Large-scale particulate air pollution and chemical fingerprint of volcanic sulfate aerosols from the 2014–15 Holuhraun flood lava eruption of Bárðarbunga volcano (Iceland)

Volcanic sulfate aerosols play a key role on air quality and climate. However, the oxidation of sulfur dioxide (SO2) precursor gas to sulfate aerosols (SO2− 4 ) in volcanic plumes is poorly known, especially in the troposphere. Here we determine the chemical speciation as well as the intensity and temporal persistence of the impact on air quality of sulfate aerosols from the 2014–15 Holuhraun flood lava eruption of Bárðarbunga icelandic volcano. To do so, we jointly analyze a set of SO2 observations from satellite (OMPS and IASI) and ground-level measurements from air quality monitoring stations together 5 with high temporal resolution mass spectrometry measurements of Aerosol Chemical Speciation Monitor (ACSM) performed far from the volcanic source. We explore month/year-long ACSM data in France from stations in contrasted environments, close and far from industrial sulfur-rich activities. We demonstrate that volcanic sulfate aerosols exhibit a distinct chemical signature in urban/rural conditions, with NO3:SO4 concentration ratios lower than background aerosols. These results are supported by thermodynamic simulations of aerosol composition, using ISORROPIA II model, which show that ammonium 10 sulfate aerosols are preferentially formed at high concentration of sulfate, leading to a decrease in the production of particulate ammonium nitrate. Such chemical signature is however more difficult to identify at heavily-polluted industrial sites due to a high level of background noise in sulfur. Nevertheless, aged volcanic sulfates can be distinguished from freshly-emitted industrial sulfates according to their contrasting degree of anion neutralisation. Combining AERONET (AErosol RObotic NETwork) sunphotometric data with ACSM observations, we also show a long persistence over weeks of pollution in volcanic 15 sulfate aerosols while SO2 pollution disappears in a few days at most. Finally, gathering 6 month-long datasets from 27 sulfur monitoring stations of the EMEP (European Monitoring and Evaluation Programme) network allows us to demonstrate a much

Volcanic sulfate aerosols in the troposphere, the topic of this paper, also have a detrimental impact on air quality and human health, as they represent a dominant component of fine particulate matter characterized by a diameter less than 2.5 µm. Owing to their small size, these aerosols have slow settling velocities and thus can accumulate in the boundary layer and penetrate deeply into the lung, exacerbating symptoms of asthma and cardiorespiratory diseases (Delmelle, 2003;Thordarson and Self, 2003;Longo et al., 2008;van Manen, 2014). They also adversely affect the environment, with deleterious effects on vegetation, 5 agriculture, soils and groundwater (Delmelle, 2003;van Manen, 2014;Thordarson and Self, 2003;Oppenheimer et al., 2011).
Last but not least, sulfate aerosols can damage aircraft engines (Carn et al., 2009), a poorly-known impact especially under repeated aircraft encounters with diluted volcanic clouds as recently tolerated by legislation (ICAO, 2016).
Volcanic sulfate aerosols can be divided in two categories, either of primary or secondary nature. Primary sulfate aerosols 10 are directly emitted at the vent, as observed at a few volcanoes worldwide (e.g. Allen et al. (2002); Mather et al. (2003bMather et al. ( , 2004; Zelenski et al. (2015)). On the other hand, secondary sulfate aerosols result from in-plume oxidation of sulfur dioxide (SO 2 ), one of the most abundant gas species emitted by volcanoes, during transport downwind Pattantyus et al., 2018). Dominant pathways have been identified for this SO 2 -to-sulfate conversion in the troposphere via both gas-and aqueous-phase processes. In the gas phase, SO 2 oxidation predominantly occurs by reaction with hydroxyl radicals (OH) to 15 form sulfuric acid (H 2 SO 4 ) according to the reactions (Seinfeld and Pandis, 2012 where M is another molecule (usually N 2 ) that is required to absorb excess kinetic energy from the reactants. In presence of water vapour, SO 3 is then rapidly converted to H 2 SO 4(g) : SO 3 + H 2 O + M H 2 SO 4(g) + M 25 Due to its highly hygroscopic nature, H 2 SO 4(g) is efficiently taken up to the aqueous phase to form sulfate aerosols (Seinfeld and Pandis, 2012)  As shown in volcanic clouds, H 2 SO 4(g) can also nucleate to form new particles (Boulon et al., 2011). Gas-phase SO 2 oxidation takes place on a timescale of weeks in the troposphere.
Much faster oxidation occurs, over hours or days, through heterogeneous reactions in the aqueous phase if SO 2 is taken up to particles, either aerosols or cloud droplets. SO 2 easily dissolves in water and can form three different chemical species depending on pH values: 1-bisulfite ion (HSO − 3 ), the preferential sulfur species for pH values in [2][3][4][5][6][7]; 2-hydrated SO 2 (SO 2 .H 2 O), for low pH values (pH < 2); and 3-sulfite ion (SO − 3 ) for basic pH values (pH > 7), according to equilibrium reactions (Seinfeld and Pandis, 2012 These three species have a sulfur oxidation state equal to 4, referred to as S(IV). Oxidation of these S(IV) species to sulfate aerosols (SO 2− 4 ), whose sulfur oxidation state is equal to 6 (S(VI)), is mainly known to occur by reaction with dissolved ozone (O 3 ) for pH > 5.5 and with hydrogen peroxide (H 2 O 2 ) as follows (Seinfeld and Pandis, 2012;Stevenson et al., 2003): 15

S(IV) + O 3 S(VI) + O 2 S(IV) + H 2 O 2 S(VI) + H 2 O
In volcanic plumes as in other environments, S(IV) can also be oxidized in the aqueous phase by dissolved oxygen (O 2 ) cat- 20 alyzed by iron and manganese (Seinfeld and Pandis, 2012) and halogen-rich species (HOBr or HOCl) as shown by von Glasow and Crutzen (2003). More recently, the importance, if not dominance, of O 2 -catalyzed oxidation in volcanic environments has been highlighted (Galeazzo et al., 2018).
Therefore, SO 2 oxidation to sulfate within volcanic clouds involves complex processes in the gas-and aqueous-phases 25 depending on many variables including solar insolation, relative humidity, temperature, pH of aerosol/cloud droplets and concentrations of the co-existent ash particles and various gas species. As such, the rate of production of volcanic sulfate aerosols is still poorly known, with a large range of rates observed near-source in different volcanic environments in the world, as summarized in Pattantyus et al. (2018). 30 The chemical speciation of volcanic sulfate aerosols has been poorly studied until now and is also barely known. Some observations have been occasionally collected, using filter packs or cascade impactors, near the vent of a few volcanoes worldwide (e.g. Mather et al., 2003a;Martin et al., 2011;Ilyinskaya et al., 2011). However, such methods only provide an average representation of the chemical composition of aerosols over the duration of instrument exposure to volcanic emissions, which is usually limited to short campaigns. In addition to the low temporal resolution of these sparse and limited-time observations, 35 a tedious and careful post-collection laboratory analysis is required to avoid biases. To our knowledge, one single study of Kroll et al. (2015) explored through near real-time quasi-continuous measurements the partitioning between SO 2 and sulfate aerosols taking place near-source at the strongly degassing Kilauea volcano in 2013, showing the wide variability of sulfur partitioning linked to the complex atmospheric dynamics of the plume.

