Chemical mass balance of refractory particles ( T = 300 ◦ C ) at the tropospheric research site Melpitz , Germany

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Abstract
In the fine particle mode (aerodynamic diameter < 1 µm) refractory material has been associated with black carbon (BC) and low-volatile organics and, to a lesser extent, with sea salt and mineral dust.This work analyses refractory particles at the tropospheric research station Melpitz (Germany), combining experimental methods such as a mobility particle size spectrometer (3-800 nm), a thermodenuder operating at 300 • C, a multiangle absorption photometer (MAAP), and an aerosol mass spectrometer (AMS).The data were collected during two atmospheric field experiments in May/June 2008 as well as February/March 2009.As a basic result, we detected average refractory particle volume fractions of 11 ± 3 % ( 2008) and 17 ± 8 % ( 2009).In both periods, BC was in close linear correlation with the refractory fraction, but not sufficient to quantitatively explain the refractory particle mass concentration.Based on the assumption that BC is not altered by the heating process, the refractory particle mass fraction could be explained by the sum of black carbon BC (47 % in summer, 59 % in winter) and a refractory organic contribution estimated as part of the Low-Volatility Oxygenated Organic Aerosol (LV-OOA) (53 % in summer, 41 % in winter); the latter was identified from AMS data by factor analysis.Our results suggest that organics were more volatile in summer (May-June 2008) than in winter (February/March 2009).Although carbonaceous compounds dominated the sub-µm refractory particle mass fraction most of the time, a cross-sensitivity to partially volatile aerosol particles of maritime origin could be seen.These marine particles could be distinguished, however, from the carbonaceous particles by a characteristic particle volume size distribution.The paper discusses the uncertainty of the volatility measurements and outlines the possible merits of volatility analysis as part of continuous atmospheric aerosol measurements.

Introduction
Atmospheric aerosol particles are made of a large variety of organic and inorganic compounds.They affect global climate through direct and indirect radiative forcing (IPCC, 2007), the ecosystem (e.g.Bohlmann et al., 2005;Jickells et al., 2005;Molina and Molina, 2004) as well as human health (e.g.Gurjar et al., 2010;Ostro et al., 2007;Pope, 2000).Moreover, aerosol chemistry and thermodynamic properties (e.g.heterogeneous interactions and chemistry, condensation or evaporation of semi-volatile compounds on the particle phase) directly contribute to modify the chemical composition of both atmospheric gas and particle phases (e.g.Kolb and Worsnop, 2012;Pöschl et al., 2007).In a worldwide overview of sub-µm chemical particle composition, Zhang et al. (2007) reported organic mass fractions between 20 and 90 % in PM 1 depending on location and season.This organic fraction is difficult to characterize because it includes thousands of organic compounds, many of which have not been identified analytically.Organic aerosol particles are of biogenic and/or anthropogenic origin and can be emitted directly from these sources (primary organic aerosol -POA), or formed as a secondary organic aerosol (SOA) by chemical reactions of gas phase precursors such as Volatile Organic Compounds (VOCs) to form low volatility reaction products which can partition into particles (e.g.Kanakidou et al., 2005;Hallquist et al., 2009).
The aerosol particle composition is strongly depending on the gas-to-particle partitioning of their constitutive compounds.
In studies related to the atmosphere, volatility analysis has proved to be useful to differentiate classes of inorganic compounds, such as particulate ammonium nitrate, ammonium sulfate, sulfuric acid, and sodium chloride (Schmid et al., 2002;Smith and O'Dowd, 1996).After the evaporation of high-volatile particulate compounds, volatility analysis leaves refractory particle cores containing material that is non-volatile at the given temperature.Hence, the volatilization temperature is, among others, a parameter that can be characteristic for certain classes of organic, but also inorganic compounds.In the fine particle mode (aerodynamic diameter < 1 µm) non-volatile material has been Figures Back Close Full associated with black carbon particles and low-volatile organic, and to a lesser extent with sodium chloride and crustal material (e.g.Engler et al., 2007;Huffman et al., 2009Huffman et al., , 2008)).While the volatilization temperatures of inorganic compounds are relatively well known (cf.Engler et al., 2007), they are more uncertainties for the organic phase; the ratio of more and less volatile compounds, in particular, strongly depends on the source of the particulate matter, and the stage of the aerosol within its atmospheric lifecycle.For example, the chemical composition as well as the thermodynamic properties of SOA are not constant over time but evolve during the aging processes of the aerosol (e.g.Donahue et al., 2012a).Therefore, Donahue et al. (2006) developed the Volatility Basis Set (VBS) approach, a model where organic aerosols formation and aging processes is described by their saturation vapor pressure (C*).SOA volatility depends on the cascade of reactions that happen during the aging processes, with functionalization leading to compounds with lower volatility, and fragmentation contributing to a higher volatility (Jimenez et al., 2009).A quantitative relationship between C* and the organic aerosol oxidation state was introduced in an improved VBS model (2D-VBS, Donahue et al., 2011Donahue et al., , 2012b)).
A practical device to investigate aerosol particle volatility is the thermodenuder (TD).Here, aerosol particles are evaporated in the airborne state inside a heated flow tube at defined temperature.See Burtscher et al. (2001) for a survey of the design of different TDs.The TD can, in principle, be combined with most particle sampling techniques and on-line particle sizing and classification instruments.Volatility tandem differential mobility analyzers (V-TDMA; Rose et al., 2006;Philippin et al., 2004) have been used to measure the shrinkage of monodisperse particles after volatilization.The volatility twin differential mobility particle sizer (V-TDMPS; V-SMPS), in contrast, measures complete particle number size distributions upstream and downstream of the TD (Ehn et al., 2007;Engler et al., 2007).Comparing integral particle properties such as number and volume upstream and downstream of a TD allows estimating the refractory fraction at the given temperature.In order to investigate the refractory part of atmospheric aerosol particles on a long term basis, continuous V-TDMPS or V-SMPS measurements have Introduction

