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Due to very few reports of [delta].sup.13 CO.sub.2 (the stable carbon isotopic ratio of CO.sub.2) observations in the stratosphere, its variations are not well understood. In order to elucidate stratospheric [delta].sup.13 CO.sub.2 variations and their governing mechanisms, we have collected stratospheric air samples using balloon-borne cryogenic samplers over Japan since 1985 and analyzed them for [delta].sup.13 CO.sub.2 . To obtain precise [delta].sup.13 CO.sub.2 values, we incorporated the mass-independent fractionation of .sup.17 O and .sup.18 O in the [delta].sup.13 CO.sub.2 calculation. [delta].sup.13 CO.sub.2 has decreased through time in the mid-stratosphere with an average rate of change of -0.026 ± 0.001 0/00 yr.sup.-1 for the period 1985-2020, consistent with that in the troposphere. However, mid-stratospheric [delta].sup.13 CO.sub.2 values did not show a time delay compared to the tropical tropospheric values. This could be explained by the production of CO.sub.2 by CH.sub.4 oxidation and the gravitational separation of .sup.13 CO.sub.2 and .sup.12 CO.sub.2 . To confirm this hypothesis, we used a two-dimensional model to simulate the stratospheric [delta].sup.13 CO.sub.2 values while accounting for these processes. The results indicate that these two effects strongly impact the vertical distribution of [delta].sup.13 CO.sub.2 . We newly defined \"stratospheric potential [delta].sup.13 C\" ([delta].sup.13 C.sub.P) as a quasi-conservative parameter incorporating the kinetic isotope effect of CH.sub.4 oxidation and gravitational separation, and we found that [delta].sup.13 C.sub.P in the mid-latitude mid-stratosphere decreases over time with about a 5 yr lag relative to the tropical upper troposphere. This fact strongly supports that stratospheric [delta].sup.13 CO.sub.2 variations are governed by the airborne production of .sup.13 C-depleted CO.sub.2 by CH.sub.4 oxidation, the gravitational separation, and the propagation of the decreasing tropospheric [delta].sup.13 CO.sub.2 trend into the stratosphere.

The Asian Summer Monsoon (ASM) is a seasonal weather pattern characterized by heavy rains and winds, mainly affecting South and Southeast Asia during the summer months. The deep convection within the ASM is an important transport process for pollutants from the planetary boundary layer up to the tropopause region. This study uses in situ observations of CH.sub.2 Cl.sub.2 from the PHILEAS (Probing High Latitude Export of Air from the Asian Summer Monsoon) aircraft campaign in late summer 2023 to examine the transport pathways and timescales for polluted air from the ASM to the extratropical upper troposphere and lower stratosphere (UTLS). CH.sub.2 Cl.sub.2 mixing ratios of up to 300 ppt (â 500 % of the Northern Hemisphere background) were measured in the upper troposphere in the subarctic region. The three largest observed pollution events were analyzed with the help of the Lagrangian particle dispersion model FLEXPART, both in terms of their origin and their potential entry into the lower stratosphere. The results show that the East Asian Summer Monsoon (EASM) is the key pathway for transporting uncontrolled short-lived chlorinated substances (Cl-VSLSs) into the tropopause region, which contributes to an increase in tropospheric background levels with the potential to enter the lower stratosphere. The transport analysis of elevated mixing ratios shown here suggests that transport to the upper troposphere in the subarctic region did not occur through transport into the Asian Summer Monsoon Anticyclone (ASMA) with subsequent eddy-shedding events but rather by large convective transport contributions from the EASM. The projected entry into the lower stratosphere in the following days amounts to a few percent, indicating that the direct influence of these particular events on the lower stratosphere is probably minor.

Three years of multi-axis differential optical absorption spectroscopy (MAXDOAS) measurements (2011-2013) have been used for estimating the NO.sub.2 mixing ratio along a horizontal line of sight from the high mountain subtropical observatory of Izaña, at 2370 m a.s.l. (NDACC station, 28.3° N, 16.5° W). The method is based on horizontal path calculation from the O.sub.2 -O.sub.2 collisional complex at the 477 nm absorption band which is measured simultaneously to the NO.sub.2 column density, and is applicable under low aerosol-loading conditions. The MAXDOAS technique, applied in horizontal mode in the free troposphere, minimizes the impact of the NO.sub.2 contamination resulting from the arrival of marine boundary layer (MBL) air masses from thermally forced upwelling breeze during middle hours of the day. Comparisons with in situ observations show that during most of the measuring period, the MAXDOAS is insensitive or very slightly sensitive to the upwelling breeze. Exceptions are found for pollution events during southern wind conditions. On these occasions, evidence of fast, efficient and irreversible transport from the surface to the free troposphere is found. Background NO.sub.2 volume mixing ratio (vmr), representative of the remote free troposphere, is in the range of 20-45 pptv. The observed seasonal evolution shows an annual wave where the peak is in phase with the solar radiation. Model simulations with the chemistry-climate CAM-Chem model are in good agreement with the NO.sub.2 measurements, and are used to further investigate the possible drivers of the NO.sub.2 seasonality observed at Izaña.