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Volcanic aerosols may also affect the troposphere at a long distance. Various volcanic eruptions or persistent passive degassing activities (e.g. Laki/Iceland in 1783-84, Miyake-jima/Japan in 2001, Erebus/Antarctica, Holuhraun eruption of Bárðarbunga volcano/Iceland in  have been shown to trigger, at a large scale, modifications of the atmospheric chemistry and air pollution episodes in SO 2 (Tu et al., 2004;Schmidt et al., 2015;Ialongo et al., 2015;Steensen et al., 2016;Boichu et al., 2016) and sulfate aerosols (Radke, 1982;Thordarson and Self, 2003;Aas et al., 2014Aas et al., , 2015Twigg et al., 2016). These 10 studies demonstrate that volcanic SO 2 and SO 4 coexist in the troposphere at long distances indicating that the oxidation of SO 2 to secondary sulfates operates on long timescales of several days. However, the kinetics of SO 2 -to-SO 4 oxidation remains poorly constrained, especially within volcanic plumes transported over large distances in contrasted environments. Understanding the lifecycle of sulfur in volcanic plumes is fundamental to better 1) understand the rate of SO 2 depletion (review in Pattantyus et al. (2018)) to robustly describe it in volcanic plume dispersal models and rigorously evaluate volcanic SO 2 15 emissions from satellite observations (e.g. Theys et al., 2013;Boichu et al., 2013;Flemming and Inness, 2013;Moxnes et al., 2014), 2) assess the rate of production of sulfate to rigorously estimate the intensity, geographical influence and temporal persistence of long-range volcanogenic particulate pollution and the impact of tropospheric eruptions on climate.
Understanding the factors controlling the oxidation of SO 2 within volcanic plumes requires sampling the chemical com-20 position the volcanic plume over a broad range of plume residence time, which is only accessible by collecting observations over a vast spatial region. Furthermore, as chemical interactions of sulfate with co-existent aerosols of different type also affect the speciation and chemical partitioning of sulfur, these observations should include monitoring of inorganic and organic aerosol concentrations. A multi-parameter chemical analysis is also indispensable for distinguishing a specific signature of volcanogenic pollution, in particular in contexts where anthropogenic pollution may interfere. 25 In this paper, we propose to fill this gap by exploring the chemical signature of volcanic sulfate aerosols after long-range transport and by assessing the intensity of air pollution that these particles may generate at a large scale. We benefit here from a recently developed technology based on near real-time mass spectrometry, named Aerosol Chemical Speciation Monitor (ACSM), which provides mass and chemical composition of the non-refractory fraction of submicron particles at high temporal 30 resolution.
By gathering a large set of ground level in-situ gas and aerosol data jointly analyzed with satellite remote sensing observations from OMPS/Suomi NPP and IASI/MetOp-A sensors, this study aims first to quantify the intensity of air pollution in sulfur-rich particles caused by the Holuhraun eruption of Bárðarbunga volcano (Iceland) in France (Sections 4.1 and 4.2).
Secondly, we propose to explore whether the chemical signature of volcanic sulfate aerosols is distinct from those of background aerosols in industrial or urban environments, comparing observed patterns with ISORROPIA II thermodynamic model simulations of aerosol composition (Section 4.3). To achieve these goals, along with satellite SO 2 observations, we exploit ground-level in-situ observations of SO 2 from regional air quality monitoring stations and ACSM measurements performed at two French research sites in contrasted environments, near or far from industrial sulfur-rich emitting activities. Both sites 5 were indeed impacted by sulfur dioxide and sulfate aerosols in relation with the Holuhraun eruption of Bárðarbunga volcano (Iceland) on repeated occasions in September 2014.
In a third stage, the joint analysis of in-situ ACSM measurements with sunphotometry column-integrated observations from co-located stations of the AERONET AErosol RObotic NETwork allows to evaluate the temporal persistence of particulate pollution in sulfur (Section 4.4). 10 Fourthly, to provide a broader picture, we explore 6-month long sulfur monitoring datasets (September 2014-February 2015 from 27 stations of the EMEP (European Monitoring and Evaluation Programme) network. Using a multi-site concentrationweighted trajectory analysis for selected EMEP stations, we evaluate the intensity of the large-scale chemical fingerprint of the Holuhraun eruption on gaseous SO 2 and particulate sulfate in Europe, compared to other anthropogenic industrial sources (Section 4.5). 15 Finally, we assess the range of variability of SO 2 -to-SO 4 ratios according to the volcanic cloud history and derive for the first time an estimation of the oxidation rate from the eruption site to stations located few thousands kilometers away (Section 4.6). The chemical composition of non-refractory submicron aerosols (NR-PM 1 ), including sulfate (SO 2− 4 ), nitrate (NO − 3 ), ammonium (NH + 4 ) and organic (Org) species, are monitored with a time resolution of about 30 min and detection limits of 0.2 µg m −3 , using quadrupole Aerosol Chemical Monitors (ACSM) at two French sites with contrasting background conditions (Dunkirk and SIRTA). Note that charges of inorganic species, determined as ions by ACSM, are not systematically 25 indicated in text and figures hereafter, to ease readability.
For a detailed description of the ACSM, developed by Aerodyne Research Inc., the reader is referred to Ng et al. (2011).
Briefly, aerosols are sampled into the instrument through a critical orifice mounted at the inlet of a PM 1 aerodynamic lens and focused under vacuum to an oven at the temperature of 600 • C. Flash vaporized molecules are then ionized at 70eV electron 30 impact before being detected and quantified by the mass spectrometer. Raw data are corrected for aerosol collection efficiency following the protocol defined by Middlebrook et al. (2012). A specific ionization efficiency (relative to nitrate, RIE) should also be defined for each species. For the Dunkirk ACSM, a constant 0.55 SO 4 RIE has been used, based on results obtained from calibrations performed regularly (typically, every 2 months) during the campaign. By the time of the measurement, a default SO 4 RIE value was preferably taken as equal to 1.20 for the SIRTA ACSM (Ng et al., 2011;Crenn et al., 2015). Therefore, figures hereafter display ACSM data processed using these values of 0.55 and 1.20 for the Dunkirk and SIRTA datasets, respectively. However, it may be noted that recent optimizations of the ACSM calibration procedure are currently 5 allowing to reassess SO 4 RIE values (Xu et al., 2018;Freney et al., 2019). In particular, a value of 0.86 was obtained in spring 2016 when applying the new calibration procedure for the first time to the ACSM at SIRTA (Freney et al., 2019). Note that the more recent calibrated RIE value (0.86) may not be relevant to correct older measurements, and standard practice is to keep the original value (1.2) for older measurements, which includes 2014 (our period of study). For the sake of completeness, impacts of the choice of the RIE value on SO 4 concentrations used in the present study are evaluated in Sections 4.3.2 and 10 4.3.3. Such differences are still in the range of uncertainties (15-36%) estimated for the measurements of major submicron chemical species using ACSM (Budisulistiorini et al., 2014;Crenn et al., 2015).
Standard diagnostics were used to clean up the ACSM data, such as spikes in the air beam and/or water signals, drop of inlet pressures indicative of clogging. No averaging was needed to compare the species obtained with the same instrument and therefore the original time resolution was kept. 15 Dunkirk located in northern France (latitude 51.052 • N, longitude 2.348 • E, map in inset of Fig. 1) hosts a large harbour, ranking 7 th in Europe, with a developed manufacturing industry (map in Fig. A1) accounting for more than 80% of total particulate matter (PM) emitted locally over 2009(Clerc et al., 2012. About 97% of primary PM 1 are emitted by metallurgy, steel and smelter activities ( Fig. 1-7 of Zhang (2016)). A remarkable 14 month-long 30 min-resolved ACSM dataset has been 20 collected at Port-East site (map in Fig. A1), with collocated ground-level SO 2 measurements, from 15 July 2013 to 11 Sept 2014 (Zhang, 2016), allowing us to compare the chemical signature of industrial and volcanic sulfate aerosols.
The SIRTA facility (Site Instrumental de Recherche par Télédétection Atmosphérique, http://sirta.ipsl.fr, Haeffelin et al. (2005), latitude 48.713 • N and longitude 2.214 • E), is located about 20 km southwest of the Paris city center (map in inset of 25 Fig. 2). This atmospheric observatory is notably part of the European Aerosol, Clouds and Traces gases Research InfraStructure (ACTRIS, www.actris.eu) as a peri-urban station for remote sensing and in-situ measurements representative of background particulate matter (PM) levels of the Paris region. ACSM data have been routinely collected there since the end of 2011 (Petit et al., 2015). A 2-month hourly-resolved dataset (Sept-Oct 2014) has been used for the purpose of the present study to investigate the speciation of volcanic sulfate aerosols, especially during the largest event of volcanogenic air pollution affecting 30 France in late September 2014 (Boichu et al., 2016).