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Full been implemented at several atmospheric observation sites in Germany: the rural research site Melpitz (Engler et al., 2007), the HGMU urban monitoring site Augsburg (Birmili et al., 2010), and six other stations inside the German Ultrafine Aerosol Network (GUAN; Birmili et al., 2009).In all cases, a temperature of 300 • C was selected in the thermodenuder.The reason for choosing this temperature is that sulfates, nitrates and most organic compounds would be stripped off while the charring of organic matter would be prevented in the presence of atmospheric oxygen (cf.Engler et al., 2007).
The motivation to monitor black carbon and other refractory particle compounds in atmospheric networks involves, on the one hand, the necessity to help quantify the regional and global effect of light absorption (Cheng et al., 2009;Koch et al., 2009;Stier et al., 2007;Bond and Bergstrom, 2006).On the other hand, atmospheric soot particles and refractory compounds have been associated with certain adverse health effects by epidemiological and toxicological studies (Attfield et al., 2012;Janssen et al., 2012;Mazzarella et al., 2007;Totlandsdal et al., 2012).A monitoring strategy such as pursued in GUAN attempts to provide experimental data that can be used to address both issues mentioned above.As an additional issue, volatility instruments have also been used to examine the evolution of the atmospheric nucleation mode.A substantial observation has been that each particle of the evolving atmospheric nucleation mode seems to contain a refractory core at 300 • C, even though these particles were initially formed purely from gas-to-particle conversion at ambient temperatures (Ehn et al., 2007;Wehner et al., 2005).
Recently, an Aerosol Mass Spectrometer (AMS, e.g.Canagaratna et al., 2007) was combined with the TD, allowing a direct investigation of the chemical composition of the refractory fraction (Faulhaber et al., 2009;Huffman et al., 2009Huffman et al., , 2008;;Wu et al., 2009).These measurements point out that the refractory fraction is not only made of black carbon and crustal material (which cannot be detected by the AMS), even at temperatures higher than 200 • C, but that the presence of a refractory organic fraction has to be considered as well.These remaining organic compounds represent an important description parameter for a better estimation of the atmospheric organic aerosol (OA) Introduction

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Full since it should be only negligibly influenced by dilution and gas-to-particle equilibrium.
Improving our understanding on this low-volatile organic fraction is necessary in order to improve the current SOA models, to better estimate the impact of the OA to the global aerosol mass loading and to finally better predict the organic contribution to both climate and human health.
In this study, a mass closure study based on parallel Thermodenuder (TD), Aerosol Mass Spectrometer (AMS), and MAAP measurements to identify the composition of refractory fraction at 300 • C during two intensive campaigns in May/June 2008 and February/March 2009 at the TROPOS Central European regional atmospheric research station Melpitz (Germany) is performed.A simple method will be used to estimate the refractory chemical composition, principally based on laboratory measurements performed using a similar TD running in comparable conditions as well as on comparisons with similar measurements available in the literature.The influence of the meteorological conditions as well as of the air mass origin on the non-volatile fraction will be discussed.

The TROPOS research station Melpitz (Germany)
Atmospheric aerosol measurements were performed at the TROPOS research station Melpitz (51.54 • N, 12.93 • E, 86 m a.s.l.), 50 km to the east of Leipzig, Germany.The station has been used since 1992 to examine the effect of atmospheric long range transport on local air quality.The site itself is mainly surrounded by agricultural pastures and forests.The atmospheric aerosol observed at Melpitz can be regarded as representative for Central European regional conditions, as confirmed by multiple site comparisons within the programme EMEP (co-operative programme for monitoring and evaluation of the long-range transmissions of air pollutants in Europe, Aas et al., 2012) and the GUAN Network (Birmili et al., 2009).Introduction

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Full All on-line instruments were set up in the same container laboratory and utilized the same air inlet.This inlet line consisted of a PM 10 Anderson impactor located approximately 6 m a.g.l. and directly followed by an automatic aerosol diffusion dryer to keep the relative humidity inside the sampling line below 30 % (Tuch et al., 2009).After entering the container laboratory, the sampling flow was divided among the different instruments.For a basic overview of the physical and chemical aerosol characterization methods see e.g.Birmili et al. (2008); Spindler et al. (2010Spindler et al. ( , 2004Spindler et al. ( , 2012)).

