# Organics in the stratosphere: new insights from weather balloon flights conducted in France

Roland Benoit, Hazel Vernier, Jean-Paul Vernier, Lilian Joly, Nicolas Dumelié, Gisèle Krysztofiak, Loïc Vieille, Sandrine Delpeux, Cyril Crevoisier, Philippe Dagaut, et al.

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Roland Benoit, Hazel Vernier, Jean-Paul Vernier, Lilian Joly, Nicolas Dumelié, et al. Organics in the stratosphere: new insights from weather balloon flights conducted in France. npj climate and atmospheric science, 2025, 8 (1), pp.221. 10.1038/s41612-025-01106-9. hal-05114851

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# Organics in the stratosphere: new insights from weather balloon flights conducted in France

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Roland Benoit<fnref n="1" /> ☑, Hazel Vernier²,³, Jean-Paul Vernier⁴,⁵, Lilian Joly³, Nicolas Dumelié³, Gisèle Krysztofiak², Loïc Vieille², Sandrine Delpeux⁶, Cyril Crevoisier⁷, Philippe Dagaut<fnref n="1" /> & Gwenaël Berthet²

Atmospheric aerosols significantly influence the Earth's climate, yet they remain a major source of uncertainty in climate projections. Here, we present a set of unique offline analysis of organics collected from the ground to the stratosphere with ultimate sensitivity using a new balloon-borne sampling technique deployed on simple weather balloons. Activated and functionalized carbon fabrics were used to target chemical compounds in the gaseous and aerosol forms during three flights over France. Ultra-high-resolution mass spectrometry, enabling the detection of thousands of organic compounds, and highlighted high-carbon-number organics, indicating oligomerization in long-lived tropical stratospheric air masses. Our measurements suggest that large stratospheric sulfate aerosols from volcanic eruptions could play a role in the formation of secondary aerosols in internally mixed particles and may explain the presence of organics with higher carbon numbers. These measurements revealed a drop in the number of unsaturated carbons at higher altitudes.

A large number of studies have shown that atmospheric chemistry is affected by anthropogenic and biogenic emissions, as well as by releases caused by extreme events such as wildfires or volcanic eruptions<fnref n="1" />,². These emissions contribute to the formation of aerosols which, depending on their size and chemical composition, can be liquid or solid but can also be externally-mixed. This implies that one can observe simultaneous presence of pure solid and liquid particles or internally-mixed, i.e., included in or coated by a liquid phase³. They can also be in the form of semi-volatile particles, such as some secondary organic aerosols, or volatile liquid particles formed from gaseous precursors with dissolved materials. Gaseous aerosol precursors and primary particles injected into the atmosphere evolve as a function of physical (transport, thermodynamics, humidity), microphysical (condensation, coagulation, fragmentation, sedimentation) and chemical (photolytic and oxidation reactions) conditions⁴,⁵. As a result, various complex properties of atmospheric aerosols, mainly in terms of composition, shape, and conditions of removal from the atmosphere via evaporation/sublimation and sedimentation, were reported⁶. These aerosols present a diameter ranging from around 2 nm to 100 µm. Over time, changes in aerosol chemistry, size and morphology associated with dynamical processes and cloud formation contribute to their mixing in the troposphere while their longer lifetime in the stratosphere makes them more stable and

homogeneous. The structure of the atmospheric layers and the formation of aerosols in these different strata are well documented, including the catalytic effect of acidic environments⁷, but only broadly understood⁸. These layers need to be studied simultaneously and frequently to link chemical and morphological data with time, thermodynamic and dynamical parameters.

The upper troposphere and lower stratosphere (UTLS) represent a critical atmospheric region where complex interactions between aerosols, trace gases, and dynamic transport processes influence both climate and stratospheric chemistry. Aerosols in the UTLS, originating from natural and anthropogenic sources, play a key role in radiative forcing, cloud microphysics, and ozone layer stability. While the dominant component of stratospheric aerosols has long been considered to be sulfuric acid, recent studies highlighted the increasing presence of carbonaceous particles, primarily from wildfires and industrial emissions, which significantly alter aerosol composition and properties in this atmospheric region. Aerosols in the UTLS originate from a combination of direct injections (i.e., immediate release of substances, such as gases, aerosols, or particles, directly into a specific layer of the atmosphere, without undergoing intermediate processing or mixing in lower layers) and upward transport mechanisms. Volcanic eruptions⁹⁻¹², intense wildfires, and anthropogenic emissions introduce both primary particles and gaseous precursors, which undergo oxidation and

<footnote n="1">¹ICARE, UPR 3021, CNRS, Orléans, France. ²LPC2E, UMR 7328 CNRS-Université d'Orléans-CNES, Orléans, France. ³GSMA, UMR 7331 CNRS-Université de Reims Champagne-Ardenne, Reims, France. ⁴NASA Langley Research Center, National Institute of Aerospace, Hampton, VA, USA. ⁵National Institute of Aerospace, Hampton, VA, USA. ⁶ICMN, UMR 7374 CNRS-Université d'Orléans, Orléans, France. ⁷Laboratoire de Météorologie Dynamique (LMD/IPSL), CNRS, École Polytechnique, Université Paris-Saclay, Palaiseau, France. ☒e-mail: roland.benoit@cnrs-orleans.fr</footnote>
condensation to form secondary aerosols. Wildfire smoke, in particular, has become an increasingly significant source, with extreme pyro convection events directly injecting carbonaceous particles into the UTLS. These events contribute to the formation of organic-rich aerosol layers that persist for months. Transport processes, including deep convection, stratospher-troposphere exchange (STE), and isentropic mixing, govern the distribution of aerosols in the UTLS<sup>13,14</sup>. Strong convective updrafts in tropical regions efficiently transport tropospheric pollutants into the tropical lower stratosphere<sup>15,16</sup> while exchange of air masses between the tropical and extratropical UTLS is facilitated by quasi-isentropic transport<sup>17,18</sup>. Once in the UTLS, aerosols are subject to long-range transport, with residence times ranging from weeks in the upper troposphere to several years in the lower stratosphere, depending on altitude and latitude<sup>19,20</sup>.

In the stratosphere, it is commonly accepted that sulfuric acid droplets<sup>21-24</sup> coexist with non-sulfated materials such as black carbon, organic matter and nitrates<sup>19,25-28</sup>. Airborne analyses carried out at 19 km altitude confirm this coexistence<sup>27,29</sup>. Furthermore, sulfuric acid vapor, often cited in nucleation mechanisms, remains insufficient to explain the totality of aerosols observed in the lower stratosphere, making organic vapors a possible candidate<sup>30</sup>.

The phenomena of nucleation/condensation, photo-dissociation, oxidation, fragmentation, and transport are the main channels for chemical evolution of aerosols. Wildfires and volcanic eruptions contribute to this diversity and complexification of chemical compositions<sup>2,31-33</sup>. Under-standing changes in the chemical composition of the atmosphere is becoming essential since aerosol formation has an impact on the atmo-spheric radiative balance and global warming<sup>19,34,35</sup>.

In these atmospheric layers, the formation and chemical evolution of aerosols is mainly described in terms of four types of interaction, such as chemical reactions with NOx, SOx, OH, and O<sub>3</sub><sup>36,37</sup>. However, these che-mical interactions are also affected by changes in aerosol size. For aerosols smaller than 1 µm in size, particularly those of the order of a few hundred nanometers, the chemical composition is considered to be homogeneous, i.e., mainly driven by the condensation of organic gaseous compounds. For aerosols larger than 1 µm, chemical inhomogeneity is assumed to increase with size<sup>5</sup>. Recent modeling has shown that for aerosols smaller than 1 µm, the proportion of organic matter can be as high as 90%<sup>38-40</sup>. For hetero-geneous aerosols, again according to these models, the organic phase seems to be preferentially present on the aerosol surface<sup>1</sup>. One can understand that the data concerning this atmospheric chemistry are likely to vary according to the means used (ground-based platforms, airplane, satellite, big zero-pressure stratospheric balloon, light weather balloon, etc.) and the char-acterization techniques (mass spectrometry, spectroscopy, electron micro-scopy, impactor, etc.).