SO 2 mass concentrations from French air quality monitoring network
Ground-level mass concentrations of SO 2 are routinely monitored using ultraviolet fluorescence analyzers by regional air quality monitoring networks, with a detection limit of 5.3 µg m −3 and an uncertainty never exceeding 15%. For the present study, data from Atmo Hauts-de-France and Airparif could be explored, corresponding to the following stations: Dunkirk (Port East site), Calais-Berthelot, and Malo-les-Bains on the one hand, and Neuilly-sur-Seine and Vitry-sur-Seine on the other hand (maps in inset of Figures 1 and 2). Hourly mean data have been used here for all stations but the Port-East one in Dunkirk with 15-min time resolution.

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The EMEP (European Monitoring and Evaluation Programme, http://ebas.nilu.no) network, in charge of monitoring air pollution and surface deposition with harmonized measurements across Europe, gathers ground stations that are weakly affected by local sources of pollution (Tørseth et al., 2012). We focus here on stations where measurements provide at the same temporal resolution ground-level mass concentrations of both gaseous SO 2 and particulate SO 4 . More precisely, we exploit here data of the corrected sulfate concentration, i.e. the total sulfate minus sulfate originating from sea-salt particles, of the PM 10 fraction 10 of samples. Such observations are collected on a daily or hourly basis, using respectively either filter-pack measurements, the most common method, or online ion chromatography with a MARGA instrument. These latter observations, presenting the best time resolution, are only available at two stations in Great Britain at the time of the Bárðarbunga Holuhraun eruption in 2014-15 (Twigg et al., 2016). Unfortunately, measurements providing concentrations of both SO 2 and SO 4 species simultaneously are not performed anymore at that time in many North-Western European countries including France, Belgium, and 15 the Netherlands. The network still adequately covers Scandinavia (Finland, Sweden, Norway and Denmark) and only a few stations are left in Germany, Ireland, Poland, Slovakia and Slovenia. We consequently explore in this study data from 27 stations based in 11 countries (Great Britain, Finland, Norway, Sweden, Denmark, Germany, Ireland, Poland, Slovakia, Slovenia and Russia) as listed in Table 1 (Carn et al., 2015), allow tracking the large-scale dispersal of the Holuhraun SO 2 -rich cloud and identifying the dates it is transported over specific ground stations. According to IASI (Infrared Atmospheric Sounding Interferometer) satellite observations described below, 25 the altitude of Holuhraun SO 2 is most often lower than 6 km over France (see the Animation in the Supplementary Material).
Consequently, the Level-2 planetary boundary layer (PBL) products for the SO 2 total column are chosen to study the dispersal of the Holuhraun cloud over France.
IASI observations from polar-orbiting MetOp-A satellite, with a pixel footprint at nadir of 12 km diameter, full swath width 30 of 2200 km and Equator crossing time at 9:30 and 21:30 local time are also presented. IASI observations are generally less sensitive than OMPS to SO 2 below 5 km of altitude as shown in the study of the Holuhraun cloud dispersal (Boichu et al., 2016). However, IASI benefits from a shorter revisit interval (i.e. 12 hours) and provides both column amount and altitude of SO 2 . After the retrieval of the SO 2 altitude using the algorithm described in detail in Clarisse et al. (2014), an optimal estimation scheme with generalized noise covariance is used for SO 2 column retrieval (Bauduin et al., 2014).