AMS
The AMS is an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) here simply referred to as AMS (DeCarlo et al., 2006).Briefly, the AMS allows two alternative detection modes to measure the particle size distribution (PTOFmode) and the chemical composition of the particle (MS-mode).Before being detected by the time of flight mass spectrometer, the particles are flash vaporized by impaction on a heated surface (600 • C) and the vapors are ionized by an electron impact ionization source at 70 eV.Because soot, crustal material and sea salt cannot be detected, the AMS is commonly considered to only provide non-refractory PM 1 aerosol chemical composition.Different AMS intercomparison exercises performed by Bahreini et al. (2009), Crippa et al. (2013b) Freutel et al. (2013) estimated the total AMS variability around 30 % (including 10 % for the different inlets, 20 % for the ionization efficiency calibration and 20 % for the bounce efficiency).Further details on the AMS data analysis and results for these two measurements periods can be found in Poulain et al. (2011).
Source apportionment of the organic aerosol fraction was conducted using the unit mass resolution (UMR) organic mass spectra and performed using the multi-linear Engine algorithm (ME-2) developed by Paatero (1999).Its results were analyzed according to the ME-2 graphic interphase developed by the Paul Scherrer Institute and de-Introduction

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Full tailed in Canonaco et al. (2013).Further details on the factor analysis results can be found in Crippa et al. (2013a).
This instrument internally converts the absorption coefficient measured at a wavelength of 637 nm to the soot mass concentration by applying a constant mass absorption coefficient of 6.6 m 2 g −1 .This value was determined from a combination of independent measurements of the absorption coefficient using the MAAP and the soot mass concentration applying a thermographic method for urban aerosol samples (Petzold and Schönlinner, 2004).However, model calculations and experimental literature results suggest that the mass absorption coefficient of atmospheric soot particles is not constant, but may depend on the state of mixture with other compounds (Fuller et al., 1999).The analysis of graphitic carbon from MAAP filter samples at various GUAN sites using Raman-spectroscopy yielded an average mass absorption efficiency of 5.3 m 2 g −1 for Germany (Nordmann et al., 2013), which differs from the default mass absorption efficiency of than 6.66 m 2 g −1 .This implies a systematic uncertainty of BC derived from the MAAP measurements, depending on which mass absorption efficiency is used.
An additional source of error is the measurement accuracy of an individual MAAP instrument.Direct intercomparison of multiple MAAP instruments yielded a variability of less than 5 % around the mean value (Müller et al., 2011).Another correction was necessary in this work: at Melpitz, the MAAP is operated downstream of a PM 10 inlet.
Our subsequent calculations, however, refer to PM 1 .We converted BC (PM 10 ) to BC (PM 1 ) by multiplication with a factor of 0.9, which was obtained by running two MAAPs at Melpitz side by side with different inlets (cf.Poulain et al., 2011).Considering instrumental, mass absorption efficiency and size cutting uncertainties, a global uncertainty of approximately 15 % on the PM 1 BC mass concentration can be expected.Introduction

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Ambient and refractory particle number size distributions
Particle number size distributions (3-800 nm) were continuously measured upstream and downstream of a TD using a volatility twin differential mobility particle sizer (V-TDMPS).The TD follows the design of Wehner et al. (2002).Briefly, it consists of a heating section (50 cm) followed by a cooling section filled with active carbon to remove evaporated material and to cool down the sample to room temperature.The TD was used at a flow rate between 2.5 and 3.0 L min −1 (corresponding to a residence time on the heating section of approximately 1.0 and 0.8 s, respectively) and at a constant temperature of 300 A Twin Differential Mobility Particle Sizer (TDMPS, Birmili et al., 1999) was deployed to measure particle number size distributions from 3-800 nm mobility diameter with a time resolution of 10 min, changing its sampling inlet between atmosphere aerosol and this aerosol after passing through the thermodenuder.The system consisted of two Differential Mobility Analyzers (DMA, Hauke-type) and two Condensation Particle Counters (CPC, TSI model 3010 and TSI model 3025).The sheath air was circulated in closed loops for both DMAs.The evaluation of particle number size distributions includes a multiple charge inversion, the CPC efficiency and diffusional losses in the DMA and all internal and external sampling lines according to the recommendations in Wiedensohler et al. (2012).The V-TDMPS data was additionally corrected for particle losses due to diffusion inside the TD.The particle volume concentrations are calculated from the measured number size distribution assuming spherical particles.The volume fraction remaining (VFR) was determined by dividing the refractory volume concentration measured downstream of the TD by the total particle volume measured at ambient temperature, i.e. by-passing the TD.

