Measurement of semivolatile carbonaceous aerosols and its implications: A review

Environment International 35 (2009) 674–681

Contents lists available at ScienceDirect

Environment International j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e n v i n t

Review article

Measurement of semivolatile carbonaceous aerosols and its implications: A review Y. Cheng a, K.B. He a,⁎, F.K. Duan a, M. Zheng b, Y.L. Ma a, J.H. Tan a a b

Department of Environmental Science and Engineering, Tsinghua University, People's Republic China School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, United States

a r t i c l e

i n f o

Article history: Received 25 September 2008 Accepted 27 November 2008 Available online 7 February 2009 Keywords: Semivolatile organic carbon Sampling artifact Denuder Carbonaceous aerosol

a b s t r a c t Measurement of carbonaceous aerosols is complicated by positive and negative artifacts. An organic denuder with high efficiency for removing gaseous organics is an effective approach to eliminate the positive artifact, and it is a precondition for the accurate determination of SVOC by an adsorbent backup filter. Evaluations of different configurations of the organic denuder, and SVOC determined by different denuder-based samplers, both integrated and semi-continuous, are reviewed. A new equation for determination of the denuder efficiency is estimated, considering the efficiency of removing both the gaseous organics that could be adsorbed by the quartz and the gaseous passing through the quartz that could be subsequently adsorbed by the backup adsorbent filter. The origin of OC on the backup quartz filter, behind either quartz or Teflon filter, is quantitatively evaluated by the denuder-based method based on the data published. The backup-OC is shown to be dominated by either gaseous organics passing through the front filter or the evaporated particulate organic carbon depending on the sampling environment. © 2008 Published by Elsevier Ltd.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . Evaluations of organic denuder . . . . . . . . . . . . . . 2.1. Particle loss . . . . . . . . . . . . . . . . . . . . 2.2. Evaporation of particulate organic carbon . . . . . . 2.3. Denuder breakthrough and efficiency . . . . . . . . 2.3.1. Influence of backup adsorbent filter. . . . . 2.3.2. Efficiency of activated carbon-based denuder 2.3.3. Efficiency of XAD-based denuder . . . . . . 3. Integrated measurement of SVOC . . . . . . . . . . . . . 3.1. Results from CIF denuder-based sampler. . . . . . . 3.2. Results from ACF denuder-based sampler . . . . . . 3.3. Results from ACM denuder-based sampler. . . . . . 3.4. Results from ACI denuder-based sampler . . . . . . 3.5. Results from XAD denuder-based sampler . . . . . . 4. Semi-continuous measurement of SVOC . . . . . . . . . . 5. Implications for measurement of carbonaceous aerosols. . . 5.1. Origin of QBQ-OC and QBT-OC . . . . . . . . . . . 5.1.1. Positive artifact-contributed OC dominant . . 5.1.2. Negative artifact-contributed OC dominant . 5.2. High-volume sampling . . . . . . . . . . . . . . . 5.3. Rethinking denuder efficiency . . . . . . . . . . . 6. Summary . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . .

⁎ Corresponding author. Tel.: +86 10 6279 4331; fax: +86 10 6278 5687. E-mail address: [email protected] (K.B. He). 0160-4120/$ – see front matter © 2008 Published by Elsevier Ltd. doi:10.1016/j.envint.2008.11.007

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Y. Cheng et al. / Environment International 35 (2009) 674–681

