On the e ff ect of water-soluble compounds removal on EC quantification by TOT analysis in aerosol samples

A. Piazzalunga, V. Bernardoni, P. Fermo, G. Valli, and R. Vecchi Department of Inorganic, Metallorganic and Analytical Chemistry, Università degli Studi di Milano, Via Venezian 22, 20133 Milan, Italy Department of Physics, Università degli Studi di Milano, Via Celoria 16, 20133 Milan, Italy now at: Department of Environmental and Territorial Sciences, Università di Milano-Bicocca, Piazza della Scienza 1, 20126 Milan, Italy


Introduction
At the state of the art the identification of organic (OC) and elemental (EC) carbon in aerosol samples using thermal protocols is ambiguous; therefore, they are operationally defined.Indeed, part of the EC thermally evolves in oxidising atmosphere and part of the OC can char especially in an oxygen-poor atmosphere giving origin to a refractory component (pyrolytic carbon, PyC) similar to EC (Watson et al., 2005 and therein cited literature).The presence of organic compounds prone to pyrolysis (e.g.water soluble compounds) and of tar-like organic compounds (produced to a great extent by wood burning) can interfere with EC determination.The presence of inorganic catalytic compounds in the samples further complicates the understanding of the thermal behaviour of aerosol carbonaceous samples.Among the different approaches used for OC/EC quantitative evaluation, thermaloptical analyses are the most widespread (Chow et al., 1993;Birch and Cary, 1996).Different heating ramps both in He and He/O 2 phase are reported in the literature for thermal-optical analyses.It is noteworthy that temperature and duration of the plateaus have been already identified as the main factors influencing OC/EC separation (Chow et al., 2001;Schmid et al., 2001;Conny et al., 2003;Schauer et al., 2003;Chow et al., 2004;Subramanian et al., 2006;Zhi et al., 2009); a twofold difference in the EC quantification by different thermal-optical approaches is quite usual (e.g.Schmid et al., 2001;Watson et al., 2005 and therein literature).
As already mentioned, PyC is considered one of the main interferent components in the EC quantification.Previous works (Chow et al., 2001;Subramanian et al., 2006) evidenced that EC (and/or PyC) pre-combustion can occur in the He phase when it is carried out at high temperature -i.e. about 850 • C -especially when inorganic catalytic compounds are present in the sample (Novakov and Corrigan, 1995;Chow et al., 2001;Yu et al., 2002;Wang et al., 2010).Moreover, the same authors singled out the presence of heavy, tar-like organic compounds which do not evolve until the highest temperature in He atmosphere is reached.
The water-soluble organic compounds (WSOC) removal has been identified as an effective procedure for a better EC quantification as WSOC have an important role in PyC formation (Novakov and Corrigan, 1995;Yu et al., 2002); nevertheless -as far as we know -a systematic analysis of the results obtained by the thermal-optical analyses on real samples after WSOC removal has never been carried out.Also inorganic catalytic compounds as well as some polymeric, partially aromatic, coloured organic products of combustion which evolve only at high temperature are water soluble; thus, they can be removed by washing the filter before analysis in order to reduce possible catalytic effects or interferences in the EC determination (Andreae and Gelencs ér, 2006) .
The influence of the sample composition on the thermal behaviour of carbonaceous species makes difficult finding out a universal thermal method for OC/EC separation Introduction

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Full suitable for aerosol samples collected in different environments.
In this work, tests were carried out with the aim of identifying the most suitable protocol for OC/EC measurements on samples collected at an urban area in the Po Valley, which is one of the major pollution hot-spots in Europe.Three protocols mainly differing for the highest temperature in the inert atmosphere (i.e.870 • C, 650 • C, and 580 • C) were tested.The novelty of this work is that the tests were performed both on untreated and water-washed samples.In addition, water-soluble organic compounds (WSOC) extracted from our samples were also analysed to study their thermal behaviour and gain further information on the different carbonaceous aerosol components.

Samplings
The sampling campaigns were carried out at an urban background station in the Milan university campus at about 10 m above ground level.PM 10 was sampled on two parallel quartz fibre filters (2500 QAO-UP, Pall Corporation, 47 mm diameter) pre-fired at 700 • C for 1 h (Vecchi et al., 2009) using low volume samplers (flow rate 2.3 m 3 h −1 ).
28 parallel samplings were carried out from 17 January to 9 February 2010.The sampling strategy was to perform 9-h samplings (from 09:00 to 18:00 and from 21:00 to 06:00) in order to limit the filter loading and to operate the carbon analyses in optimal conditions (see Sect.