Aerosol particles with diameters larger than 40 nm were collected during the flight campaign StratoClim 2017 within the Asian tropopause aerosol layer (ATAL) of the 2017 monsoon anticyclone above the Indian subcontinent. A multi-impactor system was installed on board the aircraft M-55 Geophysica, which was operated from Kathmandu, Nepal. The size and chemical composition of more than 5000 refractory particles/inclusions of 17 selected particle samples from seven different flights were analyzed by use of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with energy dispersive X-ray (EDX) microanalysis. Based on chemical composition and morphology, the refractory particles were assigned to the following particle groups: extraterrestrial, silicates, Fe-rich, Al-rich, Hg-rich, other metals, C-rich, soot, Cl-rich, and Ca-rich.

Atmospheric CO.sub.2 retrievals with peak sensitivity in the mid- to lower troposphere from the Atmospheric Infrared Sounder (AIRS) have been assimilated into the GEOS-5 (Goddard Earth Observing System Model, Version 5) constituent assimilation system for the period 1 January 2005 to 31 December 2006. A corresponding model simulation, using identical initial conditions, circulation, and CO.sub.2 boundary fluxes was also completed. The analyzed and simulated CO.sub.2 fields are compared with surface measurements globally and aircraft measurements over North America. Surface level monthly mean CO.sub.2 values show a marked improvement due to the assimilation in the Southern Hemisphere, while less consistent improvements are seen in the Northern Hemisphere. Mean differences with aircraft observations are reduced at all levels, with the largest decrease occurring in the mid-troposphere. The difference standard deviations are reduced slightly at all levels over the ocean, and all levels except the surface layer over land. These initial experiments indicate that the used channels contain useful information on CO.sub.2 in the middle to lower troposphere. However, the benefits of assimilating these data are reduced over the land surface, where concentrations are dominated by uncertain local fluxes and where the observation density is quite low. Away from these regions, the study demonstrates the power of the data assimilation technique for evaluating data that are not co-located, in that the improvements in mid-tropospheric CO.sub.2 by the sparsely distributed partial-column retrievals are transported by the model to the fixed in situ surface observation locations in more remote areas.

The mid-infrared lightweight tunable diode laser hygrometer, \"Pico-Light H.sub.2 O\", the successor to Pico-SDLA H.sub.2 O, is presented and its performances are evaluated during the AsA 2022 balloon-borne intercomparison campaign conducted at the CNES Aire-sur-l'Adour (AsA, 43.70° N; 0.25° W) balloon launch facility and the Aeroclub d'Aire-sur-l'Adour in France. The Pico-Light instrument has primarily been developed for sounding of the upper troposphere and stratosphere, although during the AsA 2022 campaign we expand the range of comparison to include additionally the lower troposphere. Three different types of hygrometer and two models of radiosonde were flown, operated by the French Space Agency (CNES) and the NOAA Global Monitoring Laboratory (GML) scientific teams: Pico-Light H.sub.2 O, the NOAA Frost Point Hygrometer (FPH), the micro-hygrometer (in an early phase of development), and M20 and iMet-4 sondes. Within this framework, we intend to validate measurements of Pico-Light H.sub.2 O through a first intercomparison with the NOAA FPH instrument. The in situ monitoring of water vapour in the upper troposphere-lower stratosphere continues to be very challenging from an instrumental point of view because of the very small amounts of water vapour to be measured in these regions of the atmosphere. Between the lapse rate tropopause (11-12.3 km) and 20 km, the mean relative difference between water vapour mixing ratio measurements by Pico-Light H.sub.2 O and NOAA FPH was 4.2 % ± 7.7 %, and the mean tropospheric difference was 3.84 % ± 23.64 %, with differences depending on the altitude range considered. In the troposphere, relative humidity (RH) over water comparisons led to agreement between Pico-Light and NOAA FPH of -0.2 % on average, with excursions of about 30 % RH due to moisture variability. Expanding the comparison to meteorological sondes, the iMet-4 sondes agree well with both Pico-Light and FPH between the ground and 7.5 km (within ± 3 % RH), as do the M20 sondes, up to 13 km, which are wet-biased by 3 % RH and dry-biased by 20 % in cases of saturation.