Overall, the extent to which these physico-chemical parameters con-tribute to the stratospheric organic aerosol load and composition remains largely uncertain. Furthermore, the microphysical and chemical transfor-mations that aerosols undergo during their stratospheric transport are still poorly characterized and not well understood. Lastly, despite being key factors in determining the chemical reactivity and radiative properties of organic aerosols from the UTLS to the stratospheric “overworld” (i.e., the altitude levels above the UTLS and the 380-K isentropic surface encom-passing most of the stratospheric ozone layer, which can also be defined as “Lower stratosphere”), their chemical composition and mixing state remain among the least documented aspects. These knowledge gaps hinder our ability to evaluate the impact of wildfire events and human-induced dis-turbances on stratospheric organic aerosols, and consequently, on the ozone layer and climate.

Addressing these gaps requires in situ sampling and analysis, since satellite measurements, although providing excellent temporal and spatial coverage, cannot easily distinguish organic from sulfuric acid aerosols<sup>41,42</sup>. Any identification of chemical compositions from satellite observations remains partial and limited to specific chemical functions or light chemicals, mainly in the presence of identified wildfire smoke<sup>13,44</sup>. They have been limited attempts to study stratospheric aerosol composition using balloon-

borne platforms. Rosen et al.<sup>45</sup> used an inlet heated connected to an Optical Particle Counter (OPC) up to an altitude of 30 km and found that the boiling temperature of stratospheric aerosols was consistent with a solution of 75%/ 25% of sulfuric acid and water. More recently, Vernier et al.<sup>26</sup> used a balloon-borne aerosol impactor to study the ionic composition of UTLS aerosol up to an altitude of 18 km during the Summer Asian Monsoon. The novelty of our work lies in the use of a lightweight sampling device to collect aerosols up to the stratosphere using a simple weather balloon while previous work was performed using large plastic balloons.

Aircraft are pertinent means to probe the lower stratosphere in situ, with various types of instruments onboard, but despite their significance, current measurement techniques lack the capability to fully capture the complex physical and chemical properties of organic aerosols in the stra-tosphere, with some altitude range restrictions, i.e. limited to ~19 km and ~12 km for high-altitude and commercial aircraft, respectively (Murphy et al.; Martinsson et al. Detailed organic chemical functions (e.g., carboxylic acids) and formulas must be provided to identify the of sources and che-mical transformations, rather than only elemental composition previously published in the literature from aircraft-based measurements (Murphy et al., 2007; 2014). A complete set of chemical and physical properties of the organic matter, i.e., particle morphology, chemical composition, oxidation state, oligomerization, and unsaturation, from the upper troposphere to the stratospheric “overworld”, has to be determined. This is necessary to pre-cisely point out the contribution assigned to specific emissions on these parameters, comparatively to previous studies, which has never been reported to date. Moreover, in situ measurements by aircraft remain difficult due to high speeds and experimental conditions that result in the cut-off diameter effect<sup>46</sup>. Balloon samplings, which reduce the size of the studied areas, are hampered by the limited weight of the on-board measuring sys-tems. On the other hand, aerosol dispersion and low chemical concentra-tions of compounds require high-performance measurement systems with weight incompatible with balloon flights.

Nevertheless, the vast majority of field studies on the chemical com-position of organic aerosols have been conducted at the surface in urban and rural environments, or during biomass burning events. Various techniques have been used to derive the chemical composition of organic aerosol in the troposphere either using offline (i.e., particle collection) and online analysis as summarized in Laskin et al. (2015)<sup>47</sup>. Mass Spectrometry (MS) associated with techniques like electrospray ionization has been used extensively for the chemical characterization of organic aerosols (OA) using offline aerosol analysis. High-resolution mass spectroscopy (HR-MS) is particularly useful for detailed molecular-level identification of various organic compounds as well as their chemical structures, providing information on the degree of saturation of the organic compounds (Smith et al., 2009) and on the for-mation of oligomers (Kourtchev et al., 2016)<sup>48</sup>. High-performance liquid chromatography (HPLC) coupled with UV-vis detection and electrospray ionization mass spectrometry (HPLC/UV-vis/ESI-MS) is used for offline analysis of organic matter collected on substrates or into liquids (Kitanovski et al., 2012)<sup>49</sup>. Techniques like Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) are used to examine the mor-phology and internal structures of submicron atmospheric particles (Hand et al., 2005). They are often combined with Energy-dispersive X-ray spec-troscopy (EDX) and electron energy loss spectroscopy (EELS) to provide elemental composition and chemical bonding information.

In this paper, we formulate a new in situ methodology to study the chemical and physical properties of organic compounds across atmospheric layers, up to 30 km using light weight instrumnts during three flights (Table 1). With this methodology one can analyze the chemicals compounds collected on functionalized carbon filers at very low concentrations<sup>20,51</sup> by ultra-high-resolution mass spectrometry (UHRMS)<sup>52-54</sup>. The detailed che-mical characterization by UHRMS of organic compounds is complemented by aerosol size information and other meteorological parameters. Moreover, we perform partial Energy Dispersive X-ray Spectroscopy (EDX) chemical and Scanning Electron Microscopy (SEM) morphological analyses of aerosols. We assess the general transport pathways and origin of air masses
<table><caption>**Table 1 | Chemical properties of ACC-27 determined potentiometric titration and temperature programmed desorption from ambient to 1473 K under helium flow (10 ml/min)**</caption><thead><tr><th rowspan="2">pH<sub>PZC</sub></th><th colspan="4">Oxygen evolution from TPD</th><th colspan="3">Functionnal groups pK<sub>a</sub> distribution (mmol/g)</th></tr><tr><th>CO<sub>2</sub> (µmol/g)</th><th>CO (µmol/g)</th><th>H<sub>2</sub>O (µmol/g)</th><th>O (w/w%)</th><th>3 &lt; pK<sub>a</sub> &lt; 7</th><th>7 &lt; pK<sub>a</sub> &lt; 11</th><th>Total</th></tr></thead><tbody><tr><td>6.9</td><td>280</td><td>1400</td><td>340</td><td>3.7</td><td>0.47</td><td>0.53</td><td>1.00</td></tr></tbody></table>

BET Brunauer-Emmett-Teller theory, DFT density functional theory method, DR Dubinin-Raduskevitch theory.

<figure><img src="image_4.png" /><figcaption>**Fig. 1 | Satellite observations of stratospheric aerosols over the periods covered by the balloon flights.** Zonally-averaged cross-sections of Scattering Ratio (SR) from CALIOP/CALIPSO for: a 01–16 January 2022; b 01–16 October 2022; c 01–16 December 2022. Orange line corresponds to the tropopause. d OMPS aerosol</figcaption></figure>

extinction at 675 nm from 2020 to 2023 with a zonal average between 30 and 50°N. White dashed lines refer to the three balloon flight dates. Thin horizontal lines represent potential temperature levels.

sampled in the stratosphere using satellite observations and trajectory modelling, especially emphasizing the transport of volcanic plumes in the North Hemisphere in winter and their influence on organics. This study indicates a drop in the number of unsaturations on organics collected in the stratosphere associated with clear signatures of oxidation and oligomerization.

## Results

### Stratospheric aerosol context in 2022 from in situ and satellite observations

The balloon flights in 2022 were conducted in periods with different aerosol loadings as shown from Cloud-Aerosol Lidar and Orthogonal Polarization (CALIOP) onboard the Cloud-Aerosol Lidar and InfraRed Pathfinder Satellite Observations (CALIPSO)⁵⁵ and cloud-filtered Limb observations from Ozone Mapping Profiler Suite OMPS. CALIPSO observations were derived from level 1 V5.1 following an approach from⁵⁶. Figure 1 shows latitudinal cross-sections from CALIPSO between 1–16 January 2022, 1–16

October 2022 and 1–16 December 2022 and a zonal mean daily profile [30°N-50°N] evolution from OMPS between January 2020 to December 2023.

On the influence of volcanic activity, on January 2022, the aged la Soufriere volcanic plume (Saint Vincent Island, eruption on April 2021) had been advected from tropical latitudes to the northern hemisphere, reaching the latitudes of balloon launching site in Reims and impacting the stratospheric aerosol content below ~21 km, as discussed by Dumélié et al.⁵⁷. Stratospheric aerosols were enhanced in 2022 after the Hunga Tonga Hunga Ha'apai (HTHH) which erupted on January 15th, being the most explosive volcanic eruption ever observed in the satellite era⁵⁸. While a large fraction of the plume remained in the southern hemisphere and the tropics⁵⁹,⁶⁰, some filaments were observed to reach the northern hemisphere⁶¹. In October 2022, the Hunga Tonga volcanic plume was visible in the Southern hemisphere and started to partially propagate to northern mid-latitudes, slightly reaching the latitudes of the balloon launching site as shown in Fig. 1. The plume was further transported in the northern hemisphere during the first
<figure><img src="image_5.png" /><figcaption>**Fig. 2** | Vertical profiles of flow rate from the chemical sampler along with atmospheric temperature for the three balloon flights. a 18 January 2022 (REAS-F#16); b 12 October 2022 (MAGIC-F#3); c 13 December 2022 (MAGIC-F#7).</figcaption></figure>

Dark blue, light blue, green and orange colors correspond to the altitude ranges probed for samples N2, N3, N4 and N5 respectively. Temperature is represented by the red line.

two weeks of December 2022 with a peak around New Year. Overall, those satellite observations consistently suggest that tropical stratospheric aerosol masses enriched with volcanic aerosols were more efficiently transported in wintertime to northern mid-latitudes where our balloon flight took place.