Column-integrated aerosol properties from the AERONET ground-based remote sensing network
Time series of daily averaged Aerosol Optical Depth (AOD) at 500 nm, derived from Direct Sun photometer measurements (Version 3, Level 2.0, in cloud-free conditions) from the AErosol RObotic NETwork (AERONET) (Holben et al., 2001), are 5 exploited at the two French sites of Dunkirk (map in Fig. A1 of the precise location of the station on Dunkirk Port) and SIRTA.

Thermodynamic modeling of aerosol composition and pH
Simulations with the thermodynamic model ISORROPIA II (Fountoukis and Nenes, 2007) are performed to evaluate inorganic aerosol composition and pH under our study conditions at SIRTA. The model is run in forward mode (Fountoukis and 10 Nenes, 2007) along with an aerosol system of NH + 4 -SO 2− 4 -NO − 3 -H 2 O and corresponding gas-phase species, including ammonia (NH 3 ) and nitric acid (HNO 3 ). The total concentrations of those inorganic species (i.e., NH 3 + NH + 4 , HNO 3 + NO − 3 , and SO 2− 4 ) are set up as the model inputs for the calculation of gas-particle equilibrium concentrations. The particle NH + 4 , SO 2− 4 , and NO − 3 mass concentrations were measured by the PM 1 ACSM in 2014, however gaseous NH 3 and HNO 3 were not collected during the same period of time. To evaluate possible concentration range of NH 3 and HNO 3 , we use the data

Multi-site concentration-weighted trajectory analysis
In order to evaluate the influence of the Holuhraun eruption on the ground-level concentrations of SO 2 and SO 2− 4 over Northern Europe, a trajectory analysis work has been undertaken for a selection of EMEP stations, whose coordinates are detailed in Table 1. First, Concentration-Weighted Trajectory (CWT, Cheng et al. (2013)) has been applied separately at each site for both pollutants, as follows: where n ij is the residence time of backtrajectories in (i, j) cell, and m ij the sum of concentrations going through each trajectory. Five-day backtrajectories, starting at an altitude of 500 m above ground level, were calculated every 3 hours for each site using the Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT, Stein et al. (2015)), with 1 • × 1 • Global Data Assimilation System (GDAS). Because of the statistically low representativeness of one backtrajectory to a daily 10 concentration value, the data coverage has been increased by taking more backtrajectories into account for a particular day (Waked et al., 2014). Wet deposition has been estimated by cutting the trajectory where significant precipitation (≥ 1 mm.h −1 ) occurred. For graphical purpose, a Gaussian smoothing has been applied.
Secondly, a multi-site (MS) approach was applied in order to take the spatial and temporal variabilities of all sites at once, 15 which has been proven to take spatio-temporal variabilities of all sites into account (Biegalski and Hopke, 2004): where m l and n l are the m and n matrices of site l. In order to retrieve quantitative information from the multi-site analysis, an edge-detection algorithm allows to integrate CWT values over a particular hotspot. Compared to the total integration, this provides an estimation of the contribution of the selected zone for particulate SO 4 and gaseous SO 2 .

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This whole work has been performed with ZeFir (Petit et al., 2017), a user-friendly tool for wind and trajectory analysis.

Results and discussion
First, we evaluate the intensity of air pollution in sulfur-rich particles induced by the Holuhraun eruption in France. We also propose to explore whether the chemical signature of sulfate aerosols is specific or not within volcanic plumes, by comparison with sulfate aerosols of industrial origin. We then define a methodology to discriminate volcanic versus local industrial sulfur- 25 rich compounds. To do so, we study several events of air pollution observed in France in September 2014 at two locations nearby (Dunkirk) and distant (SIRTA) from industrial activities. We show the volcanogenic origin of these episodes of atmospheric pollution that are characterized by elevated ground-level concentrations of both SO 2 and SO 4 . Then, we investigate whether similar events of air pollution are also detected more broadly, at the European scale, by exploiting in-situ data from the EMEP ground network. Finally, we identify, using a multi-site concentration-weighted trajectory analysis, the sources of gas and particulate pollution in sulfur and examine whether the sulfur partitioning in volcanic samples collected in France is similar at various other EMEP stations in Europe.

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SO 2 is commonly used as a marker of volcanic plumes. Hence, OMPS satellite SO 2 observations allow to detect when the volcanic cloud passes over the two French sites equipped with ACSM (i.e. Dunkirk and SIRTA), bearing in mind that satellite ultraviolet observations of SO 2 , aside from their detection limit, have a lower sensitivity especially in the lower troposphere and the planetary boundary layer (Krotkov et al., 2008). Top of Fig. 1 indicates that a branch of the Holuhraun SO 2 cloud passes close to Dunkirk in northern France on 7 Sept 2014 and air masses containing volcanic SO 2 are still detected over Dunkirk To conclude, this joint analysis of complementary observations, from space and from the ground at a regional scale, allows to demonstrate the volcanogenic origin of the two events of air pollution associated to elevated ground-level concentration in both SO 2 and SO 4 , recorded in Dunkirk on 7 Sept between 07:36 and 23:19 UTC (hereafter named "DK volcanic event 1") 25 and the second between 10 Sept 20:00 and 11 Sept 2014 05:50 UTC (hereafter named "DK volcanic event 2") (grey shaded areas in Fig. 1).
Similarly, exploiting OMPS satellite maps and Airparif SO 2 measurements at various air quality monitoring stations of the Paris region (only Vitry-sur-Seine and Neuilly-sur-Seine are shown here) demonstrates the volcanic origin of the largest event 2). This particulate pollution is concomitant with a pronounced air pollution in SO 2 , with a ground-level concentration up to Boichu et al. (2016). Nevertheless, despite these high SO 2 ground-level concentrations measured regionally on 22-24 Sept (Bottom of Fig. 2), it is interesting to point out that, on 24 Sept, neither OMPS nor OMI satellite observations are sensitive enough to detect any SO 2 over the northern part of France encompassing the Paris region (OMI satellite data not shown here). This demonstrates the necessity to combine both space and ground observations, especially when SO 2 is confined in the boundary layer. Note that the two simultaneous anomalies observed on 9 and 10 Sept 2014 in both SO 4 at SIRTA and 5 SO 2 concentrations at Airparif stations may also be volcanogenic. Nevertheless, this 2-day long episode of air pollution is not selected for further analysis as it is of lower intensity compared to the three other volcanogenic events already selected.

