Accuracy and precision of 14 C-based source apportionment of organic and elemental carbon in aerosols using the Swiss_4S protocol

. Aerosol source apportionment remains a critical challenge for understanding the transport and aging of aerosols, as well as for developing successful air pollution mitigation strategies. The contributions of fossil and non-fossil sources to organic carbon (OC) and elemental carbon (EC) in carbonaceous aerosols can be quantiﬁed by measuring the radiocarbon ( 14 C) content of each carbon fraction. However, the use of 14 C in studying OC and EC has been limited by technical challenges related to the physical separation of the two fractions and small sample sizes. There is no common procedure for OC/EC 14 C analysis, and uncertainty


Introduction
Carbonaceous aerosols play an important role in the Earth system by influencing many biogeochemical and climate processes (Pöschl, 2005;Jimenez et al., 2009).By absorbing and scattering solar and terrestrial radiation, carbonaceous aerosols directly affect Earth's radiation budget (Andreae and Gelencsér, 2006).As cloud condensation nuclei, they influence cloud formation (Novakov and Penner 1993;Dusek et Published by Copernicus Publications on behalf of the European Geosciences Union.al., 2006;Spracklen et al., 2011) and microclimate (Bond et al., 2013).Carbonaceous aerosols also affect atmospheric chemistry by reacting with photooxidants, such as ozone and nitrogen dioxide, and acids (Lary et al., 1997;Ammann et al., 1998).
Emissions of carbonaceous aerosols from fossil-fuel combustion and biomass burning have significantly increased since preindustrial times (Griffin and Goldberg, 1983;Mc-Connel et al., 2007) and account for a major fraction of fine particulate matter in polluted urban environments and in the global atmosphere (Pöschl, 2005).Numerous studies show that high concentrations of fine air particulate matter are correlated with severe health effects, such as enhanced cardiovascular, respiratory and allergic diseases and mortality (Gauderman and Avol, 2004;Mauderly and Chow, 2008;Janssen et al., 2011;Johnston et al., 2012).Due to their direct and indirect effects on climate, as well as their strong influence on the air quality and human health, carbonaceous aerosols have become a major environmental concern worldwide.
Carbonaceous aerosols encompass all particles containing carbon, excluding carbonates.This highly variable mixture of compounds is traditionally divided in two fractions: weakly refractory, light polycyclic or polyacidic hydrocarbons (organic carbon, OC) and strongly refractory, highly polymerized and light-absorbing carbon (elemental or black carbon, EC) (Pöschl, 2005).Both fractions play decisive, yet very different roles in the global climate system.The two fractions also originate from distinct sources and undergo different aging processes (Pöschl, 2005;Hallquist et al., 2009;Jimenez et al., 2009;Szidat et al., 2009;Bond et al., 2013).Particulate OC originates from either primary (i.e., direct) emissions or secondary formation (i.e., by oxidation of volatile organic compounds (VOCs)), whereas EC derives from incomplete combustion of fossil fuels or biomass.Assessing the respective contribution of fossil and biomass burning emissions is necessary in many air quality and climate mitigation applications for the development of efficient abatement strategies (Penner et al., 2010;Zhang et al., 2012, Bond et al., 2013;Szidat et al., 2013).
