# Atmospheric formaldehyde at El Teide and Pic du Midi
remote high-altitude sites

Cristina Prados-Roman, Miguel Fernández, Laura Gómez-Martín, Emilio Cuevas, Manuel Gil-Ojeda, Nicolas Marusczak, Olga Puentedura, Jeroen E. Sonke, Alfonso Saiz-Lopez

<figure><img src="image_1.png" /></figure>

► To cite this version:

Cristina Prados-Roman, Miguel Fernández, Laura Gómez-Martín, Emilio Cuevas, Manuel Gil-Ojeda, et al.. Atmospheric formaldehyde at El Teide and Pic du Midi remote high-altitude sites. Atmospheric Environment, 2020, 234, 10.1016/j.atmosenv.2020.117618. insu-03661439

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Atmospheric formaldehyde at El Teide and Pic du Midi
remote high-altitude sites

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Cristina Prados-Roman<sup>a,1,*</sup>, Miguel Fernández<sup>a,2</sup>, Laura Gómez-Martín<sup>b</sup>, Emilio Cuevas<sup>d</sup>, Manuel Gil-Ojeda<sup>b</sup>, Nicolas Marusczak<sup>c</sup>, Olga Puentedura<sup>b</sup>, Jeroen E. Sonke<sup>c</sup>, and Alfonso Saiz-Lopez<sup>a,*</sup>

<sup>a</sup> Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano (CSIC), Madrid, 28006, Spain.

<sup>b</sup> Atmospheric Research and Instrumentation Branch, National Institute for Aerospace Technology (INTA), Madrid, 28850, Spain.

<sup>d</sup> Izaña Atmospheric Research Center (AEMET), Santa Cruz de Tenerife, 38001, Spain.

<sup>e</sup> Observatoire Midi-Pyrénées, Laboratoire Géosciences Environnement Toulouse, CNRS/IRD/Université de Toulouse, Toulouse, 31400, France.

<sup>1</sup> Currently at: Atmospheric Research and Instrumentation Branch, Spanish National Institute for Aerospace Technology (INTA), Madrid, 28850, Spain.

<sup>2</sup> Currently at: Department of Space Sciences and Payloads, Spanish National Institute for Aerospace Technology (INTA), Madrid, 28850, Spain.

*Corresponding author: Cristina Prados-Roman (pradosrc@inta.es) and Alfonso Saiz-Lopez (a.saiz@csic.es)

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ABSTRACT

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Formaldehyde (CH₂O) is a tracer of the photochemical activity of the atmosphere. Linked to air quality, CH₂O is an ozone (O₃) precursor and serves as a proxy for natural and anthropogenic reactive organic emissions. As a product of the photooxidation of methane (CH₄) and other hydrocarbons (e.g., isoprene), CH₂O represents an important source of radicals in the remote free troposphere. This work aims at improving the characterization of this part of the troposphere where data are scarce. In particular, this study assesses the presence of CH₂O at two high-altitude remote sites: El Teide (TEI, 3570 m a.s.l., Tenerife, Canary Islands, Spain) and Pic du Midi (PDM, 2877 m a.s.l., French Pyrenees). Through ground-based remote sensing measurements performed during two field campaigns in July (TEI) and September (PDM) 2013, this study presents the vertical distribution of CH₂O at both locations. Results at PDM show that CH₂O mixing ratios follow a decreasing vertical profile with a mean maximum of 0.5 ± 0.2 nmol mol⁻¹ (i.e., ppbv) at the instruments' altitude. At TEI, observations indicate an uplifted layer of CH₂O with a mean maximum of 1.3 ± 0.3 nmol mol⁻¹ at 3.8 km a.s.l. (i.e., 300 m above the instrument's altitude). At both remote sites, the observed CH₂O levels are higher than expected for background methane oxidation (a threefold increase in the case of TEI). Air mass back trajectory analysis links CH₂O
35 observations with abundant natural (e.g. forests) and/or anthropogenic isoprene emissions from the
36 region nearby PDM, while the high CH₂O levels detected at TEI indicate in-plume formation of
37 CH₂O resulting from its precursors emitted from west-African and Canadian fires. Finally, as a key
38 trace gas for O₃ and HOₓ chemistries, we estimate the upper limit of bromine monoxide (BrO) in
39 the free troposphere at TEI and PDM to be 0.8 and 1.5 pmol mol⁻¹ (i.e., pptv) respectively.

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## 1. Introduction

Formaldehyde ($CH_2O$) is a volatile organic compound (VOC) that plays an important role in the tropospheric chemistry and budget of $O_3$, $HO_x$ ($OH + HO_2$) and $NO_x$ ($NO + NO_2$). Ubiquitously present in the Earth's atmosphere, formaldehyde's vertical and geographical distribution is not homogeneous and has been the subject of investigations for decades. Kitchens et al. (1976) presented a review of $CH_2O$ in polluted environments and its related health risks, while Lowe and Schmidt (1983) addressed the relevance and challenges of measuring $CH_2O$ in the non-urban atmosphere. Currently, $CH_2O$ is considered one of the *Essential Climate Variables* by the WMO (GCOS, 2016) and measurements of its atmospheric abundance are a key for a better understanding of the oxidizing mechanisms in the troposphere, for tracking $CH_2O$ emissions and their embedded hydrocarbon reactions as well as for parametrizing atmospheric chemical and transport models.

Although a small fraction of $CH_2O$ (< 2%) enters the troposphere directly from biomass burning or from anthropogenic or vegetation emissions (Andreae and Merlet, 2001; Olivier et al., 2003; Lathière et al., 2006), most of the $CH_2O$ in the atmosphere is a secondary product of the oxidation of methane (60 %, with a production rate of 1600 Tg/yr) and non-methane hydrocarbons (NMHC), mainly isoprene (~20 %, Stavrakou et al., 2009a; Bates and Jacob, 2019). As for the $CH_4$ oxidation channel, one of its intermediate products is methyl peroxy radical ($CH_3O_2$), which proceeds reacting primarily with $NO_x$ ($NO + NO_2$) in the semi- and polluted atmosphere, yielding methoxy radical ($CH_3O$) and then $CH_2O$ (Lowe and Schmidt 1983; Wolfe et al., 2016). In unpolluted environments ($NO < 20$ pmol mol$^{-1}$), $CH_3O_2$ reacts with $HO_2$ forming methyl hydroperoxide ($CH_3OOH$), eventually yielding $CH_2O$. In addition to reactions with OH and photolysis (e.g., von Kuhlmann et al., 2003), $CH_3OOH$ can be lost through heterogeneous reactions, thus aerosols may indirectly regulate the presence of $CH_2O$.