Background air pollution in sulfur-rich gas and aerosol species
At SIRTA, a 2-month average SO 4 mass concentration of 1.0 µg m −3 is recorded with hourly-resolved ACSM data during the Although investigated here on a shorter period of 2 months (Sept-Oct 2014), variations in submicron particle concentrations at the SIRTA platform are much more limited with peak values of 16, 13, 11 and 4 µg m −3 for SO 4 , organic, NO 3 and NH 4 aerosols respectively (Fig. 4). At SIRTA, unlike nitrate and organics, the highest concentrations in ammonium aerosols are and 2, respectively). 15 As Dunkirk is a much more polluted site than SIRTA, with various types and sources of aerosols, we start by comparing the signature of volcanic aerosols to SIRTA background.We observe that volcanic aerosols at both sites can be clearly distinguished from SIRTA (SI) background aerosols (in blue), especially in the scatter plots of SO 2 (bottom of Fig. 5-A), NO 3 (bottom of Globally, we observe that volcanic aerosols at both sites display a lower NO 3 :SO 4 concentration ratio than background aerosols 25 at SIRTA, thus exhibiting a clearly distinct pattern. Similarly, it could be noted that a forecasted ammonium nitrate pollution event did not eventually occurred when Eyjafjallaj'okull volcanic emissions significantly impacted air quality over France in Spring 2010 (Colette et al., 2011).
In contrast to NO 3 , a narrower range of concentration in organics is observed during volcanic events (< 9 µg m −3 ) than during background conditions at SIRTA with Org concentrations up to 13 µg m −3 (bottom of Fig. 6-D). Again, volcanic 30 aerosols present a distinct behavior with a much lower Org:SO 4 mass concentration ratio compared to SI background aerosols.
Similarly, volcanic aerosols display a much lower SO 2 :SO 4 concentration ratio than background aerosols (bottom of Fig. 5-A).
However, isolating volcanic aerosols from SI background is less obvious in the scatter plot of NH 4 versus SO 4 concentrations (bottom of Fig. 5-B). This will be further analyzed next in the text with thermodynamical simulations of aerosol composition.
Whereas higher NH 4 concentrations up to 7 µg m −3 are recorded during volcanic events, concentrations are about twice lower 35 in SI background conditions. Nevertheless, volcanic aerosols do not present a NH 4 :SO 4 concentration ratio significantly different from SI background characteristics (bottom of Fig. 5-B).

Specific signature of freshly-emitted industrial sulfate-rich aerosols
Concentrations at Dunkirk display a more complex behavior with widely scattered values compared to SIRTA. We are espe- We demonstrate in the following that cyan data points, shown to be industrial aerosols, are not neutralized but acidic. To do 15 so, we compare the predicted concentration of NH 4 with the measured concentration of NH 4 (Fig. 7). According to Seinfeld and Pandis (2012), the preferred form of sulfate is the neutral (NH 4 ) 2 SO 4 form in an ammonia -nitric acid -sulfuric acid -water system rich in ammonia and presenting a relatively elevated relative humidity. Under these assumptions, NH 4 , pred, the predicted concentration of NH 4 , is calculated assuming that NH + 4 has completely neutralized available sulfate, nitrate and chloride ions to form (NH 4 ) 2 SO 4 , NH 4 NO 3 and NH 4 Cl aerosols, which writes: ACSM data associated to volcanic events and to background conditions in Dunkirk are roughly aligned in the scatter plot of measured versus predicted concentrations of NH 4 along the first bisector indicating their neutralization (Fig. 7). However, industrial aerosols colored in cyan are widely scattered below the first bisector. This result demonstrates that, regarding these 5 industrial aerosols, NH + 4 ions have not neutralized surrounding sulfate and nitrate ions. We assess in the following whether this absence of neutralization results from a lack of background NH 3 or a lack of time available for neutralization.
The industrial sector in Dunkirk -where two main sulfur emitters (a refinery and a coke power plant) are located -expands between 500 m and 3 km from the sampling site. Winds blowing from this industrial sector often exhibit speeds above 5 m.s −1 10 (top left of Fig. A3), thus residence times of industrial plumes in the atmosphere are generally well below one hour, and often only a few minutes, before reaching the sampling site.
On the other hand, wind sector analysis of the predicted versus measured NH 4 levels, or anion neutralization ratio (ANR), demonstrates that under urban or marine emissions, there is enough NH 3 to neutralize both sulfate and nitrate aerosols on the same site, but that industrial emissions disturb the equilibrium (bottom of Fig. A3). Bottom of Fig. 4 shows the extent of 4.0 ± 2.8 µg m −3 , reaching maxima of 11-12 µg m −3 , respectively. In the Dunkirk area, we expect that local emissions -50% originating from the "manufacturing industries, waste treatment and construction" according to the latest available inventory of AtmoHDF (2012), compared to 96% from the agricultural sector when considering the entire Hauts-de-France region -will even increase this background level by a few µg m −3 . Dunkirk atmosphere can consequently be considered to be sufficiently 25 rich in NH 3 to produce the concentration of ammonium required to neutralize the concentrations of industrial sulfate the most commonly measured. Local NH 3 may generally not be lacking, but rather short residence times between the plume emission points and the sampling site are responsible for the acidity of these observed aerosols.
To summarize, we show that the group of ACSM data very poor in particulate nitrate while rich in sulfate originates from 30 the industrial sector, are acidic and display short residence time. We conclude that they represent freshly-emitted aerosols of industrial origin, likely emitted by metallurgy and steel activities. We note that these aerosols are also relatively poor in ammonium and very poor in organic compared to background aerosols (bottom of Figures 5 and 6).