One way to investigate the sources of OC and EC is to measure their radiocarbon ( 14 C) content.Radiocarbon is a naturally occurring radioisotope that is produced in the atmosphere by cosmic ray interaction with nitrogen gas.Radiocarbon is oxidized to carbon dioxide (CO 2 ) and enters the food chain through photosynthesis so that all living things are intrinsically labeled with a characteristic radiocarbonto-carbon ratio ( 14 C / 12 C).The carbon content of materials with 14 C / 12 C ratios similar to the present 14 C / 12 C ratio of atmospheric CO 2 is described as modern.As 14 C decays, 14 C / 12 C ratios approach zero, as compared to a modern standard ( 14 C / 12 C sample 14 C / 12 C standard ), and the carbon content is then described as fossil.In addition to natural production of 14 C, atmospheric nuclear weapons testing produced large quantities of 14 C in the mid-20th century.Since the partial test cessation in 1963, atmospheric 14 C levels have been declining as this bomb-14 C is mixed with older CO 2 outgassing from the oceans and soils and is further diluted by increasing levels of emissions of fossil CO 2 (Levin et al., 2010).Thus, modern and fossil sources of OC and EC in carbonaceous aerosols can be quantified by measuring their 14 C content.For aerosols, biomass burning and organic aerosols derived from biogenic VOCs produce carbon with a modern 14 C signature, while fossil fuel combustion generates carbon depleted in 14 C (Clayton et al., 1955).
Since OC and EC fractions differ in their origins and often show very different 14 C signatures (Szidat et al., 2004(Szidat et al., , 2009)), 14 C-based source apportionments require a clear and physical separation between OC and EC.However, there is no defined sharp boundary between OC and EC, but a continuous increase of thermochemical refractiveness and specific optical absorption going from non-refractive and colorless organic compounds to graphite-like structures (Pöschl, 2005).Further, OC can be pyrolized (charred) into EC during analytical procedures.Therefore, the OC/EC split is defined based on the applied optical or thermochemical methods used (absorption wavelength, temperature gradient, etc.), which can lead to substantially different results and strongly limits the comparability and suitability of OC and EC data for the determination of mass balances and physicochemical properties of aerosols.
Thermal-optical analysis (TOA) is one of the most commonly applied techniques for OC/EC measurements (Schmid et al., 2001;Chow et al., 2004;Cavalli et al., 2010).However, moderate changes in the thermal evolution protocol for an OC/EC analysis can have a large impact on the OC/EC split (Schauer et al., 2003).A protocol allowing the isolation of OC and EC for accurate 14 C measurements requires a complete removal of interfering fractions with maximum CO 2 recovery from each fraction.Recently, Zhang et al. (2012) developed such a TOA protocol ("Swiss_4S") with optimized thermal-optical conditions to minimize OC charring, untimely removal of EC, and the potential positive artifacts leading to co-evolution of EC with residual OC.
Here, we describe an investigation of the precision and accuracy of the Swiss_4S protocol for 14 C analysis of OC and EC using the W. M. Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory (KCCAMS) at the University of California (UC), Irvine.We coupled a thermal-optical aerosol analyzer (OC/EC analyzer, Sunset Laboratory Inc., Portland, OR, USA), which oxidizes each fraction to CO 2 , to a vacuum line, allowing us to cryogenically trap each fraction's CO 2 for 14 C analysis.We quantified the blank associated with our setup by analyzing both modern and fossil OC and EC standard materials ranging in size from 4 to 43 µg C. The goals of this study were to evaluate the performance of our analytical setup, quantify the uncertainties and compare the consistency of our results with those from the Laboratory of Radiochemistry and Environmental Chemistry at the University of Bern, Switzerland (Zhang et al., 2012).This study is an important step towards developing a common procedure for the 14 C analysis of OC and EC in fine air particulate matter.

