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AtmosphericChemistryandPhysics Chemical composition of free tropospheric aerosol for PM1 and coarse mode at the high alpine site Jungfraujoch

. The chemical composition of submicron (ﬁne mode) and supermicron (coarse mode) aerosol particles has been investigated at the Jungfraujoch high alpine research station (3580 m a.s.l., Switzerland) as part of the GAW aerosol monitoring program since 1999. A clear seasonality was observed for all major components throughout the period with low concentrations in winter (predominantly free tropospheric aerosol) and higher concentrations in summer (enhanced vertical transport of boundary layer pollutants). In addition, mass closure was attempted during intensive campaigns in March 2004, February–March 2005 and August 2005. Ionic, carbonaceous and non-refractory components of the aerosol were quantiﬁed as well as the PM1 and coarse mode total aerosol mass concentrations. A relatively low conversion factor of 1.8 for organic carbon (OC) to particulate organic matter (OM) was found in winter (February– March 2005). Organics, sulfate, ammonium, and nitrate were the major components of the ﬁne aerosol fraction that were identiﬁed, while calcium and nitrate were the only two measured components contributing to the coarse mode. The aerosol mass concentrations for ﬁne and coarse mode aerosol measured during the intensive campaigns were not typical of the long-term seasonality due largely to dynam-ical differences. Average ﬁne and coarse mode concentrations during the intensive ﬁeld campaigns were 1.7 (cid:22) g m (cid:0) 3 and 2.4 (cid:22) g m (cid:0) 3 in winter and 2.5 (cid:22) g m (cid:0) 3 and 2.0 (cid:22) g m (cid:0) 3 in summer, respectively. The mass balance of aerosols showed higher contributions of calcium and nitrate in the coarse mode during Saharan dust events (SDE) than without SDE.


Introduction
Atmospheric aerosol particles play an important role in the global radiation balance. The chemical composition and size distribution of the aerosol are important in quantifying these radiative effects, through influencing absorption and scattering. The size and composition also influence the hygroscopic properties of the aerosol particles and their ability to act as cloud condensation nuclei, and therefore affect cloud formation. Aerosol particles can also participate in heterogeneous reactions in the atmosphere, which are both composition and morphology dependent.
Measurements of size resolved chemical information provide information on the sources of aerosol particles. Primary/secondary and anthropogenic/natural aerosol particles can have significant chemical and physical differences. Aerosols are often separated into a coarse fraction (particles with a diameter d>1 µm) and a fine fraction (PM1; d<1 µm) during sampling. Primary aerosol refers to particles that are directly released into the atmosphere in the particle phase. Primary natural aerosol such as sea-salt, dust or volcanic ash particles are typically found in the coarse mode. Primary anthropogenic particles have various sources (black carbon (BC) for example is emitted by combustion processes) and is often found primarily in the fine mode. Secondary aerosol is formed by the transformation of gaseous precursors of either natural or anthropogenic origin into condensable material. For example, sulfate can be found as natural secondary aerosol originating from the oxidation of dimethyl sulfide released from marine plankton (Charlson et al., 1987), or anthropogenic secondary aerosol from the oxidation of SO 2 (Colbeck, 1998) which is emitted in large concentrations from fossil fuel combustion. Automobiles, industry, cooking and combustion are examples of anthropogenic sources of aerosol precursor gases. Secondary aerosol typically represents the majority of the fine particle mass (Turpin and Huntzicker, 1995).
The main constituents of fine atmospheric aerosol are inorganic ions, organic compounds and to a lesser extent black carbon (BC) otherwise known as elemental carbon (EC) (Krivacsy et al., 2001). While inorganic ion species and their concentrations have been determined at many locations around the world, data for carbonaceous compounds are rather limited due to the lack of instrumentation allowing for an artifact free quantification of the organic fraction.
Recently new instrumentation (aerosol mass spectrometry, thermal-optical carbonaceous analyzers) has become available, allowing for the quantitative determination of the organic fraction. These measurements show that organic material comprises the dominant fraction of the fine aerosol in various environments .
Knowledge of the chemical composition of atmospheric aerosol is important to assess its impact on the environment. Here we present long-term measurements of the chemical composition of coarse and fine mode aerosol particles from 1999 to 2006 at the Jungfraujoch high-alpine site in the Swiss Alps. The mass closure for PM1 and coarse mode aerosol was also investigated during a series of intensive field measurements. Following instrument intercomparisons, mass fractions of the various components for different seasons and aerosol types are discussed as well as their partitioning into the different size fractions.

Site, sampling and analysis
Measurements have been performed as part of the Global Atmosphere Watch (GAW) aerosol program of the World Meteorological Organization since 1999 and more intensively during Cloud and Aerosol Characterization Experiments (CLACE) at the Jungfraujoch high-alpine research station in Switzerland. The intensive field campaigns took place in March 2004, from mid July to end of September 2004, from mid February to mid March 2005 and in August 2005.