Regarding $CH_2O$ loss processes, its main sink is photolysis at $\lambda \le 350$ nm (Crutzen 1988), resulting in an increase of atmospheric carbon monoxide (CO) which is an ozone precursor. Also, reaction with the hydroxyl radical ($OH$) and washout and dry deposition can contribute to $CH_2O$ losses (Solberg et al., 2001). As for the $CH_2O$ lifetime, although it can span up to 2 days, in the sunlit atmosphere it is only a few hours (Lowe and Schmidt, 1983; Sander et al., 2006). Due to this short lifetime, $CH_2O$ is often used as a constraint for determining direct emissions of non-methane VOC. During the last two decades, global tropospheric columns of $CH_2O$ have been intensively monitored by different satellites (GOME/ERS2, SCIAMACHY/Envisat, GOME2/MetOp, OMI/Aura, TROPOMI/S5P; e.g., Chance et al., 2000; De Smedt et al., 2018) aiming at characterizing the tropospheric chemical processes, mainly those linked to air quality or climate change (Schroeder et
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al., 2016). In fact, CH₂O is one of the very few VOC that can be detected by satellite and several studies have aimed at determining VOC emissions at a global scale using CH₂O satellite observations (Palmer et al., 2003; Stavrakou et al., 2009a, b; Bauwens et al., 2016) often assisted by numerical models. Also, by means of satellite observations of CH₂O along with chemical models, the distribution of OH in the remote troposphere can be inferred (Wolfe et al., 2019). The downscaling effort of the satellite and model community to understand CH₂O and the oxidizing mechanisms within emission plumes makes essential the characterization of pristine areas since in those regions the low signal-to-noise ratio of the satellite observations results in worse data quality and the VOC inventories are scarce.

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Despite all the years that CH₂O has been a scientific target, very few studies have been performed in remote areas (Platt et al., 1979; Lowe and Schmidt, 1983; de Serves 1994; Arlander et al., 1995, Riedel et al., 1999; Mahajan et al., 2010; Vigouroux et al., 2018) since determining its presence and vertical distribution in the remote troposphere is indeed an instrumental challenge. Several studies have addressed the budget of CH₂O in the free troposphere from airborne observations (Lowe et al., 1980; Arlander et al., 1995; Singh et al., 2001; Nicely et al., 2016; Anderson et al., 2017; Wolfe et al., 2019; Kluge et al., 2020) and also from measurements at high-altitude sites in The Andes (Pico Espejo, 4765 m, Schreier et al., 2016), in Mexico (Altzomoni, 3980 m; Vigouroux et al., 2018), in Reunion Island (Maïdo, 21600 m; Vigouroux et al., 2018), in the USA (Mauna Loa, 3397 m, Heikes et al. 1992; Zhou et al 1996; Cantrell et al., 1996; Vigouroux et al., 2018, and Idaho Hill, 3000 m, Fried et al., 1997; Cantrell et al., 1997; Harder et al, 1997), in The Alps (Jungfraujoch, 3580 nm, Legreid et al., 2008; Balzani Lööv et al., 2008; Franco et al., 2015, and Zugspitze, 2962 m, Leuchner et al., 2016; Schreier et al., 2016; Vigouroux et al., 2018), in the Apennines (Mt. Cimone, 2165 m, Fischer et al., 2003) and in the Canary Islands (Izaña-IZO, 2360 m, Fischer et al., 1998; de Reus et al., 2005; Salisbury et al., 2006; Vigouroux et al., 2018). Overall, these studies set the values of CH₂O in the free troposphere between 0.1 and 1 nmol mol⁻¹, with high variability depending on season (minimum in winter and maximum in summer), location and altitude, and reaching up to 5 nmol mol⁻¹ in case of upslope pollution transported from the planetary boundary layer (PBL). The above-mentioned studies expanding beyond the PBL into free troposphere, report a vertical distribution of CH₂O with a decreasing profile and a weak diurnal variation in the free troposphere.

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This study aims at a better understanding of the chemical composition of the free troposphere by providing observations from two different environments: from an island in the subtropical region and from the Pyrenees, more exposed to European pollution. In particular, this work reports ground-based remote sensing observations of CH₂O and BrO from two remote high-altitude sites: El Teide

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109 (TEI, Tenerife, Canary Islands, Spain; 3570 m) and Pic du Midi (PDM, French Pyrenees; 2877 m).
110 During two field campaigns (AMISOC-TEI and Pic du Midi-PDM) in July (TEI) and September
111 (PDM) 2013, a MAX-DOAS instrument (Multi-Axis Differential Optical Absorption Spectroscopy;
112 Hönninger et al., 2004; Wagner et al., 2004; Platt and Stutz, 2008) scanned the troposphere
113 vertically in order to gain insights regarding the atmospheric composition at the measurement
114 locations. This work first describes the field campaigns as well as the measurement sites and
115 methodology. Then, it presents the observations to, later on, discuss the presence of CH₂O at the
116 two mountain sites and address the upper limit of BrO at the sites during each field campaign.

## 2. Measurements and methods

118 This section describes the two field campaigns and observational sites referred to in this study,
119 along with the measurement method during both campaigns.

### 2.1. Field campaigns and sites description

120 During July and September/October 2013, two different one-month field campaigns took place at
121 two different remote high-altitude sites (El Teide and Pic du Midi, see Fig. 1). Given the scarcity of
122 observations in the free troposphere (i.e., above 2000 m a.s.l.; Chevalier et al., 2007), the goal of the
123 campaigns was to characterize the composition of that part of the atmosphere. Indeed, the
124 possibility of performing consecutive ground-based observations from El Teide and Pic du Midi
125 presented itself as a unique opportunity to investigate the free troposphere in two different
126 environments (marine and continental) and in different latitudinal locations (sub-tropics and mid-
127 latitudes). In the following, each field campaign and characteristics of the observational sites are
128 described.
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<figure><img src="image_3.png" /><figcaption>**Fig. 1:** Mean formaldehyde tropospheric column density during 2013 as observed from space. The observational sites referred to in this work are indicated in black (El Teide-TEI and Pic du Midi-PDM). The satellite data belong to the Ozone Monitoring Instrument (OMI) on the Aura platform (http://h2co.aeronomie.be, last access 09/11/2019).</figcaption></figure>

### 2.1.1. AMISOC campaign at El Teide (28.27° N, 16.64° W, 3570 m a.s.l.)

Within the framework of the AMISOC research project led by INTA (Spanish National Institute for Aerospace Technology), a field campaign took place in July 2013 at the island of Tenerife (Canary Islands, Spain) in the Atlantic Ocean. The major target of the campaign was the study of the tropospheric vertical distribution of minor species related to the ozone chemistry in an oceanic environment. The specific goal of the Spanish National Research Council (CSIC) within AMISOC was to explore the presence and vertical distribution of UV-absorbing trace gases such as halogen oxides (BrO) and VOCs (CH₂O) in the remote free troposphere in the sub-tropics.

