Source apportionment and impact of long-range transport on carbonaceous aerosol particles in Central Germany during HCCT-2010

Abstract. The identification of different sources of the carbonaceous aerosol (organics and black carbon) was investigated at a mountain forest site located in central Germany from September to October 2010 to characterize incoming air masses during the Hill Cap Cloud Thuringia 2010 (HCCT-2010) experiment. The near-PM1 chemical composition, as measured by an Aerosol Mass Spectrometer (HR-ToF-AMS), was dominated by organics (OA, 41 %), followed by sulfate (19 %) and nitrate (18 %). Source apportionment of the OA fraction was performed using the Multilinear Engine approach (ME-2), resulting in the identification of five factors: Hydrocarbon-like OA (HOA, 3 % of OA mass), biomass burning OA (BBOA, 13 %), semi-volatile-like OA (SVOOA, 19 %), and two oxygenated OA (OOA) factors. The more-oxidized OOA (MO-OOA, 28 %) was interpreted as being influenced by aged polluted continental air masses, whereas the less-oxidized OOA (LO-OOA, 37 %) was found to be more linked to aged biogenic sources. Equivalent black carbon (eBC) measured by a multi-angle absorption photometer, MAAP, represented 10 % of the total PM. The eBC was clearly associated with the three factors HOA, BBOA, and MO-OOA (all together R2 = 0.83). Therefore, eBC's contribution to each factor was achieved using a multi-linear regression model. More than half of the eBC (52 %) was associated with long-range transport (i.e. MO-OOA), whereas liquid fuel eBC (35 %) and biomass burning eBC (13 %) were associated with local emissions leading to a complete apportionment of the carbonaceous aerosol. The separation between local and transported eBC was well supported by the mass size distribution of elemental carbon (EC) from Berner-impactor samples. Air masses with the strongest marine influence based on back trajectory analysis corresponded with a low particle mass concentration (6.4–7.5 µg m−3) and organic fraction (&approx; 30 %). However, they also had the largest contribution of primary OA (HOA &approx; 4 % and BBOA 15–20 %), which was associated with local emissions. Continental air masses had the highest mass concentration (11.4–12.6 µg m−3) and a larger fraction of oxygenated OA (&approx; 45 %) indicated highly processed OA. The present results emphasize the key role played by long-range transport processes not only on the OA fraction but also on the eBC mass concentration and the importance of improving our knowledge on the identification of eBC sources.



Introduction 35
Atmospheric aerosol particles affect global climate through direct and indirect radiative forcing (IPCC, 2013), human health (Lelieveld et al., 2015;Burnett et al., 2014;Pope et al., 2011), as well as the ecosystems (Bohlmann et al., 2005;Jickells et al., 2005). Chemical composition of atmospheric particles at a specific sampling place (e.g. rural, urban, or marine environment) is not only depending on the local environment and sources but is also influenced by the history of the particles reaching the sampling site. During transport, so-called aging processes not only modify the chemical composition of the particles but also 40 affect their physical properties (e.g. size distribution, volatility, hygroscopicity, CCN activity, optical properties, Donahue et al., 2014;Farmer et al., 2015;Moise et al., 2015). As a consequence, aerosol particles at a specific location result from a complex mixture of different sources combined with complex processing.
Carbonaceous aerosol particles are a dominant fraction of total particle mass and are made of a large number of chemical 45 species, which can be divided into organic aerosol (OA) and black carbon (BC) (e.g. Cabada et al., 2002). One of the most significant aerosol particle components influenced by atmospheric aging processes is the OA fraction, which can represent up to 90 % of the fine aerosol particle mass (e.g. Zhang et al., 2007). To better understand the origins of OA, source apportionment analysis is commonly applied to distinguish primary organic sources (e.g. related to fossil fuel, biomass, or coal combustion) from secondary organic aerosol (SOA) sources based on either on-line measurements (e.g. Zhang et al., 2011;Canonaco et al., 50 2013), off-line chemical analysis (e.g. van Pinxteren et al., 2016;Srivastava et al., 2018) or a combination of both (Srivastava et al., 2019). Black carbon is associated with primary emissions from combustion processes of either anthropogenic (car, house heating, industry) or biogenic (e.g. wildfires) origins. In contrast to OA, source identification of BC remains sparse and only the recent development of an aethalometer model approach allows now to distinguish equivalent BC (eBC) related to traffic emissions from wood combustion eBC (e.g. Sandradewi et al., 2008;Laborde et al., 2013;Zhu et al., 2018;Martinsson et al., 55 2017;Liakakou et al., 2020). Not only local sources drive the aerosol particle chemical composition; long-range transport, influenced by air mass origin, also plays an important role in local number size distribution and aerosol particle chemical composition (e.g. van Pinxteren et al., 2016;van Pinxteren et al., 2019;Waked et al., 2018). Therefore, not all the eBC mass concentration has to be linked to local sources, and a significant fraction can be attributed to long-range transport (e.g. Healy et al., 2012;van Pinxteren et al., 2019). 60 The present work investigates the aerosol particle chemical composition and the different sources of carbonaceous particles reaching a site close to the village of Goldlauter in the Thuringian forest in Central Germany. The measurements were part of the "Hill Cap Cloud Thuringia 2010" (HCCT-2010) experiment, which aimed to investigate the impact of cloud processing to https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License. aerosol physico-chemical properties. The Goldlauter site served as upwind site to study air masses before entering hill cap 65 clouds at the Schmücke mountain. The present study focusses on an in-depth characterization at this site, while companion papers have related upwind site data to the other experiment sites (see https://www.atmos-chemphys.net/special_issue287.html and e.g. Harris et al., 2013). The presented results stand for themselves, but can also be used to further interpret HCCT-2010 results and guide associated modeling and future experimental studies in the Schmücke area.