Jungfraujoch station
The Jungfraujoch (JFJ) site is located on an exposed mountain saddle in the Swiss Alps at 3580 m a.s.l. Atmospheric aerosols and gases have been measured at the JFJ within the GAW program for over a decade. The station is regularly engulfed in clouds (37% of the time based on a year long survey by Baltensperger et al. (1998)). Due to its elevation, the site is only weakly influenced by local anthropogenic sources. These characteristics make the Jungfraujoch site a well suited ground based location to investigate continental background aerosols and clouds.
Measured aerosol parameters at the Jungfraujoch exhibit a strong seasonal cycle with a maximum in summer and a minimum in winter. In summer, the site is influenced by injections of planetary boundary layer air during afternoons when solar insolation is high or during periods of frontal activity, leading to variations in the concentrations and properties of the aerosol measured. Reduced solar heating in winter reduces the extent of vertical mixing and results in substantially lower aerosol concentrations for longer periods (Lugauer et al., 1998;Weingartner et al., 1999;Henne et al., 2005). Under these circumstances, the site is deemed representative of the lower free troposphere above a continental area. More information on the Jungfraujoch site and the long-term aerosol measurements can be found in Baltensperger et al. (1997) andCollaud Coen et al. (2007). Table 1 summarizes the different types of instruments operated during the different campaigns.

Inlet
The standard GAW total aerosol inlet was used at the JFJ. It is heated to 25 • C to evaporate cloud droplets and ice crystals at an early stage of the sampling process (GAW, 2003) and has been designed to sample cloud droplets smaller than 40 µm at a wind speed up to 20 m s −1 (Weingartner et al., 1999). The aerosol sample thus consists of aerosols incorporated into cloud droplets, ice crystals, and un-activated (interstitial) aerosols. Behind the inlet, the aerosol particles are sampled at laboratory room temperature under dry conditions (relative humidity <20%).

Black carbon measurements
The Multi-Angle Absorption Photometer (MAAP, Thermo ESM Andersen) (Petzold and Schonlinner, 2004) measures the transmitted and backscattered light intensity from a defined source (λ=630 nm) that is incident on a fiber filter through which air is also drawn. The particle light absorption coefficient b abs is obtained from a radiative transfer scheme which corrects for artifacts caused by the interaction of the light with the filter material. The instrument is used to measure the black carbon mass concentration (BC) in real time. Black carbon (BC) is the most efficient lightabsorbing aerosol species in the visible spectral range. Thus, the aerosol light absorption in the visible spectral range is highly correlated with the concentration of black carbon. The relationship between the aerosol absorption coefficient b abs (m −1 ) and the corresponding black carbon mass concentration BC (g m −3 ) is established by a mass absorption efficiency σ abs,BC (m 2 g −1 ) via the relationship b abs = BC · σ abs,BC where σ abs,BC and b abs are wavelength dependent. σ abs,BC depends on the type of aerosol, the aging and the size of the BC particles (Liousse et al., 1993) and thus needs to be determined for each site. The values used throughout this work  . Another light absorbing aerosol component is hematite (Fe 2 O 3 ), a minor component of mineral dust, which usually makes a small contribution but can be important during Saharan dust events (Collaud Coen et al., 2004). Because of the relatively low mass absorption efficiency of mineral dust (σ abs (660 nm) ∼0.02-0.01 m 2 g −1 ) (Alfaro et al., 2004;Linke et al., 2006), the overall contribution of this component to the absorption coefficient is expected to be small during periods that are not influenced by Saharan dust events. Such Saharan dust episodes occur at the Jungfraujoch site on average 24 times a year (ranging from 10 to 34 per year in the last 5 years) with different duration (Collaud Coen et al., 2004). Light absorbing organic material is another component that is suspected to contribute to the measured absorption coefficient. Again, the low mass absorption efficiency associated with organic compounds (e.g. for humic like substances (HULIS) σ abs (532 nm) ∼0.03 m 2 g −1 (Hoffer et al., 2005)) would require unrealistically high concentrations of these compounds to significantly contribute to the measured absorption coefficient. We therefore assume that the measured absorption coefficient is entirely due to absorption by BC except during Saharan dust episodes.

Semi-continuous OC/EC thermo-optical analyzer
Organic carbon (OC) and elemental carbon (EC) in the total suspended particles (TSP) were measured using a semi-continuous OC/EC thermo-optical transmission analyzer (Sunset Laboratory) (Birch and Cary, 1996;Bae et al., 2004). Sampled particles are accumulated for 5 hours 45 minutes (flow rate=7.5 L min −1 ) on a quartz filter housed within the instrument after the air has passed through a gas phase denuder (charcoal-impregnated filter strips, Sunset Laboratory) (Bae et al., 2004). After sampling, the filter is heated in an oxygen-free ultra high purity helium atmosphere in four increasing temperature steps, which permits the detection of various organic carbon fractions. During these heating steps some organic compounds may be pyrolytically converted to EC. This pyrolytic conversion is continuously monitored by measuring the transmission of a laser beam (λ=660 nm) through the filter. The organic compounds are vaporized and oxidized to carbon dioxide by a manganese dioxide catalyst held at a temperature of 800-900 • C. The gas is then switched to a 2% O 2 /He mixture and the filter heated in two increasing temperature steps for determination of EC. In both cases, the evolved CO 2 is measured using a continuous, non-dispersive infrared absorption (NDIR) method. At the end of each analysis an automatic internal calibration is performed by using a known volume and a fixed concentration of methane (5% CH 4 ; 95% He). A further, off-line calibration was conducted at the beginning and the end of each campaign with an external source of methane gas injected during the He/O 2 phase.
The determined EC includes both the original EC in the particles and that produced by the pyrolysis of organics. The point where the laser beam transmission through the sample returns to the original sample transmission defines the split between organic and elemental carbon (the instrument response prior to this point is assigned to OC, and after this point is assigned to EC). This split point is determined automatically and is used to distinguish between EC and pyrolized OC. This discrimination is the major difficulty of the method: several temperature programs and results of intercomparison exercises are described in the literature Schmid et al., 2001;Schauer et al., 2003). These studies generally show that the sensitivity of the separation between EC and OC using a TOT (thermal optical transmission) analysis method depends mainly on the temperature program and on the type of samples analyzed. Previous observations from the JFJ have shown that the split point was quite often in the last OC fraction and the pyrolized fraction was small.
The analyses were based on a temperature program provided by NIOSH (National Institute for Occupation Safety and Health). This consisted of a temperature increase to 840 • C for the analysis of OC in 100% He in a series of stages (1st step: 310 • C for 60 s, 2nd step: 480 • C for 60 s, 3rd step: 615 • C for 60 s; 4th step: 840 • C for 90 s), and up to 850 • C for the analysis of EC in 98% He+2% O 2 (1st step: 550 • C for 35 s; 2nd step: 850 • C for 105 s).