Methods and materials
To achieve precise and reproducible 14 C measurements of OC and EC aerosol carbon fractions, we performed three consecutive steps.First, we introduced a sample into a thermal-optical OC/EC analyzer coupled to a vacuum line.The aerosol carbon fraction of interest (either OC or EC) was isolated via the OC/EC analyzer with the Swiss_4S protocol and cryogenically trapped on the vacuum line in the form of CO 2 and sealed into a Pyrex tube.Second, we converted the CO 2 into a solid target (graphite) that was subsequently analyzed for its 14 C content with accelerator mass spectrometry (AMS).Samples were graphitized via hydrogen reduction, specifically optimized for small samples (Santos et al., 2007a).Third, the total uncertainty associated with OC/EC separation, cryo-trapping, graphitization and 14 C analysis was estimated as the sum of modern and fossil carbon contaminations.This information was used to adjust the fraction modern values of our samples (Santos et al., 2010).This last step is critical for the analysis, as each step introduces extraneous carbon, which if not accounted for, could yield biased results.Additional uncertainties, associated with sample collection in the field, were not considered in this study.It should be noted, however, that field blanks have the potential to contribute significantly to the overall uncertainty for ambient aerosol studies (Zotter et al., 2014).

Isolation of OC and EC
A thermal-optical OC/EC analyzer (Sunset Laboratory Inc.) was used for the combustion and recovery of the OC and EC fractions.The instrument is specifically developed to separate the carbon content of ambient atmospheric samples collected on quartz fiber filters into OC and EC fractions by thermal evaporation and/or oxidation at separate temperature steps.As the carbon fragments pass through a manganese dioxide (MnO 2 ) oven, they are quantitatively converted to CO 2 gas and measured directly by a self-contained nondispersive infrared (NDIR) detector system, placed upstream the instrument's outlet.At the end of each run, a known volume of methane (CH 4 ) is injected into the oven and oxidized to CO 2 to ensure accurate quantification of OC and EC concentrations.
When operated with standard analytical protocols (e.g., NIOSH (NIOSH, 1999), IMPROVE-A (Chow et al., 2007), EUSAAR_2, (Cavalli et al., 2010), the Sunset OC/EC analyzer evaporates the OC fraction under an inert helium (He) atmosphere, while the EC fraction is subsequently oxidized in a He/O 2 mix (Cavalli et al., 2010, Chow et al., 2001).Under these conditions a fraction of the OC can undergo sub-stantial pyrolysis (charring) during the O 2 -free step.When measuring concentrations, the amount of charring-derived EC is accounted for internally by the OC/EC analyzer.A tunable diode laser measures the light transmittance of the sample throughout the heating ramp cycle.Since the transmittance decreases as OC is pyrolized to EC, but increases as EC and the pyrolized OC are removed during the second heating cycle, the OC/EC split point for quantification is defined to be the point at which the laser transmittance returns to its initial value (Bauer et al., 2009).This widely accepted procedure for the determination of OC and EC concentrations may not be suitable for 14 C analysis of the EC fraction, especially when analyzing ultra-small samples, because accurate measurements of 14 C require a physical separation of the EC from the OC fraction and it is not possible to mathematically correct the 14 C data for charring.
For 14 C measurements, the Sunset OC/EC analyzer was modified and equipped with a solenoid valve and a gas flow sensor for controlling O 2 flow, which allows the use of pure O 2 (99.999 %) during oxidation (Zhang et al., 2012).The use of pure O 2 reduces OC charring and allows the use of alternative TOA protocols.The O 2 flow is controlled in the same way as the default carrier gases -He (> 99.999 %) and O 2 /He mix (10 % O 2 in He) and the reference gas -CH 4 (5 % CH 4 in He), all of which are controlled by the instrument's gas flow program.The gas flow rate through the OC/EC analyzer is adjusted and stabilized at 60 mL min −1 .
To achieve a clear physical separation of OC and EC for 14 C-based source apportionment, we used the Swiss_4S protocol, which involves four consecutive steps: (S1) pure O 2 at 375 • C for evaporation/oxidation of OC without premature EC evolution, (S2) pure O 2 at 475 • C followed by (S3) in He at 650 • C, both of which aim to achieve complete OC removal before EC isolation and lead to better consistency with TOA protocols like EUSAAR_2, and (S4) pure O 2 at 760 • C for desorption and recovery of EC (Zhang et al., 2012).Note that our analysis differed from the Zhang et al. (2012) method, in which OC is trapped only during (S1).
Here, we trapped OC evolving during both (S1) and (S2) to achieve maximum OC recovery.The material evolving during ( S3) is defined as a mixture of OC and EC and is not included in the analysis of either carbon fraction and therefore discarded.
Each run was completed by an injection of reference gas (5 % CH 4 in He), allowing for a conversion of sample CO 2peak sizes to µg C cm −2 of sample filter.The OC/EC analyzer is purged with He prior to every run to ensure that there is no cross contamination from a prior run.
During routine operation, fine air particulate matter (PM 2.5 ) samples, collected on quartz microfiber filters (Pallflex Tissuquartz, 2500 QAT-UP, Pall, Port Washington, NY) were introduced into the Sunset OC/EC analyzer as 1.5 cm 2 filter punches (dimensions 1 cm × 1.5 cm, Sunset) on a flat quartz spoon.Under the Swiss_4S protocol, separate punches were used for OC and EC analysis.The EC punch was treated with Milli-Q (MQ) water prior to the analysis to remove the water-soluble OC and minimize charring as follows: a filter disk (23 mm diameter) was placed on a plastic filter holder (25 mm diameter, Sartorius GmbH, Germany) between two sealing rings with the laden side upwards, and 20 mL of MQ water was passed through from the top using a syringe (Zhang et al., 2012).After drying at 60 • C for up to 1 h, a 1.5 cm 2 punch was taken from the disk for analysis.
Standards for calibration of the OC/EC analyzer (e.g., water-soluble OC, such as sucrose) are typically applied as solution, pipetted on a pre-baked filter punch, dried and analyzed through the desired TOA protocol.However, since EC is water insoluble, this procedure cannot be applied when running EC standards for 14 C analysis or method verification.To introduce OC and EC reference materials alike, we developed and customized quartz boats (Jelight Company Inc., Irvine, CA, USA), which were placed into a specifically designed round-bowled (instead of flat) quartz spoon (Jelight Company Inc., Irvine, CA, USA).This enabled us to introduce solids into the OC/EC analyzer.Boats were pre-baked at 900 • C for 2 h and stored in a baked, closed porcelain crucible to avoid contamination.Spoons were baked in O 2 inside the OC/EC analyzer for 10 min at 920 • C.

