Intercomparison of Thermal–Optical Methods for the Determination

ARTICLE pubs.acs.org/est

Intercomparison of Thermal
Optical Methods for the Determination of Organic and Elemental Carbon: Influences of Aerosol Composition and Implications Yuan Cheng,*,† Feng-kui Duan,† Ke-bin He,† Mei Zheng,‡ Zhen-yu Du,† Yong-liang Ma,† and Ji-hua Tan§ †

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China ‡ College of Environmental Sciences and Engineering, Peking University, Beijing, China § Key Laboratory of Computational Geodynamics, College of Earth Science, Graduate University of Chinese Academy of Sciences, Beijing, China

bS Supporting Information ABSTRACT:

An intercomparison of organic carbon (OC) and elemental carbon (EC) measurements was conducted based on ambient aerosol samples collected during four seasons in Beijing, China. Dependence of OC and EC values on the temperature protocol and the charring correction method is presented and influences of aerosol composition are investigated. EC was found to decrease with the peak inert mode temperature (Tpeak) such that EC determined by the IMPROVE (the Interagency Monitoring of Protected Visual Environments)-A protocol (Tpeak was 580 °C) was 2.85 ( 1.31 and 3.83 ( 2.58 times that measured by an alternative protocol with a Tpeak of 850 °C when using the transmittance and reflectance correction, respectively. It was also found that reflectance correction tends to classify more carbon as EC compared with transmittance; results from the IMPROVE-A protocol showed that the ratio of EC defined by reflectance correction (ECR) to that based on transmittance (ECT) averaged 1.50 ( 0.42. Moreover, it was demonstrated that emissions from biomass burning would increase the discrepancy between EC values determined by different temperature protocols. On the other hand, the discrepancy between ECR and ECT was strongly associated with secondary organic aerosol (SOA) which was shown to be an important source of the organics that pyrolyze during the inert mode of thermal
optical analysis.

1. INTRODUCTION Carbonaceous aerosol, an aggregate of thousands of poorly characterized species with a wide range of chemical, thermal, and optical properties, has been the focus of extensive studies in the past decade due to its complex effects on human health and the environment. It is commonly classified as organic carbon (OC) and elemental carbon (EC) by thermal
optical (or thermal) methods in chemical speciation monitoring. OC and EC have long been routinely measured by both individual investigators and national networks such as the Interagency Monitoring of Protected Visual Environments (IMPROVE) network, the Speciation Trends Network (STN) in the Untied States,1 and the Canadian National Air Pollution Surveillance (NAPS) network in Canada.2 A variety of thermal
optical methods have been developed; the conventional ones include the IMPROVE method,3 the NIOSH (National Institute for Occupational Safety and r 2011 American Chemical Society

Health) method,4 and the methods developed based on NIOSH such as STN,5 ACE-Asia (Aerosol Characterization ExperimentsAsia,6 and MSC (Meteorological Service of Canada).7 Importantly, measured EC concentrations have been shown to differ significantly among the various methods,6,8
15 which introduces substantial difficulties and complexities in direct comparison and integration of OC and EC measurement across studies and regions, and thus limits the ability to understand the origin, distribution, and role of carbonaceous aerosols. The modern thermal
optical methods differ mainly with respect to (i) temperature protocol, including the temperature Received: July 31, 2011 Accepted: November 1, 2011 Revised: October 18, 2011 Published: November 01, 2011 10117