Thermal-optical transmittance analysis
In this work, a thermal-optical transmittance (TOT) analyser by the Sunset Laboratory Inc. was used to quantify EC, OC, and TC in aerosol samples.The carbonatic carbon component was not considered in this work as previous studies reported that in PM 10 carbonate is negligible in most European areas except for coastal sites in south Europe (ten Brink et al., 2004;Sillanp ä ä et al., 2005;Perrone et al., 2011).Briefly, in the first part of the TOT analysis the sample is heated in an inert atmosphere (He) using different thermal ramps depending on the protocol in use.Then, the second part of the analysis is carried out in an oxidising atmosphere (He/O 2 mixture, 90/10 %) (Birch and Cary, 1996).The carbon evolving during heating is completely oxidised to CO 2 by a MnO 2 catalyst and then reduced to CH 4 to be quantified by a flame ionisation detector (FID).
To account for PyC formation, the transmission of a laser beam through the sample is constantly monitored during the analysis.Transmittance usually decreases throughout the He-step, indicating the formation of light-absorbing PyC.In the the He/O 2 phase, an increase of the laser signal is registered and the PyC evolution is conventionally assumed completed when the transmittance reaches its initial value.Carbon evolving after this point (called split-point) is then considered as EC.
In this work, three thermal protocols mainly differing for the highest temperature in the He atmosphere were tested (see Table 1 for details).The protocol called He-870 is very similar to NIOSH2 (Maenhaut and Clayes, 2007) and ACE-Asia base (Schauer et al., 2003).The He-580 is a low temperature and time variable protocol which is a proxy of the Desert Research Institute IMPROVE A protocol (Chow et al., 2007) implemented on a thermal-optical transmittance instrument.The third protocol is EUSAAR 2 (Cavalli et al., 2010), which has been recently proposed as a standard for carbon analysis on samples collected at European regional background sites.It is noteworthy that few samples analysed by EUSAAR 2 required the last step in oxidising atmosphere to be prolonged in order to obtain a complete carbon evolution, as previously reported by Introduction

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The protocols differ for temperature and duration of the heating steps.It is important to point out that the plateau temperatures and step time-lengths in He-870 and EUSAAR 2 are fixed.As for He-580, the plateau temperatures are fixed but the step time-lengths are variable; indeed, this protocol allows the complete evolution of the carbon at each step (i.e. each plateau lasts till the FID signal approaches to zero).
Of the two quartz filters sampled in parallel, one was analysed as-is (in the following named untreated sample) and the other was water-washed to remove water soluble compounds, as explained in Sect.2.3.The protocols were tested on a 1 cm 2 punch taken from the untreated and the water washed samples.

Washing procedure
To set up the filter washing procedure, PM 10 samples were collected at the previously described location (see Sect. 2.1) using a high-volume sampler (flow-rate: 30 m 3 h −1 ) on 150 mm diameter quartz fibre filters (QAT-UP, Pall Corporation), which allowed multiple tests on the same material.
Water soluble compounds were removed washing portions of the high volume sampled filters using MilliQ (Millipore) water (TOC <5 ppb).Each portion of the filter was placed in a filtration assembly -similar to the one reported by Yttri et al. (2009) -suitably designed for this application.The washed area was 37 mm diameter wide (i.e.smaller than the total deposit area) to avoid the possible loss of sampled particles from the filter edge.
Tests were carried out to determine the minimum amount of water needed for a complete water soluble compounds removal so that different punches of the same sampled filter were washed using increasing water quantities.Residual total carbon (TC) concentrations measured by TOT were used to check the washing efficiency with different water quantities.It was noticed that the residual TC on the filter decreased when the water used for the washing increased, until a minimum TC quantity (TC w ) was reached.The water quantity (V H 2 O ) necessary to reach TC w was different for each Introduction

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Full filter and depended on the initial TC load.A linear relationship (R 2 =0.99) was found between the TC load of the sample (in the range 20-100 µg cm −2 ) and V H 2 O : 7.5 µg mL • washed area (cm 2 ).
This relation was used to estimate the water quantity to be used for the washing of the LV samples collected for testing different protocols.It is noteworthy that the minimum water quantity used corresponded to the amount necessary for washing a 20 µg cm −2 TC loaded sample.After washing, filters were placed in open but dust-protected sievetrays and air dried at room temperature for 24 h.The uniformity of the washed filter was tested measuring the TC concentration on three different punches (area: 1 cm 2 ) taken from the same filter and washed with V H 2 O .
Differences among the three values were lower than 10 %.