OC/EC-vacuum line
The outlet of the OC/EC analyzer was connected to a vacuum line (Fig. 1), which allows for the physical trapping of the CO 2 released during the different steps of the analysis for subsequent 14 C measurement.In addition to trapping the CO 2 gas, the vacuum line is also used to remove noncondensables and O 2 .The vacuum line exhibits a very simple design and allows fast and efficient cooling, trapping and release of the gasses.In the following section, all numbers in brackets correspond to the vacuum line parts described in Fig. 1.
Prior to each OC/EC run, the vacuum line was evacuated to 1 × 10 −4 torr (0.013 Pa) on the Pirani vacuum gauge (8) using a turbomolecular oil-free diaphragm pump system (9).If no trapping was in progress, the line was kept isolated from the Sunset analyzer and air by keeping valves V2 and V3 closed, while V1 was kept open so the exhaust from the Sunset was open to air (1).The rest of the line stayed open to the turbo pump (9), ensuring the vacuum in the line, which was monitored by both the Pirani vacuum gauge (8) (1 × 10 −4 torr at vacuum) and the pressure transducer of the calibrated volume (6) (±0.1 torr at vacuum).While evacuated, we cleaned the inner surfaces of the break-seal tube (7) of any trapped exogenous C by heating along its length with a blowtorch (O 2 /natural gas).
Once the Swiss_4S TOA protocol started, the trapping procedure consisted of the following steps: a liquid nitrogen (N 2 (l)) dewar was placed so it covered the cryogenic trap (2) completely a minute prior to Swiss_4S step (S1)-for OC trapping or (S4)-for EC trapping.The cryogenic trap was then closed to the pump via V4.When CO 2 was evolving from the OC or EC fractions (steps S1 and S2 or S4, respectively), the gas stream was re-directed to the vacuum line by first closing the exhaust (via V1), waiting for the pressure in the line to build to 1 psig (∼ 6895 Pa), as shown by the OC/EC analyzer's program, and opening the flow to the cryogenic trap (V2 is opened; pressure drops).After waiting for the pressure to reach 1 psig for a second time, the second exhaust (4) was opened via V3 and stayed open while the Swiss_4S protocol was in progress.During that time, CO 2 accumulated in the cryogenic trap.At the end of S2 for OC or S4 for EC, the line was closed off from the OC/EC analyzer.First V3 was closed, and once the pressure again reached 1 psig, V1 was opened and V2 was immediately closed.After this step the CO 2 from the desired fraction was in the cryogenic trap.Note that waiting for the pressure to build up before opening the line to either exhaust (1) (at the end of the trapping procedure) or exhaust (4) (at the beginning of the trapping procedure) is crucial in preventing backflow in the system, which could introduce ambient CO 2 and O 2 from the room air into the N 2 (l)-cooled sample trap (2).However, the pressure buildup causes the O 2 carrier gas to condense in the trap.Once either exhaust is open, the reduced pressure prevents the O 2 from condensing in the trap.Releasing the pressure allowed us to avoid the need for an additional chemical trap for O 2 trapping, as compared to the setup in Bern.
Next, the O 2 that accumulated in the trap during the periods of increased pressure in addition to the sample CO 2, was pumped away, together with other non-condensables: N 2 (l) was left in place (2), while V4 was opened and closed, after which V7 was opened until vacuum was established again.These steps were repeated until the pressure on the pressure gauge (6) was below 300 torr (the upper limit of the pressure transducer) and then the line was finally opened fully to the turbo pump (9).The repeated closing and opening of V4 and V7 was necessary, because the turbo pump cannot operate at high pressures.
After the line pressure at the vacuum gauge (8) was less than 1 × 10 −4 torr, the cryogenic trap was closed to the pump via V4, and the N 2 (l) was replaced with a dry ice-ethanol slurry to release the sample CO 2 , but not water.The sample was transferred to the calibrated volume (5), where the pressure, corresponding to the carbon mass of the sample, was recorded.Finally, the measured CO 2 was transferred to the pre-cleaned 4 mm O.D. Pyrex tube (7) and sealed with the blowtorch for future graphitization and 14 C analysis.