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Environmental Science & Technology plateaus and residence time at each plateau, and (ii) charring correction by light reflectance or transmittance. Several intermethod and interlaboratory studies have been conducted and the results indicate the following: (i) The temperature of the last step in the inert mode, which is usually called the peak inter mode temperature, has substantial influence on the split of OC and EC such that measured EC concentration typically decreases with the peak inter mode temperature. Results from North America and Europe showed that EC measured by the IMPROVE temperature protocol was about 1.2
1.5 times of that measured by NIOSH when both using the transmittance correction.6,12,13,16 (ii) The reflectance correction tends to classify more carbon evolved in the oxidizing atmosphere as EC compared with the transmittance correction, which has been attributed to the charring that takes place within the filter.9 The ratio of EC measured by the reflectance correction to that determined by transmittance was estimated to be about 1.3
1.8 for ambient samples when using the IMPROVE temperature protocol.8,9,16,17 (iii) The discrepancy between EC values defined by different methods would be even larger, if one method implemented the transmittance correction with a high peak inert mode temperature (e.g., NIOSH and NIOSH-derived method) whereas the other used the reflectance correction with a low peak inert mode temperature (e.g., IMPROVE method). For example, EC measured by the IMPROVE and NIOSH method might differ by a factor of up to about 5.9,18 However, the factors that contribute to the discrepancy described above (e.g., origin of the OC that chars during the inert mode of the analysis) remain unclear. This paper reports the results from an intercomparison study conducted in Beijing, China to investigate the influences of aerosol composition on the discrepancy between EC values determined by different thermal
optical methods.

2. EXPERIMENTAL SECTION 2.1. Field Sampling. Ambient PM2.5 samples were collected at the Tsinghua University campus in Beijing by a five-channel Spiral Aerosol Speciation Sampler (SASS, MetOne Inc.). Tsinghua University (40°190 N, 116°190 E) is located in the urban area of Beijing, about 12 km northwest of the city center. There are no major industrial sources around the campus. A total of 161 daily samples were collected using denuded quartz filters during four seasons: 29 were collected from January 9 to February 12, 2009; 45 were collected from March 1 to April 14, 2010; 37 were collected from June 25 to July 31, 2010; and 50 were collected from September 27 to November 15, 2010. These samples will be referred to as winter (WI), spring (SP), summer (SU), and autumn (AU) samples, respectively, in this paper. The activated carbon denuder (provided by MetOne) is 20 mm long and 38 mm in diameter with about 1000 1 mm  1 mm channels. The denuder was designed such that the residence time of aerosol in the denuder is 0.18 s at the operating flow rate (6.7 L/min), less than 0.2 s which was suggested to minimize offgassing of particulate organic carbon.19 A new denuder was used for each season. Evaluation of the denuder, including its efficiency for removing the positive artifact, was reported elsewhere.17 The quartz filters (2500 QAT-UP) were from Pall Corp. (Ann Arbor, MI) and were baked at 550 °C in air for 24 h before use. All of the quartz filters used throughout each season were taken from the same lot.20 A total of 61 filters were kept as blank. The OC concentration of the blank filters averaged 0.37 ( 0.11 μgC/cm2 and no EC was detected.

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Table 1. Temperature Protocols Used in the Present Study temperature (°C) step

gas

IMPROVE-A

IMCA-850a

IMCA-850-simplified

OC1

He

140

140

140

OC2

He

280

280

280

OC3

He

480

480

480

OC4

He

580

580

580

OC5

He

n.a.

650

850

OC6

He

n.a.

700

n.a.

OC7

He

n.a.

750

n.a.

OC8 OC9

He He

n.a. n.a

800 850

n.a. n.a.

EC1

He/O2b

580

850
580c

850
580c

EC2

He/O2

b

740

740

740

EC3

He/O2b

840

840

840

IMCA refers to the “Improved Measurement of Carbonaceous Aerosol in Beijing” study. b Actual gas composition: 98%He + 2%O2. c The temperature is reduced from 850 to 580 °C. a

