Comment on acp-2022-274

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

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.

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.

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.

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. 

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.

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.

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

<p>The cumulative impacts of frequent moderate-magnitude eruptions on stratospheric aerosols were identified among the factors in recent decadal climate trends. Moderate volcanic eruptions are a recurrent source of sulfur dioxide (SO2) in the Upper Troposphere and Lower Stratosphere (UTLS) region and the resulting formation of sulfuric acid aerosol particles from the SO2 emitted provides sites for chemical reactions leading to enhancement of stratospheric optical depth (SAOD) and ozone depletion. Modelling properly the volcanic aerosol content and its evolution in this region is important for radiative impact issues. In this work, we explore the variability of the tropical UTLS aerosol content between 2013 and 2019, a period which was particularly impacted by moderate tropical and mid-latitude volcanic eruptions. For that purpose, space-borne observations from OMPS (version 2, datasets from GES DISC), and IASI, together with simulations by the Whole Atmosphere Community Climate Model (WACCM) coupled with the Community Aerosol and Radiation Model for Atmospheres (CARMA), are used. Different model sensitive experiments, particularly for the injection altitude and timing, have been conducted to evaluate how the model captures the aerosol plume in terms of content, optical and microphysical properties, transport and residence time. We find that the decay of the Calbuco and Kelud plumes observed by OMPS version 2 is well reproduced by the model. Comparisons with unique datasets in the tropical southern hemisphere from the NDACC Maïdo observatory (Reunion Island, France, 20.5°S, 55.5°E) show good agreement between the lidar SAOD observations and WACCM-CARMA SAOD simulations although we observe a difference in the altitude of the maximum aerosol concentration between the model and the in situ profile after Calbuco eruption in April 2015. A particular focus is also made on recent eruptions like Raikoke, Ambae and Ulawun. The plume of the Ambae volcano (15°S, 167°E) which erupted in July 2018 is shown to propagate to the northern hemisphere with some influence until summer 2019 in the Asian monsoon region. For the year 2019, we investigate how the Ulawun (5°S, 151°E; ~0.14 Tg of SO2) tropical eruption and the Raikoke mid-latitude eruption (48°N, 153°E; ~1.5Tg of SO2), have influenced the aerosol burden in the tropics.</p>

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.

Both the scientific and operational communities are increasingly interested in subseasonal to seasonal variations of weather and climate. The Semi-Annual Oscillation (SAO) has been studied extensively at the surface as well as in the middle atmosphere (upper stratosphere and the lower mesosphere). However, the SAO in the upper troposphere and lower stratosphere (UTLS) has been less discussed. Here we find evident SAO of temperature in the UTLS (250–175 hPa) from the subtropics to mid-latitudes (22.5–42.5º) using high-quality satellite measurements, reanalysis data, and model simulations. We show the mechanism of its formation by an energy budget analysis. The temperature in the Northern Hemisphere (NH) UTLS shows the first peak in February according to the dynamical heating and shows the second peak in July due to the dynamical heating and moist processes. Similar to the NH, the winter time maximum of temperature in the SH is related to dynamical heating and the summer time maximum is related to both moist and dynamical heating in the UTLS. Model simulations indicate that the SAO in the UTLS is partly modified by the SAO of sea surface temperatures (SSTs).

Abstract. The complex distribution of CO2 in the upper troposphere and lower stratosphere (UTLS) results from the interplay of different processes and mechanisms. However, in such difficult-to-access regions of the atmosphere our understanding of the CO2 variability remains limited. Using vertical trace gas profiles derived from measurements with the balloon-based AirCore technique for validation, we investigate the UTLS and stratospheric CO2 distribution simulated with the ECHAM/MESSy Atmospheric Chemistry (EMAC) global chemistry-climate model. By simulating an artificial, deseasonalised CO2 tracer, we disentangle the CO2 seasonal signal from long-term trend and transport contribution. This approach allows us to study the CO2 seasonal cycle in a unique way in remote areas and on a global scale. Our results show that the tropospheric CO2 seasonal cycle propagates upwards into the lowermost stratosphere and is most modulated in the extra-tropics between 300–100 hPa, characterised by a 50 % amplitude dampening and a 4-month phase shift in the Northern Hemisphere mid-latitudes. During this propagation the seasonal cycle shape is also tilted, which is associated with the transport barrier related to the strength of the subtropical jet. In the stratosphere, we identified both, a vertical and a horizontal “tape recorder” of the CO2 seasonal cycle. Originating in the tropical tropopause region this imprint is linked to the upwelling and the shallow branch of the Brewer-Dobson-circulation. As the CO2 seasonal signal carries information about transport processes on different timescales, the newly introduced tracer is a very useful diagnostic tool and would also be a suitable metric for model intercomparisons.