Site
For the "Hill Cap Cloud Thuringia 2010" (HCCT-2010) experiment, the same places were used as for the FEBUKO/MODMEP experiments in 2001/02 (Herrmann et al., 2005). This work is focused on measurements performed at the upwind site (10•45'20'' E, 50•38'25'' N, 605 m a.s.l.) and approximately 6 km from the nearest city of Suhl. The sampling place is located on the outskirts of the forest in front of the mountain site "Schmücke" under the prevailing SW wind direction and about 350 m 75 below the mountain site "Schmücke" (see Tilgner et al. (2014) for more details). In the following, the upwind-site will be referred to as Goldlauter (GL). All times in the manuscript are given in Central European Summer Time (CEST).

Instrumentation
A large setup of on-line and off-line instruments was deployed during HCCT-2010, covering both gas and particle phases. Online instruments were sitting in two nearby laboratory containers and were operated continuously during the entire campaign. 80 On the other hand, off-line sampling systems were applied only during specific intensive observation periods (IOPs) associated with two different conditions: 1-SW wind direction and presence of cloud at the Mt. Schmücke site for the full cloud events (FCE) and 2-SW or NE wind direction and no clouds or fog at any site for the non-cloud events (NCE). A detailed overview of these events is given by Tilgner et al. (2014).

Gas-phase measurements 85
The trace gases O3, SO2, and NOx (NO and NO2) were quantified continuously using standard gas monitor systems for the whole duration of the experiment (Table SI-1). Non-methane hydrocarbon sampling (NMHC) was carried out with stainless steel containers every 2 hours during IOP events only. Ambient air was sucked into the container for three minutes before closing the valves. Analysis of the NMHC was carried out by the German Federal Environmental Protection Agency (UBA) at their laboratory at Schmücke (UBA and GAW site) within 72 hours after sampling, using a GC-FID gas chromatographic 90 analysis (Rappenglück et al., 2006). The cleaning of the stainless-steel container was carried out overnight through heating and evacuating.
Additionally, a Monitor for AeRosol and Gases in Ambient Air (MARGA 1S ADI 208, Metrohm AG, Switzerland, Rumsey et al., 2014;Twigg et al., 2015;Stieger et al., 2017 ) connected to a Teflon-coated PM10 inlet was deployed for the detection of https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License. the major water-soluble inorganic compounds in both gas phase and particle phase. A good correlation for the MARGA SO2 95 and the UV-fluorescence monitor was reported in agreement with previous works from Makkonen et al. (2012);Stieger et al. (2017).

Online aerosol particle measurements
The online physico-chemical characterization of the ambient aerosol particles was performed using a large set of instruments. 100 A High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS, Aerodyne Research Inc, DeCarlo et al. (2006), later referred to as AMS), a dual mobility particle size spectrometer (TROPOS-Type T-MPSS, Birmili et al., 1999), a Multi-Angle Absorption Photometer (MAAP, Model 5012, Thermo Scientific, Petzold and Schönlinner, 2004), a three-wavelength nephelometer (TSI Model 3563, Heintzenberg et al., 2006). All these instruments were located in the same laboratory container and connected to the same sampling inlet consisting of a PM10 inlet located approximately 6 m above ground level directly 105 followed by an automatic aerosol diffusion dryer system maintaining the relative humidity in the sampling line below 30 % (Tuch et al., 2009). Moreover, water-soluble PM10 inorganic ions were also measured by MARGA.