In collaboration with INTA and the Izaña Atmospheric Research Center (IARC) from the State Meteorological Agency of Spain (AEMET, Cuevas et al., 2017), the CSIC team installed a MAX-
183 DOAS instrument at El Teide (TEI) from 7th July until 1st August 2013. The instrument was
184 mounted on a hut nearby the upper cable car station of El Teide National Park, approximately 1200
185 m above the Izaña Observatory (IZO, IARC), and ~150 m below the summit of the volcano El
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187 TEI sits at a very dry environment under high insolation most of the year. Although TEI itself is not
188 a permanent measurement site, multiple atmospheric observations have been performed for decades
189 at IZO (i.e., ~1 km below TEI). These investigations describe IZO as free troposphere background
190 conditions. The site is usually above the temperature inversion layer, and periodically impacted by
191 the Saharan Air Layer, mainly in summer (Cuevas et al., 2013, 2017; Rodríguez et al., 2011; García
192 et al., 2014). Indeed, IZO is a well-established and characterized research center that is part of the
193 World Meteorological Organization-Global Atmospheric Watch Programme (WMO-GAW) and its
194 predecessor Background Atmospheric Pollution Monitoring Network (BAPMoN) since 1984. IZO
195 also contributes to several international research networks such, e.g., the AErosol RObotic
196 NETwork (AERONET), the Network for the Detection of Atmospheric Composition Change
197 (NDACC) or the Total Carbon Column Observing Network (TCCON) (Cuevas et al., 2015).

198 2.1.2. Pic du Midi campaign (42.93° N, 0.13° E, 2877 m a.s.l)

199 From 15th September until 13th October 2013, the same MAX-DOAS instrument from AMISOC
200 was deployed at the Pic du Midi Observatory (PDM) in the French Pyrenees. The goal of the
201 campaign, organized by the University of Toulouse, was to investigate the oxidation pathways of
202 mercury (Hg) in the free troposphere. As in AMISOC, the specific goal of the CSIC-DOAS
203 observations was to assess the presence of reactive compounds such as halogens (BrO) or aldehydes
204 (CH2O) in the free troposphere, this time in the mid-latitudes. During the campaign, the MAX-
205 DOAS measurements were performed from the facilities of the Atmospheric Research Laboratory
206 at PDM, along with observations of Hg, CO and O3 (Marusczak et al., 2017).

207 The PDM (http://p2oa.aeroobs-mip.fr/) is part of the *Observatoire Midi-Pyrénées* and is a GAW
208 station since 2018, offering long-term astronomical and atmospheric observations since the 19th
209 century. From an air quality point of view, the site is defined as "mostly remote" (Henne et al.
210 2010) and, since PDM sits on top of the Pic du Midi de Bigorre and hence 1300 m above the
211 surrounding area, the location has often been used as a remote location for free-tropospheric
212 observations (Marenco et al. 1994). Under predominant westerly winds, the PDM generally
213 receives humid air masses from the Atlantic Ocean. Warming of the surrounding plains leads to up-
214 slope winds during daytime, carrying pollutants (Hg, CO, black carbon) up to the PDM (Fu et al.,
215 2016). During nighttime, the PDM katabatic winds bring free tropospheric air to the PDM,
providing a window into the middle and upper free troposphere (2-8 km, Marenco et al. 1994;

Gheusi et al., 2011; Fu et al. 2016).

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## 2.2. Remote sensing method

The MAX-DOAS instrument deployed during both campaigns consisted of two units (indoor and outdoor units) and it was previously described in the work of Prados-Roman et al. (2015) and references therein. Thus, only a brief description is provided here. In summary, through a scanning telescope and a 10 m long optic fiber bundle, the scattered skylight of the sunlit atmosphere was directed from the weatherproof outdoor unit to the indoor unit. The latter consisted of a Princeton CCD camera and a 0.5 m Czerny-Turner spectrometer with mobile mirrors and a 600 groove/nm grating on a rotating turret. That turret was automatically moved every 25 min in order to change the diffracting angle so we could gather spectra in the UV and in the Vis range. In this work, only UV data will be referred to. The set-up yielded an instrumental field of view (FOV) of 0.5° and a spectral resolution of 0.5 nm (FWHM). Figure 2 shows the outdoor units as deployed at each site.

Although the concept of the measurement routine scanning the atmosphere at discrete elevation angles from the limb to the zenith was the same during both campaigns, the details were slightly different depending on the location. In the case of the deployment at TEI, the instrument FOV azimuth angle was fixed at 62° (0° is North) and the chosen scanning elevation angles above the instrument plane were -1, 0, 1, 2, 3, 5, 10, 30, 70 and 90 degrees (~3minutes/scan). In the case of PDM set-up, the azimuth angle of the MAX-DOAS instrument was 195° while the elevation angles were set to -2, -1, 0, 1, 2, 20 and 90° (~2 minutes/scan). Note that in both cases, the MAX-DOAS observations were performed only for solar zenith angles (SZA) lower than 70° (SZA 90° is overhead sun), minimizing possible contribution of stratospheric trace gases to the retrieval of the aimed tropospheric constituents.

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<figure><img src="image_4.png" /><figcaption>**Fig. 2:** Outdoor unit of the MAX-DOAS instrument installed at the two research sites. At Tenerife (a), the outdoor unit of the instrument was mounted on the roof of a hut at 3570 m a.s.l., just below El Teide's summit (3718 m a.s.l., shown in the photograph). At the Pyrenees (b), the instrument was placed on the roof of the Atmospheric Chemistry Platform at the PDM (2877 m a.s.l.), with the field of view towards the South (195°). In both cases, through a rotating mirror inside the outdoor unit, the atmosphere was scanned vertically from the limb to the zenith.</figcaption></figure>

### 2.2.1. Spectral retrieval of trace gases

The procedure for the DOAS (Differential Optical Absorption Spectroscopy; Platt and Stuzt, 2008) spectral retrieval of the trace gases was the same for both datasets and the software used for the spectral retrieval in both campaigns was QDOAS (Danckaert et al., 2017). Following the settings suggested by Pinardi et al. (2013) where the authors performed a thorough comparison and validation exercise for MAX-DOAS observations, the spectral retrieval of CH₂O was performed in the 336.5-359 nm spectral window (further details are provided in Table 1). An example of the spectral fit is shown in Fig. 3.