<strong class="journal-contentHeaderColor">Abstract.</strong> <span id="page2050"/>Efficient transport pathways for ozone-depleting very short-lived substances (VSLSs) from their source regions into the stratosphere are a matter of current scientific debate; however they have yet to be fully identified on an observational basis. Understanding the increasing impact of chlorine-containing VSLSs (Cl-VSLSs) on stratospheric ozone depletion is important in order to validate and improve model simulations and future predictions. We report on a transport study using airborne in situ measurements of the Cl-VSLSs dichloromethane (CH<span class="inline-formula">₂</span>Cl<span class="inline-formula">₂</span>) and trichloromethane (chloroform, CHCl<span class="inline-formula">₃</span>) to derive a detailed description of two transport pathways from (sub)tropical source regions into the extratropical upper troposphere and lower stratosphere (Ex-UTLS) in the Northern Hemisphere (NH) late summer. The Cl-VSLS measurements were obtained in the upper troposphere and lower stratosphere (UTLS) above western Europe and the midlatitude Atlantic Ocean in the frame of the WISE (Wave-driven ISentropic Exchange) aircraft campaign in autumn 2017 and are combined with the results from a three-dimensional simulation of a Lagrangian transport model as well as back-trajectory calculations. Compared to background measurements of similar age we find up to <span class="inline-formula">150</span> % enhanced CH<span class="inline-formula">₂</span>Cl<span class="inline-formula">₂</span> and up to <span class="inline-formula">100</span> % enhanced CHCl<span class="inline-formula">₃</span> mixing ratios in the extratropical lower stratosphere (Ex-LS). We link the measurements of enhanced CH<span class="inline-formula">₂</span>Cl<span class="inline-formula">₂</span> and CHCl<span class="inline-formula">₃</span> mixing ratios to emissions in the region of southern and eastern Asia. Transport from this area to the Ex-LS at potential temperatures in the range of 370–400 K takes about 6–11 weeks via the Asian summer monsoon anticyclone (ASMA). Our measurements suggest anthropogenic sources to be the cause of these strongly elevated Cl-VSLS concentrations observed at the top of the lowermost stratosphere (LMS). A faster transport pathway into the Ex-LS is derived from particularly low CH<span class="inline-formula">₂</span>Cl<span class="inline-formula">₂</span> and CHCl<span class="inline-formula">₃</span> mixing ratios in the UTLS. These low mixing ratios reflect weak emissions and a local seasonal minimum of both species in the boundary layer of Central America and the tropical Atlantic. We show that air masses uplifted by hurricanes, the North American monsoon, and general convection above Central America into the tropical tropopause layer to potential temperatures of about 360–370 K are transported isentropically within 5–9 weeks from the boundary layer into the Ex-LS. This transport pathway linked to the North American monsoon mainly impacts the middle and lower part of the LMS with particularly low CH<span class="inline-formula">₂</span>Cl<span class="inline-formula">₂</span> and CHCl<span class="inline-formula">₃</span> mixing ratios. In a case study, we specifically analyze air samples directly linked to the uplift by the Category 5 Hurricane Maria that occurred during October 2017 above the Atlantic Ocean. CH<span class="inline-formula">₂</span>Cl<span class="inline-formula">₂</span> and CHCl<span class="inline-formula">₃</span> have similar atmospheric sinks and lifetimes, but the fraction of biogenic emissions is clearly higher for CHCl<span class="inline-formula">₃</span> than for the mainly anthropogenically emitted CH<span class="inline-formula">₂</span>Cl<span class="inline-formula">₂</span>; consequently lower CHCl<span class="inline-formula">₃</span> : CH<span class="inline-formula">₂</span>Cl<span class="inline-formula">₂</span> ratios are expected in air parcels showing a higher impact of anthropogenic emissions. The observed CHCl<span class="inline-formula">₃</span> : CH<span class="inline-formula">₂</span>Cl<span class="inline-formula">₂</span> ratio suggests clearly stronger anthropogenic emissions in the region of southern and eastern Asia compared to those in the region of Central America and the tropical Atlantic. Overall, the transport of strongly enhanced CH<span class="inline-formula">₂</span>Cl<span class="inline-formula">₂</span> and CHCl<span class="inline-formula">₃</span> mixing ratios from southern and eastern Asia via the ASMA is the main factor in increasing the chlorine loading from the analyzed VSLSs in the Ex-LS during the NH late summer. Thus, further increases in Asian CH<span class="inline-formula">₂</span>Cl<span class="inline-formula">₂</span> and CHCl<span class="inline-formula">₃</span> emissions, as frequently reported in recent years, will further increase the impact of Cl-VSLSs on stratospheric ozone depletion.

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.