Chemical composition of aerosol filter samples
The filter sampling system was designed to sample two size fractions, total suspended particles (TSP) and particles with an aerodynamic diameter d<1 µm (PM1) (Henning et al., 2003). The two sizes fractions were selected according to the recommendations of the Global Atmosphere Watch (GAW) Scientific Advisory Group for Aerosol, reflecting the fact that the mass scattering efficiency is quite different for submicron and supermicron aerosols (Finlayson-Pitts and Pitts, 2000;GAW, 2007;IPCC, 2007). Four stages of a cascade impactor (Maenhaut et al., 1996) were used to generate the cut-off of d=1 µm with a flow rate of 11 L min −1 . The flow was controlled by a mass flow controller (5851E Brooks Instrument, Fisher-Rosemount AG). The impactor stages were treated with high-vacuum grease to prevent bouncing. More details on the sampling system can be found in Henning et al. (2003).
Teflon filters (PTFE, Sartorius AG) with a pore size of 1.2 µm and 47 mm diameter were used to sample the aerosol particles, and Nylon filters (Nyabsorb, PALL/Gelman Sciences), with a pore size of 1 µm, were used for the collection of nitrate evaporating from the first filter (Zhang and Mc-Murry, 1992;Cheng and Tsai, 1997). As explained by Henning et al. (2003), gaseous nitric acid was lost to the walls of the inlet system (4.5 m length, 11 s residence time) during sampling and so any nitrate measured on the backup Nylon filter was assumed to originate from ammonium nitrate. In the following, NO − 3 data are presented as the sum of nitrate from the Teflon and the Nylon filters in order to obtain the total aerosol nitrate. For NH + 4 , the total ammonium concentration was obtained by the addition of the ammonium determined on the Teflon filter plus an equivalent amount of am-monium estimated from the associated nitrate on the backup filter.
It was found that cleaning of the filters with deionized water, NaOH or methanol was unnecessary because of the low blank values of the filters. This assumption was tested regularly. All filters were stored in Petri dishes lined with baked aluminum foil liners and placed in sealed polyethylene bags. All filters were stored at −18 • C until their analysis.
The samples were analyzed for water soluble ionic components with ion chromatography in a clean room after aqueous extraction of the filters in SCHOTT bottles. For the Teflon filters this was done using 0.3 mL of methanol (Baker CMOS grade) in order to first wet the Teflon filters, followed with 8.7 mL of deionized (DI) water, while for the Nylon filters the extraction was performed with 5mL of DI water. Extracts were analyzed within 1 hour after extraction in order to limit possible oxidation reactions in the liquid phase. The cations (Na + , NH + 4 , K + , Ca 2+ ) were analyzed with a DIONEX DX320 chromatograph using a CS16 column 4 mm with a CG16 guard column, and chemical regeneration was performed with a CSRS ULTRA II autosuppressor. The injection loop volume was 0.6 mL. The analyses were performed under isocratic MSA conditions, at 40 mM and with a flow rate of 1.5 mL min −1 . Analysis of the anions was performed in parallel with a DIONEX ICS 2500 chromatograph using an AS11 column (4 mm) with an AG11 guard column and an ASRS ULTRA II autosuppressor at a flow rate of 1.5 mL min −1 . Injections were made via a 1-mL injection loop. The eluent (KOH) was generated with an EG50 eluent generator. Gradient conditions were adapted from Ricard et al. (2002) but no methanol was used in the composition of eluents. The 15-min runs allowed for the detection of the major inorganic anions (Cl − , NO − 3 , SO 2− 4 ) and of oxalic acid. Blank levels for each chemical species were calculated from the analysis of 30 procedural blanks and were subtracted from the measured sample concentrations to obtain the actual concentrations. Atmospheric detection limits were calculated as twice the standard deviation of the blank sample concentrations, using a typical sampling duration of 24 h. Major components (sulfate, chloride, sodium, ammonium, nitrate, potassium, and calcium) were almost always above detection limit (SO 2− 4 (8.5), Cl − (5.4), Na + (9.3), NH + 4 (2.8), NO − 3 (17.6), K + (3.0) and Ca 2+ (10.7) in ng m −3 ). The chemical composition of the coarse mode particles was calculated by the difference between the TSP and PM1 data.