In the case of TEI observations, due to saturation issues, the spectra acquired at elevation angles of 90° and at SZA < 20° were filtered out and not used at all (i.e., no observations between noon and 2:30 pm). As for the analysis procedure, the spectra were analyzed against the spectra with the higher elevation angle of each scan (i.e., 70° in TEI, 90° in PDM). In the case of the PDM campaign in the Pyrenees, where weather conditions were highly variable, the PDM-DOAS data were cloud-filtered. This was not needed in the cloud-free conditions of the TEI campaign. Further details on the data quality filtering are provided in the Supplementary Material.
<table><caption>266 **Table 1:** Summary of the settings used for the spectral retrieval of CH₂O (after Pinardi et al., 2013).</caption><thead><tr><th>Parameter</th><th>Specification</th></tr></thead><tbody><tr><td>Absorption cross sections</td><td></td></tr><tr><td>CH<sub>2</sub>O</td><td>Meller and Moortgat (2000)</td></tr><tr><td>O<sub>3</sub></td><td>Bogumil et al. (2003)</td></tr><tr><td>NO<sub>2</sub></td><td>Vandaele et al. (1996)</td></tr><tr><td>BrO</td><td>Fleischmann et al. (2004)</td></tr><tr><td>O<sub>4</sub></td><td>Thalman and Volkamer (2013)</td></tr><tr><td>Ring</td><td>QDOAS (Chance and Spurr, 1997)</td></tr><tr><td>Closure term</td><td>5<sup>th</sup> order polynomial</td></tr><tr><td>Wavelength calibration</td><td>Based on reference solar spectrum (Chance and Kurucz, 2010)</td></tr><tr><td>Intensity offset</td><td>Linear</td></tr></tbody></table>

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269 In addition to CH₂O, the presence of BrO in the free troposphere was also investigated at both high-altitude sites. Using the same absorption cross sections as those for the CH₂O spectral retrieval (Table 1), the BrO DOAS analysis was performed in three different spectral windows: in the classical two BrO absorption bands (346 - 359 nm) suggested by Aliwell et al. (2002), in the 336.5-359 nm range covering 4 bands (i.e., window of the CH₂O retrieval) and, based on the work of Franco et al. (2015), also in the region of 328.5-359 nm (i.e., 6 BrO absorption bands) where the correlation between the BrO and CH₂O absorption cross sections is minimum. Moreover, the BrO retrieval in those windows was also tested by fixing CH₂O to the average slant column density inferred at each station. Despite several tests, there was no positive spectral detection of BrO (i.e., no signatures above the residual of the DOAS fit). Hence in this work only upper limits of BrO at each site are reported (Sect. 3.1). Note that excluding the BrO cross section of the CH₂O spectral fit decreased the CH₂O differential slant column densities (dSCD) in only 1 % (within the 15 % mean error of the CH₂O dSCD, see Supplementary Material). Similarly, including the water absorption (Polyansky et al., 2018) in the spectral fit of CH₂O does not affect the retrieved CH₂O dSCD (refer to the Supplementary Material).

284 In order to invert the retrieved dSCD at the different elevation angles into the vertical distribution of the target gases, additional information regarding the light path is needed. The standard method for MAX-DOAS observations is to retrieve the O₂-O₂ collision induced absorption (i.e., O₄; Thalman and Volkamer et al., 2013) since its vertical distribution is known in the atmosphere. In this work, the spectral retrieval of O₄ was performed between 339-367 nm using the same absorption cross sections as for CH₂O (Table 1).

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<figure><img src="image_5.png" /><figcaption>**Fig. 3:** Example of the DOAS spectral fit of CH₂O. The measurement was taken on 19th July 2013 at TEI (10:30 UTC) for an elevation angle of 0°. The fit of the absorption cross sections is provided in red while the measured features are shown in black. The retrieved CH₂O dSCD correspond to 3.76·10¹⁶ molec cm⁻² (RMS = 1.88·10⁻⁴).</figcaption></figure>

### 2.2.2. Inversion of vertical profile distribution

The inversion of the vertical profile distribution of CH₂O was performed by means of the “O₄ method” (Wagner et al., 2004; Frieß et al., 2006) in a two-step approach: (1) the vertical profile of the aerosol extinction coefficient (AEC) was first inverted from the retrieved O₄ dSCD, and (2) the obtained aerosol profile is used as input in the inversion of the trace gas vertical profile distribution. Note that only positive elevation angles (≥ 0°) were used in the vertical profile retrievals. These retrievals were performed with the Linearized Discrete Ordinate Radiative Transfer (LIDORT) radiative transfer model (Spurr et al., 2008) as part of the BePRO inversion algorithm (BIRA, Clémer et al., 2010), based on the Optimal Estimation Method (OEM, Rodgers, 2000). In order to use this method, measurements (Sₑ) and a prior (Sₐ) covariance matrices must be provided. Sₑ is a diagonal matrix whose diagonal terms correspond to the squared error coming from the DOAS fit. Diagonal elements of Sₐ correspond to 100% of the a priori profile when the CH₂O profiles are estimated. Diagonal values of Sₐ for aerosol retrieval were calculated following (Clémer et al., 2010), using a scaling factor (β) of 0.4. This method allows to capture large variations of the aerosol concentrations, as those observed at Tenerife (Cordoba-Jabonero et al., 2016) when Saharan dust intrusions take place. In both cases (CH₂O and aerosols), Sₐ non-diagonal elements correspond to a
Gaussian distribution (Hendrick et al., 2004) with a correlation length of 100 m. The averaging kernel matrix of the retrieval, as defined in Rodgers (2000), describes the sensitivity of the retrieved profile to the true state. Its trace provides the degrees of freedom of the retrieval. In this work, only inversions with degrees of freedom equal or higher to 1 were considered (i.e., at least a piece of information of the profiles came from the measurements and not from the a priori). The atmospheric characterization of TEI and PDM was obtained from the standard atmosphere (Anderson et al., 1986) for tropical and mid-latitudes, respectively. The considered atmospheric vertical grid consists of layers 100 m thick up to 6 km, layers 500 m thick between 6 and 8 km and equal to the standard atmosphere above. The surface albedo has been set to 0.07 for both stations. This value is typical of sea water for the UV-A spectral range (Chadysiene and Aloyzas, 2008) and it is also an intermediate value between pasture (0.0243) and limestone (0.11) for the UV radiation (Turner et al., 2018), corresponding to the observed scenarios at TEI and PDM respectively. In the radiative transfer model, aerosols were characterized through their phase moments and single scattering albedo (SSA). For TEI, these parameters were obtained from available AERONET data corresponding to the studied days. For PDM, there were no available AERONET data for the studied period, thus several reasonable values for SSA and the asymmetry parameters (γ) were tested, finally choosing those providing the best fit between simulated and calculated dSCDs (SSA=0.95, γ=0.65). Then, phase moments were calculated using Heyney-Greenstein phase functions.