AMS data analysis and positive matrix factorization
The AMS data was processed under the Squirrel version 1.52L and the PIKA version 1.13B (downloaded from 110 http://cires.colorado.edu/jimenez-group/ToFAMSResources/ToFSoftware) using the IGOR Pro software package (Wavemetrics Inc, Portland, USA). A Chemical Dependent Collection Efficiency (CDCE) correction was applied on the AMS mass concentration according to Middlebrook et al. (2012). Quality assurance on the AMS data was achieved by comparing it to the MARGA and Berner impactors (sum of the first three stages) for individual species, while mass closure of the PM1 Source apportionment was performed on the high-resolution organic mass spectra dataset using the multi-linear engine (ME-2) model developed by Paatero (1999) and using the Source Finder tool (Sofi4.9, Canonaco et al., 2013) developed at the Paul Scherrer Institute (PSI, Switzerland). Prior to analysis, the high-resolution organic mass spectra matrix was prepared according 120 to the recommendations of Ulbrich et al. (2009). Isotope ions, which are calculated as a constant fraction of the parent ion, were removed. A minimum counting error was applied and ions with a signal-to-noise (SNR) ratio between 0.2 < SNR < 2 were down-weighted by a factor of 2, and ions with SNR < 0.2 were down-weighted by a factor of 10. Finally, ions related to CO2 + were also down-weighted since they are calculated as a fraction of the ion CO2 + (Allan et al., 2004). The source apportionment was made following the recommendation of Crippa et al. (2014): in a first step, a non-constrained model 125 approach was investigated. Since primary factors were not properly resolved during this first approach, a partially constrained approach was then investigated. Elemental analysis of the identified mass spectra was performed using the Analytic Procedure https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License.
for Elemental Separation (APES 1.06) based on Aiken et al. (2008) and including the improved approach from Canagaratna et al. (2015). A detailed description of the source apportionment analysis can be found in the supplementary information SI-5. 130

Off-line aerosol collection by 5-stage Berner-impactors and laboratory analysis
In parallel to the on-line measurements, a five-stage low-pressure Berner-impactor (LPI 80/0.05/2.9; Hauke GmbH und Co. KG, Austria, Berner and Lurzer, 1980) was used to collect PM size segregated during the IOPs using a humidity-controlled inlet (RH < 80 %). Water-soluble ions, organic carbon (OC), elemental carbon (EC), as well as sugar and anhydrosaccharide analyses were performed. Details on the sampling conditions, sample preparation, as well as analytical methods can be found 135 in the supplementary information (section SI-2). In the following, only results of IOPs longer than 2 h are considered in order to collect enough material on each stage of the impactor. Details on the sampling period for each considered IOP is provided in a dedicated section on the supplementary Information and in Table SI-2.

Back-trajectories and cluster calculations
The 96 h back trajectories were used to determine the influence of the air mass origin on aerosol.  Carslaw and Ropkins, 2012;Ropkins and Carslaw, 2012). 145

Results
Data analysis will first focus on the overall aerosol particle chemical composition and mass closure. The second part will discuss the source apportionment of both organic aerosol and eBC. Finally, the third section will investigate the influence of the air mass origins on aerosol particle chemical composition and size distribution.

Overall AMS-MAAP time series
Aerosol particle chemical composition (mass concentration and mass fraction) as measured by AMS and MAAP as well as the particle number size distribution over the entire time-period are shown in Figure 1. On average, the near-PM1 particulate chemical composition was principally made-up of organic aerosol, OA (41 % of the total mass, Fig. 1). Sulfate and nitrate have quite similar contributions (19 % and 18 %, respectively). The rest of the aerosol particle mass concentration was made 155 of ammonium (11 %), eBC (10.0 %), and chloride (1 %). Despite their similar contribution to the particle mass fraction, sulfate and nitrate showed a clear time dependency (Fig. 1). Although sulfate dominates the inorganic fraction at the beginning of the https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License. measurement period, nitrate becomes more important over time. This can be directly linked to a decrease of temperature during the sampling period ( Fig. SI-5), inducing a change in nitrate partitioning between gas and particle phase. A last factor that must be considered is the decrease of solar radiation from summer to winter, influencing the photochemical formation of 160 sulfate. Variations of the organics and eBC mass concentration over the sampling period will be discussed in sections 3.2 and 3.3, respectively.

