Reply on RC1

The lower tropospheric ozone enhancement over Central and Eastern China (CEC) was reported by Hayashida et al. (Atmos Chem Phys 15(17):9865–9881, 2015) using the Ozone Monitoring Instrument (OMI) multiple-layer product retrieved by Liu et al. (Atmos Chem Phys 10(5):2521–2537, 2010), which first showed the lower tropospheric ozone enhancement from ultraviolet and visible (UV-Vis) spectra measurements from space. However, to clarify the enhancement in the concentration of the lowermost ozone using spaceborne measurements, it is necessary to understand the effect of ozone variation in the upper troposphere and lower stratosphere (UT/LS), because of large smoothing errors in the retrieval scheme. In this study, a scheme was developed to eliminate the artificial effect of UT/LS ozone enhancement on lower tropospheric ozone retrieval using OMI. By applying the UT/LS screening scheme for June 2006, we removed the artificial effect of the UT/LS ozone enhancement on the lower tropospheric ozone. Even after UT/LS screening, we were able to show a clear enhancement in the lower tropospheric ozone over CEC in June 2006 and confirmed the conclusion derived by Hayashida et al. (Atmos Chem Phys 15(17):9865–9881, 2015). To clarify the reason for ozone enhancement in June, the effects of emissions from open crop residue burning (OCRB) in the North China Plain on lower tropospheric ozone were also examined using a comparison with model simulations. On the scale of the vertical resolution of OMI observations, the effect of OCRB on ozone enhancement does not seem to be significant, although it may be more significant when focusing on ozone in the planetary boundary layer.

Airborne in situ measurements of short-lived chlorocarbons and investigation of their pathways from northern hemispheric source regions into the lowermost stratosphere

The impact of transient tropospheric forcing on the deep vertical mountain-wave propagation is investigated by a unique combination of in situ and remote sensing observations and numerical modeling. The temporal evolution of the upstream low-level wind follows approximately a [Formula: see text] shape and was controlled by a migrating trough and connected fronts. Our case study reveals the importance of the time-varying propagation conditions in the upper troposphere and lower stratosphere (UTLS). Upper-tropospheric stability, the wind profile, and the tropopause strength affected the observed and simulated wave response in the UTLS. Leg-integrated along-track momentum fluxes ([Formula: see text]) and amplitudes of vertical displacements of air parcels in the UTLS reached up to 130 kN m−1 and 1500 m, respectively. Their maxima were phase shifted to the maximum low-level forcing by ≈8 h. Small-scale waves ([Formula: see text] km) were continuously forced, and their flux values depended on wave attenuation by breaking and reflection in the UTLS region. Only maximum flow over the envelope of the mountain range favored the excitation of longer waves that propagated deeply into the mesosphere. Their long propagation time caused a retarded enhancement of observed mesospheric gravity wave activity about 12–15 h after their observation in the UTLS. For the UTLS, we further compared observed and simulated [Formula: see text] with fluxes of 2D quasi-steady runs. UTLS momentum fluxes seem to be reproducible by individual quasi-steady 2D runs, except for the flux enhancement during the early decelerating forcing phase.

We use the global three-dimensional Goddard Earth Observing System chemical transport model with the Universal tropospheric–stratospheric Chemistry eXtension mechanism to examine the contributions of the chemical processes to summertime O3 in the upper troposphere and lower stratosphere (UTLS) over the Tibetan Plateau and the South Asian monsoon region (TP/SASM). Simulated UTLS O3 concentrations are evaluated by comparisons with Microwave Limb Sounder products and net chemical production of O3 (NPO3) are evaluated by comparisons with model results in previous studies. Simulations show that the chemical processes lead to an increase in O3 concentration, which is opposite to the effect of O3 transport in the UTLS over the TP/SASM region throughout the boreal summer. NPO3 in UTLS over the TP/SASM region is the largest in summer. Elevated values (0.016–0.020 Tg year−1) of the seasonal mean NPO3 are simulated to locate at 100 hPa in the TP/SASM region, where the mixing ratios of O3 are low and those of O3 precursors (NOx, VOCs, and CO) are high. The high concentrations of O3 precursors (NOx, VOCs, and CO) together with the active photochemical reactions of NO2 in the UTLS over the TP/SASM region during summertime could be important reasons for the enhancement of $${\text{NP}}_{{{\text{O}}_{3} }}$$ over the studied region.