For the vertical profile retrieval of aerosols extinction and CH₂O volume mixing ratio (vmr) at both stations, an a priori vertical profile exponentially decreasing with altitude was considered. This assumption is based on the fact that its photolysis (CH₂O main sink) increases with altitude as well as with the distance from the emission sources in the PBL. In addition, the concentration of OH decreases with altitude and so does the oxidation of CH₄ (CH₂O main source in the free troposphere) (Lowe et al., 1980; Arlander et al., 1995; Singh et al., 2001; Lawrence et al., 2001; Nicely et al., 2016). Aerosol optical depth (AOD) corresponding to the aerosol a priori profile was 0.01 and 0.005 for TEI and PDM, respectively. Vertical column density (VCD) values for the a priori profiles of CH₂O were 0.7·10¹⁵ molec cm⁻² and 1.0·10¹⁵ molec cm⁻² for TEI and PDM, respectively. These values of AOD and VCD provided the best fit between simulated and observed dSCDs of O₄ and CH₂O, respectively. The degrees of freedom of the vertical profiles retrieved in this work were 1.9 ± 0.2 and 1.6 ± 0.2 for CH₂O at TEI and at PDM, respectively (i.e., mean ± standard deviation); and 2.8 ± 0.4 and 2.4 ± 0.3 for the aerosol retrieval (TEI and PDM, respectively).
362 An example of a CH₂O vertical profile retrieved at TEI and at PDM, referred to also later on, is
363 shown in Fig. 4. The statistics of all the CH₂O and AEC inverted vertical profiles at each site are
364 summarized in Fig. 5.

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<figure><img src="image_6.png" /></figure>

366
367 **Fig. 4:** Examples of inferred CH₂O vmr vertical profiles. The example (a) corresponds to
368 observations performed at El Teide (8th July 2013, 9 am UTC) and (b) to observations performed at
369 PDM on 26th September, 2013 (9.45 UTC). The dotted line on both plots indicates the average
370 detection limit. Note that the vertical scales are different in both plots.
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<caption>**Fig. 5:** Box charts of the vertical profiles of CH₂O vmr (left: a, c) and AEC (right: b, d) at TEI (up: a, b) and PDM (down: c, d) at each retrieval grid layer. Note the same vertical scales at each site. The a priori profiles for the inversion are provided in cyan (CH₂O) and green (AEC). Only cloud free data are considered.</caption>
## 3. Results and discussion

This section presents the vertical distribution of CH₂O and aerosol extinction coefficient retrieved at each site. Also, this section reports the BrO upper limit and discusses CH₂O observations at each site.

### 3.1. CH₂O and aerosol vertical distribution

The time series of the CH₂O and AEC vertical profiles during the two research campaigns are shown in Fig. 6 (TEI) and Fig. 7 (PDM). From the aerosol load point of view, both stations presented rather clear conditions although the aerosol optical depth was generally higher at TEI where a strong intrusion of Saharan dust took place on the last day of the campaign. Overall, both stations presented a mean aerosol extinction coefficient of ~0.05 km⁻¹ at the instrument's altitude (0.04 km⁻¹ at 3570 m at TEI, and 0.06 km⁻¹ at 2877 m at PDM). At PDM, most of the extinction was located between the research site and an altitude of 4 km. However, at TEI, a higher AEC was often found above the instrument's location. As for CH₂O, the median of the retrieved mixing ratio vertical profiles indicated that, while the maximum vmr is at the instrument's altitude during the Pic du Midi campaign (0.5 ± 0.2 nmol mol⁻¹ at 2.9 km a.s.l.), during AMISOC the maximum CH₂O vmr was located hundreds of meters above the instrument's altitude (0.7 ± 0.2 nmol mol⁻¹ at TEI and 1.3 ± 0.3 nmol mol⁻¹ at about 3.8 km a.s.l.). The mean CH₂O detection limit (i.e., double of the error of the retrieval) at the instrument's altitude was between 0.1 and 0.3 nmol mol⁻¹ at both sites. Further details of the vertical profile inversion at each site and its statistics are provided in the Supplementary Materials.