1. Introduction In recent years, continuing attention has been paid to the accurate measurement of ambient fine particulate matter, both mass concentration and chemical composition. Differential-TEOM (Patashnick et al., 2001), which was a combination of SES–TEOM (TEOM with Sample Equilibration System, Meyer et al., 2000) and ESP (Electrostatic Precipitator, Yi et al., 2004) technologies, and FDMS–TEOM (TEOM with Filter Dynamic Measurement System, commercial version of differential-TEOM) were designed for real-time measurement of the total PM2.5 mass, including the components lost by the traditional TEOM. However, the speciation monitoring of PM2.5, especially the measurement of the carbonaceous component, is still complicated by the sampling artifacts discussed below. Quartz filters are typically used for collection and subsequent thermal or thermal–optical analysis of carbonaceous aerosols. However, the quartz filter has a large surface area upon which adsorption of gaseous organics could occur, leading to overestimation of particulate organic carbon (POC). On the other hand, volatilization of particulate organic carbon caused by the pressure drop across the filter would result in underestimation of the POC. These errors are known as positive and negative artifact, respectively. The organic denuder removes gaseous organics by diffusion to an adsorbent surface, such as activated carbon (activated carbon-based denuder) and polystyrene-divinylbenzene resin (XAD-based denuder). As a result, this denuder could be placed upstream of quartz filter to eliminate the positive artifact. But the use of organic denuder may induce a larger negative artifact, since the removal of gaseous organics disturbs the gas-particle equilibrium and enhances evaporation of POC collected on the quartz filter. As a result, a highly adsorbent backup filter, such as activated carbon impregnated cellulose filter (CIF, Eatough et al., 2001), activated carbon impregnated glass fiber filter (CIG, Eatough et al., 2001), or XAD impregnated quartz filter (XAD-Q, Fan et al., 2003; Swartz et al., 2003) should be included. PUF (Lane et al., 2000; Peters et al., 2000; Eiguren-Fernandez et al., 2003) and XAD resin (Gundel et al., 1995a,b; Temime-Roussel et al., 2004a) could also be used to collect the POC evaporated. However, these resins are typically used for the determination of individual organics (such as PAH), and are not suitable for the OC/EC analysis. Then, the “true value” of POC is estimated as the sum of OC on the front quartz filter (nonvolatile organic carbon, NVOC) and OC on the adsorbent backup filter (semivolatile organic carbon, SVOC). It should be pointed out that there is no generally accepted definition for SVOC. Different vapor-pressure-based definitions were summarized by Turpin et al. (2000), but these definitions were inconsistent and as a result not suitable for the routine monitoring. In this paper, SVOC is defined as the OC collected by the backup filter in an organic denuder-based sampler. SVOC may be formed by the daytime photochemical reactions between oxidants, most of which are OH radicals caused by the photodissociation of ozone, and reactive gaseous organics or primary particulate organic carbon (Seinfeld and Pandis, 1998). And additional SVOC may be formed by the reaction between oxidants and POC collected by the front filter. As shown in an urban area of Pittsburgh, the diurnal pattern of SVOC concentrations coincided well with that of ozone and peaked at mid-day (Modey et al., 2004). The secondary nature of SVOC was also confirmed by the source apportionment results reported for the Wasatch Front, Utah area (Long et al., 2002; Grover et al., 2006a) and Pittsburgh (Eatough et al., 2006a, 2007; Anderson et al. 2006), where the secondary sources including SVOC are associated with ozone and contain little EC, transition elements, or crustal elements. The concentration of SVOC is also temperature dependent. As shown in Toronto, the SVOC concentration was much higher during the night; the elevated ozone concentration episodes coincided with higher nighttime SVOC concentrations; and there was no correlation between OC and EC during the sampling period (Fan et al., 2004a). Then it was concluded that SVOC was formed and then remained in