Berner impactor data
Parallel to online measurements, Berner-impactor provides size-resolved chemical composition up to 10 µm (Fig. 2). Over the samples, PM1.2 mass concentration (sum of the first three stages) represents more than 75 % of the PM10. The only exceptions 165 are for FCE 1.1, NCE 0.1, NCE 0.2, and NCE 0.3, which are all associated with back trajectory cluster C1-West and maritime air masses (see section 3.4). However, it is important to note that such aged maritime air masses do not systematically include an important super-µm fraction (for example, FCE 22.1). A simple reason for that is a washout of the air mass during rain events before it reaches the sampling site. A systematic look at the PM1.2 to PM10 ratio for the main aerosol component shows that EC and sulfate are principally present on the PM1.2 (> 80 % each), while OC-PM1.2 represents more than 60 % of the PM10 170 mass concentration. Nitrate is the only species that has a strong variability on the PM1.2 to PM10 ratio ranging from 20 to > 90 %).

AMS comparison with MARGA
According to the large contribution of the PM1.2 mass concentration to PM10 discussed above and despite their respective size cutting, AMS and MARGA were successfully compared for ammonium, nitrate, and sulfate ( Fig. 3, SI-2 and SI-3), indicating 175 that these compounds were principally present in the PM1 size range. In spite of the observed agreement, certain limitations must be considered. For example, the presence of organo-nitrates (ONs) and organo-sulfates (OSs) can interfere with the quantification of inorganic nitrate and sulfate by the AMS (Farmer et al., 2010;Bruns et al., 2010). The presence of such compounds might explain the slight overestimation of the AMS nitrate ( Additionally, some specific periods were also found in which the nitrate mass concentration measured by the MARGA appears to be higher and not related to the AMS's one (Fig. 3). This clearly indicates a larger contribution of super-µm particles that are not detected by the AMS as previously reported for the Berner-impactor samples. During these periods, MARGA's nitrate contains a larger fraction of sodium nitrate, which originates during the aging of marine aerosol when it crosses polluted areas (e.g. Dasgupta et al., 2007), as confirmed by both the concomitant increase of sodium mass concentration, and the attribution 185 of these periods to marine influence clusters (C1-West and C2-Northwest) with RTIwater above 0.5 ( https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License.

Overall result of PM analysis 190
AMS, MARGA, and MAAP measurements provide complementary information on the aerosol particle chemical composition. Therefore, eBC from MAAP, organics from AMS, and inorganic ions from MARGA were combined to provide a comprehensive picture of the ambient PM10 particle composition (Fig. 4) in a similar way as Schlag et al. (2016). The main advantage of combining these three instruments is to provide a more detailed description of the ambient particles than if they were used individually. However, some limitations must be considered: i) crustal material is detected neither by MARGA nor 195 AMS and therefore will not be considered at all in the following discussion; ii) the use of different upper-size cutting for the OA might lead to an underestimation of the total OA mass as expected from the OC distribution. However, the resulting PM10 estimation correlates very well with the PM10 mass concentration measured by the Berner-impactor (slope of 0.98, R² = 0.96, Fig. 4), indicating a minor contribution of non-considered species (i.e. dust, calcium, magnesium, and trace metals) to the total PM10 mass, as well as a lower organic contribution to the super-µm size range. Although the estimated PM10 mass was validated 200 only during the IOPs, it appears to be reasonable to consider it as accurate over the entire experiment within the previously mentioned limitations.

Organic aerosol source apportionment
An investigation of the organic aerosol source apportionment highlights the presence of five different factors, which were identified based on their individual time series, mass spectra, diurnal variability, and comparison with external measurements 205 (Fig. 5). A detailed description of the different steps of the analysis as well as the identification of the different factors is given in the supplementary information (Section SI-5). Briefly, in a first step, a non-constrained model was run, and, in a second step, a series of partly constrained runs were investigated in order to better distinguish the different primary organic factors.
The selected final solution results in a partially constrained model with two primary organic factors: Hydrocarbon-like organic aerosol (HOA) and biomass burning organic aerosol (BBOA). HOA was constrained using the mass spectra reported by Mohr 210 et al. (2012) in Barcelona (Spain) and is available on the AMS mass spectra database (http://cires.colorado.edu/jimenezgroup/HRAMSsd/, Ulbrich et al., 2009). For BBOA, the mass spectra identified during the source analysis process itself was used (see details in section SI-5). In addition to these two primary OA factors, three distinct oxygenated organic aerosols (OOA) were identified as semi-volatile OOA (SV-OOA), low-oxidized OOA (LO-OOA) and more-oxidized OOA (MO-OOA) (Fig. 5). It is important to note that the three OOA factors were already identified in the first step of the source apportionment 215 analysis during the non-constrained approach.