As previously mentioned, unless ventilation from the PBL takes place, the vertical profile of CH₂O in the troposphere is usually assumed to decrease with altitude. While in this work this is the case for the vertical profiles retrieved at PDM, it is not the case for TEI observations. The unique distribution of CH₂O at TEI with a maintained uplifted maximum throughout AMISOC is unlikely related to upslope transport of air masses from the PBL since in summer a thick (~200 m) and strong temperature inversion (ΔT~+3°C) associated to the top of the marine boundary layer is found between 1500 and 1800 m a.s.l. (Carrillo et al., 2016), far below TEI altitude. During AMISOC, the meteorological vertical profiles from the radiosondes launched at Güímar (Tenerife) showed these characteristics. As an example, Fig. 8 shows the Skew t -Log P diagram of the radiosonde corresponding to the 8th July 2013 (CH₂O vertical profiles of this day are shown in Fig. 4a). On this day, there was a very strong temperature inversion (ΔT~+10°C) between ~ 860 m a.s.l. (base of the temperature inversion) and ~ 1400 m a.s.l. (top of the temperature inversion), preventing ventilation from the PBL (see Supplementary Materials with the Skew t -Log P diagram plots of the 12 UTC
WMO- 60018 radiosonde station corresponding to the period July 1–August 3, 2013). Although TEI itself is an arid site, a possible nearby emission of CH₂O (and/or its precursors) above the PBL could be the pine forest below TEI that expands from 600 m up to 2000 m a.s.l. (Cuevas et al., 2013) and could well be a source of isoprene, a biogenic VOC that produces 18% of the CH₂O found globally (Bates and Jacob, 2019). However, if upslope transport of air masses takes place and the origin of the CH₂O observed at TEI was solely this forest, given the high irradiation at this altitude in this subtropical region, a decreasing CH₂O vertical profile would be expected with its maximum located around IZO (i.e., 1.2 km below TEI), not 1800 m above the tree line (and over 300 m above TEI). Note that during similar time of the year as AMISOC, previous studies performed at IZO reported on CH₂O vmr similar to the ones observed in this work at TEI's altitude (i.e., no decreasing CH₂O vertical profile between IZO and TEI). With in-situ analyzers placed at IZO, de Reus et al. (2005) and Salisbury et al. (2006) reported 0.65 nmol mol⁻¹ mean CH₂O daily values for non-dust conditions. Similar findings were made by Fischer et al. (1998), whose study already pointed out the high CH₂O values observed at IZO as compared to box model results and to observations at the remote site of Mauna Loa. Also, the MAX-DOAS observations do not represent in-situ measurements but an average of the properties of air masses tens of kilometers away from the instrument's location (Gomez et al., 2014). Thus, the CH₂O uplifted maximum detected during AMISOC agrees with an advected layer around El Teide that sits at ~4 km altitude throughout July 2013 (i.e., just above the volcano's summit). As depicted in the 22-year study of Cuevas et al. (2013), the air masses arriving at IZO during the month of July come from the west (Atlantic Ocean or Canada), or from the east (from the Sahel region and the northern savannas of Africa). This is also shown by the backward trajectories calculated with the HYbrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT, Stein et al., 2015, https://www.arl.noaa.gov/hysplit/, last access: 09/11/2019) at TEI during AMISOC (e.g., Fig. 9, note that the backward trajectories for each day are provided in the Supplementary Material). During AMISOC, during few days (7th, 15th, 20th and 21st July) the air masses arriving at TEI's altitude crossed the Atlantic Ocean. However, most of the time the air masses arriving at TEI originated from the northern savannas of Africa (e.g., 8th July) or from the Sahel region (e.g., 22nd July). In July, these two African regions are within the Inter Tropical Convergence Zone (ITCZ) influence (see Supplementary Material, Fig. S3), with deep convection lifting air masses straight up into the free troposphere and transporting them poleward (Nicholson, 2009). Measurement and models point out these northern savannas as a source region of isoprene emissions (Marais et al., 2014). As shown in Fig. 10a, during July 2013 the Sahel and the northern savannas of Africa suffered numerous fires, known to be strong direct sources of CH₂O and its precursors. Moreover, as shown in Fig. 10b, during AMISOC absorbing
aerosols (related to biomass burning, dust and/or volcano activity, e.g., Torres et al., 1998) were
detected over the North Atlantic Ocean as a result of strong Canadian fires and also carbonaceous
aerosols and/or desert dust from African outflow.

The history of the vertical distribution of the air masses arriving at Tenerife during summertime is indeed quite complex. As an example, Fig. 11 (a) shows the backward trajectories of the air masses arriving at Tenerife on 31st July, 2013. While the air masses arriving at 2.5 km altitude (~IZO station) came from the free troposphere of the Sahel region, those arriving at 4 km (~TEI site) came from isoprene-rich northern savannas of Africa (with fires), and the air masses above the Teide summit came from the clean upper troposphere over the Atlantic Ocean. Note that the different air masses above and below 5 km altitude for this same day were previously reported by Dyroff et al. (2015). This sort of vertical stratification of the origin of the air masses reaching Tenerife, directed the unique behaviour of CH₂O observed during AMISOC with maximum CH₂O observed up to 500 m above the instrument's altitude (e.g., Fig, 12 b).

As for the trajectories of the air masses reaching Pic du Midi, in general they come from the Atlantic Ocean, from France or from Spain (e.g. Fig. 12, note that the backward trajectories for each day are provided in the Supplementary Material). Regardless, the air masses arrived PDM after passing over through the Pyrenees. Note that the Pyrenees and Spain are known to be strong sources of isoprene from vegetation and/or pollution (Simon et al., 2001; Simpson et al., 1995, 1999; Jiang et al., 2019). In addition, as shown in Fig. 13, the Iberian Peninsula suffered biomass burning during the measurement period at the Pyrenees. Overall, unlike at TEI, at PDM the retrieved CH₂O presented decreasing vertical profiles with a maximum value located nearby the altitude of the instrument. An example of the CH₂O retrieved at PDM is shown in Fig. 4b and the statistics of the CH₂O retrieved at PDM are provided in Fig. 5c. Further discussion on the evolution of CH₂O at both sites will be addressed in Sect. 3.2.

Regarding BrO, there was no positive detection after the DOAS retrieval and only an upper limit (i.e., double of the measurement error) of the halogen compound could be set at each high-altitude site. This was made using the RTM calculations made for Rayleigh conditions for the limb viewing direction (0°). Results indicate that the BrO upper limit at TEI during July is of 0.8 pmol mol⁻¹ (at 3.5 km a.s.l.). At PDM the BrO upper limit during September was 1.5 pmol mol⁻¹ (at 2.9 km a.s.l.).
Note that this higher upper limit at PDM relates to worse lighting conditions compared to TEI campaign. Both BrO vmr upper limits in the free troposphere are consistent with previous studies (Volkamer et al. 2015; Wang et al. 2015).

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<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 6:** CH₂O (a) and AEC (b) at El Teide. The horizontal scale shows the time period of the measurement campaign. The vertical scale indicates the altitude in the troposphere while the colour code depicts de CH₂O vmr (a) and the Aerosol Extinction Coefficient (b). The night periods when no observations were performed are shown in grey and values below detection limit in black.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 7:** CH₂O (a) and AEC (b) at Pic du Midi. The horizontal scale shows the time period of the measurement campaign. The vertical scale indicates the altitude in the troposphere while the colour code depicts de CH₂O vmr (a) and the Aerosol Extinction Coefficient (b). The night periods are shown in grey, the periods with lack of measurements due to bad weather are dashed and the values below detection limit are shown in black.</figcaption></figure>