Graphitization and AMS measurements
Our sample CO 2 was graphitized and analyzed for its 14 C content at the KCCAMS facility.Graphite for AMS measurements is typically produced by reduction of CO 2 gas over a catalyst.The production of high quality, uniform graphite targets, suitable for the AMS ion source, is important for the optimal performance of any AMS system.The quality of the graphite influences the level of background contamination, the mass fractionation, and the beam current (Santos et al., 2007c, Kim et al., 2010).
In this study, blanks and standards were chosen to resemble regular fine particulate matter aerosol samples collected over periods of 1-7 days (usually ∼ 2-50 µg C) and were much smaller than regular sized 14 C samples (∼ 300-1200 µg C).A modified protocol was used to maximize yields for our ultra-small samples and to improve target processing and measuring (Santos et al., 2007a).Cryogenically purified CO 2 was transferred into Pyrex tube reactors of 1.6 cm 3 volume and reduced to graphite at 450 • C using hydrogen gas over pre-cleaned iron powder as a catalyst.A magnesium perchlorate trap was used to remove water from the reaction.Graphite targets were analyzed for their 14 C content using a modified compact AMS system (NEC 0.5 MV 1.5SDH-1) (Beverly et al., 2010).

Background correction
During the processing of samples for 14 C analysis, contamination with extraneous carbon (C ex ) always occurs independently of sample size.Its presence can significantly affect the 14 C signature of smaller samples and puts a practical lower limit on the minimum 14 C sample size that can be reliably measured (Santos et al., 2007a(Santos et al., , 2010;;Ziolkowski and Druffel, 2009).In some cases, the mass and 14 C signature of the C ex can be evaluated directly, and the measured 14 C data of unknown samples can be corrected for it, using a mass balance approach.However, when the mass of C ex is too small to directly measure, the extraneous carbon contamina-tion cannot be quantified using this approach.In this study, we adopted an indirect mathematical approach (Santos et al., 2007a), which treats C ex as a mixture of two components; (a) modern carbon -mostly introduced during sample handling by tools and in equipment, and (b) fossil carbon -originating from 14 C-free carbon in the iron powder used to catalyze the reduction of CO 2 to graphite and/or in our OC/EC isolation and trapping setup.The mass of the C ex (m ex ) can then be expressed as (Eq.1): where m modern is the mass of the modern component and m fossil the mass of the fossil component of C ex .
The average contribution of modern carbon can be quantified by measuring blanks ( 14 C-free materials) of different sizes through each step of the sample processing and is expressed as (Eq.2): where m meas . is the mass measured on the graphitization line, FM blank is the measured 14 C signature of the blank, expressed as fraction modern carbon, and FM ref is the fraction modern of a normal-sized, modern reference materials (in this study we used 1 mg C of OX-I).This latter term is directly measured by the AMS system, and reported normalized to six time-bracketed aliquots of the reference material, and corrected for isotope fractionation (Santos et al., 2007b Finally, the 14 C content of a sample can be corrected for the mass and 14 C contribution of C ex and expressed as (Eq.4): where FM cons. is the canonic fraction modern value (the standard multiplier) of the reference material, used to produce the six time-bracketed graphite targets measured in a single batch or wheel, and FM meas. is the 14 C content of the unknown sample, directly measured by the AMS system.To determine the uncertainty in our calculation of FM sample , we mathematically propagated the uncertainty, applying a 50 % error in m fossil and m modern , based on long-term measurements of m ex variance in small samples (Santos et al., 2007a).Further detailed description of this approach for correcting ultra-small mass samples and its use at the KCCAMS/UCI can be found in Santos et al. (2007a).
Here, we evaluate the contribution of modern and fossil carbon, introduced both during the TOA analysis with the Swiss_4S on our vacuum line (C swiss_4S ) and during the graphitization procedure (C graph ).To assess the overall mass and 14 C signature of C ex , we measured a set of blanks and standards through all steps of the analysis for both OC and EC.To quantify the relative contribution of C ex introduced during either processing step, we measured another set of blanks and standards, which only underwent graphitization.The C swiss_4S contributions were estimated as the difference between C ex and C graph .