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the gas phase during the day, and condensed onto pre-existing particles during the night due to the decreased temperatures (Fan et al., 2004a). Formation of SVOC has a potential influence on climate and the hydrological cycle (Ramanathan et al., 2001), mainly through its role in cloud condensation nuclei (CCN), because photochemical reactions would result in water soluble compounds (such as organic acids) in the SVOC due to the polar functional groups formed by oxidation (Maria et al., 2004). SVOC is of interest also because of its possible effects on human health (U.S. EPA 2004), and visibility (Long et al., 2005), besides its secondary nature (Eatough et al., 2003a) and potential influence on climate. 2. Evaluations of organic denuder Removal of gaseous organics by an organic denuder is a precondition for the accurate measurement of SVOC. Many types of organic denuder, both activated carbon-based and XAD-based, have been developed. Their configurations are summarized in Table 1. However, the use of an organic denuder may cause additional artifacts that complicate the measurement of carbonaceous aerosols: (1) particle loss, caused by diffusion to the walls of denuder, (2) evaporation of particulate organic carbon during transportation through the denuder, due to the depletion of gaseous organics, and (3) breakthrough of gaseous organics. 2.1. Particle loss Particle loss in the denuder could be evaluated by comparing concentrations of nonvolatile components, such as EC, measured by the quartz filter downstream of an organic denuder (DQ-EC) and a bare quartz filter (BQ-EC). DQ-EC and BQ-EC were compared in Fig. 1 for the different types of organic denuder described in Table 1. No significant loss of EC is seen in any type of denuder. Comparable concentrations of DQ-sulfate (downstream of CIF denuder and XADannular denuder) and BQ-sulfate are reported by Solomon et al. (2003), also suggesting that the loss of particles is minimal. Temime-Roussel et al. (2004b) used another approach to evaluate the particle loss, in which mass and number concentration of particles emitted by a diesel vehicle were measured alternatively downstream and upstream of the denuder. It is demonstrated that the denuder has no significant effect on the transmission efficiency of particles. 2.2. Evaporation of particulate organic carbon The evaporation of particle phase organic carbon during transit in the denuder is difficult to quantitatively evaluate. To avoid the off-gassing of particulate organic carbon, some organic denuders, such as the ACM denuder, the ACI denuder and the XAD-honeycomb denuder were Table 1 Summary of organic denuders and their configurations Denuder

Configuration 17 strips of CIF filter (Schleicher and Schuell, Germany), 4.5 × 58 cm, separated at the long edges by 2 mm Two tubes from copper net coaxially placed in a stainless tube, the space within of the inner cupreous net and the space between outer cupreous net and the stainless tube are filled with activated carbon ACM Activated carbon monolith tube (Mast Carbon, UK), 250 mm long and 30 mm in diameter with 230 channels/sq. inch ACI Activated carbon impregnated foam, composed of fine activated carbon particles that are incorporated uniformly into open-cell PUF Annular 8 coaxial glass tubes spaced 1 mm apart and coated with washed XAD resin (URG, USA) Honeycomb Stainless honeycomb disk coated with XAD resin

Activated CIF/BOSS carbon-based ACF

XAD-based

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Y. Cheng et al. / Environment International 35 (2009) 674–681

Fig. 1. Comparison of DQ-EC and BQ-EC. Data from Cabada et al. (2004), Viana et al. (2006a,b), Pang et al. (2002a), Fan et al. (2003, 2004a,b), Mader et al. (2001).

designed such that the residence time of the particles in denuder was less than 0.2 s, a value suggested by Kamens and Coe (1997) and Strommen and Kamens (1999). For some other denuders, such as the BOSS denuder, though a residence time of more than 1 s was adopted, no evidence was seen for loss of organics from particles (Eatough et al., 1993). 2.3. Denuder breakthrough and efficiency Denuder breakthrough is determined by placing a quartz filter upstream of the denuder (this is termed the breakthrough configuration — a sampler operated without the upstream quartz filter is defined as the monitoring configuration). Thus, organic carbon collected by the quartz filter and the backup adsorbent filter downstream of the denuder is the breakthrough OC. Denuder efficiency is then defined as: denuder efficiency = 100k−

breakthrough OC gas phase OC

ð1Þ

where the gas phase OC is usually determined by OC collected by the backup adsorbent filter in a collocated filter pack sampler (Eatough et al., 1999; Lewtas et al., 2001; Modey et al., 2001; Ding et al., 2002a; Pang et al., 2002b). 2.3.1. Influence of backup adsorbent filter The type of backup adsorbent filter has a substantial influence on the breakthrough SVOC and the gas phase OC, due to the different collection efficiency for gaseous organics between different types of adsorbent filters. The CIF and CIG filter have a similar collection efficiency (usually assumed as 100%), because gaseous organics are removed mostly by the activated carbon rather than the cellulose or glass fiber. The collection efficiency of XAD-Q filter is 46%, as compared with the CIF filter (Lewtas et al., 2001). But the breakthrough SVOC of the XAD-based denuder could only be determined by the backup XAD-Q filter. Exceptionally high SVOC concentrations have been measured by the backup CIF filters placed downstream of XAD denuders, compared with that measured after the BOSS denuders. The SVOC concentrations were also twofold higher than the gas phase OC concentrations, as shown in Fig. 2, indicating that denuder breakthrough could not explain the abnormally high SVOC concentrations. This high SVOC was also measured by Mader et al. (2001, 2002, 2003). Lewtas et al. (2001) suggested that this high SVOC concentration was caused by residual solvents (hexane,