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<figure><img src="image_14.png" /><figcaption>Fig. 8: Meteorological sounding corresponding to 8th July, 2013 (12 UTC, WMO- 60018 radiosonde station). Note the strong temperature inversion of ΔT~+10°C between 1000 and 850 hPa (indicated in red). The vertical profiles of CH₂O observed this day are shown in Fig. 4a. Horizontal dashed grey lines correspond to standard pressure levels (hPa), isotherm lines (°C) are shown in right slanted dashed black, dry adiabats (°C) are given in curved brown lines, saturation adiabats (°C) are shown in blue, and saturation mixing ratios (g/kg) are shown in right slanted dashed blue lines. The left and right black lines represent the dewpoint (°C) and temperature (°C), respectively. The wind bars are provided in green.</figcaption></figure>

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<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 9: 10-day backward trajectories at TEI for the 8th July (a) and for the 22th July (b), 2013. Red lines represent the backward trajectories arriving at 4 km altitude and blue lines at 500 m (a.s.l.).</figcaption></figure>

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<figure><img src="image_17.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Fig. 10:** Conditions during the AMISOC 2013 campaign. (a) Fire counts during July 2013. The TEI site is shown in white (Fire Information for Resource Management System, FIRMS, NASA). (b) Time averaged UV Aerosol Index (Torres et al., 1998) during AMISOC 2013 (OMTO3d, NASA Goddard Space Flight Center, Goddard Earth Sciences Data and Information Services Center, GES DISC, https://giovanni.gsfc.nasa.gov/giovanni/, last access 02/02/2020).</figcaption></figure>
(a)
NOAA HYSPLIT MODEL
Backward trajectories ending at 1000 UTC 31 Jul 13
CDC1 Meteorological Data

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /><figcaption>**Fig. 11:** (a) 10-day backward trajectories of the air masses arriving at 10 UTC at TEI (green), at 4 km (blue) and at 5.5 km (red) on 31st July, 2013. (b) Vertical distribution of the CH<sub>2</sub>O mixing ratio retrieved on that day (10.30 UTC, the dot line shows the average detection limit).</figcaption></figure>

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<figure><img src="image_22.png" /><figcaption>Fig. 12: 3-day backward trajectories at PDM for the 15th (a) and 26th September (b), 2013. Red lines represent the backward trajectories arriving at 3.5 km altitude and blue lines at 500 m (a.s.l.).</figcaption></figure>

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<figure><img src="image_23.png" /><figcaption>Fig. 13: Fire counts during the Pic du Midi campaign (September-October 2013). The PDM site is shown in green (Fire Information for Resource Management System, FIRMS, NASA).</figcaption></figure>

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## 3.2. Formaldehyde at each site

For a better understanding of the evolution of CH₂O in the free troposphere during each campaign, Fig. 14 shows the CH₂O mixing ratios retrieved at each site (as shown in Fig. 6 and Fig. 7), averaged over each site's location up to 1 km higher. Thus, in Fig. 14 the CH₂O vmr are averaged from TEI up to 4.5 km (mean altitude 4035 m a.s.l.) and from PDM up to 4 km (mean altitude 3438 m a.s.l). Note that these altitude ranges cover the location of the CH₂O maximum at each station and measurement time, simplifying therefore an overview of the CH₂O mixing ratio in the free troposphere at each location. Additionally, Fig. 14 also includes (in red) the amount of CH₂O expected from a simplified scheme where the main source of CH₂O in a pristine troposphere is the oxidation of background CH₄ by OH, and the main loss processes are photolysis and oxidation by OH (Equation 1-3; Platt et al., 1979):

$$CH_2O + hv \rightarrow H + HCO (1)$$

$$CH_2O + hv \rightarrow H_2 + CO (2)$$

$$CH_2O + OH \rightarrow H_2O + HCO (3)$$

For the steady state calculation given by

$$[CH_2O] = \frac{k_4[OH][CH_4]}{k_3[OH]+J_1+J_2} (4)$$

and shown in Fig. 14, the OH concentration, photolysis rates (J₁, J₂) and CH₄ oxidation rate (k₄) used are those reported by Zhou et al. (1996) for the high-altitude site of Mauna Loa (latitude 19.47° N)

in summer (TEI) and autumn (PDM) values. The coefficient rate k₃ used is based on Platt et al. (1979) and the concentration of CH₄ used is of 1851 nmol mol⁻¹, mean value measured at IZO in 2013 with a standard deviation of less than 1% throughout the year (Cuevas et al., 2015). The concentration of OH in the troposphere is very dependent of latitude, altitude and season (e.g., Lelieveld et al., 2016; Wolfe et al., 2019). Thus, given the stability of CH₄ in a remote troposphere, OH oxidation of CH₄ and CH₂O is the main uncertainty (Δ) in this simplified estimation of steady-state CH₂O concentration. In this sense, sensitivity tests indicate that, e.g., an overestimation of 20 % in the concentration of OH used in our work (which is feasible given, for instance, the OH values reported for a mid-latitude mountain site of similar altitude as PDM, Fried et al. 1997), would lead to an overestimation of ~14 % in the amount of CH₂O in the steady-state calculation (i.e., Δ[CH₂O] ~ 0.7 x Δ[OH]). Thus, in this case, the steady-state CH₂O values provided in Fig. 14 would represent simplified upper limits.
Regarding the CH₂O mixing ratio, Fig. 14 shows the difference between both remote sites. While the mean free tropospheric CH₂O mixing ratio in the free troposphere of the Pyrenees was 0.35 ± 0.12 nmol mol⁻¹ (PDM), at the Canary Islands (TEI) it was 0.97 ± 0.26 nmol mol⁻¹ (i.e., more than double). This difference could be due to the fact that the AMISOC campaign took place during summer (when CH₂O typically shows its seasonal maximum) and the Pic du Midi campaign took place in summer/autumn. However, the photolysis of CH₂O at TEI (i.e., in the subtropics and at higher altitude) is expected to be faster than at PDM. Thus, if only local sources of CH₂O are considered, the high CH₂O values observed at TEI are unexpected for a pristine high-altitude location.

As shown in Fig. 14, overall, the observed CH₂O at both sites exceeded the values expected for a pristine troposphere with background methane as main CH₂O source, which could only explain 40 % and 0.4 % of the observations at PDM and TEI, respectively. Moreover, the mean observed CH₂O values at TEI were over 3 times higher than predicted from CH₄ oxidation. But, in addition to this background CH₄, isoprene is also known to be a relevant CH₂O source producing about 20 % of the global CH₂O budget (Stavrakou et al., 2009a; Bates and Jacob, 2019). Therefore, if one considers not only background CH₄ but also that 20 % of the observed CH₂O could come from isoprene, then both background sources (i.e., methane and isoprene) could explain the observations in the Pyrenees but they could only partially explain the CH₂O values measured at TEI (51%).