A new approximation is proposed to estimate O.sub.3 and NO.sub.2 mixing ratios in the northern subtropical free troposphere (FT). The proposed method uses O.sub.4 slant column densities (SCDs) at horizontal and near-zenith geometries to estimate a station-level differential path. The modified geometrical approach (MGA) is a simple method that takes advantage of a very long horizontal path to retrieve mixing ratios in the range of a few pptv. The methodology is presented, and the possible limitations are discussed. Multi-axis differential optical absorption spectroscopy (MAX-DOAS) high-mountain measurements recorded at the Izaña observatory (28° 18' N, 16° 29' W) are used in this study. The results show that under low aerosol loading, O.sub.3 and NO.sub.2 mixing ratios can be retrieved even at very low concentrations. The obtained mixing ratios are compared with those provided by in situ instrumentation at the observatory. The MGA reproduces the O.sub.3 mixing ratio measured by the in situ instrumentation with a difference of 28%. The different air masses scanned by each instrument are identified as a cause of the discrepancy between the O.sub.3 observed by MAX-DOAS and the in situ measurements. The NO.sub.2 is in the range of 20-40 ppt, which is below the detection limit of the in situ instrumentation, but it is in agreement with measurements from previous studies for similar conditions.

Global positioning system (GPS) radio occultation (RO) observations, first made of Earth’s atmosphere in 1995, have contributed in new ways to the understanding of the thermal structure and variability of the tropical upper troposphere–lower stratosphere (UTLS), an important component of the climate system. The UTLS plays an essential role in the global radiative balance, the exchange of water vapor, ozone, and other chemical constituents between the troposphere and stratosphere, and the transfer of energy from the troposphere to the stratosphere. With their high accuracy, precision, vertical resolution, and global coverage, RO observations are uniquely suited for studying the UTLS and a broad range of equatorial waves, including gravity waves, Kelvin waves, Rossby and mixed Rossby–gravity waves, and thermal tides. Because RO measurements are nearly unaffected by clouds, they also resolve the upper-level thermal structure of deep convection and tropical cyclones as well as volcanic clouds. Their low biases and stability from mission to mission make RO observations powerful tools for studying climate variability and trends, including the annual cycle and intraseasonal-to-interannual atmospheric modes of variability such as the quasi-biennial oscillation (QBO), Madden–Julian oscillation (MJO), and El Niño–Southern Oscillation (ENSO). These properties also make them useful for evaluating climate models and detection of small trends in the UTLS temperature, key indicators of climate change. This paper reviews the contributions of ROobservations to the understanding of the three-dimensional structure of tropical UTLS phenomena and their variability over time scales ranging from hours to decades and longer.

The complex distribution of CO.sub.2 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 CO.sub.2 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 CO.sub.2 distribution simulated with the ECHAM/MESSy Atmospheric Chemistry (EMAC) global chemistry-climate model. By simulating an artificial, deseasonalised CO.sub.2 tracer, we disentangle the CO.sub.2 seasonal signal from long-term trend and transport contribution. This approach allows us to study the CO.sub.2 seasonal cycle in a unique way in remote areas and on a global scale. Our results show that the tropospheric CO.sub.2 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 CO.sub.2 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 CO.sub.2 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.

Assessing global models in the upper troposphere (UT) and in the lowermost stratosphere (LS) is an important step toward a better understanding of the chemical composition near the tropopause. For this purpose, the current study focuses on an evaluation of long-term simulations from four chemistry-climate/transport models, based on In-service Aircraft for a Global Observing System (IAGOS) measurements. Most simulations span the period from 1995 to 2017 and follow a common protocol among models. The assessment focuses on climatological averages of ozone (O.sub.3 ), water vapour (H.sub.2 O), carbon monoxide (CO), and reactive nitrogen (NO.sub.y). In the extra-tropics, the models reproduce the seasonality of O.sub.3, H.sub.2 O, and NO.sub.y in both the UT and LS, but none of them reproduce the CO springtime maximum in the UT. Tropospheric tracers (CO and H.sub.2 O) tend to be underestimated in the UT, consistently with an overestimation of cross-tropopause exchanges. Most models systematically overestimate ozone in the UT, and the background of nitrogen oxides (NO.sub.x) appears to be the main contributor to ozone variability across the models. The partitioning between NO.sub.y species changes drastically across the models and acts as a source of uncertainty in the NO.sub.x mixing ratio and on the impact of these species on atmospheric composition. However, we highlight some well-reproduced geographical variations, such as the Intertropical Convergence Zone (ITCZ) seasonal shifts above Africa and the correlation of extratropical ozone (H.sub.2 O) in the LS (UT) with the observations. These features are encouraging with respect to the simulated dynamics in both layers. The current study confirms the importance of separating the UT and the LS with a dynamical tracer for the evaluation of model results and for model intercomparisons.