Off-line chemical characterization
During the 2008 campaign, daily PM 2.5 High Volume DIGITEL filter samples (Digitel Elektronic AG, Hegnau, Switzerland) were collected from midnight to midnight, while Introduction

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Full A 5-stage BERNER impactor (Hauke, Austria, Berner and Lurzer, 1980) was deployed on individual days (from midnight to midnight) during February/March 2009.All filters were analyzed according to the same analytical methods.Filters were weighted for the total particle mass and inorganic cations and anions as well as OC and EC were determined.Details on the different analytical methods and results for these two specific periods can be found in Spindler et al. (2010).

Mass closure
Before discussing the volatility measurements, the balance of PM 1 particle mass obtained from AMS and MAAP on the one hand, and the TDMPS on the other hand is examined.To convert the particle volume concentration from the TDMPS into a mass concentration, we estimated the gravimetric particle density on the basis of measured chemical composition using Eq.(1) (Salcedo et al., 2006).In this equation, the density of ammonium nitrate and ammonium sulfate was assumed to be 1.75 g cm −3 (Lide, 1991), the density of ammonium chloride as 1.52 g cm −3 (Lide, 1991), the density of organic matter as 1.2 g cm −3 (Turpin and Lim, 2001) and the density of BC as 1.77 g cm −3 (Park et al., 2004).
Applying the density to measured particle composition, a mean density of 1.4 g cm It needs to be noted that the mass concentrations derived from the TDMPS are on average 20 % higher than those derived from AMS and MAAP (Fig. 1) which is close to previous comparisons (e.g.Mensah et al., 2012;Setyan et al., 2012).This deviation is within the range of instrumental uncertainties provided by the methods used: TDMPS total particle volume (±10 %), BC PM 1 mass concentration (15 %), AMS mass concentrations (30 %), density estimation.In addition, another source of uncertainties related to the different size cut offs of the AMS and TDMPS has to be considered.The transmission efficiency of the AMS aerodynamic lenses starts decreasing towards 700 nm (mobility), so that the mass concentration of non-refractory compounds measured by the AMS might underestimate the one measured by the TDMPS (upper size range 800 nm mobility).Differences due to different amounts of water present in the particulate phase as a result of particle hygroscopicity can be neglected because the relative humidity in the sample aerosol was always below 30 %.

Ambient and refractory particle volume concentrations
Figure 2 shows the time series of the ambient and refractory particle volume concentrations obtained from the TDMPS, in conjunction with the volume fraction remaining (VFR) at 300 • C. The VFR is higher in the February/March 2009 period (17 ± 8 %; mean ± standard deviation) compared to the May/June 2008 period (11 ± 3 %).It can also be seen that the VFR shows considerable variations with time, although to a lesser extent than the total particle volume concentrations.Such seasonality may be due to a change in the aerosol chemical composition as discussed in the following sections.
In order to directly compare the refractory particle volume concentration (RVC) measured after the TD with the aerosol chemical composition, it is essential to convert it into refractory mass concentration (RMC).As discussed above, the particle density directly depends on its chemical composition.However, in contrary to ambient particles, the chemical composition of the refractory fraction is unknown.Therefore, in the following, a constant density of 1.6 g cm −3 was applied to convert the TD measurements into mass concentration.Introduction

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Inorganic contribution to the refractory mass concentration
In the continental troposphere, the inorganic sub-µm particulate fraction consists of major amounts of ammonium nitrate and ammonium sulfate.This was confirmed for Melpitz by Poulain et al. (2011).Ammonium nitrate is a semi-volatile compound that starts evaporating at 30 • C, while ammonium sulfate is a less volatile compound, evaporating only at around 150 • C from the particulate phase in a thermodenuder of our type (Wu et al., 2009).Consequently, no ammonium nitrate or sulfate are expected to remain in the particulate phase at 300 It can be thought that other inorganic compounds, such as chloride, sodium, calcium, potassium, or magnesium remain in the particulate phase at 300 • C.However, their concentrations were rather low compared to the concentrations of ammonium sulfate and nitrate, as suggested by Digitel filter samples taken simultaneously with our study (Daily PM 2.5 filter samples were taken during May/June 2008, while daily PM 1 filter samples were taken during February/March 2009).Specifically, chloride, sodium, calcium, potassium, and magnesium accounted for around 13 % of the total inorganic Introduction

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Full PM 2.5 mass and for around 4 % of the total inorganic PM 1 mass (Poulain et al., 2011;Spindler et al., 2010).

