent region

AbstractThe distribution characteristics of cloud-top and tropopause height in the tropics and subtropics in boreal summer are analyzed based on CALIPSO data for the period 2008–2012. The maximum values of cloud-top vertical cumulative frequency above the tropopause (CTAT) are concentrated in three tropical regions: the Asian summer monsoon region, Central America, and western Africa. The contributions to the area-weighted CTAT frequency in the three regions from the Northern Hemisphere are 49.0%, 13.5%, and 12.4%, respectively. Moreover, the contribution of troposphere-to-stratosphere transport (TST) in the Asian monsoon region to global TST can be far greater than 50%, according to analysis of the continuous equation, velocity potential, and divergent wind from ERA-Interim data. Furthermore, the Asian summer monsoon circulation system controls the distribution of the cloud top. On the south side of the Tibetan Plateau, the maximum frequency of the cloud top, more than 10% per 500 m vertically, is most l...

Long-term trends of surface ozone and its influencing factors at the Mt Waliguan GAW station, China – Part 2: The roles of anthropogenic emissions and climate variability

Abstract. Inter-annual variability and long-term trends in tropospheric ozone are both environmental and climate concerns. Ozone measured at Mt Waliguan Observatory (WLG, 3816 m a.s.l.) on the Tibetan Plateau over the period of 1994–2013 has increased significantly by 0.2–0.3 ppbv yr−1 during spring and autumn but shows a much smaller trend in winter and no significant trend in summer. Here we explore the factors driving the observed ozone changes at WLG using backward trajectory analysis, chemistry–climate model hindcast simulations (GFDL AM3), a trajectory-mapped ozonesonde data set, and several climate indices. A stratospheric ozone tracer implemented in GFDL AM3 indicates that stratosphere-to-troposphere transport (STT) can explain ∼ 60 % of the simulated springtime ozone increase at WLG, consistent with an increase in the NW air-mass frequency inferred from the trajectory analysis. Enhanced STT associated with the strengthening of the mid-latitude jet stream contributes to the observed high ozone anomalies at WLG during the springs of 1999 and 2012. During autumn, observations at WLG are more heavily influenced by polluted air masses originating from South East Asia than in the other seasons. Rising Asian anthropogenic emissions of ozone precursors are the key driver of increasing autumnal ozone observed at WLG, as supported by the GFDL AM3 model with time-varying emissions, which captures the observed ozone increase (0.26 ± 0.11 ppbv yr−1). AM3 simulates a greater ozone increase of 0.38 ± 0.11 ppbv yr−1 at WLG in autumn under conditions with strong transport from South East Asia and shows no significant ozone trend in autumn when anthropogenic emissions are held constant in time. During summer, WLG is mostly influenced by easterly air masses, but these trajectories do not extend to the polluted regions of eastern China and have decreased significantly over the last 2 decades, which likely explains why summertime ozone measured at WLG shows no significant trend despite ozone increases in eastern China. Analysis of the Trajectory-mapped Ozonesonde data set for the Stratosphere and Troposphere (TOST) and trajectory residence time reveals increases in direct ozone transport from the eastern sector during autumn, which adds to the autumnal ozone increase. We further examine the links of ozone variability at WLG to the quasi-biennial oscillation (QBO), the East Asian summer monsoon (EASM), and the sunspot cycle. Our results suggest that the 2–3-, 3–7-, and 11-year periodicities are linked to the QBO, EASM index, and sunspot cycle, respectively. A multivariate regression analysis is performed to quantify the relative contributions of various factors to surface ozone concentrations at WLG. Through an observational and modelling analysis, this study demonstrates the complex relationships between surface ozone at remote locations and its dynamical and chemical influencing factors.

Mechanisms Governing Interannual Variability of Stratosphere‐to‐Troposphere Ozone Transport

AbstractFactors governing the strength and frequency of stratospheric ozone intrusions over the Pacific‐North American region are considered for their role in modulating tropospheric ozone on interannual timescales. The strength of the association between two major modes of climate variability—the El Niño–Southern Oscillation (ENSO) and the Northern Annular Mode (NAM)—and the amount of ozone contained in stratospheric intrusions are tested in the context of two mechanisms that modulate stratosphere‐to‐troposphere transport (STT) of ozone: (StratVarO3) the winter season buildup of ozone abundances in the lowermost stratosphere (LMS) and (JetVar) Pacific jet and wave breaking variability during spring. In essence, StratVarO3 corresponds to variability in the amount of ozone per intrusion, while JetVar governs the frequency of intrusions. The resulting analysis, based on two different reanalysis products, suggests that StratVarO3 is more important than JetVar for driving interannual variations in STT of ozone over the Pacific‐North American region. In particular, the abundance of ozone in the LMS at the end of winter is shown to be a robust indicator of the amount of ozone that will be contained in stratospheric intrusions during the ensuing spring. Additionally, it is shown that the overall strength of the winter season stratospheric NAM is a useful predictor of ozone intrusion strength. The results also suggest a nuanced relationship between the phase of ENSO and STT of ozone. While ENSO‐related jet variability is associated with STT variability, it is wave breaking frequency rather than typical ENSO teleconnection patterns that is responsible for the ENSO‐STT relationship.

Ozone Variability and Anomalies Observed during SENEX and SEAC4RS Campaigns in 2013.

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 anti-correlated (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 mid-troposphere 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%.