HOA
HOA is commonly considered a surrogate for fossil fuel combustion emissions, especially related to traffic emissions. The HOA mass spectrum is characterized by a larger contribution of hydrocarbon-like ions (CxHy + fragments, Fig. 5), resulting in a low O:C (0.04) and high H:C (2.00), which is in agreement with previously reported values from Canagaratna et al. (2015). 220 https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License.
A clear dependency of HOA contribution on OA and temperature was observed (Fig. 6), ranging from around 1 % during the warmest period up to 5 % for the coldest period. This temperature dependency indicates that HOA should be mostly associated with local residential house heating rather than car exhaust. 230

BBOA
The BBOA factor (13.3 ± 10.0 % of the total OA) is related to biomass burning emissions and its mass spectrum is characterized by the presence of two specific fragments: C2H4O2 + at m/z 60 and C3H5O2 + at m/z 73 (Fig. 5); they are known to be related to anhydrous sugars like levoglucosan (e.g. Schneider et al., 2006). This is confirmed by the correlation observed in off-line levoglucosan (R² = 0.84, Fig. SI-16). The elemental ratios of the BBOA factor correspond with the lower range of all the 235 reported values in Canagaratna et al. (2015). Although BBOA mass spectrum is influenced by the type of wood used, the combustion conditions, as well as the wetness of the wood (Ortega et al., 2013), the low O:C value of the BBOA factor indicates that it is mostly freshly emitted. BBOA also correlates with specific NMHC compounds like ethane (R² = 0.34), ethylene (R² = 0.55), m,p-xylene (R² = 0.32), and more generally with the total alkenes (R² = 0.40), as previously reported by Gaeggeler et al. (2008) and Schauer and Cass (2000). Similarly to HOA, the BBOA fraction to total OA increases with the 240 decrease of the temperature, representing almost 20 % of the OA during the coldest period (Fig. 6). The correlation between BBOA and HOA (R² = 0.86) clearly indicates that these two factors are emitted from a similar origin. This supports our conclusions on the HOA as being mostly associated with residential house heating.
Wood combustion used for residential house heating dominates the local anthropogenic emissions in the surrounding area of the sampling place. This is in agreement with the reported BBOA contribution of 20 % for a similar place in Germany in winter 245 (Poulain et al., 2011). The predominance of biomass burning emissions compared to liquid fuel is also supported by the benzene to toluene ratio value during the IOPs (mean: 1.1, min.: 0.47, max.:2.65), which is comparable to the ratio reported by Gaeggeler et al. (2008) for a similar location in Switzerland.
Consequently, taking all of them together, the C1-West and C2-Northwest clusters might be impacted by the village of Goldlauter, as discussed later in section 3.4. https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License.

SV-OOA
The SV-OOA was identified according to the relative similarity of its time series with nitrate (R² = 0.40) and its diurnal profile, 255 which shows the highest concentrations during nighttime, a decrease in the early morning hours, and a minimum during daytime (Fig. SI-17). The SV-OOA mass spectrum is characterized by a higher contribution of the ions C2H3O + (m/z 43) compared to CO2 + (m/z 44). The elemental analysis of the SV-OOA mass spectra shows an O:C of 0.40 and a H:C of 1.70 (Fig. 5), which is in the lowest range of the reported SV-OOA values from Canagaratna et al. (2015). Its MS is also similar to the median SV-OOA mass spectra based on 25 AMS measurements over Europe (R² = 0.69) described by Crippa et al. (2014). 260 The difference was mainly attributed to a smaller contribution of the CO2 + ion (m/z 44) on our factor. On average, throughout the entire sampling period, SV-OOA represented 19 ± 11 % of the total OA, ranging from 12 to 22 %, with a clear temperature dependency similar to the POAs (Fig. 6). Nevertheless, this is in agreement with the fact that SV-OOA is generally associated with gas-to-particle partitioning of semi-volatile organic compounds (e.g. Ulbrich et al., 2009;Zhang et al., 2011).
Consequently, it has to be related to a more local and /or regional influence. Interestingly, SV-OOA also correlates with 265 anthropogenic NMHC gases like toluene (R² = 0.55), i-pentane (R² = 0.52), benzene (R² = 0.46), as well as the sum of aromatics NMHC (R² = 0.60) (Fig. SI-18). This correlation may be a consequence of the presence of semi-volatile organic compounds and intermediate volatility organic compounds (VOCs and IVOCs, respectively) either directly emitted from anthropogenic sources or resulting from the dilution of the POA (e.g. Lipsky and Robinson, 2006;May et al., 2013). Ambient relative humidity and, subsequently, aerosol liquid water content seem to be two of the essential parameters driving the partitioning of the 270 anthropogenic VOCs and IVOCs in the particle phase (Murphy et al., 2017). Therefore, correlations between SV-OOA and NMHC might be results from the condensation of semi-volatile compounds emitted by anthropogenic sources.