Organic contribution to the refractory mass concentration
In this section, we discuss the possible organic contribution to the refractory particle mass concentration (RMC), based on the AMS measurements.Figure 4 illustrates the mass balance for the refractory particle mass, which is described in detail in the following.
As described in Sect.2.2.1.,factor analysis of organic aerosol matter was performed using the ME-2 approach to describe the organic aerosols.A detailed description of the ME-2 analysis can be found in Crippa et al. (2013a) and is summarized in Supplement SI-1.Only the main results will be discussed here.In May/June 2008, three factors were identified to explain the total organic aerosol (OA).These factors correspond to Low-Volatility Oxygenated Organic Aerosol (LV-OOA, 46.7 % of total OA), Semi-Volatile Oxygenated Organic Aerosol (SV-OOA, 45.7 % of total OA) and Hydrocarbon-like Organic Aerosol (HOA, 7.5 % of total OA).In February/March 2009, an additional factor was found, and identified as biomass burning emissions (Biomass Burning Organic Aerosol or BBOA, 13.3 % of total OA).During this period, LV-OOA, SV-OOA and HOA represent 43.4,32.9 and 10.3 % of total OA, respectively.HOA and BBOA are related to primary organic aerosol sources: BBOA represents biomass burning while the HOA is more related to traffic and liquid domestic fuel combustion.LV-OOA and SV-OOA are more related to secondary organic aerosol.LV-OOA is known to be associated with more oxygenated OA with the lowest volatility, while the SV-OOA is commonly considered to be representative for freshly generated OA made of less oxygenated OA, with a higher volatility than LV-OOA (e.g.Jimenez et al., 2009;Lanz et al., 2007;Ulbrich et al., 2009).

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Full a MFR of 70-30 %, depending on the location.Because LV-OOA has a similar mass spectrum as Humic Like Substances (HULIS), Wu et al. (2009) investigated the temperature dependency of the volatility of fulvic and humic acids as surrogate for HULIS, using a similar TD-AMS setup as in the present study.Within that study it was found that these two compounds present a MFR of 47 % for fulvic acid and 58 % for humic acid after being heated at 300 • C, in agreement with the field observations made by Huffman et al. (2009).Kondo et al. (2011) reported an evaporation of 10-30 % for laboratory generated monodisperse particles of different surrogates of atmospheric HULIS heated at 300 • C and 400 • C with a residence time inside the heated part of 0.3 s.
Although direct comparisons between the different TD measurement can be strongly influenced by the residence time of the particle inside the heated section (An et al., 2007), the previous works are in good agreement and confirm that a large fraction of the HULIS are not evaporated even after being heated at high temperatures.
Since Wu et al. (2009) used the same TD setup as in the present work, the refractory organic aerosol (ROA) mass concentration was estimated to represent 52 % of the measured LV-OOA, which is the mean value of the measured surrogate of HULIS used by the authors.This is also in agreement with the mean value of the Huffman et al. (2009) observations.Time series of the estimated ROA fraction for each season are presented in Fig. 4 and the relationship with the measured refractory mass concentration is shown in Fig. .The relationship between ROA and the measured refractory mass shows a correlation slope of 0.74 (R 2 = 0.51) and 0.24 (R 2 = 0.34) for 2008 and 2009, respectively (Fig. SI-2a and d).These results will be discussed in the following.and ROA (Fig. 5a).In this case, the ROA represents 66 ± 8 % of the reconstructed refractory mass.In contrast, only 60 % of the measured refractory mass during February/March 2009 can be explained by the same approach (Figs. 4 and 5d).In that second case, the ROA represents 41 ± 13 % of the reconstructed refractory mass.

Assessment of the refractory particles chemical composition
It should be noted that the discrepancy between estimated and measured RMC was not constant over the whole measurement period in February/March 2009.On some days, balance was matched (e.g. 16, 17, 25, 26, 27 March) while on others, a large discrepancy could be observed (e.g.18-19 March 2009, and the first days of the measurement period).Interestingly, the largest differences usually occurred when particles consisted of a particularly large refractory fraction (Figs. 2 and 4).For example, on 18 March 2009, the VFR was 60 %.In order to better understand the limits of our approach and to draw some hypotheses to explain the discrepancies, it is essential to go into more details on the different factors that could influence each time period.

Discussion
Our results suggest that the refractory particle mass fraction (RMF) could be fully explained by BC and ROA during May/June 2008, while about one third of this balance remained unexplained in February/March 2009.We now scrutinize two reasons for this deviation: particle size distribution, and chemical particle composition.particle mass concentrations.The basic effect is that the overall particulate volume decreases, and the particle diameters shrink after passage through the TD.This is consistent with earlier observations (Birmili et al., 2010;Ehn et al., 2007;Engler et al., 2007).However, for certain periods, the refractory volume size distribution increases in concentration above 400 nm.This effect is visible in Fig. 6, and even better in Fig. 7.