TC measurements
TC was measured on untreated and water washed samples using the three protocols.In Fig. 1 the comparison between TC measured by the protocols is shown.As expected, a good agreement was found among the different protocols; indeed TC quantification is not dependent on the thermal treatment as previously found in different inter-comparison exercises (Schmid et al., 2001;Watson et al. 2005; and therein cited literature).
The TC results variability on each sample was evaluated as the ratio between the standard deviation of the three TC measurements and the average concentration value.
In the untreated samples, a 4 % variability was detected on average (range of TC concentration: 17.6-57.0µg cm −2 ).This estimate is consistent with the ±5 % Introduction

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Full measurement precision on TC values and 0.2 µg cm −2 as minimum uncertainty reported for this instrument (Subramanian et al., 2006).Nine field blanks were also analysed.The TC content in untreated field blanks was in the range 0.4-3.5 µg cm −2 .The average TC variability in the washed samples was 9 %.Four field blanks were also washed and their TC content was in the range 0.7-3.2µg cm −2 .These values are comparable to those measured on untreated field blanks; therefore, our washing procedure did not introduce any systematic filter contamination.Moreover, the comparison between the TC variability on untreated and washed samples indicates that our washing procedure did not significantly affect the filter uniformity (i.e.no more than 5 %).

EC measurements
EC results obtained by the three protocols were compared for both the untreated and washed samples.There was one outlier in the dataset with an EC concentration exceeding 15 µg cm −2 .Previous works evidenced that the transmittance variation through heavily loaded filters (i.e.samples with EC >15 µg cm −2 ) cannot be correctly monitored because the initial laser signal is too low (Subramanian et al., 2006;Wall én et al., 2010), thus the outlier was excluded from the database.It is noteworthy that EC concentration was higher than 5.5 µg cm −2 in about half of the 9-h samples collected at our station.Therefore, if 24-h instead of 9-h samplings were carried out, EC concentration would have been higher than 15 µg cm −2 in one half of the samples, preventing the detection of transmittance variations during the He step.This observation is of particular interest because it evidences that traditional 24-h samplings in heavily polluted areas can limit the possibility to obtain reliable EC concentrations by the TOT method.EC results obtained by the three protocols on both untreated and washed samples showed a good correlation (R 2 >0.87).However, significant differences (slope >1.48) in absolute EC concentrations were detected on untreated filters when comparing the results obtained by the He-870 protocol to those by other protocols (Fig. 2).These Introduction

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Full differences were lower in the results obtained on water-washed filters.As expected, these results show that the removal of water soluble compounds from the filter is effective in reducing the differences among the EC values measured by different protocols.However, washing filters is not enough to obtain a full agreement among protocols as EC quantification depends also on other parameters, as it will be shown later in the text.
An unexpected finding was that EC concentrations were generally (in 83 %, 67 % and 79 % of the cases for He-870, EUSAAR 2 and He-580, respectively) higher in the washed than in the untreated samples; the increases were up to 54 % (with He-870), 24 % (with EUSAAR 2), and 43 % (with He-580) of the EC measured on untreated filters.This result suggests that measurements on untreated filters can lead to an EC underestimation.
The role of the carbon fraction evolving in He at high temperature (see Sect. 3.3) and PyC formation (see Sect. 3.5) were explored to better understand differences among protocols and between washed and untreated samples, in order to choose the most suitable thermal treatment for our samples.

Matching the different protocols: the nature of carbon evolving at high temperature in the inert atmosphere
As mentioned before, EC results were protocol-dependent in both untreated and washed samples.One of the main differences among the tested protocols is the highest temperature in the He step.Subramanian et al. (2006) identified the carbon evolving at high temperature (about 850 • C) in the He atmosphere as responsible for the disagreement observed in EC results.
In our tests, a good agreement was found comparing the sum of EC and the carbon fraction evolving in the He4 step in the He-870 protocol (in the following called C He4 870 ) to the EC values obtained by EUSAAR 2 and He-580 for both untreated and washed samples (Fig. 3).This result suggests that to obtain the most reliable EC estimate, it is mandatory to understand the chemical nature of the C He4 870 fraction 19859 Introduction