Materials
To evaluate the mass of the C ex and its associated 14 C signature for proper background corrections and to assess the performance of our OC/EC analyzer and vacuum line, we measured a set of 14 C fossil (blanks) and modern (standards) reference materials, described in Table 1.They were chosen to have strongly contrasting 14 C contents and (when possible) to consist of international reference materials.We chose adipic acid as a blank and oxalic acid (OX-I) as a standard for OC.Coal (POC #3 USGS (United States Geological Survey) coal) and rice char (Oryza sativa L.) were used as blank and standard for EC, respectively.Both OC and EC reference materials were introduced into the OC/EC analyzer as solids.The EC reference materials were pre-cleaned using the ABA (acid-base-acid) method (Santos and Ormsby, 2013).
To quantify the efficiency of our setup at trapping each carbon fraction, we measured seven samples with a range of sizes from each reference material, with the exception of OX-I (n = 6).For better assessment of the C ex and its 14 C content, more samples from the same reference materials were included from two additional AMS runs.A detailed description of all reference material samples can be found in Appendix A.
A second set of similar-sized 14 C reference materials (coal, n = 2-4 and OX-I, n = 3-5) were graphitized in parallel with each batch of samples (separately for each of the three AMS runs) to quantify any contamination introduced only during the graphitization procedure and to distinguish it from the contamination introduced by the OC/EC analyzer and vacuum line.
In addition, we analyzed the 14 C content of five mixed OC and EC in ambient particulate matter on quartz fiber filters, which were previously collected and analyzed for their 14 C content under the Swiss_4S protocol at the University of Bern, Switzerland.Samples were collected during winter with high-volume air samplers in Switzerland (Bern: 8 January 2009, Chiasso: 11 February 2012, Massongex: 13 February 2012 and Solothurn: 13 February 2012; for details see Zotter et al., 2014) and Poland (Zabrze: Winter 2012) (Appendix B).

Sample recovery
On average, 91 % of the carbon in each sample was cryogenically trapped following the Swiss_4S TOA on the Sunset OC/EC analyzer and attached vacuum line (Fig. 2a).Here, the OC content was directly estimated as the total carbon content of the reference materials, based on its molecular weight.The EC content could only be assessed with our TOA setup, and for each EC run the measured EC content was compared to the average EC content of the reference materials (n = 7).
Sample recovery, or yield, during this step reflects the homogeneity of each reference material, the accuracy of weighing samples into the boats, and the efficiency of the cryotrapping procedures.In general, the OC materials were more pure than the EC materials and consequently showed a better linear relationship between expected and recovered yields (Table 1).
About 5 % of each sample was lost on the graphitization line during the second cryogenic trapping of CO 2 (Fig. 2b).One sample was excluded from this analysis, because only 58 % of sample CO 2 was recovered -implying improper handling.Sample recovery during this step was influenced by CO 2 lost on the break-seal and vacuum line inner surfaces, the efficiency of the cryo-trapping, as well as differences in the calibration of the pressure gauges.

Background assessment
The amount of C ex introduced during each step of the analysis was estimated indirectly by measuring it as the sum of the modern and fossil 14 C background contamination (Table 2).We found no difference in the amount of C ex introduced during the analysis of OC compared to EC.
The amount of modern carbon introduced during the analysis was 0.8 ± 0.4 µg C (Fig. 3b, d).A comparison of the results for samples that only underwent combustion and graphitization, but not TOA and cryo-trapping, indicated that the Swiss_4S TOA with the OC/EC analyzer plus the attached vacuum line introduced a smaller amount of modern carbon (46 % of modern C ex ) than the graphitization procedure (Table 2).Our modern carbon blank estimate was based on the analysis of three separate AMS runs and the exclusion of one coal replicate, which showed an excess modern carbon amount of 1.5 µg C and was not comparable to the rest of the blanks.
The total amount of fossil C ex introduced by our method was estimated to be 0.67 ± 0.34 µg C (Fig. 3a, c), primarily (80 % of fossil C ex ) due to graphitization, with much smaller contribution from the TOA and the cryo-trapping procedure (Table 2).Note that to have all 14 C results from small reference materials fall within ±2σ of the expected value, an error of 50 % was then imposed into their background subtractions and propagated into their final uncertainties, as described in Sect.2.4.This approach accounts for any long-term variability on blank measurements on ultra-small samples, as shown in Santos et al. (2010).

Intercomparison of aerosol sample 14 C measurements
The comparison of 14 C measurements of the OC and EC content of PM 2.5 filter samples at UC Irvine and the University of Bern showed a very good agreement, with samples measured in Bern being slightly enriched in 14 C compared to UC Irvine (Fig. 4).A Pearson's major axis type II regression, minimizing both x and y residuals simultaneously with all data given equal weight, showed a strong linear relation with  , with FM modern ≈ 1 and FM fossil = 0. a slope of 1.24 ± 0.17 and a R 2 value of 0.85.It is worth noting that the EC samples are much smaller in size and consequently have a higher uncertainty level, which can explain the standard deviation of the slope.The steepness of the slope we observed is primarily driven by the smallest measured EC sample, which consequently has highest uncertainty.If this sample is excluded, the new Type II regression slope is equal to 1.07 ± 0.01 and is much closer to the 1 : 1 line with R 2 = 0.95.Further, to obtain a better comparison between the two labs, we performed a paired sample t-test, assuming unequal variances in the measurements of the two labs and two tails.The results showed that the null hypothesis ("means are equal") cannot be rejected at the 5 % significance level for OC, EC and OC and EC together.The p-value or probability of observing the given result was 0.79.
The fraction modern values shown in Fig. 4 were corrected for background contamination during the sample preparation at each lab, but no field blank corrections were applied.This inter-laboratory comparison confirmed that the Swiss_4S TOA protocol can be used successfully with our proposed modifications (cryo-trapping and graphitization procedures) to reproduce OC and EC 14 C measurements of unknown aerosol samples.