Aerosol Mass Spectrometer
An Aerodyne Quadrupole Aerosol Mass Spectrometer (Q-AMS, Jayne et al. (2000)) was used for measuring on-line chemically resolved mass concentrations and size distributions of non-refractory aerosol components (sulfate, nitrate, ammonium, organics), in a vacuum aerodynamic diameter range d va =50-700 nm. In the Q-AMS, the particles are drawn into vacuum through an aerodynamic lens sampling inlet system (Zhang et al., 2002), which focuses aerosol particles into a narrow, collimated beam that impacts on a porous tungsten surface (the vaporizer) heated typically to 500 • C under high vacuum (∼10 −8 torr). The non-refractory fraction of the particles, mostly the volatile and semi-volatile components, flash vaporizes upon contact with the vaporizer surface on a time scale of a few microseconds. The resultant gaseous molecular constituents are ionized using a 70-eV electron impact ionization source positioned such that the maximum electron density and the centre of the vaporized plume are co-located in the extraction zone of the mass spectrometer. A quadrupole mass spectrometer (QMA 410, Balzers Instruments, Balzers) is utilized to analyze the positive ions with unit mass-to-charge (m/z) resolution. For more details see Allan et al. (2003).

Particle size distribution measurements
A scanning mobility particle sizer (SMPS, TSI 3934), comprising a differential mobility analyzer (DMA, TSI 3071) and condensation particle counter (CPC, TSI 3022), was used to measure the particle size distribution between 17 and 900 nm (dry) diameter . The SMPS had a closed loop configuration for the sheath and excess air, which was held constant at 3 L min −1 by a critical orifice, while the sample flow rate was 0.3 L min −1 . A correction was applied to the measured diameters to account for the reduced pressure at the Jungfraujoch (650 mbar on average). Latex spheres, nebulized into the SMPS were used to confirm that the instrument was correctly sizing. The SMPS was frequently compared to another SMPS and systematic differences in integrated concentrations <5% were encountered. Aerosol volume concentrations with d<900 nm were derived from the measured SMPS number size distributions, assuming that the particles were spherical. Additionally, an optical particle counter (OPC, Grimm Dustmonitor 1.108) was operated to measure the particle size distribution in the optical diameter range d=0.3-20 µm. The comparison of the size distribution spectra of the GRIMM and the SMPS showed a very good agreement for February-March 2005 whereas for March 2004 the OPC diameters would need to be slightly shifted (multiplied by 1.2) to larger sizes to get a good agreement. The slight discontinuity in the combined number size distribution is likely to result from the fact that in the OPC the individual particles are classified according to their light scattering behavior, which depends on the particle size, morphology and refractive index. Since the OPC is calibrated with spherical Latex particles the observed shift is most probably due to the different refractive index (and complex morphology) of the measured particles in the range d=300-1000 nm.

Betameter TSP and PM1
A beta-attenuation monitor (Thermo ESM Andersen FH62 I-R) was used for continuous measurements of TSP and PM1 mass concentration . A glass fiber filter collects sampled particles whilst being exposed to a continuous flux of beta particles. The beta particles are emitted with a continuum energy distribution by a radioisotope source and are attenuated by a reference section and the sample. The transmitted fraction of the beta particles is measured using an electron counter. The number of beta particles transmitted through the sample decreases exponentially as the thickness of the deposited material increases according to Evans (1955), where I 0 is the incident flux, µ is the mass absorption coefficient for β radiation absorption (cm 2 g −1 ), and x is the mass thickness of the sample (g cm −2 ). The mass absorption coefficient is determined through a calibration procedure involving the measurement of a series of known standards (calibration foils), which bracket the mass range of interest (Jaklevic and Gatti, 1981).

High-volume sampler for TSP
A high-volume sampler (HIVOL DHA80, Digitel, Hegnau) was used to determine 48-h averages of total suspended mass concentrations. TSP was collected for 2 days on glass fiber filters (Ederol 227/1/60, diameter 15 cm, Digitel) with a flow rate of 45 m 3 h −1 . The mass concentrations were determined gravimetrically by weighing the filters before and after sampling (Mettler, AE200). From January 2006 onwards, the sampling intervals were reduced to 24 h and the glass fiber filters were replaced by quartz fiber filters (QMA20, Whatman, Dassel).

Nephelometer
The aerosol light scattering coefficients of TSP were simultaneously measured at three wavelengths (λ=450, 550, and 700 nm) by an Integrating Nephelometer (IN, TSI 3563). The sampled aerosol is illuminated over an angle of 7 to 170 • by a halogen light source directed through an optical pipe and an opal glass diffuser. The sample volume is viewed by three photomultiplier tubes through a series of apertures set along the axis of the main instrument body. Aerosol scattering is viewed against the dark backdrop of a very efficient light trap.

Mass absorption efficiency
The mass absorption efficiency σ abs,BC of the Jungfraujoch aerosol was determined from the EC measurements of the OC/EC analyzer and the light absorption measurements (MAAP). σ abs,BC is wavelength dependent and is determined at λ=630 nm. A large range of values (2-25 m 2 g −1 ) has been reported in the literature (Bond and Bergstrom (2006) and references therein). This variability is partly caused by differences in the aerosol mixing state, with external mixtures favoring smaller values, while larger values indicate internal mixing of BC (Liousse et al., 1993). Consequently, σ abs,BC depends on the aerosol composition, mixing state and to a lesser extent on particle size. However, it is believed that a major contribution to this wide range is caused by the variability in the different methods to determine EC and BC (Schmid et al., 2001). Figure 1 shows the comparison between the absorption coefficient b abs measured by the MAAP with the EC concentration determined by the OC/EC analyzer in February-March 2005 and in August 2005. A reduced major axis regression was performed on the data to account for the error of each instrument (Ayers, 2001). Noise was introduced into the data to estimate the error on the mass absorption efficiency. The mass absorption efficiency is higher in summer (11.1±0.2 m 2 g −1 ) compared to winter (7.6±0.2 m 2 g −1 ). The reasons for this difference are currently unclear but could be due to a greater coating of BC particles by e.g. organic compounds due to increased photochemical activity during summer. Such coatings may lead to increases in the mass absorption efficiency (Fuller et al., 1999). This seasonality in the mass absorption efficiency has been seen in other studies such as Sharma et al. (2002) in the Canadian Arctic, but not in a previous study at the Jungfraujoch (Lavanchy et al., 1999). The latter may be due to the fact that different procedures were applied in the different campaigns: Lavanchy et al. (1999) used a 2-step thermal method which did not correct for potential pyrolysis and the Aethalometer data were not corrected for scattering effects.