OOAs
Finally, the two OOAs referred to as low-oxidized oxygenated organic aerosol (LO-OOA) and more-oxidized oxygenated organic aerosol (MO-OOA) were identified during an early stage of the source apportionment analysis, as discussed in the 275 supplementary information (section SI-5). They present two distinct time series and mass spectra, indicating two different sources rather than an artificial splitting by the model (Fig. 5). Both are characterized by a high contribution of mass m/z 44 (mostly CO2 + ), while only LO-OOA has a strong contribution of m/z 43 (mostly C2H3O + ). Their elemental ratios reflect this difference. MO-OOA is more oxygenated (O:C = 0.89) than LO-OOA (O:C = 0.58). The difference between the two OOAs might be related to either different precursors or aging processes. 280 The two OOAs are the two most important contributors to the total OA fraction (28 ± 12 % and 37 ± 18 % of the OA for MO-OOA and LO-OOA, respectively). However, their individual relative contributions strongly vary over time. LO-OOA dominated at the beginning of the measurement period and contributed up to ca. 60 % of OA (Fig. 5). The fragment C7H7 + (m/z 91) was frequently associated with biogenic SOA, even though it cannot be considered as a specific tracer (Lee et al., 2016). C7H7 + arises from the fragmentation of aromatic compounds and can, therefore, have several sources. Here, the 285 https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License. contribution of C7H7 + to total OA (f91) is higher in LO-OOA than MO-OOA, which might indicate a larger contribution of biogenic SOA to LO-OOA. Opposite to the anthropogenic related factors, the mass fraction of LO-OOA decreases with the decrease of temperature (Fig. 6).
This can be associated with the decrease of the biogenic VOC emissions from late summer to early winter (Helmig et al., 290 2013). The impact of biogenic sources is also supported by the air mass cluster analysis, which associated the highest fraction of LO-OOA with a cluster with the highest RIT over natural vegetation (Table 1 and discussion in section 3.4).
In contrast, MO-OOA does not show a pronounced temperature dependency, but it strongly correlates with eBC (R² = 0.79, Fig. 5 and SI-16), which is higher than the coefficient correlation for the POA factors. MO-OOA also correlates better with oxalic acid (R² = 0.81) than LO-OOA (R² = 0.65) (not shown). Moreover, MO-OOA correlates moderately with SO2 (R² = 295 0.35), suggesting an anthropogenic influence. It is known that the aging of primary OA leads to mass spectra with a similar pattern to OOA (Jimenez et al., 2009). Consequently, MO-OOA can be identified as being related to processed polluted/anthropogenic air masses from long-range transport.

Equivalent black carbon (eBC) source apportionment
As mentioned before, eBC correlated with three different organic factors (HOA, BBOA, and MO-OOA) identified during 300 source apportionment analysis. Taken together, the sum of these factors correlates strongly with eBC (R² = 0.83) as shown in Figure 7-a. Therefore, using a multilinear regression model, the different sources of eBC were assessed for each factor following Laborde et al. (2013) and Zhu et al. (2018). The assumption made here is that the eBC mass is attributed to individual contribution of each OA factor (i.e. eBCHOA, eBCBBOA, and eBCMO-OOA) at any time as following:

305
( 1 ) The eBC emission related to each source is assumed to be proportional to the individual source mass concentration released (mHOA, mBBOA, and mMO-OOA, respectively). Therefore, the multilinear regression model could be explained as follows:

310
( 2 ) Where a, b, and c respectively represent the linear regression coefficient for mHOA, mBBOA, and mMO-OOA, which will be used to estimate the respective eBC contribution towards each OA factor.
A very good correlation between measured and modeled eBC was obtained (Figure 7-b), and modeled eBC explained 96 % of 315 the measured one. Based on this approach, long-range transport particles associated with MO-OOA are the largest source of eBC during the measurement period, contributing to half of it (52 %), while eBC associated with local emissions of HOA and BBOA represents 35 % and 13 %, respectively. Considering only local eBC sources, fossil fuel combustion dominates the https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License. eBC fraction (73 % for eBCHOA and 27 % for eBCBBOA), which in agreement with previous works (e.g. Healy et al., 2012;Herich et al., 2011). 320 Using single-particle mass spectrometer measurements, Healy et al. (2012) reported that size distribution of EC can also directly be used to apportion soot sources: a local EC source was related to particles with a vacuum aerodynamic diameter (dva) of < 400 nm, while continental transported soot was related to particles with dva > of 400 nm. A similar cut-off diameter was applied to the Berner-impactor measurements to split the EC into freshly emitted and transported one, assuming that the 325 first two stages (i.e. aerodynamic diameter ranging from 50 to 420 nm) were associated with the local EC, and the three larger ones (i.e. diameter ranging from 420 nm to 10 µm) with continental transport (Fig. 8). The resulting EC classification provides quite similar results than the one discussed before and using the multilinear regression approach (Fig. 8) with an uncertainty of approximately 20 % (mean ratio between the two approaches excluding FCE 11.2) supporting our conclusions. Only FCE 11.2 provides completely different results between the two approaches without any clear explanation. In summary, 330 comparisons between the two approaches (multilinear regression and impactor size cutting) support each other and both confirm the importance of a minimum near 400 nm (in aerodynamic diameter) in EC size distribution for distinguishing freshly emitted from long-range transported soot. The importance of long-range continental transport of soot is also in agreement with the measurements made by Roth et al. (2016) at the summit station by the ALABAMA. The authors reported that soot was mainly found in particles with diameters larger than 450 nm, which correspond with aged/processed soot. 335

Influence of air mass origin to chemical composition and particle number size distribution
A total of 6 clusters was obtained based on 96 h backward air mass trajectories (Fig. 9) and they are characterized in Table 1 by their residence time index (RTI) over different types of ground before reaching the sampling place, based on the approach described in van Pinxteren et al. (2010), and their meteorological conditions. Cluster C1-West and C2-Northwest correspond to two different types of marine-influenced air masses with RTIwater of 0.34 and 0.47, respectively. C1-West starts near Iceland, 340 while C2-Northwest comes from the Norwegian Sea. These trajectories occurred during 31 % and 17 %, respectively, of the measurement period. Although the cluster C3-Southwest (18 % of the time) also contains a maritime component at the starting point of the air masses (RTIwater = 0.12), it is dominated by an RTI associated with continental areas (France and South-Germany). Cluster C4-South is characteristic of southern Europe, coming from an industrial and polluted area of northern Italia via Austria and the south of Germany. It also presents the highest RTI (0.51) related to natural vegetation (i.e. forest). 345 Furthermore, it corresponds to the warm period. However, this cluster only occurs for a short period (8 % of the total sampling period). Cluster C5-Northeast is an issue of the boreal area (north of Sweden), spending some time over the Baltic Sea (near Finnish, Latvian, and Polish coasts) before entering the north of Germany. Although it might also contain a small maritime component (RTIwater = 0.18), this cluster mostly follows coastal areas. Therefore, it should present an important continental and polluted aspect. Similar to C4-South, it is not very common (9 % of the sampling period). Finally, C6-East clearly 350 represents continental air masses coming from the east side of Europe (crossing Russia, Ukraine, Poland, and the Czech https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License. Republic). The sampling site was under its influence 17 % of the time, mostly during the second part of the experiment. Not surprisingly, the air mass clusters with the highest continental background (C3-Southwest, C4-South, and C6-East) also correspond with the ones with the highest aerosol particle mass concentrations (Table 1).