About the Impact of wildfire aerosol extinction from OMPS in the northern hemisphere mid-latitudes reveals that the most significant signal resulted from wildfire emissions in North America during summer 2021 as observed in the lower stratosphere (up to ~17 km). Such episodes are much less evident in summer 2022 compared to 2021 at the same period⁵⁷. We noted also that summer-fall 2023 was much more intense in terms of fire emissions than 2022 as reflected in Fig. 1d. Dumelié et al. have proposed that smoke particles emitted from wildfire episodes throughout the summer-fall period in 2021 may have still impacted the aerosol burden at these altitude levels in January 2022 and consequently the observations of the REAS-F#16 flight⁴⁰. A fire smoke signal seems visible in summer 2022 but is limited⁵⁷ to lower altitudes (i.e., very close to the tropopause) and does not extend beyond this period. Rather, the aerosol burden in fall 2022 shows a minimum throughout the 2020–2023 period showing that the MAGIC-F#3 flight in October was conducted with a low stratospheric aerosol content, i.e., very close to background conditions. In the UTLS region, measurements of the CO fires tracer over France by the MLS space-borne instrument do not show evidence of any remaining smoke signature in January, October and December 2022 (Fig. S1), likely because of the limited lifetime of this species (~60 days) with fire episodes having occurred well before the present in-situ aerosol collections.

Samples were taken up to an altitude of 29 km and distributed according to different altitude regimes on several filters numbered N1 to N5 according to the atmospheric zones studied (Figs. 2 and 3).

## Coincident balloon-borne aerosol measurements

Figure 3 (top) shows total aerosol concentrations from the Particle plus Optical Particle Counter⁵⁷ (POPC, observations as well as the mean volumetric size distribution for the 3 sampling regions (N3, N4, N5) on 18th January, 12th October, and 13th December 2022. Simultaneous in situ observations on 12 October by the Printed Optical Particle Spectrometer (POPS) optical particle counter clearly confirm this vertical partitioning with excellent agreement with POPC in terms of structuration of the aerosol content and absolute values of total concentrations as shown for sizes ≥ 0.3 µm (Fig. S2).

In the troposphere, the aerosol content is highly variable from one flight to another, reflecting different emission sources, humidity conditions, evolution along transport, etc. The temperature profiles (Fig. 2) indicate that N4 samples were mainly collected in the lowermost stratosphere (LMS) with some contribution of the upper troposphere. The 15–16 km altitude level

(i.e. the bottom altitude for N5 samples for REAS-F#16 and MAGIC-F#3) clearly corresponds to changes in the vertical structure of the aerosol content, marking a change in the aerosol sources, with a clear impact of volcanic plumes for REAS-F#16⁵⁷ and MAGIC-F#7, and/or in the stratospheric dynamical regime (see Section 3.4). The sampling strategy for those two flights was driven by previous-night balloon-borne aerosol backscatter measurements. The MAGIC-F#3 flight in October shows the lowest aerosol content in the stratosphere (Fig. 3) as confirmed by the POPC instrument (Fig. S2). The higher aerosol content, on average, above ~15 km in December 2022 resulting from the transport of some part of the HTHH plume as inferred from the satellite data is confirmed by the POPC in situ observations of aerosol concentrations for the MAGIC-F#7 flight. While the size distribution (Fig. 3, bottom) shows a consistent decrease of $dV/dLogD$ as a function of increasing radii for REAS-F#1, a mode of larger aerosols was observed during MAGIC-F#7 associated with HTHH volcanic aerosols. SAGE III /ISS showed that the effective radius of stratospheric aerosol was near 200 nm in January 2022 but increased to 300 nm in December 2022 within the HTHH plume at 20 km (Fig. S3).

## Characterization of collected material on filters

SEM and EDX characterizations were carried out on these filters. The first measurement campaign (January 2022) analyzed by DESI-UHRMS revealed a major presence of organic compounds on the surface of the elements analyzed (filter and particles). Sulfur, in the form of sulfate, was only detected in trace amounts, even though it is one of the components of the atmosphere⁶². The two additional campaigns covered by this study (October 2022 and December 2022) confirmed the trace presence of sulfur. Normally, given the size of the particles collected, down to several hundred nanometers, the presence of sulfur should have been better detected. This is probably due to the fact that in DESI mode, the desorption of surface layers is estimated to be only a few nanometers thick (1–10 nm). The difficulty of ionizing sulfur, and probably its low presence on surface layers, explain this low signal intensity and the need to use elemental analysis techniques (EDX) with a greater analysis depth (1 µm). To confirm these results, SEM and EDX analyses were performed to characterize the morphology and chemistry of the deposits and particles. This study was carried out using the N2, N3, N4, and N5 filters from the December 2022 campaign, which met the most favorable conditions for aerosol preparation and collection. Cooling the sample surface to –140 °C minimized evaporation of organic substances during SEM and EDX analyses. On all four filters, we observed particle sizes ranging from a few 10 nm to several microns. Particles of a few tens of nanometers (maximum resolution given the experimental cooling conditions) are stuck to the surface, while micron-sized particles appear to be deposited on the filter surface. Figure 4 illustrates particles diversity, which is
<figure><img src="image_6.png" /><figcaption>Fig. 3 | Vertical profiles of total aerosol concentrations and corresponding volumetric size distributions for the three sampling regions (N3, N4, N5) of the aerosol sampler. a, d 18 January; b, e 12 October; c, f 13 December 2022. Concentrations profiles are provided for 12 aerosol sizes between 0.3 and 1.0 µm.</figcaption></figure>

Volumetric size distributions are shown within the different atmospheric layers corresponding to samples N3 in green, N4 in orange and N5 in blue. La Soufriere volcanic plume is visible above 15 km for the January flight and HTHH plume between 15 and 23 km for the October and December flights.

<figure><img src="image_7.png" /><figcaption>Fig. 4 | SEM images and EDX analyses of Virgin, N3, and N4 filters. Top - SEM photographs (measurement campaign 12/13/22) of virgin, N3, and N4 filters. (Bottom) EDX analysis.</figcaption></figure>

well represented by the N3 and N4 filters (compared with the Virgin reference). These particles correspond to the collected aerosols. Although the number of SEM observations is small, the particle size of N4 seems larger than for N3.

Particles with just a few microns in size are a mixture of crystal-like matter and small aggregates. This organized matter has a crystal-like appearance, with sharp edges and geometric surfaces. Aggregates are assemblies of particles of different sizes, preferably smaller than 100 nm.
<figure><img src="image_8.png" /><figcaption>**Fig. 5** | Venn diagram of the three measurement campaigns. Venn diagrams summarizing current data with the number of chemical formulas detected per measurement campaign, exclusive to one filter or common to several filters.: a 18 January 2022; b 12 October 2022; c 13 December 2022. Annotations: spe: specific, ∩: intersection.</figcaption></figure>

EDX analyses (with an interaction volume of 1 µm³) chemically confirm morphological heterogeneity (Fig. S4). We note the absence of contamination on the Virgin filter (chemical composition carbon and oxygen, with no particles present on the surface), while for the other filters, e.g., N3 and N4, we observed different elemental chemical components (Na, Al, Mg, Si, S, K, Fe, Cl). Surface charge effects have limited the acceleration voltage to 5 keV. Based on these elemental chemical data, several chemical maps have been established. These elemental chemical maps confirm the presence of sulfur within the particles. The presence of sulfur, organized both within the refractory particles and as a matrix which spreads out on the filters, seems to be favoured with increasing altitude, although the surface roughness does not allow quantification (line $S_{\text{Kalpha1}}$: N3 vs N4). Material similar in appearance to crystals corresponds to SiOx grains, either coming from the troposphere or from chondritic meteoric particles⁶³. These are most likely SiOx silica grains (probably sand). Carbon is mostly present in the form of small aggregates, mainly in the N4 filter (N4, line $C_{\text{Kalpha1,2}}$). However, the similarity with soot cannot be established, when comparing these photographs with those of SEM observations reported in the literature⁶⁴.