There are two key modes of the South Asia high (SAH) during the boreal summer: the Iranian Plateau (IP) and Tibetan Plateau (TP) modes. The anomalies of chemical constituents in the upper troposphere and lower stratosphere (UTLS) region within the SAH area largely depend on this bimodality. To better understand the underlying mechanisms of this dependence, the ensemble of 30-day backward trajectories, initialized in the UTLS region within the SAH, was simulated by a Lagrangian model FLEXPART. The comparative diagnostic was performed from the perspective of atmospheric ventilation. The results show that vertical transport from the lower troposphere to the UTLS during the TP mode was very efficient, resulting in tropospheric air mass transported into the UTLS within a shorter timescale than during the IP mode. Furthermore, the effect of SAH isolation during the TP mode was stronger than during the IP mode. This stronger trapping is likely to force the tropospheric air mass to reside in the SAH area for a longer period of time. In addition, compared to the IP mode, near-surface air mass sources during the TP mode overlapped more with areas of severe air pollution (CO emissions). The above three factors associated with the processes of atmospheric ventilation, i.e., the efficiency of vertical transport, the strength of the SAH isolation, and the boundary layer sources, provide potential explanations as to why the anomalies of atmospheric constituents in the UTLS are different between the TP and the IP mode.

As one of the most important greenhouse gases, water vapor in the upper troposphere and lower stratosphere (UTLS) has a significant impact on the global earth-atmosphere system. The Tibetan Plateau (TP) is an important high terrain which exerts a profound impact on the change of weather and climate, and mass exchange. Tropopause folds occur frequently over the TP due to the impact of the subtropical westerly jet, which affects water vapor transport between the stratosphere and the troposphere. In this paper, the spatial and temporal distribution characteristics of tropopause folds over the TP are examined by applying an improved three-dimensional (3D) labeling algorithm to the ERA5 reanalysis data (1979 to 2019). The effects of different fold depths in various regions over the TP on the variations of UTLS water vapor are further studied. The results of a case study (25 February 2008) suggest that there is a good continuity in identification of the fold depth for the same fold event using the improved 3D labeling algorithm. The fold depth and height are consistent with the results of radiosonde data and ERA5 reanalysis data. The fold frequency over the TP shows an increasing trend in the last 41 years, with slightly lower frequency of medium folds than that of shallow folds, and lowest frequency of deep folds. There is increasing water vapor in the UTLS over the TP due to tropopause folds. The results indicate that tropopause folds enhance the horizontal divergence of water vapor in the UTLS and increase the vertical water vapor flux in the UTLS region. The folding over the plateau leads to increased moisture in the UTLS. It is argued that vertical velocity anomalies in the vicinity of the fold and subgrid perturbations have a significant impact on the increase of UTLS water vapor over the TP. The results of this work provide a scientific basis for a better understanding of the stratosphere-troposphere exchanges due to tropopause folds over the TP.

Sulfur compounds in the upper troposphere and lower stratosphere (UTLS) impact the atmosphere radiation budget, either directly as particles or indirectly as precursor gas for new particle formation. In situ measurements in the UTLS are rare, but are important to better understand the impact of the sulfur budget on climate. The BLUESKY mission in May/June 2020 explored an unprecedented situation. 1) The UTLS experienced extraordinary dry conditions in spring 2020 over Europe, in comparison to previous years and 2) the first lockdown of the COVID-19 pandemic caused major emission reductions from industry, ground, and airborne transportation. With the two research aircraft HALO and Falcon, 20 flights were conducted over Central Europe and the North Atlantic to investigate the atmospheric composition with respect to trace gases, aerosol, and clouds. Here, we focus on measurements of sulfur dioxide (SO2) and particulate sulfate (SO42-) in the altitude range of 8 to 14.5 km which show unexpectedly enhanced mixing ratios of SO2 in the upper troposphere and of SO42- in the lowermost stratosphere. In the UT, we find SO2 mixing ratios of (0.07 ± 0.01) ppb, caused by the remaining air traffic, reduced SO2 sinks due to low OH and low cloud fractions, and to a minor extend by uplift from boundary layer sources. Particulate sulfate showed elevated mixing ratios of up to 0.33 ppb in the LS. We suggest that the eruption of the volcano Raikoke in June 2019, which emitted about 1 Tg SO2 into the stratosphere in northern midlatitudes caused these enhancements, in addition to Siberian and Canadian wildfires and other minor volcanic eruptions. Our measurements can help to test models and lead to new insights in the distribution of sulfur compounds in the UTLS, their sources and sinks. Moreover, these results can contribute to improve simulations of the radiation budget in the UTLS with respect to sulfur effects.