Discussion
The classification of aerosols into OC and EC components is a well-established paradigm, but the split point is not clearly defined and highly dependent on the separation procedure.Therefore, our ability to quantitatively measure and apportion OC and EC is method-specific (Currie et al., 2002).The TOA protocols used to measure OC and EC in partic-ulate matter by mass can only be mathematically corrected for charring (e.g., IMPROVE_A).However, a TOA protocol allowing for the isolation of OC and EC for accurate 14 C measurements, especially for ultra-small samples, requires a complete physical removal of interfering fractions with maximum recovery of each fraction.This is largely because the OC and EC fractions differ in their 14 C signature and charring would transfer OC with an unknown 14 C signature into the EC fraction and thus change the 14 C signature of both fractions.Our results confirmed that the Swiss_4S protocol successfully isolates OC and EC with very different 14 C contents when implemented in a new laboratory setting.We obtained high yields and reproducible and accurate 14 C measurements of both the OC and EC fractions.
We were further able to successfully quantify the amount and 14 C signature of the extraneous carbon (C ex ) introduced during each step of the analysis, and apply appropriate corrections.For the OC fraction, we chose to combine the first two steps S1 and S2 of the Swiss_4S protocol to ensure complete OC recovery.It is important to note that this modification to the Swiss_4S protocol increases the trapping time and thus the probability for trapping prematurely evolving EC.The results from the comparison between Bern and UC Irvine show that there is no detectable difference in the 14 C measurements of OC when OC is trapped at both S1 and S2 (UC Irvine) or S1 only (University of Bern).However, our estimate for the blanks of the system based on the OC data is therefore an upper limit of the blank due to the S2 extension and could potentially be reduced if OC trapping is only done during S1.
The EC reference materials (coal and rice char) used for quantifying the background were introduced into the OC/EC analyzer as solids and unlike regular EC filter samples, they did not undergo water extraction pretreatment, which could allow for water soluble gas or particles to attach to the EC surface.This could contaminate the EC measurement with small but dateable amount and means that the EC blanks estimated here are also an upper limit of the EC blank.
The modern carbon blank of the graphitization procedure measured in this study is in good agreement with a previous estimate (0.2-1 µg C) for ultra-small samples at the KC-CAMS facility (Santos et al., 2007a).The fossil carbon contamination expected at the KCCAMS is 0.1-0.5 µg C (Santos et al., 2007a), which is slightly lower than what we measured.By quantifying the background corrections necessary at each step of the procedure, we are now in a position to accurately track any C ex introduced through the procedure and correct for it when interpreting 14 C measurements of environmental samples.It is important to note that the C ex analysis described here should be carried out in parallel with each batch of ambient samples, as it is affected by the performance of the AMS system (Santos et al., 2007a).
The modern and fossil contributions of C ex for the Swiss_4S TOA and cryo-trapping corresponded to an overall C ex of 0.55 ± 0.27 µg C with a FM of 0.74 ± 0.37 at UC Irvine.This value is similar to measurements made in Bern (0.4 ± 0.2 µg C with a FM of 0.76 ± 0.17) that were obtained www.atmos-meas-tech.net/8/3729/2015/from a simplified aerosol system (Jenk et al., 2007) and used for background correction of the Bern results shown here.