Best strategy to isolate volcanic sulfate from other types of aerosols
We have shown in Sections 4.3.1 and 4.3.2 that exploring the detailed chemical speciation of aerosols provided by ACSM measurements allows us to isolate the signature of aged volcanic sulfate aerosols (e.g. aerosols already transported over a long distance from the eruption site) from those of freshly-emitted industrial sulfate or background aerosols in various urban, marine or agricultural-influenced environments. As summarized in Fig. 8, angular sectors, which highlight the broad range of 5 values associated to each type of aerosols, are more distinctively separated in the scatter plots of NO 3 or Org vs SO 4 mass concentrations, which are thus more informative to identify the aerosol source.
To combine in a single plot the information on both the chemical signature of aerosols from these scatter plots as well as their degree of neutralization or acidity, we represent the variations of the NO 3 :SO 4 (top of Fig. 9)  All aerosols present values of the NH 4 , meas:NH 4 , pred mass concentration ratio, or anion neutralization ratio (ANR) close 15 to 1 indicating their neutralization, except freshly-emitted industrial aerosols in Dunkirk (in cyan) with most values < 0.75 indicative of their strong acidity (left of Fig. 9). Nevertheless, we note a few values of the neutralization ratio exceeding 1 (up to 1.5) for both the largest volcanic event at SIRTA (in red) and some background aerosols in Dunkirk (in blue) (left of Fig. 9).
This phenomenon could be linked with NH 3 uptake onto particulate organic acids, as previously observed in northwestern Europe (Schlag et al., 2017). It may also partly result from possible bias in the evaluation of the SO 4 relative ionization efficiency  are weakly influenced by such a change (Fig. 9), it weakly impacts aerosol acidity as ANR values are lower with a RIE equal to 0.86, independently of the type of aerosols (Figures 7 and 9). ANR values do not greatly exceed anymore the value of 1 reducing the bias above mentioned.
Concerning the NO 3 :SO 4 mass concentration ratio, whichever the sulfate RIE coefficient, volcanic aerosols (in red and 30 green) present values between 0.1 and 3, while background aerosols at SIRTA (in blue) are associated to the highest values (> 3) and freshly-emitted industrial aerosols in Dunkirk (in cyan) the lowest values (< 0.15) (top of Fig. 9).
Concerning the Org:SO 4 mass concentration ratio, background aerosols at SIRTA are characterized by ratios greater than 2.5. In contrast, low values (mostly < 1.6) are observed during the volcanic event (bottom of Fig. 9). Accordingly, these low ratios are primarily explained by a high concentration of SO 4 (denominator). Nevertheless, we note that the volcanic event coincides with a period of relatively low concentration of organics (numerator). Although similarly low concentrations are observed in the months prior or following the volcanic event (Fig. 4), one cannot exclude that this coincidence may also re-5 flect a causal relationship between the low organic concentration and the high SO 4 concentration. Indeed, bottom of Fig. 6 B shows that the Org:SO 4 mass concentration ratio at Dunkirk is remarkably impacted by the occurrence of industrial pollution events carrying acidic freshly-emitted aerosols (detected by means of their anion neutralization ratio and trajectory analysis, see Section 4.3.2). Hence, such sulfur-rich industrial pollution events are generally characterized by a very low concentration of organics at Dunkirk, if not a quasi-complete depletion. To summarize, both NO 3 :SO 4 and Org:SO 4 mass concentration ratios allow to distinguish at SIRTA volcanic aerosols from background aerosols. However, the NO 3 :SO 4 ratio seems the most powerful to also isolate the chemical pattern of volcanic 25 aerosols from those of freshly-emitted industrial aerosols as shown in Dunkirk.
Nonetheless, Fig. 9 (as well as Figures 5, 6 and 8) illustrates much more data scatter for background aerosols in Dunkirk (in yellow) compared to SIRTA (in blue), independently of the ratio of interest (NO 3 :SO 4 or Org:SO 4 ). It has to be recalled that the Dunkirk dataset covers a much longer time period (more than a year) than the SIRTA one (2 months), which may partly explain 30 this observation. In addition to its coastal location implying the presence of sulfur-rich aerosols from marine or ship emissions (Zhang, 2016), that are naturally absent at SIRTA, Dunkirk hosts both intense harbor and industrial activities as previously mentioned (Section 4.2). Therefore, Dunkirk is a much more polluted site in sulfur-rich particles than SIRTA. This certainly explains the significantly broader range of both NO 3 :SO 4 and Org:SO 4 ratios observed for Dunkirk background aerosols, with values much lower than for SIRTA background aerosols that even intersect those associated to volcanic aerosols (in red and 35 green). Hence, such a result demonstrates the most challenging issue to discriminate the signature of volcanic aerosols among other types of aerosols at a heavily polluted site.