The origin of iron, again better detected at higher altitudes, may come from meteoric inputs into the atmosphere⁶⁵. The presence of aluminum in particle form may be due to the “burn-up” of satellites and rocket stages during reentry⁶³. Recent work has shown that 10% of aerosols contain aluminum and other metals. These metals appear to accumulate at altitudes between 15 and 30 km, consistent with our sampling⁶⁶. Our results align with the understanding that aerosols have a surface layer, largely composed of organic material (identified by thermo-desorption), in addition to a substantial elemental component typically observed in atmospheric aerosols⁶⁷. Various hypotheses have been proposed concerning the chemical architecture of these aerosols as influenced by atmospheric conditions, altitude and residence times³,¹⁶,⁶³,⁶⁸.

Identification and specificities of chemical formulas have been carried out. During these three measurement campaigns, data processing enabled the chemical composition of compounds present on carbon fabric filters (chemical formulas) to be subtracted from the collected data. Despite the decrease in pump efficiency with increasing altitude, adsorption still occurs, probably amplified by the drop in temperature, which favors physisorption and then chemisorption⁶⁹. We believe that the decrease in temperature with altitude up to the tropopause region limited or stopped desorption while amplifying physisorption. The combined effect of these non-linear physisorption and chemisorption phenomena remains complex, so our hypotheses are limited to observations of analytical results. We compared our data from these three measurement campaigns globally by drawing up Venn diagrams (Fig. 5). To illustrate these results, the ten most intense chemical formulas CxHyOz (MS intensity) of the chemical compounds detected on the N5, N4, N3 and N2 (campaign December 2022) filters are available in the supplements (Table S1).

Measurements from the ground (filter N1: 0 -500 m) were not included, due to the poor reproducibility of the samples during the launch phase. The chemical formulas common to the N2, N3, N4 and N5 filters are identified by the intersection of the four sets ($N2 \cap N3 \cap N4 \cap N5$). For flight REAS-F#16, the high number of chemical formulas common to all samples is consistent with the absence of surface activation on the carbon fiber fabrics the day before the flight. Without this activation step (which typically removes more surface contaminants), more compounds are likely to be present across all samples. In contrast, this number is reduced for flights MAGIC-F#3 and MAGIC-F#7 because the carbon fiber fabrics were vacuum-activated the day before the measurement campaigns. This vacuum activation removed the surface contamination and generated a new surface with controlled hydrophilic/hydrophobic properties. The removal of this contamination reduced the number of overlapping chemical formulas.

The difference in the number of chemical formulas observed for these samples is probably linked to atmospheric conditions, but also to the natural diversity of this atmospheric chemistry. This diversity coexists with the similarities of a few hundred chemical formulas present in these samplings. It should be noted that these few hundred identified chemical formulas represent only a proportion of a much larger data set³⁹. This difference can be explained by the sampling timeframe, the sensitivity, and the nature of the analysis equipment. In the data obtained during the three campaigns, the tropopause (roughly the bottom part of N4) can be used to distinguish two sets of data: subsets (spe N2, spe N3, $N2 \cap N3$) for the troposphere and subsets (spe N4, spe N5, $N4 \cap N5$) for the lower-stratosphere and the stratosphere. This choice makes it possible to purposefully separate the troposphere from the stratosphere, eliminating any similarities in chemical formulas in order to identify the specific features of these two atmospheric layers.

## Data processing

Calculating the number of carbon atoms in the chemical formulas identified, according to the addition rules previously given ($N2 + N3 + N2 \cap N3$; $N4 + N5 + N4 \cap N5$), yields the graphs in Fig. 6. For the measurement campaigns carried out in winter (January and December 2022), there is an increase in the number of carbon atoms in chemical compounds present in the stratosphere compared with those of the troposphere. The multimodal distributions with the presence of periodicities observed on these graphs, whatever the atmospheric layers and for all the campaigns, may suggest a prevalence of oligomerization phenomena. The upper limit of 60 carbon may be linked to desorption difficulties due to their lower vapor pressure and ionization challenges during mass spectrometry analysis⁷⁰. We estimate that the thickness of the desorbed layers is probably a few nanometers, without being able to identify the source of the desorption phenomenon and whether they come from collected particles of a few tens of nanometers or from material extracted from larger aerosols (greater than 0.15 µm in diameter, as detected by the POPC instrument (Fig. 3). The measurements
<figure><img src="image_9.png" /><figcaption>**Fig. 6** | Carbon number distribution of chemical formulas for the three measurement campaigns. a January 2022; b October 2022; c December 2022 with comparisons to unexposed filters taken from the glove box (grey). Sampling zones</figcaption></figure>

spe N2 + spe N3 + (N2∩N3) (blue) and spe N4() + spe N5() + (N4∩N5) (red).
Annotations: spe: specific, ∩: intersection.

<figure><img src="image_10.png" /><figcaption>**Fig. 7** | Carbon number distribution vs. oxygen number distribution with DBE number for the three campaigns. a January 2022; b October 2022; c December 2022. Carbon and oxygen densities are also plotted.</figcaption></figure>

carried out in October 2022 show different characteristics to the other two campaigns (01/18/22 and 13/12/22, Fig. 2). In October, some data from the stratosphere (N4 + N5 + N4∩N5) are mixed with those from the troposphere (N2 + N3 + N3∩N4). These observations can be linked to the influence of seasonally-varying transport processes. Figure 6, where chemical compounds are classified according to their carbon number, highlights this phenomenon. Nevertheless, this mixing remains partial, as the oxidation of these chemical compounds remains higher in the stratosphere (Fig. 7, Oxygen number and density). Finally, the distributions remain consistent in intensity and range, and display multimodal characteristics.

The hypothesis of a mixing between the troposphere and the stratosphere at the time of the measurement campaign should therefore be considered to explain these results. The literature describes the structures in internally-mixed inorganic and organic particles and a link has been found between aerosol size, chemical composition and particle aging⁵. Although some homogeneous-like structures have been found mainly in mixed inorganic and organic aerosols for sizes typically <1 µm, the presence of heterogeneity, reported as coating, core-shell, dumbbell or dispersed organic matter structures, tends to increase with particle size⁶⁴. Morphologies of

both organic (high carbon number) and inorganic (e.g. water, sulfate, ammonium) particles are complex and the distribution of organics depends on relative humidity RH and on the mass of organic matter available⁴. The phases tend to separate: the organics tend to be non-homogeneously distributed on the surface of the inorganic part for low RH (<40%) although, to our knowledge, information for typical stratospheric RH values (i.e. below 15%) is not available. It is also suggested that sulfate and organic matter are not homogeneously mixed in the individual particle⁵⁵. Studies state that a significant fraction of organic carbon in the lower stratosphere is distributed in the accumulation mode of aerosols (>100 nm) and therefore in the size ranges measured by the POPC instrument. Some of these organic particles could be in solid phase, maybe in the shape of “glasses” (hypothesis based on SEM pictures, Fig. S4).

The oligomerization process modifies the composition of secondary organic aerosols, leading to significant reduction in the volatility of these particles. The oligomerization discovered in particles N4 and N5 (representing higher altitude ranges, likely the lower stratosphere and stratosphere), clearly visible for January, October and December (Fig. 6), may indicate reduced volatility that would be an important parameter to explain
the presence of organic particles at such altitude levels. Furthermore, highly viscous organic material formed on the surface of a particle could increase the particle's lifetime by trapping semi-volatile organics³⁶.

The presence of oxidized organic matter may be the signature of particle aging resulting either from the oxidation of organics on the particle surface, e.g. by OH, O₃ or by oxidation of gases on aerosols at the low temperatures of the UTLS during their transport from their remote sources¹⁶. Organic-matter coating and core-shell structures are linked to particle ageing after long-range transport⁵. It is estimated that a 500 nm organic particle could lose half its mass through oxidation by OH after a 6-month residence time in the lower stratosphere⁷¹. On the other hand, assuming rapid heterogeneous reactions, high mass loss per reaction and much smaller particles (10–200 nm), it is estimated that OH in quantities typical of the lower stratosphere could oxidize organic aerosols after 6 days⁷¹. This is much shorter than Murphy et al.'s estimate⁶⁵. These reported estimations would suggest that most of the organic matter we have detected using an Orbitrap would be located at the surface of larger particles consisting of a heterogeneous internal inorganic/organic mixture. Note that as ozone is much more abundant than OH in the stratosphere, it could oxidize organics effectively even if the heterogeneous reactions are slower than for OH. Therefore, the simultaneous measurement of organic compounds and aerosol size distribution could be an indicator of organic coatings along their transport. It should be noted that a double distribution of carbon numbers was associated with large size aerosols from HTHH. Therefore, larger volcanic aerosols from HTHH may have contributed to capturing additional organic materials along their transport across the tropical stratosphere. But we cannot rule out that organic compounds of marine origin could have been injected with seawater during the HTHH eruption⁶¹ given the complexity of aerosols present in the HTHH plume reported by balloon-borne measurements⁷².