Q-AMS collection efficiency
It is known that the collection efficiencies of ambient aerosol particles are often less than 100% within the aerosol mass spectrometer, and so the Q-AMS data have to be corrected for this. This requirement arises because particles containing large fractions of slightly less-volatile mass (such as solid ammonium sulfate) can sometimes bounce off the Q-AMS heater before they evaporate, and so the "missing mass" of these particles is not measured. The collection efficiency compensates for this missing mass, and is usually derived by comparing the data for the non-refractory aerosol components (such as SO 2− 4 ) with simultaneously collected filter data (in this case the PM1 filter data). Figure 2 shows the comparison of the SO 2− 4 concentrations measured with the Q-AMS and on the filters. The slope of the linear correlation yields the Q-AMS collection efficiency. A collection efficiency of 0.48 was determined for sulfate. Since the aerosol in the fine mode has been shown to be mainly internally mixed at Jungfraujoch (Weingartner et al., 2002;Cozic et al., 2007), the collection efficiency of 0.48 was applied to all compounds (SO 2− 4 , NO − 3 , NH + 4 , organics) measured by the Q-AMS. The coefficient is consistent with values found in previous studies  and references therein).

Organic Matter (OM) and Organic Carbon (OC)
The OC/EC analyzer measures organic carbon (OC) and does not consider the other atoms associated with organic matter, such as O, H, N and S. In contrast, the organic mass concentration from the Q-AMS represents the total organic matter (OM) associated with all atoms present. The comparison between these two measurements yields information on the conversion factor of OC to OM for the Jungfraujoch aerosol. This comparison was only possible in February-March 2005 when the Q-AMS and the Sunset analyzer were sampling simultaneously. An offset correction was applied to the OC concentration data obtained from the Sunset analyzer, as a detailed analysis revealed an offset in the Sunset data of about 0.3 µg m −3 . Laboratory tests were performed to determine the source of this offset. All tests were conducted in nitrogen gas (quality 5.0), which was further purified with a pure air generator (AADCO Instruments Inc., 737-250 series). According to the manufacturer's specifications impurities were <1 ppb each for ozone, methane and non-methane hydrocarbons, oxides of nitrogen (NO/ NO x ), hydrogen sulfide (H 2 S), sulfur dioxide (SO 2 ), carbonyl sulfide (COS), carbon monoxide (CO), sulfur hexafluoride (SF 6 ), and fluorocarbons. In addition, a particle filter and a denuder were installed in front of the instrument inlet to remove particles and semi volatile gases from the air. The source of this offset could not be determined. It did not increase with sampling time (up to 24 h) and was only present in the first two steps of the OC temperature program. A leak or penetration of compounds through the denuder would have resulted in a blank that increased with sampling time. These offsets were estimated to 0.2 and 0.1 µg for the 1st and 2nd peaks, respectively, and were subtracted from the respective peaks measured in the field.
As can be seen in Fig. 3 there is a high correlation between the OC and the OM (r 2 =0.89) and the conversion factor of OC to OM is 1.84. This finding is in agreement with literature data where values between 1.6 to 2.1 have been reported (Turpin and Lim, 2001). An even higher OM:OC ratio might be expected for the Jungfraujoch aerosol due to its remote location. This coefficient could be slightly decreased by the size cut of 1 µm used for the Q-AMS whereas the OC/EC analyzer was running with TSP. However, it is not expected that this will have a strong influence at the JFJ as, like sulfate, the major fraction of OC is found in the fine mode. The same coefficient is used in this analysis for winter and summer data since no Q-AMS was running during the summer period. In summer this coefficient is expected to be higher due to the larger biogenic emissions and increased photochemical activity.

Betameter PM1 and SMPS mass concentration
The PM1 data from the betameter have very low signal to noise ratios and so long periods are used to average the data (typically 1 day). A higher temporal resolution of PM1 can be obtained by correlating the PM1 data (24-h averages) with the volume concentrations obtained from the measured SMPS size spectra (d=17-900 nm) and estimating an aerosol density. Although there are a large number of particles with d<17 nm, these small particles can be neglected when calculating the submicrometer aerosol volume concentration (Imhof et al., 2005). The missing aerosol volume between 900 and 1000 nm can also be considered as negligible when strong episodes of mineral dust are absent, as shown in the size distributions below (Sect. 3.4). Figure 4 shows the comparison of the PM1 mass concentration with the submicrometer aerosol volume for March 2004, February-March 2005, and August 2004. The correlation is rather weak (r 2 =0.53) as a result of the low signal to noise ratio of the betameter measurements. The conversion factor corresponds to an effective particle density of 1.5 g cm −3 which is consistent with the values observed at Monte Cimone (Putaud et al., 2004b), Hyytiälä (Virtanen et al., 2006) and those summarized by McMurry et al. (2002). Thus we conclude that it is appropriate to estimate the PM1 mass concentration from the SMPS volume concentration when SMPS measurements were available (winter campaigns) by using an effective particle density of 1.5 g cm −3 . In summer, the concentrations were higher, resulting in higher signal to noise ratios in the betameter data.