Influence of the particle volume size distribution
Refractory What is the origin of these refractory particles bigger than 400 nm?Most of these particles do not appear in the ambient particle volume distribution (Figs.6 and 7).We can therefore only think of particles originating in the particle size range above 800 nm, i.e. outside of the measurement range of the TDMPS.The entrance of such particles into the V-TDMPS system is principally possible via the PM 10 inlet.It is therefore our conclusion that the high value of measured RMC can be influenced by marine superµm particles which shrink during passage through the TD and move into the sub-µm measurement range of the TDMPS.In some sense, this is a measurement artifact because these particles can only be detected by the TDMPS downstream of the thermodenuder, not upstream.
It was a conspicuous observation that the events on 13 and 18 March 2009 were connected with maritime air masses, as indicated by 96 h back trajectories from the NOAA HYSPLIT trajectory model (Draxler and Hess, 2004).For example, on 18 March 2009, the air mass originated over the Atlantic, spending only the last 15-16 h over the continent before reaching Melpitz.By crossing polluted areas, the marine aerosol aged quickly either by mixing and/or condensation with other aerosols or by heterogeneous reactions (e.g.conversion of NaCl into NaNO 3 and/or Na 2 SO 4 ) as well as by in-cloud processes.Therefore, the air mass reaching Melpitz needs to be considered as processed marine air mass rather than fresh marine air mass.Analyses of 5-stages Berner impactor samples for this day revealed unusually high contributions of sulfate, nitrate, Introduction

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Full chloride and sodium in the coarse mode (stage 4), which are indicators of sea spray origin particles (Fig. SI-4).Similar events took place on 13 March 2009, compared to 11 March 2009, which was more under continental influence (Fig. SI-4).The nitrate mass concentration on stage 4 has to be related to the presence of sodium nitrate, a result of the reaction of NaCl and HNO 3 (e.g.Dasgupta et al., 2007).Contrary to ammonium nitrate, which is a semivolatile compound, sodium nitrate is a low-volatile compound (volatilization temperature > 300 • C, Pinnick et al., 1987), considered to represent a permanent removal pathway for atmospheric nitrate.A similar mechanism can also lead to the formation of sodium sulfate (e.g.Sievering et al., 1995).Contrary to sodium nitrate, the hydrated salt of sodium sulfate is a semi-volatile compound (melting point 32 • C) which could easily evaporate when crossing the TD.Moreover, it is known that organic acids can contribute to the chloride depletion on marine aerosol, leading to the formation of sodium salts (e.g.Kerminen et al., 1999;Zhao and Gao, 2008).Consequently, an interesting parallel can be drawn to volatility measurements of succinic acid and its disodium salts made by Wu et al. (2009).Although succinic acid is a semi-volatile compound that fully evaporates at 65 • C, in similar sampling conditions than the present study, Wu et al. (2009) demonstrated that its disodium salt shows a mass fraction remaining of 40 % after being heated at 300 • C.
Consequently, regarding the strong marine air mass influence, the presence of such compounds in the coarse mode and after crossing the TD is highly possible and should strongly influence the refractory organic mass fractions.Although a strong influence of the particles larger than 1 µm on the measured refractory particle mass fraction downstream to the TD was clearly identified, it is not possible to confirm or reject the presence of sodium nitrate and/or other organic salts in the refractory particle mass fraction, since no direct measurements of the aerosol chemical composition downstream to the TD were performed.
As mentioned above, effects of super-µm particles seem to be limited to the refractory particle size range above 400 nm (dotted lines in Figs. 6 and 7).Therefore, we Introduction

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Full recalculated RMC for a new size range corresponding to 3-400 nm mobility diameter (Figs. 3 to 6).This alternative approach reveals that most of the time the VFR is not particularly sensitive to the choice of the upper integration limit (VFR (400 nm) = 8 ± 4 % and 16 ± 6 % for 2008 and 2009, respectively).A direct consequence is the enhancement of the BC contribution of total refractory to 54 % and 58 % of the measured refractory mass for 2008 and 2009, respectively (Fig. 3b and d) and the refractory mass concentration for February/March 2009 can be fully explained by the model now (Figs. 4  and 5e).Based on the reconstructed refractory mass, it suggests that refractory aerosol at 300 • C was made, in averaged, for 59 ± 16 % of BC and 41 ± 13 % of ROA during this period.However, the reconstructed refractory mass fraction for May/June 2008 strongly over-estimates the measured one (Fig. 5b).This is mainly due to the fact that ROA already overestimates the measured refractory mass (Fig. SI-2b).Nevertheless, no significant change on the correlation coefficient (R 2 ) was observed for this period, indicating that only the absolute refractory mass concentration was affected by the adjustment of the upper size range and not the time series itself in opposite to the February/March 2009 campaign.Consequently, larger particles seem to have a lower influence in this period than during February/March 2009.