To further investigate the oligomerization phenomena described above, we have represented the data as a function of the number of carbon and oxygen atoms (Fig. 7). In the same way, we also merged tropospheric and lower stratospheric data by associating filters N2 and N3 (N2 + N3 + N3∩N4) and N4 and N5 (N4 + N5 + N4∩N5), respectively for each atmospheric layer.

This method eliminates the chemical formulas common to both atmospheric layers enabling a better understanding of the properties of each of them. We have added the density of chemical formulas to these graphs and a DBE value index (Fig. 7). On these graphs, one can see that altitude favors an increase in the number of DBEs, i.e., the presence of unsaturation (double bonds or rings). From Fig. 7, one can see that the increase in DBE is linked to the increase in carbon number. Conversely, there is no obvious link with variations in oxygen number which means that this increase cannot be attributed only to the formation of carbonyl functions. These results indicate that the organic chemical compounds present in the stratosphere are probably the result of the transport of precursors from the troposphere that then undergo oligomerization in the stratosphere which would be in line with the findings about the tropospheric origin of the organic matter

detected in the mid-latitude stratosphere. This observation is not new, but here it is reinforced by physico-chemical characterizations⁶⁵. Oligomerization in the stratosphere is compatible with the long residence times of chemical species in the lower stratosphere⁹ and the stability of aerosols once formed⁴⁸,⁷³.

When the filters are studied individually, heterogeneity in the number of chemical formulas is observed. This greater heterogeneity appears for the N2 and N3 filters over the three measurement campaigns. It probably reflects short residence times of the chemical species resulting from inconsistent mixing in the troposphere. Furthermore, the sampling altitudes of the N2 (0.5–1.8 km) and N3 (1.8–9.4 km) filters are located wholly or partly in a turbulent layer (0–3 km), which encompasses the boundary layer, with fast mixing and short-term varying sources. These conditions are not favorable for the identification of specific properties at these altitudes. Thus, the physicochemical properties observed for the N2 and N3 filters during one campaign may be reversed or not confirmed during another measurement campaign, as it is the case for the carbon number of the chemical formulas for these three campaigns.

For the filters N4 and N5, data specific to N5 and to N4 show less dispersed numbers of chemical formulas, compared to N2 and N3, but also a greater number of chemical formulas. To help visualize DBE variations during these campaigns, two representations Fig. 8 and Fig. S5 are proposed. Figure 8 shows the DBE density for each filter over the three measurement campaigns. DBE variation is highlighted by red arrows. Figure S5 shows a van Krevelen diagram with DBE on the Z axis. The decrease in DBE between N4 and N5 (for the January and December 2022 campaigns) is represented by colored circles associated with a green arrow. Looking at both sets of data, we can see that the number of DBE has increased overall from the lower troposphere (N2) to the LMS (N4), but that there is a reversal of this trend for the N5 filter. (Fig. 8a, c: red arrows). Thus, some of the chemical formulas of N5 have a lower DBE than those of N4, which is inconsistent with the oligomerization and addition phenomena characteristic of long residence times in the stratosphere. This reduction in the number of DBEs in the stratosphere is consistent with the hypothesis of higher irradiation at high altitudes (photochemical phenomena presented in previous work⁷⁴). The reduction in these double bonds, regardless of their origin, probably has an impact on chemical reactivity at these high altitudes.

The decrease in DBE number with altitude is not observed in the results of the October campaign. Oligomerization is less developed in the stratosphere, with a carbon number distribution of chemical formulas mixed with that of the troposphere (Fig. 8b). These observations can be linked to the influence of seasonally-varying transport processes.

## Influence of seasonally-varying transport processes on the organic particle chemical properties collected in the UTLS and the stratosphere

The organic matter we have detected is expected to be of tropospheric origin but shows differences between the three flights in terms of particle properties and chemical signatures as discussed above, raising the question about the

<figure><img src="image_11.png" /><figcaption>**Fig. 8** | Plot of the evolution of the DBE number as a function of altitude for the three measurement campaigns. a January 2022; b October 2022; c December 2022. The red arrows illustrate the evolution of this number as a function of sampling altitude (a and b).</figcaption></figure>
role of different source regions and transport mechanisms that have influ-
enced the organic aerosol content in the lower stratosphere and above.

The dynamics and mixing state of samples N4 and N5 were studied.
Our N4 and N5 samples span over different atmospheric layers (LMS and
stratosphere) characterized by different source regions, transport timescales
and mechanisms which are seasonally-dependent. N4 is associated with the
LMS, which bottom contains a mixing layer that follows the mid-latitude
tropopause (known as the extratropical transition layer or ExTL) with
strong chemical coupling with the local tropopause. It can therefore be
considered as a region characterised by frequent exchanges and short transit
times from the tropopause crossing¹⁸. Higher up (but still below ~16 km),
another regime with an abrupt transition, where there is less mixing and a
smaller fraction of tropospheric air, is observed. In general, it takes around
twenty days for these air masses to move from the upper troposphere (very
often in the tropics and subtropics) to the LMS is observed. N5 corresponds
to stratospheric levels (referred to as the “overworld”) with no short-term
mixing signature and long residence times for air masses, with vertical
directions that depend on the season (e.g., in the polar stratosphere). These
transport processes from higher altitude levels in winter would be in line
with increased signature of Fe and possibly Si from meteoric dust, and sulfur
from the stratospheric layer in the LMS, in comparison with the troposphere
(see N4 vs. N3 in Fig. S4). Such features in stratosphere-troposphere
exchange are reflected in CO-O3 correlation plots (Fig. S6), available for the
January (REAS-F#16) and October (MAGIC-F#3) flights, derived from
in situ observations by an ECC ozonesonde and an Aircore instrument³⁷.
Correlations clearly show a tropospheric influence, with a typical
troposphere-to-stratosphere progressive transition from around 11 km and
with a signature of significant mixing up to 16 km for REAS-F#16 and
14 km for MAGIC-F#3.

REAS-F#16 and MAGIC-F#3 shows a transition in the stratospheric aerosol vertical profile near 15–16 km (Fig. 3a, b) corresponding to the description above of the LMS and stratosphere overworld. The overworld was associated in both cases with enhanced volcanic aerosols from La Soufriere and HTHH more efficiently transported in wintertime in the northern hemisphere. As a result, the N4 sample for MAGIC-F#3 may integrate some ~2 km-thick stratospheric “overworld” contribution which could explain the population of low DBE in Fig. 8. The vertical range for N4 for the December (MAGIC-F#7) flight is wider, thus spanning contributions from both the LMS (below ~15 km) and the “overworld” (between ~15 and 20 km), the latter being overrepresented in comparison with the 2 other flights. This seems reflected in Figs. 8 and S5 with DBE in December showing two populations for N4, with strong and low DBE values likely associated with the LMS (strong tropospheric influence) and overworld (strongly impacted by UV radiation) respectively. The decrease in DBE for N5 in January and December correlates with air masses of the “overworld”.