Aerosol characteristics in the UTLS region: A satellite-based study over north India

Nitric oxide (NO) and nitrogen dioxide (NO2) are important trace gases in the upper troposphere and lower stratosphere (UTLS).  In the upper troposphere, NO is primarily formed by lightning and can react with other radicals to form a suite of reactive nitrogen (NOy) species and produce ozone.  In the lower stratosphere, NOx can be responsible for catalytic ozone destruction and can form halogen reservoirs such as ClNO2 and ClONO2.  Accurate measurements of NOx species are therefore important for understanding and diagnosing UTLS chemical regimes.  Further, understanding NO2 concentrations in the stratosphere and upper troposphere is critical for accurate satellite retrievals of tropospheric NO2. In-situ measurements of NOx species above 12-15 km are infrequent and challenging.  Where there are measurements of NOx species up to about 12 km, NO mixing ratios tend to be reasonably well-reproduced using global models.  However, NO2 measurements frequently agree well with models and photostationary state calculations up to about 8 km, but at higher altitudes routinely are significantly higher than expected relative to the measured NO.  Proposed sources of this discrepancy include measurement artifacts affecting the in-situ measurements, inaccuracies in rate constants at low temperatures, and missing chemistry.  The net result is significant uncertainty in the NO2 concentrations in the UTLS.Here we present results from the recent high-altitude aircraft campaign Asian Summer Monsoon Chemistry and CLimate Impact Project (ACCLIP) over southeast Asia in summer 2022.  We show measurements from the recently-developed NOAA NO laser-induced fluorescence instrument up to altitudes of almost 20 km.  This instrument makes direct NO measurements with precision and accuracy sufficient for measurements below one ppt with one second of integration, making it ideal for aircraft campaigns.  In order to measure NO2 with this instrument, we have developed a photolysis inlet which converts NO2 to NO using an LED centered at 395 nm.  What is novel about this NO2 conversion is that it is done in an unpressurized pylon of the aircraft and thus the sample remains at the UTLS ambient temperatures until after photolysis.  This thus significantly reduces the potential interferences from NOy species that can thermally decompose to NO2 at room temperature, eliminating a potential source of major artifact.  Our data show good agreement with photostationary state calculations performed for these flights. 

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

We describe a near-infrared in situ tunable diode laser spectrometer developed for atmospheric measurements of CH4 in the upper troposphere and lower stratosphere (UT/LS). The instrument is designed to provide fast-response (0.5–1 Hz) measurements and operate autonomously on the NASA WB-57F high-altitude aircraft. A single-mode InGaAsP distributed feedback laser diode operating at 1.6537 μm scans continuously over the R(3) rotation–vibration transition in the 2ν3 band. We use a direct absorption technique incorporating a custom-designed long path length (252 m) low-volume (3.6 L) astigmatic Herriott cell. The present detection sensitivity is 5×1010 molecules cm-3, corresponding to ∼20 ppbv in the UT/LS, with the main limit to instrument precision being background optical interference fringes. In-flight performance is demonstrated by presentation of recent data.

The Changing Atmosphere Infra-Red Tomography Explorer (CAIRT) is a satellite mission concept that was proposed within ESA's Earth Explorer 11 (EE11) programme. Although it was not selected for implementation in the EE11 competition, the CAIRT concept was assessed as scientifically and technically mature and could be implemented under another ESA framework or through international collaboration. The mission is conceived to achieve a step change in our understanding of the coupling between atmospheric circulation, composition, and regional climate. The CAIRT concept is based on limb infrared tomography of the atmosphere from the troposphere to the lower thermosphere (between ∼ 5 and ∼ 115 km altitude) with a∼400 km-wide swath, providing a three-dimensional view of atmospheric structure at unprecedented scales. This paper investigates the capability of CAIRT to resolve vertical gradients of ozone (O$_3$) and water vapour (H$_2$O) in the upper troposphere and lower stratosphere (UTLS) using an Observing System Simulation Experiment (OSSE). In this experiment, a reference atmosphere – the nature run in OSSE terminology – is built from the Copernicus Atmosphere Monitoring Service (CAMS) control simulation between October 2021 and March 2022. The nature run is used to synthesise CAIRT observations of O$_3$ and H$_2$O, employing a CAIRT orbit simulator and a simulator that accounts for CAIRT’s expected measurement errors, vertical and along-track vertical resolution. Simulated CAIRT observations are then assimilated with the Belgian Assimilation System for Chemical ObsErvations (BASCOE) to produce analyses of O$_3$ and H$_2$O (the assimilation run). To quantify the added value of CAIRT data, a BASCOE control run without CAIRT assimilation is also performed. For comparison, simulated Aura Microwave Limb Sounder (MLS) O$_3$ and H$_2$O observations are also assimilated, allowing the performance of CAIRT to be evaluated relative to this reference instrument. The results show that CAIRT (1) O$_3$ observations are able to constrain BASCOE analyses down to 7 km altitude – several kilometres lower (and thus better) than MLS – and (2) H$_2$O observations can constrain BASCOE down to the tropopause region, with slightly better performance than MLS at high latitudes. Additional sensitivity tests assess the impact of CAIRT systematic errors, cloud coverage, and reduced swath width.