2.2. Thermal
Optical Analysis. The denuded quartz filters were analyzed for OC and EC using a DRI model 2001 carbon analyzer (Atmoslytic Inc., Calabasas, CA). The analyzer allows more accurate and precise control and monitoring of the sample temperature compared with its previous version, giving rise to the IMPROVE-A temperature protocol.21,22 The temperature protocol used in the present study is shown in Table 1. The IMPROVE-A temperature protocol was the base case. The peak inert mode temperature was increased from the 580 °C used in the IMPROVE-A protocol to 850 °C in two ways, yielding the two alternative temperature protocols referred to as IMCA (temperature was increased from 580 to 850 °C through five steps in the He-mode) and IMCA-simplified (temperature was increased from 580 to 850 °C in one step during the Hemode), respectively. Charring corrections by both transmittance and reflectance, which are monitored simultaneously at 632 nm, were implemented. The transmittance-defined EC is measured as the carbon that is evolved after the filter transmittance returns to its initial value in the oxidizing atmosphere, whereas the reflectance-defined EC is the carbon measured after the filter reflectance returns to its initial value. All of the 161 denuded quartz filters were simultaneously analyzed by the IMPROVE-A and the IMCA-simplified protocol while 10 summer samples were also analyzed by the IMCA protocol. Linear regression results demonstrated that the IMPROVE-A and IMCA-simplified protocols yielded the same total carbon (TC) for the 161 Beijing samples (slope = 0.99 ( 0.00, intercept = 0.05 ( 0.10 μgC/cm2, R2 = 0.997); and the optical attenuation (ATN) measured at 632 nm was also shown to be equivalent (slope = 0.99 ( 0.01, intercept = 0.01 ( 0.01, R2 = 0.987). ATN is primarily due to the deposition of light-absorbing carbon,23 and is calculated as



Tf inal ATN ¼ ln Tinitial

 ð1Þ

where Tinitial and Tfinal are the transmittance signal before and after the thermal
optical analysis, respectively. These comparisons indicated good carbon analysis precision and optical measurement precision (both were about 3%). 10118

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Figure 1. Typical thermograms of a medium-loaded sample (summer #10, collected from July 4
5, TC = 14.5 μgC/cm2) determined by IMPROVE-A (a), IMCA-simplified (b), and IMCA (c) temperature protocol. Also shown is the thermogram of a heavily loaded autumn sample (#12, collected from October 8
9, TC = 54.1 μgC/cm2) determined by IMCA-simplified temperature protocol (d). The pink and blue dotted lines indicate the baseline of reflectance and transmittance signal, respectively, whereas the vertical dotted line indicates the introduction of O2. See Supporting Information SI-1 for examples from the other seasons.

2.3. Water-Soluble K+ Analysis. The denuded quartz filters collected during summer were also analyzed for water-soluble K+ to estimate the influence of biomass burning using ion chromatography (Dionex-600) as described by Zhao et al.24

3. RESULTS 3.1. Comparison of Thermograms Determined by Different Temperature Protocols. A typical thermogram obtained by

the IMPROVE-A, IMCA-simplified, and IMCA protocol is shown in Figure 1a
c, respectively. When using the IMPROVE-A protocol, the filter transmittance reached its minimum value partway through the 580 °C temperature plateau in the He-mode and did not increase until the introduction of O2 (Figure 1a), indicating that the premature evolution of light-absorbing carbon (including native EC and pyrolyzed OC) was negligible.17 However, if the inert mode temperature was further increased to 850 °C after the 580 °C temperature plateau (either by five steps or one step), substantial carbon was found to evolve off the filter during this process with a sharp increase of the filter transmittance and reflectance (Figure 1b and c), indicating significant loss of lightabsorbing carbon.17 Similar increase of filter transmittance at the end of the inert mode could also be detected in the published thermograms of samples from other regions, when analyzed by the NIOSH or similar protocols (the peak inert mode temperature was 850 °C or slightly higher).5,8,9,11,13,14,25 Therefore, the IMPROVE-A protocol has the advantage that it can avoid the loss of light-absorbing carbon in the inert mode as compared with NIOSH or similar protocols. Moreover, when using the IMCA-simplified protocol, a wide peak could be observed due to the evolution of carbon between 580 and 850 °C in the inert mode (Figure 1b); whereas the wide peak was replaced by five small peaks that can not be well-separated