We also studied air mass and organic particle origins. Based on mea-
surements from a commercial aircraft, Martinsson et al.²⁹ estimated that the
spring transport of carbonaceous aerosols from the highest altitudes of the
stratosphere to the lowermost stratosphere (LMS) leads to concentrations
significantly higher than those in the tropical free troposphere. This indi-
cates the existence of a reservoir of organic matter resulting directly from
injection by forest fires (probably subject to strong photochemical loss) and/
or the formation of carbonaceous aerosols in the stratosphere during the
years of transport in the upper branch of the Brewer-Dobson circulation,
with descent in winter-spring into the lower layers. The results of Mar-
tinsson et al.²⁹ reveal a seasonally-varying content of carbon-containing
particles in the LMS corresponding to the altitude levels of the N4 sample.
Their amounts are at a minimum during September-October-November
(SON) and December-January-February (DJF), albeit significant. Overall,
transport from the troposphere to the stratosphere dominates at tropical
and polar latitudes. In the lower stratosphere of the northern hemisphere,
during the SON season, air from the boundary layer mainly originates in the
tropical regions of the maritime continent (western Pacific), the Pacific and
Central America. In the DJF season, the geographical sources are mostly
over Pacific Ocean and are transported by maritime convection⁷⁵. However,

DJF maritime convection may make only a small contribution to the aerosol
content of the tropical upper troposphere (the point at which air masses
enter the stratosphere), unlike continental convection (notably present in
SON sources) which may for instance inject aerosols of biogenic origin¹⁶.
According to Froyd et al., this convection could supply the global strato-
sphere with primary and secondary aerosols via the tropical upper tropo-
sphere, where an organic accumulation mechanism is active¹⁶. Long-range
transport from any tropical reservoir of organic matter might favour oli-
gomerisation detected at mid-latitudes.

Another study shows that North Atlantic storms and lows can rapidly
transport air from the lower troposphere into the LMS (deep TST) in SON
and DJF while the Tibetan Plateau in DJF is a source region for transport
between the troposphere and the LMS⁷⁶. This is interesting because over the
oceans, studies indicate that organic aerosol precursors are important in the
production of new particles and their growth⁷⁷. These new particles are
associated with the outflow of deep convective clouds in the upper tropo-
sphere but there is no information on the nature of the oxidised organic
species. To investigate air mass histories and origins for each balloon flight,
we have computed 20-day back-trajectory calculations using the Flexpart
trajectory model⁷⁸ (see supplementary notes S7). Figure S8 shows the results
for 2 vertical levels from which trajectories have been released, i.e., 11 and
16 km to represent the bottom of N4 aerosol samples and the transition
altitude towards the stratosphere (represented by N5). Calculations also
reflect residence times of the air masses. At 16 km for all flights, the air
masses largely originate from the stratosphere and altitudes levels above
16 km, with however a small signature from the contribution of the Tibetan
plateau for October. Air masses at 11 km in January (REAS-F#16) and
December (MAGIC-F#7) cases are largely of stratospheric origin, coming
from altitudes above and reflecting expected descent effects in DJF season as
explained above. Some contribution from the Tibetan Plateau is also
simulated which is in line with Skerlak et al.⁷⁶. The October case (MAGIC-
F#3) shows a different picture with a major transport from the boundary
layer over the Atlantic Ocean, certainly as a result of a warm conveyor belt,
and transport from the lower troposphere above North America. This
feature is also visible at 12 km (not shown). This points at different aerosol
and precursor origins for the October flight with fast transport to the LMS
which might be somewhat linked to the common organic chemical prop-
erties observed between the troposphere and the LMS when comparing
N2 + N3 with N4 + N5 in term of carbon distribution (Fig. 6). If true, this
would mean that particle growth process and oligomerisation are not
dominant under such short transport scales. The long residence times for
airmasses corresponding to N5 tend to result in decreases in DBE (Fig. 8)
and increases in carbon number from oligomerisation for DJF conditions
(Fig. 6). The October flight does not show similar features for N5 which is
unexplained at this stage.

Discussion

We have performed a series of field campaigns from France in 2022 to
collect atmospheric aerosols from the ground to the stratosphere to study
their chemical composition and their organic signatures using ultra-high-
resolution mass spectrometry-DESI. This method, combined for the first
time with functionalized carbon filters, has enabled the detection of traces of
chemical compounds up to altitudes unreached by typical aircraft obser-
vations. The use of balloons has provided a wide range of altitudes, i.e., from
the lower troposphere to the stratospheric “overworld”, where samples have
been collected and has allowed us to compare the chemical properties
between the stratosphere and the troposphere. Qualitative analysis shows
that chemical characteristics of the organic compounds differ from one to
another probed atmospheric layer. In particular, oligomerisation and
unsaturation (double bonds or rings) is observed in the aged air masses of
the UTLS and the stratosphere which is novel information provided for
these atmospheric layers with respect to previously reported studies from
aircraft observations²⁷,²⁹. UV radiation (which causes breaking of double
bonds) may explain the lower level of unsaturation in chemicals detected at
the higher altitudes of the stratosphere in comparison with the UTLS.
So far, the short-term air mass origin corresponding to the organic-
compound properties detected in the stratosphere has not been precisely
investigated. Observations in January, October, and December 2022
revealed diverse chemical signatures depending on altitude, air mass origin,
and transport timescale. These variations are driven by the seasonal
dynamics of the LMS and stratosphere, providing new insights above
~12 km not captured in previous work²⁹. Wintertime aerosols above the
tropopause were associated with aged tropical air masses transported from
higher altitudes. This situation results in the presence of organics with
higher carbon numbers consistent with oligomerisation at these altitudes,
SEMD/EDX has revealed the presence of sulfur and other elements con-
sistent with meteoric dust signatures, confirming the results from Murphy
et al.⁶⁵. Fast transport processes from the lower troposphere (such as warm
conveyor belts) seem to yield on more common aerosol chemical features
between the troposphere and the LMS and much less oligomerisation, as
reflected from the October case.

The organic matter we have detected could be distributed non-
homogeneously at the surface of bigger internally mixed particles composed
of sulfuric acid (i.e. >100 nm) and simultaneously observed by optical
particle counters, although we cannot exclude the contribution of organic
particles of a few tens of nanometers that could have been captured on the
pore of the filters. The organic matter, likely of tropospheric origin, would
then survive oxidation processes (by OH and ozone) and would undergo
oligomerisation along its transport from the troposphere to the stratosphere.

Higher carbon numbers are observed under enhanced sulfuric acid burden and bigger particles following the presence of the La Soufriere and Hunga Tonga volcanic plumes in January 2022 and December 2022, as shown by satellite observations and coincident OPC measurements. The presence of sulfuric acid might favour the formation of secondary organic aerosols, as reported for those derived from isoprene in the conditions of the lower atmospheric layers, both because of the increased surface densities (directly linked to the number and size of particles, as is the case in volcanic conditions in the stratosphere) and the acidity of the particles⁷⁰. However, there is a critical lack of measurements of reaction rates between acidic organic aerosol precursors under aqueous and acidic conditions representative of atmospheric conditions, especially for the UTLS and stratosphere (low humidity, low temperatures, low quantities of organic matter).

The Unique Combination of OPC and offline organics analysis was
performed for the first time allowing to further understand the connections
between aerosols greater than 0.3 µm and organic compounds. The pre-
sence of air masses transported from the tropics including volcanic aerosols
could be identified above 15 km. However, combining and interpreting
in situ OPC and offline organics analysis remained challenging due to the
size gap between the bigger particles detected from their scattering prop-
erties to the nanometric size of thousands of organic compounds detected
with DESI-UHRMS. The giant leap between those measurements needs to
be bridged by better spatially resolved chemical imaging of organic com-
pounds in internally mixed aerosols.

Additional balloon flights spanning over other seasons, especially in
summer, and latitudes, associated with chemistry-transport modelling
driven by emission inventories of primary organic aerosols and gaseous
precursors, would be of high interest to better investigate the links between
observed organic chemical signatures and transport processes in the UTLS/
stratosphere as well as source emissions. Given the growing influence of
extreme wildfires and their impacts on the stratosphere, additional balloon
flights targeting smoke plumes would likely help assessing the impacts of
organics on chlorine activation that could delay ozone recovery in the
future¹.

Methods

Sampling system

The Sampler, also known as the CHEM Filter Sampler, is a system designed to collect particles on multiple filters for time-resolved sampling⁸⁰. In order to adapt the system to balloon flights, a micro-controller is connected via serial communication and an InterMet radiosonde is utilized to collect weather parameters (Pressure, Temperature, Relative Humidity) and Global Position System locations (Lon, Lat, Alt). The Sampler consists of eight filter cartridges that house individual filters (Fig. S9). These eight channels enable time-resolved and/or altitude-based sampling. A rotating disk with permanent magnets is positioned externally and interacts with internal valve magnets, ensuring that only one valve opens at a time based on altitude. The rotating disk is located downstream from the filters. The flow rate of the sampled air is measured by a laminar flow element at the inlet. The speed of the integrated vacuum pump is modulated to control the sampled flow. The Sampler is controlled based on the pressure readings from the InterMet radiosonde. Once initiated, the unit automatically exposes each filter to the sampled flow within a specific atmospheric layer defined by lower and upper pressure bounds. The pump is turned off during position changes. Prior to each flight, the aerosol sampler is thoroughly cleaned with isopropyl alcohol in a desiccator previously flushed with nitrogen.