Betameter TSP and Gravimetric TSP
The correlation between the two TSP measurements (Fig. 5a) is high (r 2 =0.94), with a slope slightly higher than unity (TSP betameter =1.12×TSP gravimetric ). The slightly high slope could be due to the calibration of the betameter at the Jungfraujoch. The comparison between the betameter data for TSP and PM1 (Fig. 5b) reveals that the signal of the PM1 betameter is much lower than TSP betameter as expected (TSP betameter =1.58×PM1 betameter ), but is also noisier and yields a lower correlation (r 2 =0.41). Episodes of Saharan dust (yellow points in Fig. 5b determined by the method described by Collaud Coen et al. (2004)) were excluded from the correlation, since they show much higher concentrations of coarse mode particles.

Long-term chemical composition
Since June 1999 inorganic compounds in PM1 and TSP have been determined on a semi-continuous basis (24 h sampling every 6th day) on the JFJ within the GAW aerosol program and are available at http://www.nilu.no/projects/ccc/create/ database.htm. Long-term ionic measurements at various European sites were published by Putaud et al. (2004a). However, only annual averages were presented for fewer components. Malm et al. (2004) presented a summary of monthly data for different sites within the US. They showed temporal variations over a year but no year to year variation was discussed. Long-term series are available for individual components such as sulfate (e.g., Malm et al., 2002). This paper is to our knowledge the longest time series of inorganic aerosol composition at a high altitude site presented so far. In addition the mass concentrations of PM1 and TSP have been measured since February 2004 and June 1999 respectively. Figure 6 presents the long-term measurements of the major PM1 and TSP compounds detected (SO 2− 4 , NH + 4 , NO − 3 , Ca 2+ , K + , C 2 O 2− 4 , Cl − , Na + ), as well as the TSP mass concentration determined by gravimetry and the PM1 mass determined with a betameter (since January 2004 only).
For all compounds a clear seasonality is observed with low concentrations in winter when the aerosol is typical of the undisturbed free troposphere, and maximum concentrations 34 in summer when the Jungfraujoch is influenced by injections from the planetary boundary layer (PBL). First evidence on the importance of vertical transport for the Jungfraujoch aerosol was published by Baltensperger et al. (1991). De-tailed analyses on the influence of the PBL were presented by Lugauer et al. (2000Lugauer et al. ( , 1998 showing that vertical aerosol transport by thermally driven convection, acting between late spring and late summer, is the dominant transport process at www.atmos-chem-phys.net/8/407/2008/ Fig. 7. Ratio of the measured NH 4 mass to the NH 4 calculated to reach neutralization (i.e. formation of (NH 4 NO 3 ;(NH 4 ) 2 SO 4 )) as a function of the total ion mass concentration for PM1 measured with filters (daily averages). The exponential fitted line is also presented. the Jungfraujoch. This was also shown for individual chemical components (Zellweger et al., 2000). Gehrig and Buchmann (2003) presented an overview of seasonal variations of aerosol concentrations at various sites in Switzerland and showed that the PM concentrations are higher in winter than in summer due to reduced vertical transport. For SO 2 , this reduced dilution along with enhanced emissions during winter results in higher concentrations in winter than in summer (Hueglin et al., 2005;Fisseha et al., 2006). Conversely, in summer the photochemical oxidation rates are enhanced. Therefore, the resulting SO 2− 4 is relatively constant over the year in the valleys (Hueglin et al., 2005). Thus, the higher concentrations at the Jungfraujoch in summer are mainly due to enhanced vertical transport. Such seasonal behavior was also observed by Fischer et al. (2007) at Mount Washington. In contrast, for Alert, a remote site in the Arctic, Sirois and Barrie (1999) showed an opposite seasonal cycle with higher concentrations in winter than in summer, driven by a strong seasonality in south to north transport and in wet and dry removal.
In Fig. 6 it can be seen that highest TSP mass concentrations were reached in summer 2003 when Europe encountered exceptionally high temperatures which led to increased convection in the Alps and thus enhanced injection of PBL air. These high signals in summer 2003 were also observed in other continuous aerosol measured parameters such as the light scattering coefficients (Collaud Coen et al., 2007). However, of the chemical components measured, only NO − 3 showed a significant enhancement.
No statistically significant trends in the major ionic species could be obtained from this data set in contrast to other aerosol parameters measured within the GAW program for which clear trends were observed (Collaud Coen et al., 2007). This might be explained by a lower temporal coverage of the chemical composition samples (24 h sampling 36 Figure 8:   Fig. 8. Ratio of the NH 4 mass to the NH 4 calculated to reach neutralization (i.e. formation of (NH 4 NO 3 ;(NH 4 ) 2 SO 4 )) as a function of the total ion mass concentration for PM1 during the intensive campaigns (daily averages). every 6 days instead of continuously as for the other parameters) and/or by a higher year to year variation. Both possibilities lead one to conclude that there is a need for a longer time series to detect statistically significant trends. In addition, a decrease with time was observed for the TSP mass concentration for the period 1975 to 2005, however, this trend was not significant for the last 9 years (Christoph Hüglin, personal communication). Husain et al. (2004) presented longterm trends of sulfate from 1984 to 2003 at a site comparable to the Jungfraujoch (Whiteface Mountain) and showed a decreasing trend in sulfate concentrations. Nevertheless, they also indicated that this trend was less pronounced since 1995.
The different intensive campaigns are marked as orange bands in Fig. 6. There are clear differences between the two winter campaigns. During the period February-March 2005, unusually high concentrations were observed for a winter period (e.g. ∼700 ng m −3 for SO 2− 4 instead of <400 ng m −3 for winter in other years) and concentrations similar to those found in summer were measured. The campaign in August 2005 was performed after the maximum PBL influence at the JFJ in that year, and the month of August 2005 was cool and cloudy which resulted in low influence from the PBL.