when using the IMCA protocol (Figure 1c), indicating a continuum of carbonaceous components in thermal properties. It should be pointed out that increased oxidation or catalysis at high temperatures by metals (either oxides or salts) may be also responsible for the premature evolution of native EC in the inert mode.9 Though metals were not quantified in this study, most of the Beijing samples retain a reddish (or brown) tinge rather than the white color of a clean filter after completion of the thermal
optical analysis, indicating the presence of metal oxides. 3.2. Dependence of OC and EC Values on the Temperature Protocol. Figure 2a
d illustrate the influences of peak inert mode temperature on the thermal
optical split of OC and EC. In the following discussion, OC and EC measured by the IMPROVE-A and IMCA-simplified protocol are referred to as OC580 and EC580, OC850 and EC850, respectively. With respect to results from transmittance correction, the OC580 to OC850 ratio averaged 0.87 ( 0.04 for the 161 Beijing samples whereas the EC580 to EC850 ratio averaged 2.85 ( 1.31, indicating that EC results are more significantly influenced by the split of OC and EC due to their relatively low concentrations. OC580 and OC850 correlated well (R2 = 0.992) with a slope of 0.89 ( 0.00 (intercept was set as zero), and the OC580 to OC850 ratio did not exhibit significant seasonal variation. On the other hand, linear regression of EC580 on EC850 showed a slope of 2.71 ( 0.08 (intercept was set as zero) and a much weaker correlation (R2 = 0.569). Moreover, the EC580 to EC850 ratio was found to be a little lower during autumn (2.39 ( 0.85) compared with winter (3.39 ( 1.25), spring (2.86 ( 1.22), and summer (3.04 ( 1.76). When using the reflectance correction, the OC580 to OC850 ratio and the EC580 to EC850 ratios were 0.79 ( 0.08 and 3.83 ( 2.58, respectively, indicating that the reflectance correction seems to be more sensitive to the increase of peak inert mode temperature. Influences of the peak inert mode temperature could be attributed 10119

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Environmental Science & Technology

Figure 2. OC580 to OC850 ratio and EC580 to EC850 ratio determined based on the transmittance (TOT, a and b) and reflectance (TOR, c and d) charring correction. Also shown are the OCR to OCT ratio and the ECR to ECT ratio (e and f) based on the IMPROVE-A temperature protocol. See Supporting Information SI-2 for the detailed statistical results.

to the systemic artifact of the thermal
optical method and the early split which are discussed in detail below. (i). Systemic Artifact. It has been demonstrated that pyrolyzed OC is coevolved with native EC and is more light-absorbing,9,11,25,26 which would result in the underestimation of EC by the operationally defined value. This underestimation was called the systemic artifact of the thermal
optical method by Cheng et al.17 Importantly, Subramanian et al.11 showed that the light-absorbing efficiency of pyrolyzed OC increased with the peak inert mode temperature. Thus, the systemic artifact, or the underestimation of EC by the operationally defined value, would be more significant when using the IMCA-simplified protocol as compared with the IMPROVE-A protocol, which is a likely cause of the observed phenomenon that the EC value decreased with the peak inert mode temperature (Figure 2b and d). (ii). Early Split. When analyzed by the IMCA-simplified protocol, the filter transmittance or reflectance exceeded its initial value at the end of the He-mode for more than 90% of the 161 Beijing samples, which was referred to as early split.9 When early split occurs, considerable carbon was found to evolve off the filter after the transmittance or reflectance returned to its initial value but before the introduction of O2 (Figure 1b and c). This fraction of carbon should be considered as EC but was operationally classified as OC, resulting in underestimation of EC. Therefore, the frequent early split is another likely cause of the observed