Influence of the chemical composition
Estimation of the refractory aerosol mass was only based on the chemical composition measured by the AMS, and the BC concentration, measured by the MAAP.Therefore, the presence of non-detected compounds like crustal material and/or sea-salts has to be considered.As previously mentioned, the contribution of non-measured inorganic ions represents 13 % of the total PM 2.5 mass concentration in May/June 2008 and 4 % of the total PM 1 mass concentration in February/March 2009.Additionally, identified species of daily PM 1 filter explain on average 80 ±12 % of the total filter mass in February/March 2009.Consequently, it is reasonable to consider that chemical particle composition was usually fully explained and that, finally, the non-considered compounds might have a small influence on the reconstruction of the refractory fraction.However, 26998 Introduction

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Full as mentioned earlier for the ambient mass closure, uncertainties on the MAAP and AMS measurements have also to be considered here too.The estimation of the refractory OA concentration was based on the assumption that the LV-OOA's mass fraction remaining is constant over time and using the volatility properties of the HULIS surrogate.In May/June 2008, estimated refractory mass fraction is slightly overestimated during day time (especially in the afternoon), while during nighttime; the model underestimated the refractory mass fraction (Fig. 4).It is known that the organic's oxidation state and/or particles' aging change during day time (Poulain et al., 2011); therefore, it might directly affect the volatility properties of the OA.Consequently, the over-and/or underestimation of the refractory mass fraction during May/June 2008 may highlight variations of the OA volatility properties.Another possible indication on the change of ROA volatility is the comparison between the full V-TDMPS size range and the corrected one (up to 400 nm) (Fig. 5a and b).As previously mentioned, in February/March 2009, the time series of the refractory mass concentration was strongly affected by the V-TDMPS resizing, while only a scale down of the mass concentration was observed in May/June 2008.Decreasing the upper size range of the V-TDMPS only influence the correlation slope for the 2008 campaign while the correlation coefficient remains nearly constant (Fig. 5a and b).This can also be interpreted as an overestimation of the ROA mass concentration.Therefore, change on the organic aerosol volatility was investigated by increasing the organic aerosol volatility.Assuming a MFR of 30 % for the LV-OOA (ROA 30 ), the lowest value reported by Huffman et al. (2009), instead of 52 %, strongly improves the comparison with the measured refractory mass using an upper size cutting of 400 nm (slope 1.15 and R 2 = 0.88, Fig. 5c).
ROA 30 mass concentration also better correlates with the refractory mass than before ( Another source of uncertainty is the appropriate estimation of the refractory aerosol density.A constant value of 1.6 g cm −3 was used, thus, change of the density value will consequently, influence the present results.However, the high degree of correlation obtained between the reconstructed and the measured refractory mass concentration (Fig. 5c and e) confirmed that BC and ROA can be used to fully explain the measured refractory mass concentration.Subsequently, direct measurements would be necessary to improve the refractory OA estimation and to provide exact time dependency estimations of the refractory aerosol mass concentration.

Conclusions
Refractory particles are measured continuously at the TROPOS Central European research station Melpitz (Germany) by a V-TDMPS instrument consisting of a twin mobility particle sizer and a thermodenuder operating at 300 • C. ple, e.g., by a pre-impactor.It might be desirable in the future to extend the V-TDMPS with an aerodynamic particles sizer (APS), or alternatively remove particles by a preimpactor.
Comparing the two periods, our results suggest a possible change on the LV-OOA volatility with a highest volatility in May/June 2008 (MFR = 30 %) than in February/March 2009 (MFR = 52 %).Possible reason might be change on oxidation states and/or LV-OOA origins.Nevertheless, our results emphasize a significant contribution of OA to the refractory particle mass fraction (53 % and 41 % in May/June 2008 and February/March 2009, respectively) and also suggest a possible change on their volatility properties during the run of a year.More direct measurements using aerosol mass spectrometer downstream of the thermodenuder are needed (for example with AMS during intensive period or with ToF-ACSM for longer period).Because OA do not completely evaporate at 300 • C, the refractory OA particle mass fraction would remain in the particle phase.An influence of the thermodynamic properties of atmospheric particles might be responsible for the current underestimation of SOA particle mass concentrations in the atmospheric models.2008and March 2009, Atmos. Chem. Phys., 12, 4723-4742, doi:10.5194/acp-12-4723-2012, 2012.Molina, M. J. and Molina, L. T.: Megacities and atmospheric pollution, J. Air Waste Manage.
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | during the 2009 campaign, daily PM 1 High Volume DIGITEL filter samples were taken.
/June 2008 and 1.6 g cm −3 for February/March 2009 was obtained, respectively.The reconstructed mass concentrations are presented in Fig. 1 and show a reasonable correlation with R 2 = 0.90 in 2008, and R 2 = 0.88 in 2009.Discussion Paper | Discussion Paper | Discussion Paper | carbon contribution to the refractory mass concentration Black carbon is considered as a major refractory component in sub-µmPM (e.g.Pöschl, 2005).During the GUAN project,Birmili et al. (2009) found linear relationships between the BC mass concentrations and the refractory volume concentrations for 5 different atmospheric measurement sites.It is therefore a strong hypothesis that BC constitutes a major part of the RMC.Detailed correlations between refractory particles and BC are shown in Fig.3a and c.The correlations between the calculated refractory particle mass concentration and the BC concentration yield slopes well below 1, and measures of determination (R 2 ) of 0.86 (May/June 2008) and 0.80 (February/March 2009).Similar slopes were obtained for the 2008 (0.38) and 2009 (0.36) periods.These results confirmed that BC alone cannot explain the entire refractory mass fraction.
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Figures 4
Figures 4 and 5 illustrate the time series and inter-parameter correlations of BC, ROA, and the refractory mass concentration (RMC) derived from the V-TDMPS.Overall, the May/June 2008 period exhibits higher mass concentrations than the February/March 2009 period.In both periods, the RMC proves to be highly variable on the 26994