A strong cooling trend (−0.62°C per decade) in the upper troposphere and lower stratosphere (UTLS) over China is detected based on the records of 109 radiosonde stations during the period from 1980 to 2004. In contrast, the underlying mid and lower troposphere (MLT) exhibits a relatively weak warming trend (0.17°C per decade) with the largest magnitude at the surface. Accompanying these changes in air temperature, large‐scale circulation associated with the monsoon activity in East Asia declines remarkably in both winter and summer. Meanwhile, decreasing trend of the total ozone amount has also been found in the same period. Numerical simulations using two global coupled climate models further suggest that the ozone depletion and enhanced anthropogenic greenhouse effect play an important role in the cooling of UTLS and warming of MLT.

The summer monsoon anticyclones are the dominant climatological features of the Northern Hemispheric (NH) summertime circulation in the upper troposphere and lower stratosphere (UTLS). However, the response of these anticyclones to the increased levels of remains highly uncertain, as does the impact on the distribution of UTLS ozone and other tracers. This study examines the response of the NH summertime monsoon anticyclones and UTLS ozone to the abrupt increase in forcing using output from a suite of coupled ocean–atmosphere general circulation model simulations. These models show an equatorward shift of the Asian summer monsoon anticyclone, a weakening of the North American summer monsoon anticyclone, and a stronger westerly flow penetrating deep into the tropics above the Pacific Ocean and North America. We use additional idealized experiments from atmosphere‐only general circulation models with prescribed SSTs and sea ice concentration to isolate the direct atmospheric radiative effects from the indirect effect of SST warming on the UTLS monsoon anticyclones. Comparison between atmosphere‐only and coupled ocean–atmosphere experiments shows that SST warming is the principal mechanism producing UTLS monsoonal circulation changes. The experiments result in a significant reduction up to 40%–50% of the UTLS ozone in the northern tropics, which could have an impact on radiative balance near the surface.

Annual and interannual variations of ozone in the upper troposphere and lower stratosphere (UTLS) region have been studied using ozonesonde data collected between 1994 and 2001 at several northern European stations. The climatology of ozone exhibits a prominent annual cycle in the UTLS region. The observed change in the phase of the annual cycle from late spring‐early summer at 500 hPa to spring at 200 hPa and to winter‐early spring at 100 hPa shows the switching of the ozone control from photochemical to dynamical. Traces of interannual variation in the lower stratosphere are seen not only in the upper troposphere but also in the middle troposphere (not necessarily always) indicating the dynamical influence on tropospheric ozone budget. Further, the correlation between ozone mixing ratio and potential vorticity (PV) is studied at three northern high‐latitude stations. As expected, a good correlation is found in the lower stratosphere, while the correlation is fair in the middle troposphere, except during summer over the European Arctic. This weak correlation at high latitudes indicates the dominance of photochemistry over dynamics in the presence of prolonged hours of solar illumination. The correlation coefficients derived at high latitudes are smaller than those reported at midlatitudes. This could be due to the greater number of tropopause folds at midlatitudes than at high latitudes and this eventually leads to the conclusion that the downward cross‐tropopause flux is greater at midlatitudes than at high latitudes. Absence of a significant north‐south gradient in the ozone/PV ratio in the lower stratosphere suggests that a single ozone/PV ratio (however, the ratio varies with month) can be used to convert global PV fluxes to ozone fluxes. A few cases of tropopause folds (only one case study is reported in the present study) are selected and studied in detail with the help of a very high frequency radar and meteorological analysis. The ratio between ozone and PV for these case studies agrees reasonably well with the climatological ratios.