Thermodynamic modeling of aerosol composition
While the NH 4 :SO 4 concentration ratio varies only slightly (Figures 10, A2 and B2), thermodynamic simulations of aerosol 5 composition for the atmospheric conditions met at SIRTA reproduce a large decrease in the NO 3 :SO 4 ratio with an increasing concentration of total sulfate, whichever the background level of NH 3 (Figures 10, A1 and B1). However, only the NH 3 -rich scenario allows to best fit the NO 3 observations during the volcanic event in late Sept 2014 which is characterized by large SO 4 concentrations exceeding 4 µg m −3 (Figures 10, A1 and B1), with a determination coefficient between modeled and observed NO 3 concentrations of 0.96. The NH 3 -poor scenario overestimates the decrease in particulate nitrate, with its almost complete 10 depletion for a concentration of total sulfate exceeding 12 µg m −3 (Fig. 10, B1) concomitant with a total depletion of NH 3 15 Therefore, thermodynamic model simulations suggest that the distinct chemical signature observed for Holuhraun volcanic aerosols, compared to background aerosols, results from the large abundance of sulfate within the volcanic plume. This is confirmed by model sensitivity tests addressing the impact on the production of particulate nitrate of an increasing concentration of sulfate, while all other parameters are kept constant (Fig. 11). At high concentration of sulfate aerosols, simulations show 20 that ammonia preferentially neutralizes sulfate rather than nitrate, favoring the formation of ammonium sulfate ((NH 4 ) 2 SO 4 ) rather than ammonium nitrate (NH 4 NO 3 ). In these conditions, the decrease in particulate NO 3 concentration with increasing sulfate concentration coincides with an increase in gas-phase HNO 3 , since pH has an impact on gas-particle partitioning of NO 3 -HNO 3 . In an atmosphere very rich in sulfate (e.g. a total sulfate exceeding 12 µg m −3 here), a complete depletion of gas-phase NH 3 and particulate NO 3 can occur, concomitantly with NH 4 concentration reaches a plateau value. The preferred 25 form of sulfate aerosols is not anymore SO 2− 4 but bisulfate (HSO − 4 ) and pH drastically decreases.
Thermodynamic simulations have been compared to ACSM observations with the original SO 4 RIE of 1.20 (Fig. 10). Nevertheless, investigating the influence of SO 4 RIE values, we find that while volcanic SO 4 aerosols could be overall considered neutralized with a RIE of 1.20 (left of Fig. 7), some volcanic aerosols are non-neutralized with a RIE = 0.86 (right of Fig.   30 7), industrial aerosols remaining nevertheless still always more acidic than volcanic sulfates. We find that the three periods which are affected by the presence of acidic volcanic aerosols characterized by values of the neutralization ratio < 0.7 (22 Sept 2014 from 12:00 to 21:00, 23 Sept from 11:00 to 16:00 and 24 Sept from 10:00 to 17:00 UTC) are associated to periods of elevated concentrations of SO 4 exceeding 5 µg m −3 (Fig. 2). Note that the most acidic volcanic aerosols, characterized by a weak neutralization ratio of about 0.5, are recorded on 24 Sept and are associated to SO 4 concentrations > 15 µg m −3 , the most substantial amount of volcanic SO 4 recorded at ground-level at SIRTA which is also associated to a a large SO 2 -to-SO 4 ratio ( Fig. 2). OMPS SO 2 maps (in Supplementary Material) indicate that the queue of the Holuhraun cloud arrives over Northern France on 22 Sept and do not seem to greatly move in the following days where it gets diluted according to the observed decrease of SO 2 column amounts with time. Simultaneously, an increase in concentrations of sulfur-rich species is recorded at 5 ground-level over Northern France (Fig. 2). This joint analysis of satellite and ground-level in-situ observation suggests that the volcanic plume is captured within the boundary layer, hence being more unlikely detected by any satellite sensor. This stagnation of the Holuhraun plume within the boundary layer, preventing any more displacement, may explain an exceptional lack of local NH 3 to fully neutralize volcanic sulfur-rich aerosols, which justifies the presence of remaining acidic H 2 SO 4 aerosols within the volcanic cloud according to thermodynamic simulations in Fig. 11. We can wonder whether these specific 10 transport and meteorological conditions explain the largest SO 2 -to-SO 4 mass ratio which is observed. Therefore, as suspected by model simulations of various icelandic eruption scenarios on the UK atmosphere (Witham et al., 2015), our observations show here that, despite a very long transport and dispersion over thousands of kilometers from Iceland, the Holuhraun plume may exceptionally remain so rich in sulfur that the available amount of ammonia along its way is not sufficient to neutralize all volcanic sulfate aerosols. from Holuhraun eruption: two events at Dunkirk on 7 and 10-11 Sept and two events at SIRTA, a major one on 21-25 Sept but also a more minor episode on 9-10 Sept 2014. Interestingly, these episodes of volcanogenic air pollution in SO 2 are short-lived, lasting less than a day or a few days at the most. We consequently wonder whether this persistent particulate pollution in SO 4 , that is broadly observed in France at locations a few hundreds of kilometers apart, could also be of volcanic origin. 25 To make progress on this issue, we jointly explore ACSM ground-level in-situ measurements with sunphotometer observations from the AERONET (AErosol RObotic NETwork) ground-based remote sensing network (Holben et al., 2001) at the two stations of Dunkirk and SIRTA that provide column-integrated information on aerosols (Fig. 12). On the period of the persisting exceedance anomaly in ground-level SO 4 concentration, we also observe elevated values of the aerosol optical depth  Fig. 12) and also at Dunkirk though to a lesser extent due to shorter ACSM dataset (bottom of Fig. 12). This result demonstrates that the aerosol optical depth, a column-integrated property, is mainly impacted by groundlevel sulfate aerosols in these occasions. As observed on 1 Sept at Dunkirk (Section 4.1), industrial activities can only trigger short-term peaks, lasting a few hours, in both SO 2 and SO 4 ground-level mass concentrations (Fig. 1). Therefore, we suggest that the persisting excess anomaly in both SO 4 ground-level concentration and aerosol optical depth observed in September 5 2014 at a regional scale in France may result from the broad dispersion of sulfur-rich emissions, likely originating here from the Holuhraun eruption.
As suspected by the modeling study of Witham et al. (2015), this result illustrates the much longer atmospheric persistence (a few weeks) of volcanic sulfate aerosols compared to SO 2 (a few days), even in the boundary layer, in a real case-study.   ground-level mass concentration for selected stations are displayed in Fig. 14. Note that if a station does not meet this criterion and is consequently not selected for detailed analysis, it may nevertheless be also impacted by the eruption as a daily SO 2 threshold of 3 µg m −3 is high.  (Hatakka et al., 2003;Targino et al., 2013) while Tustervatn lies in an agricultural environment poor in sulfur (Aas et al., 2013).
By comparison, stations in Denmark lie in a much more polluted environment, as shown by higher and noisier background values in ground-level sulfur concentrations (Fig. 14). This is also confirmed by concentration-weighted trajectory analysis of EMEP ground-level data over September-October 2014 applied using a multi-site approach (top left of Fig. 15) or separately at 7 out of 8 stations studied individually (left of 30 Figures A4 and A5). The strong impact of icelandic emissions of volcanic SO 2 is all the more remarkable given the relatively low number of backtrajectories leading to Iceland from each of the 8 stations, as illustrated by trajectory density maps (right of Figures A4 b, c, d and A5 a,b, c, d, e). The only exception is the result obtained at Tustervatn (Norway) (left of Fig. A4 c), indicating a pollution by SO 2 emissions from the polar Arctic region and Svalbard. Boreal biomass burning fires or industrial emissions from northern Russia may be hypothesized as distant sources of this northerly pollution (Law and Stohl, 2007), 35 but are unlikely in our case since trajectory analysis from neighbor stations (Bredkälen and Pallas) do not point to any source in the Arctic region (left of Figures A4 b and d). This suggests an inconsistency with the Tustervatn trajectory analysis. A tuning of altitude initialization in the trajectory analysis (here assumed identical for all stations) may be required to resolve this incoherence. For Denmark stations, we identify a supplementary weak influence of SO 2 emissions from Eastern Europe industry (left of Fig. A5 a, b, c). These sources correspond to SO 2 anthropogenic sources that have already been identified in 5 the the catalogue of large SO 2 emissions in 2013 derived from OMI satellite sensor observations from Fioletov et al. (2016), represented in Fig. 15. Hotspot integration provides a contribution of the Iceland area of around 25% for SO 2 over Europe, which contrasts with the 0.2% contribution of Eastern Europe (Fig. 15).
Contrary to SO 2 , the origin of sulfate aerosols measured by EMEP stations is more complex. Using a multi-site concentrationweighted trajectory analysis, emissions from the Holuhraun eruption are also identified as a major source of SO 4 at all stations (except Tustervatn again) (top right of Fig. 15). In addition to this volcanic source, we also show the significant impact on SO 4 of anthropogenic emissions from Eastern Europe (especially from Ukraine) but also from Great Britain albeit to a lesser extent. As shown in Fig. 15, these retrieved industrial sources of sulfate are in good agreement with the sources of anthropogenic SO 2 emissions in 2013 from Fioletov et al. (2016). Interestingly, both volcanic and Eastern Europe emissions contribute almost 15 equally to SO 4 over Europe (Fig. 15), which contrasts with the volcanic specificity observed for SO 2 . Retrieved sources of SO 4 are also found to be more geographically dispersed than SO 2 sources (Fig. 15), which is likely due to their longer atmospheric persistence as discussed in Section 4.4. These results attest of the interest for developing a multi-site approach, as well as the importance to jointly analyze SO 2 and SO 4 species, as performed in this study, to better isolate, among other anthropogenic sources of pollution, the volcanic impact on the concentration of aerosols. 20 Therefore, we demonstrate here the large-scale fingerprint of the Horuhraun eruption on both gas and particulate air pollution in SO 2 and sulfate aerosols, affecting broadly Europe, not only France as shown in Sections 4.1 and 4.4 but also vastly Great Britain and Scandinavia.
4.6 Evolution of SO 2 to sulfate oxidation during plume aging 25 To understand the process of SO 2 oxidation to sulfate in volcanic clouds, we investigate the SO 2 :SO 4 mass concentration ratio observed during major volcanic events for the PM 1 fraction collected by ACSM in France at SIRTA (Section 4.3, Fig. 2) and for the PM 10 fraction sampled at the 8 EMEP stations studied in detail (Section 4.5, Fig. 14). For this purpose, we select maximum values of SO 2 concentration (and corresponding SO 4 concentration values) associated to backtrajectories leading to Iceland over Sept-Oct 2014 (these values are indicated by grey circles in Fig.14). In addition, we also evaluate the age of the 30 volcanic plume for these selected volcanic events.
The scatter plot of SO 2 :SO 4 mass concentration ratio with plume age (Fig. 16) indicates a wide array of SO 2 -to-SO 4 mass ratios in the Holuhraun plume ranging in 0.8-8.0 at stations in 5 different countries of Northern Europe (France, Great Britain, Denmark, Norway, Sweden and Finland). Elevated SO 2 :SO 4 ratios observed at Northern Scandinavia stations may suggest the impact on air quality of relatively young volcanic clouds (despite the traveled distance). Indeed, IASI satellite observations of the altitude of SO 2 mostly indicate a high-altitude (up to 8 kilometers) transport of the Holuhraun cloud at high latitudes, in broad agreement with Carboni et al. (2018) (see Animation of IASI SO 2 column amount and altitude observations of the Holuhraun cloud dispersal in Sept and Oct 2014 in Supplementary Material). Such high-altitude transport is expected to be 5 faster and to cross an atmosphere poorer in solar radiation and OH-radicals favoring a lower SO 2 -to-SO 4 oxidation. On the other hand, lower SO 2 :SO 4 ratios may be associated to more aged and diluted volcanic clouds, hence providing more time for SO 2 oxidation. These aged volcanic clouds have also probably resided a longer time at lower altitude thus meeting drastically different atmospheric conditions and more likely mixing with other types of aerosols. 10 To our knowledge, this dataset of SO 2 -to-SO 4 ratios at very long distance (a few thousand kilometers) from the volcanic source is unique. The significant variability in ratios that we observe attests of the complex atmospheric history and processes that control the oxidation of SO 2 within a volcanic cloud. Nevertheless, despite this apparent complexity and the vast geographical area over which the volcanic plume is sampled, we show in Fig. 16 that the SO 2 -to-SO 4 mass ratio evolves linearly (determination coefficient of 0.89) with t, the plume age (in hours), for stations located between 1200 and 2200 km from the 15 eruption site, associated to plume age ranging between 50 and 80 hours, as follows: Hence, we estimate a nearly constant SO 2 -to-SO 4 mass oxidation rate equal to 0.23 h −1 . If we hypothesise that this linear relationship is also valid close to the volcanic source, we would expect a near-source SO 2 -to-SO 4 mass ratio of ≈ 20. This result is in agreement with measurements performed at a few hundred of kilometers from the eruption site by Ilyinskaya et al. --------- France are shown to be mostly neutralized by ammonium, except when recorded at very high concentration. As a consequence, aged (neutralized) volcanic sulfates can be clearly isolated from freshly-emitted (acidic) industrial sulfates. Hence, representing scatter plots of NO 3 :SO 4 and Org:SO 4 versus the degree of aerosol neutralization by ammonia allows for discriminating volcanic sulfate aerosols from other types of surrounding particles except in environments where a heavy sulfur-rich pollution prevails. 10 Moreover, the joint analysis of ACSM sulfate ground-level concentration and aerosol optical depth from the AERONET sunphotometer network allowed us to demonstrate in France a consecutive exceedance duration of SO 4 pollution of a few weeks, much longer than for SO 2 (a few days at most).