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phenomenon that EC850 was much lower than EC580 (Figure 2b and d). Moreover, it is interesting to notice that the OC580 to OC850 ratio based on reflectance correction was close to 1.0 for five winter samples and one autumn sample (Figure 2c). All of the six samples were heavily loaded (TC > 50 μgC/cm2). Figure 1d showed a representative thermogram of these samples determined by the IMCA-simplified protocol. Compared with common thermograms such as Figure 1b, noticeable features of Figure 1d included the following: (i) The reflectance returned to its initial value in the He-mode, suggesting that early split occurs for the filter reflectance and all of the carbon evolved in the He/ O2 mode (He/O2 carbon) would be classified as EC when using the reflectance correction. (ii) The filter reflectance remained roughly constant after returning to its initial value until the introduction of O2, indicating the premature evolution of native EC should be negligible. (iii) The transmittance was still below its initial value at the end of the He-mode, suggesting that the He/O2 carbon included considerable pyrolyzed OC besides native EC. With respect to the reflectance correction, this type of early split was quite distinctive such that EC tends to be overestimated and thus OC tends to be underestimated. As a result, the distinctive early split illustrated by Figure 1d was responsible for the six reflectance-defined OC580 to OC850 ratios which were approximately 1.0 (Figure 2c). In addition to the peak inert mode temperature, residence time is another important parameter of temperature protocol. With respect to the 10 summer samples analyzed by all of the three protocols shown in Table 1, carbon evolved between 580 and 850 °C in the inert mode was increased by 7% when using the IMCA protocol as compared with the IMCA-simplified protocol, indicating that prolonging the residence time can considerably enhance carbon evolution. EC measured by the IMCA-simplified protocol (averaging 0.88 μgC/m3) was about 1.8 times the value of that determined by IMCA (averaging 0.50 μgC/m3), and early split occurred for all of the 10 samples when using either protocol (Figure 1b and c). The significantly lower EC values obtained by the IMCA protocol are presumably due to the much longer residence time in the inert mode which enhances the premature evolution of native EC. 3.3. Dependence of OC and EC Values on the Charring Correction Method. In the following discussion, OC and EC defined by the transmittance and reflectance were referred to as OCT and ECT, OCR and ECR, respectively. When using the IMCA-simplified protocol, early split occurred frequently for both transmittance and reflectance, leading to equivalence of EC between the transmittance and reflectance correction. As a result, only results from the IMPROVE-A protocol were presented (Figure 2e and f). The OCR to OCT and ECR to ECT ratio averaged 0.90 ( 0.08 and 1.50 ( 0.42, respectively. OCR and OCT correlated well (R2 = 0.991) with a slope of 0.87 ( 0.00 (intercept was set as zero), whereas linear regression of ECR on ECT showed a slope of 1.53 ( 0.03 (intercept was set as zero) and a weaker correlation (R2 = 0.774). Moreover, the largest and smallest discrepancy between ECR and ECT was found in summer and autumn when the ECR to ECT ratio averaged 1.75 ( 0.28 and 1.29 ( 0.47, respectively. The influences of charring correction method on the split of OC and EC have been attributed to the charring that takes place within the filter.9 Briefly, the drop of filter transmittance in the He-mode is influenced by the char formed throughout the filter thickness, whereas the drop of reflectance is dominated by charring of the near-surface organics. 10120

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+

Figure 3. Daily variation of K , the EC580 to EC850 ratio determined based on the transmittance charring correction (TOT), and the ECR to ECT ratio determined based on the IMPROVE-A temperature protocol during the summer of 2010. The EC580 to EC850 ratio based on the reflectance correction (TOR) correlated well with that defined by transmittance as shown by the inner panel (linear regression results are shown with k as slope and intercept is set as zero), indicating the daily pattern of the EC580 to EC850 ratio determined by the reflectance also coincided well with that of K+.

As a result, the filter reflectance typically returns to its initial value before the transmittance in the He/O2 mode, indicating more carbon would be classified as EC by the reflectance correction.