Fig. 2. OC concentrations determined by backup CIF filter in filter pack (FP) sampler, and in denuder-based sampler, downstream of XAD and CIF denuder, respectively. Data from Lewtas et al. (2001).

dichloromethane, acetone) used in extracting the XAD denuders between sampling runs (Gundel, 1999; Stockburger and Gundel, 1999) and subsequently released during sampling. 2.3.2. Efficiency of activated carbon-based denuder Activated carbon-based denuder usually has a high efficiency for removing gaseous organics. For the BOSS denuder, a neglectable amount of breakthrough OC was detected on the front quartz filter, while a substantial amount was collected by the backup CIF filter, which accounted for a considerable fraction of the SVOC measured by a monitoring configuration. For example, breakthrough constituted 17%, 22% and 80% of the SVOC concentration, for concentrations of SVOC above 2, between 1 and 2, and below 1 μg/m3, respectively (Modey et al., 2001). For the ACM denuder, breakthrough averaged 18% of the total particulate organic carbon, which is the sum of NVOC and SVOC (Subramanian et al., 2004). But the breakthrough OC usually accounts for only a small fraction of gas phase OC, measured by the CIF backup filter in a collocated filter pack sampler. As a result, a nearly 100% efficiency is usually reported for an activated carbon-based denuder. For example, the efficiency of the BOSS denuder and ACM denuder determined by Eq. (1) were about 98% (Eatough et al., 1999; Modey et al., 2001; Ding et al., 2002a; Pang et al., 2002b) and 94% (Subramanian et al., 2004), respectively. The useful time of activated carbon-based denuder is usually a few months. For example, an averaged efficiency of 95%, varying from 91% to 97%, was obtained over a six month period for the BOSS denuder operated at 40 L/min (Eatough et al., 1999). 2.3.3. Efficiency of XAD-based denuder The efficiency of XAD-based denuder is lower than that of activated carbon-based denuder, indicated by the result that the XAD-Q backup filter measured twofold higher concentrations of SVOC downstream of the XAD denuders compared to BOSS denuder (Lewtas et al., 2001). The efficiency of XAD-coated annular denuder (8-channel, 28 cmlong) calculated from Eq. (1) was 45%, using the breakthrough concentrations (−0.19 and 3.0 μg C/m3, measured by quartz filter and backup XAD-Q filter, respectively; 23 h-average value) and the gaseous OC concentration (5.5 μg C/m3, measured by the backup XAD-Q filter in a collocated filter pack sampler) reported by Lewtas et al. (2001). The breakthrough averaged 75% of the SVOC concentration, measured by a collocated monitoring configuration. Lewtas et al. (2001) used a gas phase OC concentration of 13 μg C/m3, measured by the backup CIF

Y. Cheng et al. / Environment International 35 (2009) 674–681 Table 2 Carbonaceous components measured by organic denuder-based samplersa Denuder/filter packb

Sampling site

CIFc/Q+ CIF or CIG

Lawrence, TN Riverside, CA Bakersfield, CA Rubidoux, CA Fresno, CA Provo, UT Salt Lake City, UT Salt Lake City, UT Bountiful, UT Philadelphia, PA Pittsburgh, PA Atlanta, GA Seattle, WA ACFc/Q + Q Bily Kriz, Czech Kpuszta, Hungary Ghent, Belgium Barcelona, Spain Barcelona, Spain ACMd/Q + CIF Pittsburgh, PA ACId/ Q Seattle, WA XAD annulard/Q+XAD-Q Toronto, Canada Toronto, Canada Vancouver, Canada d XAD honeycomb / Q+Q Pasadena, CA