Filter preparation and characteristics

An activated carbon cloth (ACC), kindly supplied by Mast Carbon com-
pany (UK), referenced ACC-27, was selected as a self-standing porous filter.
This type of material, prepared from a natural viscose precursor and made of
knitted carbon fibers indeed presents good mechanical properties, unlike
activated carbon powders, making it easy to handle and avoiding particles
emission. It consists of highly porous carbon fibers having a diameter in the
range of 10–12 microns, obtained through a water steam activation process
to develop the microporosity (Fig. 9a: Cryo-Scanning Electron Microscopy
(Cryo-SEM) image of carbon fabric surface). Prior to use, the as-received
material was washed with boiling deionized water using a Soxhlet extractor
under continuous nitrogen bubbling and then dried at 393 K. The ACC-27
carbon fabric porosity was characterized, after outgassing overnight (12 h)
under vacuum up to 10⁻² mbar at 413 K, by N₂ and CO₂ adsorption, at 77 K
and 273K respectively, using an Autosorb-1 (Quantachrome). The nitrogen
adsorption isotherm (Type 1 and 4) is typical of a microporous material

<figure><img src="image_12.png" /><figcaption>Fig. 9 | SEM and BET characterization of the initial carbon fabrics. a SEM micrograph of ACC-27; b nitrogen adsorption isotherm at 77K (where *P/P*₀ is adsorption pressure and *P*₀ is saturated pressure at experimental temperature); c pore size distribution determined applying DFT.</figcaption></figure>
<table><caption>**Table 2 | Porous properties of ACC-27 determined by gas**</caption><thead><tr><th>S<sub>BET</sub><br>(m<sup>2</sup>/g)</th><th>V<sub>total</sub><br>cm<sup>3</sup>/g</th><th>V<sub>micro</sub><br>(N<sub>2</sub>,DFT)<br>cm<sup>3</sup>/g</th><th>V<sub>méso</sub><br>(N<sub>2</sub>,<br>DFT)<br>cm<sup>3</sup>/g</th><th>V<sub>ultra</sub><br>(CO<sub>2</sub>,<br>DR)<br>cm<sup>3</sup>/g</th></tr></thead><tbody><tr><td>1130</td><td>0.75</td><td>0.43</td><td>0.20</td><td>0.45</td></tr></tbody></table>

presenting a hysteresis due to the presence of mesopores (Fig. 9b). The total pore volume was estimated from the amount adsorbed at P/P₀ = 0.95. The supermicropore (0.7 < d < 2 nm) and mesopore (2 < d < 50 nm) volumes were determined by applying the density functional theory method (DFT) to the N₂ isotherms on the base of slit pore shape. The Dubinin-Raduskevitch theory (DR) was applied to the CO₂ adsorption isotherm at 273 K for estimating the volume of ultramicropores (0.4–0.7 nm). Mesopores are present and their proportion does not exceed 25% of the total pore volume. In addition, the ultramicropore volume determined from CO₂ adsorption (V<sub>DR</sub>(CO₂) = 0.45 cm³/g) is in the same range than the one determined from N₂ adsorption (V<sub>DR</sub>(N₂) = 0.44 cm³/g) showing that microporosity is mainly made of supermicropores (0.7 < d < 2 nm, Table 2). The pore size distribution (Fig. 9c) is bimodal and shows a relative narrow distribution with micropores with diameters centered at 1.2–2 nm together with some larger pores which diameters are in the range of 3.5–4.5 nm. The mean diameter of micropores L₀, calculated from Dubinin equation applied on nitrogen adsorption isotherm data, is 2.45 nm⁸¹. The surface functionality of the ACC-27 carbon fabric was determined by temperature programmed desorption (TPD) and potentiometric titration. The quantification of gases (CO, CO₂, H₂O) evolved during the thermal decomposition of the carbon material allows to measure the total oxygen content and the nature and amount of the oxygenated surface groups. The carbonaceous ACC-27 material contains 3.7 weight % of oxygen, aiming to moderate the hydrophobic character of the material and therefore to enable also the adsorption of more polar molecules through electrostatic interactions. The oxygenated surface groups (1 mmol per gram of carbon material), are mainly carboxylic groups (low pKa values) or phenolic groups (high pKa values, Table 1).

The oxygenated functional groups are therefore extending the adsorption sites for polar molecules⁸². The disks were transferred in a desiccator and stored under nitrogen until their implementation in the CHEM Filter Sampler. The efficiency of this protocol in terms of surface contamination was measured, for each campaign, on a filter taken from the glove box after the other filters had been transferred to the CHEM filter sampler. No contamination measurable by mass spectrometry was detected. These measurements were used to assess surface coverage by adsorption, by overlaying the carbon distribution fingerprints of unexposed and exposed filters (Fig. 6). The carbon fiber characteristics is given in Table 2. General properties of activated carbon fabrics are given in Supplementary notes S10 (Activated carbon characteristics and adsorption properties: general properties).

All these porous characteristics (i.e., a highly developed pore volume of 0.75 cm³/g and a broad pore size distribution, Fig. 9c) combined with specific chemical properties (i.e., hydrophilic surfaces) of the carbon filter, allow a wide range of adsorption of chemical compounds, including VOCs, due to the partially irreversible properties of the adsorption process (Fig. 9b)⁸³,⁸⁴. In particular, the bimodal porous texture enables adsorption of small molecules, volatile organic compounds, carbon chains and oligomers in the smallest pores (<2 nm), while larger molecules or aggregates can be deposited in mesopores of around 4 nm. In addition, the adsorption properties of VOCs on the activated mesoporous carbons used in this study are well known and documented⁸⁵,⁸⁶.

The macroporosity in between the fiber's braids may be responsible of the trapping of aerosol particles (100 nm–10 µm).

In adsorption processes, the size of the pores is crucial. Effective adsorbents should indeed have pore sizes of about 1.3–1.8 times larger than the kinetic diameters of the target molecules⁸⁶⁻⁸⁸. Small size and high volume of activated carbon pores are therefore important factors resulting in very

<table><caption>**Table 3 | Summary of sampler flight campaignssupp**</caption><thead><tr><th>Flight</th><th>Date (mm/dd/yy)</th><th>Time (UTC)</th><th>Loc (France)</th></tr></thead><tbody><tr><td>REAS-F#16</td><td>01/18/22</td><td>14h00</td><td>Reims</td></tr><tr><td>MAGIC-F#3</td><td>10/12/22</td><td>10h40</td><td>Reims</td></tr><tr><td>MAGIC-F#7</td><td>12/13/22</td><td>3h55</td><td>Orleans</td></tr></tbody></table>

strong adsorption forces; adsorption energies being double as on flat carbon surface (Kiselev and Yashin, gas adsorption chromatography, 1999). When very small pores are involved, close in size to the adsorbate size, the heat of adsorption reaches its maximum value. Especially, Le Cloirec et al. demonstrated the ability of activated carbons to adsorb a very broad range of VOCs and the possibilities of desorption using Joule effect but at temperatures exceeding 300 °C (Le Cloirec et al., Patent). For VOCs the gross adsorption capacity is achieved in the micropores smaller than 10 Å⁹.

Additionally, adsorption in the gas phase is exothermic and therefore adsorbed quantities are enhanced while decreasing temperature. Consequently, at high altitude, (low temperature) adsorption is favored and while decreasing altitude of the balloon, the pressure increase prevents the desorption of adsorbed species being strongly fixed in the porosity. Adsorption on activated porous carbon is dominated by process making adsorption.

Adsorption, whether chemical or physical, on the surface of a material is in equilibrium with desorption. This equilibrium can be shifted depending on temperature and pressure conditions, but also on the porosity of the material or its hydrophobic/hydrophilic properties as well as the presence of atmospheric aerosols. One can see that BET's (Brunauer, Emmett and Teller) surface treatment of carbon fabrics optimizes and simplifies adsorption properties by cleaning the surface and adjusting its hydrophobic/hydrophilic properties. In order to optimize the treatment of filter surfaces prior to sampling, two preparation methods were studied. For the first REAS F#16 campaign, filter surface activation was carried out several days before the measurement campaign. For the MAGIC-F#3 and MAGIC-F#7 campaigns, the surfaces were activated the day before the measurement campaign. The best results are obtained when carbon fiber fabrics are prepared just before the sampling phase. In this case, surface contamination is reduced, i.e., species common to all filters are minimized (Fig. 5).