4. DISCUSSION 4.1. Influences of Biomass Burning. Impacts of biomass burning on the air quality of Beijing have been investigated since early 2000s. Early studies were based on water-soluble potassium (K+)27,28 whereas recent ones typically relied on levoglucosan.29,30 High concentration of levoglucosan (above 100 ng/m3) was observed throughout the year in Beijing, and was attributed to a background contribution of biomass burning resulting from the agricultural waste and firewood consumed as domestic fuel in the rural areas of Beijing as well as in neighboring provinces.29 Moreover, the influence of biomass burning was found to be much more significant during the harvest season (e.g., late June when wheat is harvested) due to the open burning of agricultural waste in nearby provinces.27,29,31 Though the source of K+ was more complex compared with levoglucosan,32 increased concentration of K+ during the harvest season in Beijing has been demonstrated to be associated with biomass opening burning.27,31 For example, results from transmission electron microscopy with energy-dispersive X-ray spectrometry (TEM-EDX) and MODIS fire counts showed that individual salt particles rich in K were more abundant in Beijing aerosol when intensive open burning of agricultural waste was identified in neighboring provinces during June.31 As a result, it is reliable to attribute the high K+ concentrations in Figure 3, which were observed during June 25 and June 30, 2010 in this study, to the influence of biomass open burning. This assumption was also strongly supported by results from satellite observation of fire counts, because open fires identified in surrounding provinces of Beijing were much more intensive during late June as compared with early July (see Supporting Information SI-3 for details). The daily pattern of the EC580 to EC850 ratio, determined by either transmittance or reflectance, was found to coincide well with that of K+ (Figure 3). The concentration of K+ peaked on June 28 (1.6 μg/m3), and the EC580 to EC850 ratio also reached its maximum value (11.0 and

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Figure 4. Daily variation of the ECR to ECT ratio during the spring of 2010. The OCT to ECT ratio and the OCR to ECR ratio are also shown for comparison. All of the ratios are determined based on the IMPROVE-A temperature protocol. The trend was similar in the other seasons as shown in Supporting Information SI-4.

17.8 for transmittance and reflectance correction, respectively) on the same day. This finding was consistent with results from source samples such that EC concentration of the wood smoke sample was much more significantly influenced by the temperature protocol as compared with carbon black and coal fly ash.6 On the other hand, no correlation was found between K+ and the ECR to ECT ratio determined by the IMPROVE-A temperature protocol. These results (Figure 3) indicated that emissions from biomass burning would increase the discrepancy between EC values determined by different temperature protocols whereas it has little influence on the discrepancy between ECR and ECT. Moreover, both the ambient concentration of levoglucosan29 and emission inventory33 demonstrated that biomass burning emission is much higher in China compared with North America and Europe, which should be responsible for the strong dependence of EC results on temperature protocol for Beijing samples. 4.2. Influences of Secondary Organic Aerosol. OC to EC ratios include important information about the extent of secondary organic aerosol (SOA) production. Ambient OC to EC ratios greater than those characteristic of the primary emissions for a given area have long been used as an indicator for the SOA formation, giving rise to the EC-tracer method.34,35 As shown in Figure 4, the OC to EC ratios calculated by results from the transmittance correction (OCT/ECT, averaging 3.96 ( 1.20) were higher than those based on the reflectance correction (OCR/ECR, averaging 2.54 ( 0.41) and showed more significant variation. Importantly, the daily pattern of the ECR to ECT ratio was found to coincide well with that of the OCT to ECT ratio, whereas no correlation between the ECR to ECT ratio and the OCR to ECR ratio was seen. Recent work by Cheng et al.36 showed that SOA estimated by the EC-tracer method based on results from the reflectance correction exhibited no correlation with water-soluble organic carbon (WSOC) whereas predicted SOA based on transmittance correlated well with WSOC. It seems unlikely that SOA is totally non water-soluble,37 suggesting that the OCT to ECT ratio was more strongly associated with the extent of SOA formation compared with the OCR to ECR ratio. As a result, Figure 4 indicates that the discrepancy between EC values determined by reflectance and transmittance correction was mainly related to SOA. Figure 5a presents the dependence of the ECR to ECT ratio on the percentage of SOA (predicted by the EC-tracer method based on results from the transmittance correction; see Supporting Information SI-6 for detailed procedures) in organic aerosol 10121

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method in the future, which is especially meaningful for the investigators operating the Sunset analyzer (including its semicontinuous version).

’ ASSOCIATED CONTENT

bS

Figure 5. Dependence of the ECR to ECT ratio on the SOA to OA ratio (a), and the dependence of KPOC on the SOA concentration (b) during the spring of 2010 (all of the values are determined based on the IMPROVE-A temperature protocol). The horizontal dotted line in (a) indicates an ECR/ECT value of 1.0. Linear regression results are shown with K as the slope and b as the intercept in (b) (an outlier, marked by the dotted cycle, is not included in the regression). The trend was similar in the other seasons as shown in Supporting Information SI-5.