Sampling period

NVOC SVOC

EC

Jul, 1997 Sep, 1997 Feb–Mar, 1998 Jul, 2003 Dec, 2003 Dec, 1998 Dec, 2000–Jan, 2001 Jul, 2001 Dec, 2000–Jan, 2001 Jul, 1999 Aug, 2000 Aug, 1999 Apr–May, 1999 Jun–Jul, 2003 Jun–Jul, 2003 Jun–Jul, 2004 Jul–Aug, 2004 Nov–Dec, 2004 Jul–Aug, 2001 May, 2001 July, 2001 Mar, 2003 Aug, 2001 Aug, 2001

2.0 19.8 3.9 16.6 11.0 8.7 15.8 9.7 10.5 14.3 5.8 11.4 4.9 2.62 3.4 1.9 3.0 4.9 2.8 2.5 4.14 3.09 2.50 6.02

1.4 3.1 2.2 1.1 – 2.8 2.8 1.3 2.2 0.3 1.1 1.5 0.7 – – 1.0 1.3 2.3 0.5 0.1 0.87 0.4 0.49 1.00

1.5 14.3 9.1 17.5 14.9 4.6 5.8 3.0 3.1 11.1 5.3 5.3 3.8 0.04 0.06 − 0.08 0.2 0.9 0.3 – 2.38 2.22 0.30 0.05

a

See text for reference. Filter combination used downstream of the organic denuder, estimated as front + backup. c Values reported in μg/m3. d Values reported in μg C/m3. b

filter in another collocated filter pack sampler, to calculate the denuder efficiency. But this would overestimate the capacity of the XAD denuder for removal of gaseous organics. Since the additional gaseous OC collected by the backup CIF filter is not “sensitive” to XAD-Q filter, and whether it is removed by the XAD denuder or not has no effect on the breakthrough concentration determined by the XAD-Q filter. A considerable amount of NVOC (40%) was from breakthrough when using a 4-channel, 15 cm-long XAD denuder (Olson and Norris, 2005). To avoid significant breakthrough, Fan et al. (2003) declared that a 60 cm-long XAD-coated annular denuder should not be used for more than 20 h and a 28 cm-long denuder could only be used for less than 10 h, at a sampling rate of 16.7 L/min. These findings are not surprising given the fact that XAD denuders typically have been used for the research on gas-particle partitioning of PAHs (Gundel et al., 1995a,b; Lane and Gundel, 1996; Lane et al., 2000; Peters et al., 2000; Swartz et al., 2003). For this purpose, Temime-Roussel, et al. (2004a) defined the efficiency of XAD-coated annular denuder as: denuder efficiency =

mdenuder mdenuder + mfilter pack

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influence on the concentrations of both NVOC and SVOC are the temperature to which the quartz filters heated in the pretreatment; the variance in the analysis method, including thermal or thermal–optical, reflectance or transmittance correction for the thermal–optical method (Chow et al., 2001); and the temperature program used, especially the peak temperature of the inert mode (Subramanian et al., 2006). As a result, special attention should be paid to these issues when referring to the results obtained by the denuder-based samplers. Carbonaceous components determined by different denuderbased samplers are summarized in Table 2, and are discussed in detail below. 3.1. Results from CIF denuder-based sampler The CIF denuder (or BOSS denuder) developed by Eatough et al. (1999) has been successfully used in the PC-BOSS sampler (Wilson et al., 2006). The BOSS denuder has been validated in many American cities, such as Lawrence, TN (Ding et al., 2002b), Riverside, CA (Obeidi et al., 2002), Bakersfield, CA (Pang et al., 2002b), Rubidous and Fresno, CA (Wilson et al., 2005), Provo, UT (Obeidi and Eatough, 2002), Salt Lake City and Bountiful, UT (Long et al., 2002, 2003), Lindon, UT (Grover et al., 2006a,b), Philadelphia, PA (Pang et al., 2002c), Atlanta, GA (Modey et al., 2001), Seattle, WA (Lewtas et al., 2001), and Meadview, AZ (Eatough et al., 2006b). The BOSS denuder has also been used in airborne sampling campaigns, such as the ACE-Asia field study conducted during the spring of 2001 (Huebert et al., 2004) and the SAFARI field study conducted during August and September 2000 (Eatough et al., 2003b). SVOC has been measured by a PC-BOSS sampler for over a year at Pittsburgh, PA (Modey and Eatough, 2004). To our knowledge, this is the only study that focuses on the long period consecutively monitoring of SVOC. The result is shown in Fig. 3. In the PC-BOSS sampler, NVOC and SVOC are collected by a quartz and CIF (or CIG) filter respectively, and are analyzed by a thermal method named Temperature Programmed Volatilization (TPV, Ellis and Novakov, 1982; Tang et al., 1994) in which a quartz filter is heated to 800 °C in a N2/O2 atmosphere, while a CIF or CIG filter is heated to 300–400 °C in a N2 atmosphere. 3.2. Results from ACF denuder-based sampler The ACF (Activated Carbon Filled) denuder developed by Mikuška et al. (2003) has been used in a few European cities, such as Bily Kriz, Czech and Kpuszta, Hungary (Maenhasut et al., 2004), Ghent, Belgium (Viana et al., 2006a), Barcelona, Spain (Viana et al., 2006b), and Amsterdam, Netherlands (Viana et al., 2007). Sequential quartz filters