## Balloon flights

Several balloon flights were conducted in the framework of the Rapid balloon Experiments for sudden Aerosol injection in the Stratosphere (REAS) campaign during the winter 2021–2022⁵⁷ and the MAGIC (Monitoring of Atmospheric Composition and Greenhouse gases through Multi-Instruments Campaigns) campaign in fall-winter 2022. This paper is an extension of a previous work which analyzed one flight on 18 January 2022⁵⁷,⁷⁴. In this study, we present and inter-compare the results of 3 flights from the troposphere to the lower stratosphere, up to an altitude of 28 km. These data come from 3 balloon flights performed in January 2022, October 2022 and December launched from Reims and Orleans (Table 3).

For each of these three campaigns, we used 6 filters that were exposed to different altitude ranges (Fig. 2). Of the six filters used during each campaign, one was used as a reference (virgin filter) to subtract its initial chemical composition from the final results. The virgin filter has never been exposed to the atmosphere and has remained in the desiccator. These analytical data were used to determine the resident chemical composition after surface treatment.

The 3 flights were conducted to sample different regions of the atmosphere after considering the position of different aerosol layers. The transition between position 1 and position 2 was picked to match the transition between the boundary layer and the free troposphere. The tropopause (~9–10 km) was also chosen as a boundary between position 2 and position 3. Finally, a transition was also observed in the stratosphere between the lowermost stratosphere and the overworld¹⁰ when examining aerosol profiles from COBALD (Compact Optical Backscatter Aerosol
Detector) which was launched the night before each sampler flight. Based on this information, we chose around 15 km, the transition between position 4 and position 5 for F#1 and F#2. However, F#3 was treated differently to focus on sampling stratospheric aerosol from HTHH plume which was seen to be transported to the Northern Hemisphere at that time. Thus, we choose 20 km as a boundary between position 4 and position 5 for the last flight. Figure 2 shows these different boundaries and the sampling altitudes for the 3 measurement campaigns.

## Chemical analysis of samples

Cryo-SEM microscopy and EDS characterization were performed on a high-resolution JEOL JSM-IT800SHL field emission (Schottky) SEM, equipped with a Qorum Cryo system (cryo-preparation chamber, cryo-staging) at -140 °C (MACLE platform, Orléans). The Cryo-SEM is connected to an OXFORD Aztec system for EDS analysis using a 70 mm² SDD (Silicon Drift Detector). Analyses were performed at high voltages: 1, 5, and 10 kV. Filters were fixed to the analysis support with double-sided carbon tape (used in ultra-high vacuum) and cooled for one hour before analysis. No alteration of the samples was observed during these analyses (SEM and EDX).

Activated carbon (ACC) samples were analyzed by UHRMS using the Orbitrap Q-Exactive instrument manufactured by Thermo Scientific. The IonMax HESI II source (Heated electrospray ionization source) was modified to transform it into a desorption ionization source. This involved soldering a gold foil to a corona needle. Mounting brackets were used to attach the filters to this gold support. Figure S11 shows the modification of the ionization source to desorb carbon fabric. Desorption Ionization (DESI), a soft ionization method, was used to desorb the very small quantities of aerosols adsorbed on the surface of the carbon fabrics, without altering their initial chemical composition⁸⁹,⁹⁰. It is important to note that the intensity of ions generated through this method does not directly correspond to the concentration of chemicals on the filters. Despite its ultimate sensitivity (detecting low femtograms per mm², e.g. for some aromatic chemical compounds)⁹¹, the data obtained is qualitative in nature. The analysis was based on direct instillation of toluene to create a spray towards the carbon fabric surface (as flow injection analysis - FIA) for chemical ionization. The HESI source was utilized to generate this spray. Parameters for DESI included sheath gas at 20 arbitrary units (a.u.), auxiliary gas flow at 0, vaporizer temperature set to 120 °C, capillary temperature at 280 °C and a toluene flow rate of 6 µL/min recorded over a 1-minute period for data averaging. A voltage differential of 5 kV was applied between the source and the sample surface. The Orbitrap® Q-Exactive instrument was used with a mass resolution of 140,000 (at m/z 200) to ensure a mass accuracy of <0.5 ppm RMS. Positive and negative HESI mass calibrations were conducted using Pierce™ (Thermo Scientific) calibration mixtures. The maximum ion injection time was established at 200 µs with an automatic gain control of 10⁶. Each sample was subjected to analysis in both positive and negative ionization modes, with three repetitions at three distinct mass-to-charge ratio (m/z) ranges (m/z 50–750, 200–1000, and 500–2000). We used three mass ranges for each analysis to correct for the decrease in trapping efficiency at higher masses⁹². Finally, this oversampling method was used to lower the detection limit and scan the entire filter surface by overlapping the analyzed zones. No mass was detected above m/z 2000. This mass range (m/z 50–2000) may be associated with the desorption of aerosols of a few dozen nanometers or surface layers of larger aerosols (0.2 to 10 µm)⁹³.

Because of its high-sensitivity an Orbitrap Q-Exactive generates a large amount of data covering several thousand chemical formulas. To interpret these data, visualization tools have been developed to classify chemical formulas according to their degree of oxidation, number of double bonds and unsaturation. This list is not exhaustive, and new tools are regularly added to it⁹⁴⁻⁹⁷. In this study, we identified chemical formulas based on carbon, hydrogen, oxygen and nitrogen. Sulfur was detected only at trace level on some of the samples likely due to the poor ionization associated with the use of DESI or due to its absence on the surface of the samples. Although we have identified the presence of some

nitrogen compounds, they have not been studied in this work. Under these conditions, we limited our search to organic compounds of formula CxHyOz. We used the double bond equivalent (DBE) and van Krevelen diagram⁹⁸ for representing the data. Molecular formulas were assigned with a maximum mass deviation of 3 ppm and an intensity greater than 10³ (which corresponds to a dynamic range of 10⁶ with respect to the maximum amplitude). DBE represents the sum of the number of unsaturation (double bond and ring) present in a chemical compound⁹⁴⁻⁹⁹. Decreasing the number of hydrogen atoms increases DBE's value. The DBE is defined as follows:

$$DBE = 1 + c - h/2 + n/2 \quad (1)$$

(c = number of carbon atoms; h = number of hydrogen atoms; n = number of nitrogen atoms)

## Data availability

The datasets generated and/or analysed during the current study are available through the Atmospheric Science Data Center https://asdc.larc.nasa.gov/project/CALIPSO/OMPS data are available at: https://disc.gsfc.nasa.gov/datasets/OMPS_NPP_LP_L2_AER_DAILY_2/summary. Data on aerosol chemical formulas collected over the three campaigns will be supplied on request in xls format.

Received: 24 July 2024; Accepted: 29 May 2025;

Published online: 16 June 2025

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## Acknowledgements

The authors gratefully acknowledge funding from the Labex Caprysses (ANR-11-LABX-0006-01), the Labex Voltaire (ANR-10-LABX-100-01), CPER, and EFRD (PROMESTOCK and APPROPOR-e projects), the French-Indian CEFIPRA project (Grant 6409-1), the ASTuS ANR project (21-CE01-0007-01), the CNES, and the NASA UACO/SAGE III projects. In addition, we thank Johnny Mau and Amit Pandit for their technical support and Stéphane Chevrier, Thomas Lecas, Gilles Chalumeau, Emmanuel Briaud and Thierry Vincent for balloon flight preparation and operations in Orléans. We thank Dr. Neeraj Rastogi (Geosciences Division, Physical Research Laboratory, Ahmedabad, India) and Dr. François Bernard (France) for helpful discussions. We thank Travis Knepp for providing his analysis of the SAGE III/ISS Particle Size Distribution (PSD product) available at the ASDC.
**Author contributions**

R.B. conceptualization, sampling, methodology, writing the original draft, review and editing, formal analysis, Supervision. H.V. balloon experiment, Writing—review and editing; investigation; methodology; validation. J-P-V. writing—balloon experiment, review and editing; investigation; methodology; validation. L.J. balloon experiment N.D. balloon experiment. G.K. review and editing; validation. L.V. analysis of satellite data. S.D. filters preparation, writing—review and editing. C.C. balloon experiment. P.D. conceptualization, sampling, methodology, writing the original draft, review and editing, formal analysis, supervision. G.B. writing—review and editing; investigation; methodology; validation; funding acquisition; supervision.

**Competing interests**

The authors declare no competing interests.

**Additional information**

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s41612-025-01106-9.

**Correspondence** and requests for materials should be addressed to Roland Benoit.

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