(OA). When the contribution of SOA was low (e.g., less than 30%), the discrepancy between ECR and ECT was not significant such that the ECR to ECT ratio was approximately 1.0; however, the ECR to ECT ratio would increase linearly with the SOA to OA ratio once the contribution of SOA exceeded a threshold value (e.g., about 30% for spring samples shown in Figure 5a), indicating that the ECR to ECT ratio is mainly dependent on the contribution of SOA. As mentioned in Section 3.2, the discrepancy between ECR and ECT was caused by the charring that takes place within the filter.9 Cheng et al.17 suggested that the amount of pyrolyzed OC could be estimated by the attenuation caused by its formation, KPOC, which is calculated as   Tinitial K POC ¼ ln ð2Þ Tmin where Tmin and Tinitial are the minimum and initial value of the filter transmittance, respectively. As shown in Figure 5b, KPOC correlated well with the SOA concentration, indicating SOA is an important source of the organics that pyrolyze during the inert mode of the analysis. Moreover, the EC580 to EC850 ratio, determined by either transmittance or reflectance, did not exhibit apparent dependence on the OCT to ECT ratio (or the SOA to OA ratio). This indicates that the contribution of SOA should have little influence on the discrepancy between EC values determined by different temperature protocols. 4.3. Implications. A better understanding of carbonaceous aerosol requires the standardized protocols for the analysis of OC and EC in both field measurements and the determination of their emission factors. U.S. EPA has recently decided to convert the carbon analysis method of the STN network (STN method with transmittance correction, similar to NIOSH) to the IMPROVE-A method (based on reflectance correction) used by the IMPROVE network, which would have a profound influence on the development of thermal
optical method. The results of this study suggested the importance of evaluating the temperature protocol and charring correction method separately. Studies focusing on charring correction method are far fewer than those concerning temperature protocol, partly due to that the Sunset carbon analyzer could only implement the transmittance correction. More attention should be paid to the charring correction

Supporting Information. SI-1: Representative thermograms of samples from the other seasons in addition to those shown in Figure 1. SI-2: Detailed statistical results from the comparison of OC and EC values determined by different methods as shown by Figure 2. SI-3: More intensive biomass open burning in late June as compared with early July: evidence from satellite observation. SI-4: Comparison of the ECR to ECT ratio and the OC to EC ratio (including both OCT/ECT and OCR/ECR) based on samples collected in the other seasons in addition to the results shown in Figure 4. SI-5: Dependence of ECR/ECT on SOA/OA and dependence of KPOC on the SOA concentration during the other seasons in addition to the results shown in Figure 5. SI-6: Procedures of estimating SOA by the EC-tracer method. This information is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: 86-10-62781889; fax: 86-10-62771101; e-mail: duanfk@ mail.tsinghua.edu.cn and [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21107061), the National 973 Program of China (2010CB951803) and the Foundation for the Author of National Excellent Doctoral Dissertation of China (2007B57). We acknowledge visiting scholar Charles N. Freed for revising the paper. We also thank Zhen Liu and Xiao-Lu Zhang in Georgia Institute of Technology for assistances in processing hot spot results, and Dr. Lai-guo Chen in South China Institute of Environmental Science, MEP for assistance in the OC and EC analysis. ’ REFERENCES (1) Chow, J. C.; Watson, J. G.; Chen, L. W. A.; Rice, J.; Frank, N. H. Quantification of PM2.5 organic carbon sampling artifacts in US networks. Atmos. Chem. Phys. 2010, 10, 5223–5239. (2) Dabek-Zlotorzynska, E.; Dann, T. F.; Martinelango, P. K.; Celo, V.; Brook, J. R.; Mathieu, D.; Ding, L. Y.; Austin, C. C. Canadian National Air Pollution Surveillance (NAPS) PM2.5 speciation program: Methodology and PM2.5 chemical composition for the years 2003
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