Conclusions
We used an OC/EC analyzer in a new laboratory setting to quantify the precision and accuracy of OC and EC 14 C measurements with the Swiss_4S TOA protocol.A set of OC and EC 14 C reference materials, blanks and intercomparison samples, with sizes as small as 4 µg C, were analyzed to evaluate the performance of our analytical setup, quantify the uncertainties, and compare the consistency of our results for a shared set of ambient air samples from another laboratory.We were able to do the following: (1) successfully separate OC and EC fractions with the Swiss_4S protocol, (2) isolate and trap the different fractions with our newly developed setup with high yields and low carbon blanks, and (3) measure precisely the 14 C content of the separate fractions by quantifying and correcting for contamination at each set of the analysis.Our results were in good agreement with 14 C measurements from the Laboratory of Radiochemistry and Environmental Chemistry at the University of Bern.This is the first study incorporating 14 C standard materials to track and quantify background carbon introduced during aerosol OC and EC 14 C analysis.In general, the OC and EC contents of an aerosol sample are on the order of a few micrograms (the size of typical EC sample) to tens of micrograms (the size of typical OC sample).Therefore, not accounting for the extraneous carbon introduced during the analysis can significantly bias the results.This is particularly important for the 14 C measurements of the typically smaller EC fraction.For ambient aerosol studies, the correction of field blanks will also be very important.
The most recent intercomparison of 14 C analysis of carbonaceous aerosols concluded that it is not possible to agree on common procedures of OC and EC isolation among the participating labs, and that an overarching laboratory and method intercomparison quantifying both the concentration and 14 C content of OC and EC is still needed (Szidat et al., 2013).Our study presents a first step towards the development of a common protocol for OC and EC 14 C measurements.

Figure 1 .
Figure 1.Sunset/Aerosol vacuum line schematic -(1) Exhaust, coming directly from the Sunset -V1 is open whenever trapping is not in progress.(2)Cryogenic trap -allows the evolving CO 2 to accumulate during the trapping procedure, while the trap is submerged in N 2 (l).(3)Safety release Pyrex tube -in case of overpressure, this is the point of pressure release.(4) Exhaust during the trapping procedure -the tightly coiled tubing reduces the risk of air flowing inward towards of the vacuum line.(5) Calibrated volume -used to measure the volume of CO 2 trapped.(6) Pressure transducer with metal port (Silicon Microsystems SM5812, 0-5 psi) -monitors the pressure in the calibrated volume.(7) Pyrex break-seal tube -CO 2 is transferred here and sealed off with a torch.(8) Vacuum gauge -monitors the pressure at the end of the line (345 Pirani pressure vacuum sensor, MKS).(9) Turbo Pump (HiPace 80 Turbo-drag Pump, Pfeiffer Vacuum) and (10) Diaphragm pump (MVP 040-2, Pfeiffer Vacuum).The vacuum line is made of 1/8 inch OD, 0.040 inch ID stainless steal tubing and is connected to the outlet of the analyzer.Valves V1-V7 are all Swagelok SS-2P4T valves, 1/8 stainless steel.

Figure 2 .
Figure 2. recovery yield at each step of the analysis: (a) sample size measured at the calibrated volume of the Sunset vacuum line, to expected carbon yield; (b) sample size measured on the graphitization line compared to the sample size measured at the Sunset vacuum line.

Figure 3 .Figure 4 .
Figure 3. Background correction for OC standards (left panels: (a) OX-I and (b) adipic acid) and EC standards (right panels: (c) rice char and (d) coal).Black symbols represent the measured 14 C content of each standard prior to background correction.The white symbols show the 14 C content for each standard after background correction was applied.Horizontal lines show the consensus 14 C content, expressed as fraction modern (f M ) for each standard.The top panels show the modern reference materials used to quantify the fossil contamination, while the bottom panels show the 14 C-free blanks used to quantify the modern contamination.Note that the gray circle in panel (d) represents a coal sample after background correction, which was excluded from the calculation of the modern blank as an outlier.

Table 1 .
Description of reference materials used for quantifying background contamination and method accuracy of OC and EC trapping and 14 C measurements.Parameters of Fig. 2a regard the measured volume vs. expected yield and the coefficient of determination of the linear regression for the individual reference materials as explained in Sect.3.1.= not applicable, a SRM4990B (National Institute of Standards and Technology, MD, USA), b Black carbon reference material (University of Zürich, Switzerland), c Beverly et al. (2010), d The EC content was estimated by averaging repeated TOA experiments (n = 7 for char, n = 7 for coal), e Repeated measurements of the 14 C of the bulk material were performed at the KCCAMS.

Table 2 .
Summary of the amounts and 14 C signatures of extraneous carbon introduced during the isolation and graphitization of OC and EC standard materials at UC Irvine.
* The FM equivalent is calculated by mass balance as: FM ex = FM modern × m modern + FM fossil ×m fossil m modern + m fossil

Table A1 .
Radiocarbon measurements of reference materials with a known fraction modern value (standards).The table includes the results both after and before background correction.

Table A2 .
Radiocarbon measurements of 14 C free materials (blanks).Following conventional AMS data reporting, blanks are not corrected for background contamination.denotes standards and blanks, which were used only as a radiocarbon reference material during different AMS runs and are not part of the Swiss_4S yield analysis.b was excluded from the blank assessment as an outlier. a