The observations at the Pyrenees might be explained by NMHC transported from the PBL to PDM, like e.g. uplifted pollution from continental Spain and/or VOC emissions from the Pyrenees (Gomez and Baldasano, 1999). Indeed, Spain is a rather strong source of VOCs and particularly of isoprene from vegetation and/or pollution (Jiang et al., 2019) and, in addition to the above-mentioned fires, in September/October a thermal low is still present over the Iberian Peninsula yielding in a thick convective boundary layer (Hoinka and Castro, 2003). As for NMHC in Pic du Midi region, isoprene is emitted at a rate of 20-40 kt month⁻¹ during the summer/autumn season (Simpson et al., 1995). Also, Simon et al. (2001) estimated an isoprene emission of 3.6 kt y⁻¹ and a total biogenic VOC emission of 3.6 kt y⁻¹ from the Pyrenees. Thus, although at PDM observations and calculation agreed when considering that 20 % of the CH₂O could derive from isoprene, that contribution might be underestimated. Note that, to the authors' knowledge, this is the first time CH₂O is reported in the Pyrenees.

The observations at the Canary Islands suggest an unaccounted strong uplifted source of CH₂O. The isoprene emissions from the forest under the observation site could influence the values detected at TEI, but cannot explain the uplifted CH₂O layer encountered at Tenerife through one whole month. This uplifted layer might relate to long-range transport of CH₂O precursors (VOC emissions)
709 mainly from Africa due to synoptic scale conditions around the ITCZ (Marais et al., 2014). Indeed,
710 after a study of the trajectories followed by the different emissions from Africa during summer,
711 Meyer-Arnek et al. (2005) concluded that African air masses could be lifted up to the mid
712 troposphere and transported at high altitudes far away from their source region. They also
713 concluded that the air masses showing CH₂O enhancement further from the source region (distance
714 of days), where those affected by biomass burning (Janicot et al., 2008). Thus, NMHC emitted from
715 the biomass burning in Africa (and in Canada) and transported at high altitude to Tenerife seems a
716 reasonable source for the detected CH₂O during AMISOC. Such an uplifted layer of CH₂O and its
717 precursors transported from Africa and Canada is in agreement with the recent studies of Behrens et
718 al. (2019) and Alvarado et al. (2020). Further investigations with e.g. airborne observations would
719 be helpful to track the air masses and study the evolution and transformation of CH₂O and its
720 precursors from their source regions —such wildfires— into the free troposphere, as well as their
721 long-range transport. Indeed, the current effort of the scientific community on recent aircraft
722 campaigns encountering fire plumes such e.g. during the DC-8 ATom 2016-2018 in remote regions,
723 the HALO CAFE-2018 mission over West Africa or SouthTRAC mission in Southern America in
724 fall 2019, will tackle this issue and might also assist to improve the parametrization of chemical
725 transport models (CTMs) that could be used to assess the impact of wildfires worldwide.
<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /><figcaption>**Fig. 14:** Time series of the mean CH₂O mixing ratios observed at TEI (a) and PDM (b). The values are averaged within the first kilometre above each station (up to 4.5 km in TEI and up to 4 km in PDM). The night periods are given in grey, the periods with lack of measurements due to bad weather are dashed and the values below detection limit are shown in black. Formaldehyde yield from its steady state is marked in red. Note the same vertical scale at both sites.</figcaption></figure>
## 4. Summary and conclusions

This study reports on formaldehyde ($CH_2O$) vertical profiles at the high-altitude sites of El Teide (Canary Islands) and Pic du Midi (French Pyrenees). Using ground-based multi-axis differential optical absorption spectroscopy during two field campaigns in July (TEI) and September (PDM) 2013, observations indicate a mean $CH_2O$ maxima of $0.5 \pm 0.2$ nmol mol$^{-1}$ at 2.9 km altitude at PDM, and an uplifted layer of $CH_2O$ at 3.8 km at TEI (mean maxima of $1.3 \pm 0.3$ nmol mol$^{-1}$), gradually decreasing levels towards instrumental detection limit. The PDM $CH_2O$ levels reported are slightly above levels expected from pristine environment, suggesting influence from natural and/or anthropogenic isoprene emissions (i.e., $CH_2O$ precursor) from the Pyrenees and/or Spain.

The unexpected uplifted $CH_2O$ layer detected at TEI during the whole measurement campaign (~ 1 month), rather than the presence of a $CH_2O$ source in the nearby region of El Teide, points towards effective recycling and long transport (convection and advection) of hydrocarbons. In agreement with recent studies, observations at TEI support the influence of wildfires (mainly African) on emission and recycling of $CH_2O$ and its source products which, in this case, are advected and uplifted to TEI increasing the reactivity of the subtropical North Atlantic free troposphere during the summer months. The possibility of such a long-range transport of $CH_2O$ and/or its source products should be further investigated with dedicated campaigns and CTMs. Along with investigations on $CH_2O$, this study also reports on reactive BrO upper limits of 0.8 and 1.5 pmol mol$^{-1}$ at TEI and PDM, respectively. Therefore, this study contributes to broadening the knowledge of the chemical composition of the free troposphere regarding VOCs and reactive halogens in mid- and sub-tropical latitudes.

## Acknowledgements

The TEI campaign was funded through the AMISOC project (Atmospheric MInor Species relevant to the Ozone Chemistry, Spanish National RCD Funding Agency, CGL2011-24891). The PDM campaign was funded by the European Research Council Executive Agency under the European Union's Horizon 2020 Research and innovation program (ERC-2010-STG 258537) to JES. LGM would like to the MINECO support under grant VHODCA (CTM2017-83199P). The authors would like to thank the assistance of the personal working at the Izaña and at the Pic du Midi observatories. The authors acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and READY website (http://www ready.noaa.gov) used in this publication. Also, we acknowledge the use of the aerosol product (AOD) from AERONET (http://aeronet.gsfc.nasa.gov/) and the fire counts imagery from LANCE FIRMS operated by
795 NASA's Earth Science Data and Information System (ESDIS) with funding provided by NASA
796 Headquarters. The UV aerosol index visualization used in this work was produced with the
797 Giovanni online data system, developed and maintained by the NASA GES DISC. We also
798 acknowledge the OMI mission scientists and associated NASA personnel for the production of the
799 UV aerosol index data used in this study (Bhartia P. K. 2012, OMTO3d-OMI/Aura TOMS-Like
800 Ozone, Aerosol Index, Cloud Radiance Fraction L3 1 day 1 degree x 1 degree V3)
801

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