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Full (i.e.whether it is OC or EC) in the samples collected in the area of investigation (in our case a heavily polluted urban area in the Po Valley).It is useful to point out that the concentration of this fraction in the sample is comparable to the EC content, thus its wrong attribution can strongly affect EC determination.
Opposite to what reported in the literature (Chow et al., 2004;Subramanian et al., 2006), no significant increase in the laser signal was registered during the He4 870 step(see Fig. 4), especially in the case of washed filters.The laser attenuation by the sample was calculated at the starting point of the analysis, at the maximum attenuation point, and at the end of the He step as ATN= −100ln(I j /I 0 ) (I j , with j = 1. . . 3 is the laser transmission measured at the three points of interest and I 0 is the laser signal at the end of the analysis, see Fig. 4a).The increase in the laser signal registered during the He4 870 step corresponded on average to a 4 % (range 2 %-7 %) variation of the initial attenuation on the washed samples and a 6 % (range 3 %-12 %) on the untreated ones.
The slight difference between washed and untreated samples is probably due to soluble compounds (e.g.sulphates or other salts) which can alter the thermal behaviour of the light-absorbing carbonaceous species and are removed by the washing procedure (Novakov and Corrigan, 1995;Yu et al., 2002;Hitzenberger and Rosati, 2010;Wang et al., 2010; and therein cited literature).
The apparent average attenuation coefficient related to the C He4 870 (defined as the ratio between the ATN variation and the carbon evolved during the He4 870 step) was calculated.It was about 2.6 m 2 g −1 and about 3 m 2 g −1 for washed and untreated samples, respectively.These values are much lower than those reported in the literature for EC (about 20 m 2 g −1 ) and PyC (about 40-50 m 2 g −1 ) on filters (Chow et al., 2004;Subramanian et al., 2006;Boparai et al., 2008) indicating that most of the C He4 870 in our samples is not strongly light-absorbing.Therefore, the C He4 870 in this kind of samples out.Comparing this value to the measured EC concentration -and considering that also PyC can contribute to light-absorbing carbon evolution during He4 870 -an upper limit for EC pre-combustion was calculated (12 % and 5 % for untreated and washed samples, respectively).With the aim of understanding the higher EC values measured by the lower temperature protocols compared to He-870, EUSAAR 2 temperatures at the split point were analysed.As the split-point in EUSAAR 2 occurred in all cases at temperatures in the 560-710 • C range, it is possible that the organic compounds evolving in He4 870 burn together with the EC fraction when EUSAAR 2 is applied, thus leading to an EC overestimation when analysing our samples by the EUSAAR 2 protocol.However, it should be taken into account that this hypothesis needs further investigations because the thermal evolution occurs in different atmospheres (He and He/O 2 ) and the organic compounds might behave differently.Similar results were found using He-580, where the split points were always detected during the oxygen step at 580 • C.
It is noteworthy that the C He4 870 value was lower (−28 % on average) on washed than on untreated samples.The presence of soluble compounds in the C He4 870 is another indication of the role of organic compounds evolving at high temperatures in He; these organics can increase PyC and EC measured concentrations when using low temperature protocols.The reduction of this organic fraction, which contributes to the protocols disagreement (Andreae and Gelencs ér, 2006), can further explain the slight improvement in protocols comparability reported in Sect.3.2.

WSOC analysis
For a better comprehension of the differences between the results obtained on untreated and washed samples, filter extracts were also analysed to determine the thermal behaviour of WSOC.To analyse water soluble compounds the water extracts obtained by all samples were joined together, vacuum-dried, and rinsed with methanol (Piazzalunga et al., 2010a).A drop of this solution was then deposited on 1 cm Introduction

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Full punches of a pre-fired quartz fibre filter.The punches were placed in open but dustprotected sieve-trays, air dried (2 h at room temperature), and analysed using the three protocols.
As already found in previous works (Andreae and Gelencs ér, 2006;Wall én et al., 2010), the He1 and He4 carbon fractions gave the highest signal when WSOC were analysed by He-870.
It is noteworthy that a substantially higher carbon quantity (about + 77 %) evolved in the He/O 2 phase with the low-temperature protocols than with the He-870 protocol, indicating that the He4 870 step was important to allow the WSOC evolution in the He phase.
The WSOC laser signal increases in the He4 870 step.The apparent attenuation coefficient for the C He4 870 was about 5 m 2 g −1 , indicating again that most of this carbon fraction was weakly light-absorbing.Moreover, the few light-absorbing carbon evolving in He4 870 is PyC in this case, as no EC is expected in WSOC due to the extraction procedure.This is a further confirmation that the EC pre-combustion estimates given in Sect.3.3 have to be considered as upper limits.
Results on water extracts obtained by the three protocols (see Table 2) showed that no EC was measured during the analysis by the He-870 protocol, while about 1 µgC cm −2 was observed using the low-temperature protocols.
This observation further suggests that in the Milan urban atmosphere organic compounds exist, which evolve in the He step at a temperature in the 650-870 • C range (about 20 % of WSOC) and are weakly light-absorbing.In this work, these compounds have been demonstrated to interfere with EC determination when low-temperature protocols are used for the analysis, leading to a possible EC overestimation.
Part of these compounds can be ascribed to HULIS (HUmic-LIke Substances), which are weakly absorbing materials with a biogenic origin or generated during biomass burning (Andreae and Crutzen, 1997;Andreae and Gelencs ér, 2006;Iinuma et al., Introduction Conclusions References Tables Figures