Figure 6
Figure 6 shows contour diagrams of the ambient and refractory particle volume size distributions, and the ratios between AMS/MAAP-based and TDMPS-based refractory Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Fig. SI.4c).As previously mentioned, BC concentration is depending on the value of the mass coefficient absorption.Increasing this parameter by 20 % (i.e. using the default instrument value) has a small influence on the regression line (+0.1)compared to ROA.Consequently, change on the MFR of LV-OOA seems to be the dominant factor.Discussion Paper | Discussion Paper | Discussion Paper | These measurements are supplied by a MAAP instrument, from which black carbon (BC) concentrations might be derived.During specialized field campaigns in May/June 2008 and February/March 2009, chemical particle composition was determined using an AMS and MAAP.The remaining volume fraction (VFR) was 17 % in 2008 and 11 % in 2009.We estimated the refractory particle mass concentration (RMC) by two methods, based either on the TDMPS particle volume and an assumed particle density, or the sum of refractory chemical compounds identified in the AMS and the MAAP.Backward air mass trajectories as well as impactor particle mass size distribution measurements point out that the refractory mass fraction measured during the February/March 2009 period was strongly influenced by aged marine particles bigger than 1 µm.Similar influence of the super-µm particles was also observed for May/June 2008.Limiting the TDMPS range to 400 nm allowed limiting the interference of these aged marine particles, yielding a better mass balance for the refractory particles.These results highlight the potential cross-sensitivity of partially volatile coarse mode particles on sub-µm volatility measurements, as long as these particles are not removed from the sam-Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Birmili, W., Heinke, K., Pitz, M., Matschullat, J., Wiedensohler, A., Cyrys, J., Wichmann, H.-E., and Peters, A.: Particle number size distributions in urban air before and after volatilisation, Atmos.Chem.Phys., 10, 4643-4660, doi:10.5194/acp-10-4643-2010,2010.Bohlmann, N., Meissner, R., Bernsdorf, S., Bohme, F., Russow, R., and Wegener, U.: Studies of atmospheric nitrogen deposition in a mire of the German National Park Hochharz Mountains Discussion Paper | Discussion Paper | Discussion Paper | ski, Y., Fuzzi, S., Horth, J., Moortgat, G. K., Winterhalter, R., Myhre, C. E. L., Tsigaridis, K., Vignati, E., Stephanou, E. G., and Wilson, J.: Organic aerosol and global climate modelling: a review, Atmos.Chem.Phys., 5, 1053-1123, doi:10.5194/acp-5-1053-2005,2005.Kerminen, V. M., Teinila, K., Hillamo, R., and Makela, T.: Size-segregated chemistry of particulate dicarboxylic acids in the Arctic atmosphere, Atmos.Environ., 33, 2089-2100, 1999Discussion Paper | Discussion Paper | Discussion Paper | served in two intensive periods in May

Fig. 3 .Fig. 4 .Figure 5 .
Fig. 3. Black carbon (BC) vs. the refractory mass concentration (RMC) estimated from the TDMPS for May/June 2008 and February/March 2009 periods.Estimation of the refractory mass concentration was made assuming a density of 1.6 g cm −3.The left panels correspond to the total V-TDMPS size range (up to 800 nm) while the right panels refer to a size cutting of 400 nm (up to 400 nm).Correlation curves were calculated using the least orthogonal distance fit method. 6 8

Fig. 5 .Fig. 6 .Figure 7 .Fig. 7 .
Fig. 5.Comparison of the estimated (RMC est ) with the measured (RMC meas ) refractory particle mass concentration (RMC).Three different approaches were used to estimate the refractory particle mass fraction; using the full V-TDMPS size range, (up to 800 nm) or size cutting of 400 nm (up to 400 nm) and finally for May/June 2008, the refractory organic aerosol (ROA) was also estimated based on only 30 % of the measured LV-OOA (ROA 30 ).The two encased scatter plots correspond to the better solution obtained for each periods (see text for discussion).
particles bigger than 400 nm can occasionally dominate refractory volume size distribution, during events that occurred more frequent in February/March 2009 than in May/June 2008.Prominent examples are 13 and 18 March 2009.