Aerosol neutralization
The degree of neutralization of the aerosol (PM1 only) is presented in Fig. 7, which shows the ratio of the measured NH + 4 to the amount of NH + 4 that is needed to neutralize nitric and sulfuric acid as a function of the total ion mass concentration. The entire 6-year data set is shown in the figure, each data point representing a single 24-h sample. The Jungfraujoch aerosol is, to within the noise in the data, neutralized, since the average ratio is about 80%. At low mass concentrations there is a slight tendency for an increased acidity, but there is considerable scatter at these low concentrations. This tendency might be due to the fact that ammonia has a very strong gradient with altitude (Beig and Brasseur, 2000); thus a high degree of acidity may indicate an influence of free tropospheric air.
During the intensive field campaigns in March 2004, the aerosol was found to be substantially more acidic than in February-March 2005 and August 2005 (Fig. 8). As explained above, this might indicate an influence of free tropospheric air. There are more rigorous methods to determine aerosol acidity (Keene et al., 2004), but the necessary measurements were not made during these campaigns.

Chemical mass balance for PM1 and the coarse mode
Time series of the chemical mass balance for PM1 and coarse mode for three intensive field campaigns in March 2004, February-March 2005and August 2005 are presented in Fig. 9. No mass closure is given for July-August 2004 since the organic mass concentration was not measured during this period. Figure 9 presents the various measured chemical fractions as well as total PM1, total coarse and TSP mass concentrations. As explained in Sect. 3.1.4, the PM1 mass concentration was derived from the SMPS volume concentration for March 2004 andFebruary-March 2005. No SMPS data were available for the August 2005 experiment. For this campaign, the aerosol light scattering coefficient at wavelength λ=450 nm was used as a proxy for the PM1 mass concentration. The SMPS derived volume concentrations were compared to the measured aerosol light scattering coefficients at 450 nm. The analysis showed a high correlation (r 2 =0.71, slope=5.95×10 −6 m −1 cm 3 µm −3 (March 2004); r 2 =0.98, slope=6.14×10 −6 m −1 cm 3 µm −3 (July-August 2004); r 2 =0.94, slope=6.92×10 −6 m −1 cm 3 µm −3 (February-March 2005)) between these parameters. This results from the particle size distribution of the accumulation mode being relatively invariant, as shown by Weingartner et al. (1999), and justifies the use of the scattering coefficient as a PM1 proxy for August 2005. Figure 9 shows that the two mass fractions present a highly different chemical composition. The PM1 mode is mainly composed of organics and sulfate (SO 2− 4 ) along with significant fractions of BC, nitrate (NO − 3 ), and ammonium (NH + 4 ). The non-determined mass (ND) is quite small and is assumed to be composed of insoluble compounds such as silicate from mineral dust. As mentioned previously, the aerosol is sampled at laboratory temperature (i.e. under dry conditions) and the ND fraction is therefore not expected to be explained by condensed water. In contrast, the coarse mode is dominated by the ND fraction. The major determined compound in the coarse mode is calcium (Ca 2+ ), which is a known component of mineral dust. The contribution of the measured inorganic compounds is very small, and nitrate seems to be present in the same proportion as sulfate. As shown by Krueger et al. (2004), NO − 3 can be linked with Ca 2+ in the coarse mode by the reaction of mineral dust particles with nitric acid to form Ca(NO 3 ) 2 . Other compounds were not measured (e.g. silica).
The large contributions of Ca 2+ on 14 and 20 March (shown in Fig. 9) are due to distinct episodes of Saharan dust over the Jungfraujoch region. These mineral dust episodes were confirmed by the method developed by Collaud Coen et al. (2004), where the wavelength dependence of the single scattering albedo (expressed as the SSA exponent) was found to be an indicator for the presence of dust particles. This exponent becomes negative in the presence of mineral dust. For these two cases the ND fraction in PM1 can be explained by a substantial extension of the coarse mode size distribution into PM1 (Fig. 10), as also shown by Schwikowski et al. (1995).
During March 2004, the ND fraction in PM1 is surprisingly high even when Saharan dust events had been excluded. As can be seen in Fig. 9, this cannot be explained by the pres- ence of mineral dust since there is no elevated coarse mode and the SSA exponent is not negative. Figure 10 presents two typical volume size distributions from combined SMPS and OPC data during events where continental background aerosol was sampled under conditions that were and were not influenced by mineral dust particles. As explained earlier, the slight discontinuity in the combined number size distribution is likely to result from the difference of the refractive index (and complex morphology) of the measured particles with the spherical Latex particles which are used to calibrate the OPC. No correction was applied as no known calibration factor was available for the ambient aerosol.