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Full 2007; Schmidl et al., 2008).HULIS mainly evolve at temperatures higher than 600 • C in oxygen atmosphere.
It should be noted that wood burning is a not-negligible emission source in the Milan urban area during wintertime.Its primary component was estimated to account for about 14 % of the PM 10 mass in Milan during a winter period (Bernardoni et al., 2011).
Following Varga et al. (2001) preliminary HULIS measurements had been carried out by our group (Fermo et al., 2009); the results showed that HULIS can account for the 30-50 % of the OC in the Milan urban area during wintertime.
In the same samples analysed by the three protocols also levoglucosan -a marker for biomass burning (Simoneit and Elias, 2001) -was measured using the methodology reported in detail in Piazzalunga et al. (2010b).The presence of levoglucosan (concentration range: 0.6-4.0 µg cm −2 and levoglucosan carbon/TC = 3 % on average) in our samples indicated a not negligible contribution by wood burning during the investigated period, which can explain the presence of an important refractory, not strongly light-absorbing fraction.
It is noteworthy that the significant contributions due to biomass burning products makes low-temperature protocols not suitable for a correct assessment of the EC content in our samples because low-temperature TOT protocols can strongly overestimate it.

Pyrolitic carbon formation
It is well-known that PyC formation is another important source of troubles for OC/EC quantification.Indeed, literature studies (Yang and Yu, 2002) showed that neither of the following statements -necessary for a correct EC quantification -is respected in thermal-optical analysis: (a) pyrolitic carbon evolves before EC during the analysis, (b) pyrolitic carbon and OC have the same light absorption coefficient.
Therefore, if PyC formation is minimised, the analysis results are more reliable.in the carbon evolving in the washed filter can be noticed both in the He and He/O 2 phase.A reduction of the carbon signal in the He/O 2 phase was found also for the He-580 protocol, but no thermogram superimposition is possible due to the variable time lengths in the analysis performed by this protocol.The carbon evolving in the He/O 2 step was compared among the tested protocols to gain information on PyC formation.It was assumed that carbon evolving in oxygen with the EUSAAR 2 and He-580 protocols is comparable to that evolved in the He-870 but only with the addition of the C He4 870 contribution (in the following all these quantities will be called He/O 2 carbon), according to the considerations reported in Sect.3.3.He/O 2 carbon represents the sum of native EC, the refractory fraction of OC, and PyC formed during the thermal treatment.The He/O 2 carbon concentrations are represented in Fig. 7 separately for untreated and washed samples.Since the native EC on the filter does not depend on the chosen protocol, differences in He/O 2 measurements among protocols can be ascribed only to differences in PyC formation in the first part of the analysis.
Few differences are registered between He-870 and EUSAAR 2 He/O 2 carbon (on average 7 % in the untreated samples and 12 % in the washed samples), while the He-580 data are much lower (on average about −32 % in both cases).Thus, the He-870 and EUSAAR 2 protocols seem to produce comparable PyC quantities in samples collected in a polluted urban area while PyC formed by He-580 is much lower.This finding is opposite to what reported by Cavalli et al. (2010) for the EUSAAR 2 protocol but they referred to samples collected at regional background stations in Europe and likely characterised by a very different chemical composition.
The observed differences on the He/O 2 carbon could be due to the time duration of the He steps in the different protocols.In the He-580 protocol the steps are variable; the duration of each temperature plateau is such to allow the evolution of carbon at a specific temperature step until the peak approaches to zero.In the other two protocols, time steps are fixed.Introduction