15
In addition, the analysis of SO 2 and SO 4 ground-level concentrations from 27 stations of the EMEP network shows that the Holuhraun atmospheric pollution is not restricted to France but is spread more broadly in Europe up to the North of Scandinavia. Based on a multi-site concentration-weighted trajectory analysis, we identify the Holuhraun eruption as the major source of widespread persisting exceedance anomalies in SO 2 and SO 4 concentration at ground-level. This volcanogenic pollution in SO 4 is distinguished from the additional contribution of distant anthropogenic SO 4 emissions from Eastern Europe and Great 20 Britain.
We describe a wide range of volcanic SO 2 to sulfate mass ratios at EMEP stations distant of a few thousands of kilometers from the eruption site, reflecting the complex atmospheric history of volcanic clouds. In spite of an apparent spatial complexity, we highlight that the SO 2 -to-SO 4 mass ratio evolves following a simple linear dependency with the age of the plume, allowing 25 us to estimate a SO 2 to SO 4 mass oxidation rate of 0.23 h −1 .
Low-tropospheric aerosols of volcanic origin can modify the microphysical properties of clouds, as shown by several studies (e.g. Yuan et al., 2011;Schmidt et al., 2012;Malavelle et al., 2017). This volcanogenic indirect effect should be all the more important that we show here that volcanic sulfate aerosols can broadly persist over weeks in the lower troposphere, even in 30 the planetary boundary layer. While the Holuhraun eruption is of particular interest to study such atmospheric effects given its 6 month-long duration, many other tropospheric eruptions, albeit of lesser magnitude, and passive degassing activities of numerous volcanoes worldwide, are expected to collectively impact the background load of aerosols in the troposphere. More studies should address the cumulative effect of volcanoes emitting into the troposphere in order to better understand their influence on atmospheric chemistry, large-scale atmospheric pollution and climate.    Tørseth, K., Aas, W., Breivik, K., Fjaeraa, A. M., Fiebig, M., Hjellbrekke, A. G., Myhre, C. L., Solberg, S., and Yttri, K.