The high ND fractions observed in March 2004 were carefully checked for systematic biases. First, the SMPS derived volume concentrations were compared to the measured aerosol light scattering coefficients at 450 nm. As presented above these two parameters showed a very high correlation, and virtually the same ratio (V SMPS /b scat ) was determined during March 2004 compared to the other campaigns. This indicates that the aerosol volume is not a critical factor in the unexplained ND fraction. Second, to validate the chemistry data, the SO 2− 4 measured with the filters was compared with the mass concentration of sulfate determined daily within the Swiss National Air Pollution Monitoring Network by Empa. Particulate sulfate was collected for 24 h using cellulose filters with a flow rate of 2.5 L min −1 . The sulfur concentrations were determined with x-ray fluorescence which subsequently were converted to SO 2− 4 concentrations. A relatively high correlation between the two measured fractions (r 2 =0.57; SO 2− 4(PSI) =0.81×SO 2− 4(EMPA) ) was found which suggests that the concentrations of the filters were correct. Thus, the ND fraction could originate either from an unknown component or from a non-identified problem.
To analyze differences in the air mass origin of the two periods (March 2004 andFebruary-March 2005), backward air mass trajectories were calculated with the trajectory model FLEXTRA (Stohl et al., 1995) based on ECMWF (T511L60) analysis with a horizontal resolution of 1 • ×1 • . Trajectories were initialized at the JFJ every 4 h, together with 4 other trajectories, which were initialized equidistantly spaced on a circle of 0.25 • radius around the site to assess the uncertainty. The starting altitude was set to the 700-hPa level. Due to the smoothed model topography this level is about 1400 m above the model ground and about 700 m below the real station height. However, this starting altitude was found to best represent the larger scale flow conditions at the JFJ. 120-h back-trajectories are shown for the two periods in the two lower panels of Fig. 10. The trajectories were initialized every 4 h and only the trajectories arriving during the specified measurement period are shown. The different colors represent different arrival times at JFJ, thick trajectories represent the reference trajectories, while thin lines represent uncertainty trajectories. These back-trajectories confirm a potential influence of dust on 13-14 March as the air mass originated close to the surface over the north-western Saharan region. On 6-7 March the air mass came from the UK and across France at an altitude of >2000 m a.s.l. and was thus in the free troposphere.
Residence time maps (footprints) were calculated from these trajectories for both periods. Residence times were only evaluated when the trajectory remained close to the surface (1000 m above model ground level) so as to focus on possible surface sources of aerosol. The regions influencing the JFJ as indicated by the residence time maps (Fig. 11) showed distinct differences for the two periods. In general, near surface (1000 m above ground level) residence times  Table 1 for details on the employed instrumentation.
were increased during March 2004 compared to February-March 2005. There was a larger contribution from the western Alpine region and also from the upper Po valley. Furthermore, for 2004 there was a larger contribution from the south especially also from Spain and Algeria, while in 2005 the influence was more from the North. This indicates a potentially enhanced influence of fine mode mineral dust in 2004, which is a likely source of the large unexplained fraction of around 50%.
During 25 February and 10 March 2005, the concentrations of individual species were exceptionally high for the season, and close to summer values when the Jungfraujoch is influenced by injection of PBL layer air into the lower free troposphere. This is also confirmed by an observed scattering coefficient (at 450 nm) of 1.2×10 −5 m −1 compared to the long-term average of 3.2×10 −6 m −1 (years 1995 to 2006) for this time of the year. The reason for these high concentrations is presently unknown. The contribution of mineral dust to PM1 was small during this period as confirmed by the SSA exponent and OPC size distributions (not shown).
The last campaign presents a summer situation when the site is influenced by the PBL much more frequently. Much higher organic fractions in PM1 were observed than during the winter, as expected since biogenic emissions and photochemistry are enhanced in summer. The PM1 mass shows negative values due to the low signal to noise ratio of the betameter used here since no SMPS data were available. Figure 12 presents the averages of the chemical composition for the three campaigns as pie charts. These pie charts confirm the highly different composition of fine and coarse mode particles, as well as the higher organic fraction in summer compared to winter. The two winter campaigns are quite different, with a much higher non-determined fraction in PM1 in March 2004. In contrast, February-March 2005 was a period with a much higher concentration of water soluble inorganic species, as outlined above. In March 2004 the mean average concentrations of the fine and coarse mode were 1.7 µg m −3 and 2.4 µg m −3 , respectively, compared with 2.0 µg m −3 and 0.9 µg m −3 during February-March 2005.

Conclusions
The chemical composition of PM1 and TSP has been measured since 1999 at the Jungfraujoch high alpine site as part of the GAW aerosol program. A clear seasonality in all inorganic compounds was observed with minima in winter typical of the undisturbed free troposphere and maxima in summer where the site is influenced by injections of boundary layer air. In addition, intensive campaigns permitted a chem-ical mass closure of PM1 and the coarse mode. These two fractions present highly different chemical composition, with PM1 dominated by organics, sulfate, nitrate, ammonium, BC and the coarse mode composed of mainly calcium and the non-determined fraction, along with minor fractions of nitrate and sulfate. A clear influence of mineral dust episodes was observed in the coarse mode. A conversion factor from OC to OM of 1.84 in winter was found, which is within the range of values reported in the literature.