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Full Previous works (Yu et al., 2002) showed the importance of allowing a complete carbon evolution at each step to limit PyC formation.Figure 8 shows that when the He step duration in EUSAAR 2 is comparable to the He-580 one, differences between these protocols approach to zero.On the contrary, the shorter is the step time in EU-SAAR 2 compared to the He-580 one the higher is the He/O 2 carbon (i.e.PyC) formed using EUSAAR 2.
Focusing on the differences between untreated and washed samples, reduction of the He/O 2 carbon is registered after washing (−40 % for He-870 and EUSAAR 2 and −30 % for He-580).The missing fraction in the washed samples is therefore either soluble or produced by the thermal treatment of soluble compounds.Thus, the washing procedure applied to our samples reduces possible interferences in the measurements (e.g.presence of refractory organic carbon and of inorganic compounds which modify the carbon thermal behaviour, as well as PyC formation), improving the protocols agreement on EC measurements as shown in Sect.3.2.The lower reduction in the He-580 protocol can be justified considering that this protocol is less prone to pyrolysis even in untreated samples, therefore limiting the possibility of reduction of the pyrolysing carbon component.

Conclusions
This work aimed at the evaluation of the best approach to analyse EC and OC by TOT in real aerosol samples collected in a polluted urban area.Tests were carried out on both washed and untreated samples collected during a winter period in Milan (Po valley, Italy).Results obtained by TOT analysis using three different thermal protocols were discussed.
As expected, EC values measured by the three tested protocols were different.The main difference was ascribed to the carbon fraction evolving during the He step at can differently affect the EC measured by the lower temperature protocols.It was also demonstrated that the C He4 870 is mainly not light-absorbing in our samples, thus it has to be considered mainly OC.Therefore, the He-870 protocol prevents EC overestimation when analysing samples collected in polluted urban environments like Milan.WSOC were also analysed as they are significant contributors to PyC.The washing procedure used to remove WSOC from our samples resulted in a more reliable EC estimation as the PyC formation was limited.Moreover, as WSOC contribute to part of the C He4 870 their removal allowed also a slight improvement in the protocols comparability.
In summary, our tests on real samples -characterised by a chemical composition typical of an area affected by a complex mixture of pollution sources -suggest that protocols reaching high temperatures in He atmosphere are preferable to low-temperature ones to get rid of typical interferences which affect EC results.Moreover, our results indicate that the best approach to analyse carbon in urban aerosol samples should consider steps long enough for complete carbon evolution in order to reduce pyrolysis formation.Introduction

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Full  Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3.2).During the sampling period the weather was cloudy or foggy (snow was registered on 5 February 2010 and no other precipitations occurred in the period).Temperature ranged from −1 • C to 10 • C. PM 10 mass was on average 74 µg m −3 (range 35-111 µg m −3 ) and TC accounted for about 25 % of the PM 10 mass on average.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 | Discussion Paper | Discussion Paper | Discussion Paper | is likely composed mainly by organic compounds showing a weak light attenuation and only a slight evolution of the light-absorbing material is registered (i.e. about 10 % of the C He4 870 is estimated to be light-absorbing).Moreover, assuming 20 m 2 g −1 as the EC attenuation coefficient a rough estimate of the EC evolved in He4 870 step was carried Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Figure 6 shows the comparison between the thermal evolution of one untreated and one washed sample with the EUSAAR 2 and the He-870 protocols.Strong reductions 19863 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 870 • C (C He 870 ), which was demonstrated to evolve in He/O 2 atmosphere with the two lower-temperature protocols.The evolution of this fraction before or after the split-point Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Kuhlbusch, T. A. J., Borowiak, A., Gelenscer, A., Genberg, J., Gladtke, D., Maenhaut, W., Pio, C., Popoviecheva, O., Putaud, J.-P., Quincy, P., Sciare, J., ten Brink, H., Viana, M., and Yttri, K. E.: Measurement of Elemental and Organic carbon in Europe.JRC Scientific and Technical Reports (EUR collection), EUR23992 EN, available online at: http://publications. Discussion Paper | Discussion Paper | Discussion Paper | Yu, J. Z., Xu, J. H., and Yang, H.: Charring Characteristics of Atmospheric Organic Particulate Matter in Thermal Analysis, Environ.Sci.Technol., 36, 754-761, 2002.Zhi, G., Chen, Y., Sheng, G., and Fu, J.: Effects of temperature parameters on thermal-optical analysis of organic and elemental carbon in aerosol, Environ.Monit.Assess., 154, 253-261Discussion Paper | Discussion Paper | Discussion Paper |

Table 2 .
WSOC results obtained by different protocols in µg cm −2 .