355
The aerosol particle chemical composition, number size distribution (PNSD), and trace gases were averaged according to the different air mass clusters and are presented in Figure 10, and summarized in the supplementary information (Table SI-4). The highest particle mass concentrations were observed for clusters with the strongest continental influence (i.e. C3-Southwest, 11.5 µg m -3 ; C4-South, 11.4 µg m -3 , and C6-East 12.6 µg m -3 ). The largest mass concentrations of chloride, sodium, and potassium salts were associated with C1-West and C2-Northwest, which have the largest marine influence (Fig. 10 and Table  360 1). These two clusters also correspond to the periods with the lowest particle mass (7.5 µg m -3 and 6.4 µg m -3 , respectively) and trace gas concentrations (Table SI-4). Although the lowest absolute and relative mass concentrations of organics ( 30 %) and eBC (7-8 %) were also observed for these two clusters, they show the largest fraction of anthropogenic sources (HOA  4 % and BBOA 15-20 % of OA). The average particle number size distributions for these two clusters present the highest concentration of Aitken mode particles (centering on 40-50 nm), supporting the influence of local anthropogenic emissions. 365 Increasing the RTI value of the air masses over continental areas leads to an increase of the carbonaceous fraction in both absolute and relative mass concentrations. The highest mass concentrations of OA and eBC are associated with C4-South (5.4 µg m -3 and 1.0 µg m -3 , respectively) and C6-East (5.8 µg m -3 and 1.3 µg m -3 , respectively). The averaged particle size distribution for C3-Southwest and C4-South are almost unimodal, centering on 100 nm, indicating well-processed particles not impacted by local anthropogenic sources. The highest mass concentrations of eBC and OA are both linked to air masses 370 coming from eastern Europe (C6-East). This is also true in regard to gas composition since the cluster C6-East also shows the highest concentrations of SO2, HNO3, and O3 (Table SI-4). Interestingly, the different nucleation events observed during the sampling period (Wu et al., 2015) were mainly connected to clusters C5-Northeast and C6-East, as reflected by the large contribution of the smallest particle diameters (nucleation mode) to their respective PNSD profiles, which did not appear on the other clusters. Nucleation processes strongly depend on the sulfuric acid formation (Kulmala et al., 2004;Birmili et al., 375 2003), which is certainly promoted by the SO2 concentration. Finally, the two continental eastern clusters also show a large particle mode (approx. 200 nm), indicating long-range processed particles in agreement with the large fraction of MO-OOA and its associated eBC. Taken together, the aerosol particle and trace gas compositions of C6-East confirm the strong anthropogenic influence on air masses originating from this sector, which is in agreement with long-term measurements performed at the German station of Melpitz located approximately 250 km to the north-east of the Goldlauter site (Spindler et 380 al., 2013). https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License.
In the frame of the HCCT-2010 campaign, a detailed description of the aerosol particle chemical composition reaching the site of Goldlauter was made by combining continuous online measurements (AMS, MARGA, MAAP, and MPSS) with offline impactor samples performed during specific IOPs. Merging online results from the AMS, MAAP, and MARGA together 385 provides an hourly time-resolved chemical picture of the ambient PM10 composition. The consistency with the merged total PM10 mass and the one measured by the Berner-impactor highlight the fact that OA on the super-µm size range should be low, and non-considered species (i.e. dust, Calcium, Magnesium, and trace metals) should play a minor role in the total PM10 mass during our study. A total of five factors were identified from the source apportionment of the OA, including two primary organic aerosols related to fossil fuel combustion, HOA (3 % of total OA), biomass burning combustion BBOA (13 %), and 390 three oxygenated organic aerosols including a semi-volatile SV-OOA (19 %), a MO-OOA (28 %) associated with long-range transport of polluted continental air masses, and a LO-OOA (37 %) related to aged biogenic aerosol. Using the correlation between HOA, BBOA, and MO-OOA with eBC, a multilinear regression approach was applied to performed source apportionment of eBC. This analysis highlights eBC contributions related to the source of HOA (35 %), BBOA (13 %), and MO-OOA (52 %). It was therefore possible to distinguish local eBC emissions (48 % of the total eBC) dominated by fossil 395 fuel combustion (73 % of the local eBC) from long-range transported eBC. Size resolved EC from the Berner-impactor supports our founding and confirms the applicability of the size cutting at 400 nm for distinguishing local EC (< 400 nm) from transported EC (> 400 nm) as mentioned by Healy et al. (2012). Local sources of organic aerosol, appear to play a smaller role compared to long-range transport, which is responsible for 65 % of OA. These results confirm the importance of long-range transport to the total mass concentration of OA and eBC not only during high pollution events (e.g. van Pinxteren et al., 400 2019;Petit et al., 2017;Waked et al., 2018). HOA and BBOA are properly depicted by the primary anthropogenic sources of liquid fuel and biomass burning emissions, respectively. However, during their transport, aging processes will lead to signatures closer to OOA (Jimenez et al., 2009). A direct consequence is that the MO-OOA associated with long-range transport of polluted air masses, might result from such aging processes of anthropogenic sources and it highlights the complexity of the internal chemical composition of the OOAs. Consequently, further efforts should be made in the future on 405 the improvement of the OOAs' identification in order to properly distinguish between biogenic-SOA and anthropogenic-SOA, as well as between locally formed and transported SOA, which could be critical for rural and remote stations.

Figure 3: Influence of marine air masses on nitrate size distribution: (a) nitrate mass concentrations measured by AMS and MARGA as well as sodium mass concentrations by MARGA, (b) the residence time index (RTI) of the 96 h backward trajectories above different land cover as developed by van Pinxteren et al. (2010). The colored bars and numbers at the top refer to the 6 different air mass clusters (see section 3.4), and the grey bars refer to the different cloud and non-cloud events as defined in
https://doi.org/10.5194/acp-2020-626 Preprint. Discussion started: 16 July 2020 c Author(s) 2020. CC BY 4.0 License.