ð2Þ

where mdenuder and mfilter pack are the amount of a “reference compound” (only present in gas phase) collected by the denuder and the filter pack, respectively. An efficiency of approximately 100% was reported for a XAD-annular denuder over a sampling duration of 7 h, taking naphthalene as the reference compound. 3. Integrated measurement of SVOC There are many uncertainties in the measurement of NVOC and SVOC, based on the organic denuder in combination with backup adsorbent filter. (1) Denuder breakthrough may result in overestimating both NVOC and SVOC. (2) The collection capacity of the backup filter for gaseous organics has substantial influence on the concentration of SVOC measured. (3) Among the factors that have considerable

Fig. 3. Monthly averaged daily SVOC concentrations from November 1999 to December 2000. NVOC and EC concentrations are also given. Data from Modey and Eatough (2004).

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Y. Cheng et al. / Environment International 35 (2009) 674–681

were placed downstream of the denuder, and were analyzed by TOT with a temperature program described by Schmid et al. (2001). The efficiency of the ACF denuder has not been evaluated by the breakthrough configuration. Using a quartz filter to collect the evaporated OC is questionable due to its low capacity for collecting gaseous organics compared with an adsorbent filter. 3.3. Results from ACM denuder-based sampler

Table 3 Carbonaceous components measured by denuder-based semi-continuous equipments Sampling site

Carbonaceous component

Semi-continuous results

Integrated resultsa

Reference

Rubidoux, CAb

TC NVOC SVOC NVOC TC

18.8 10.2 7.6 6.3 9.1

18.8 9.2 7.7 6.2 9.3

Grover et al., 2008a

Lindon, UTb Riverside, CAc

Grover et al., 2005 Grover et al., 2008b

a

The ACM (Activated Carbon Monolith tube) denuder developed by Subramanian et al. (2004) has been validated in Pittsburgh, PA (Cabada et al., 2004). NVOC and SVOC were collected by a quartz and CIG filter respectively; the quartz filter was analyzed by TOT with the NIOSH 5040 protocol, while the CIG filter was heated to 330 °C in an He atmosphere. The efficiency of the ACM denuder has been monitored for the whole sampling period, by running a breakthrough configuration in parallel with the monitoring configuration. 3.4. Results from ACI denuder-based sampler The ACI (Activated Carbon Impregnated foam) denuder developed by Pang et al. (2002a) has been validated in Seattle, WA. Only a quartz filter was involved, and it was analyzed by TOT with the NIOSH 5040 protocol. The sampler needs further evaluation and validation, for example, a backup adsorbent filter should be included and the breakthrough concentration should be determined.

Determined by collocated PC-BOSS sampler. Values reported in μg C/m3, NVOC determined by traditional Sunset monitor with CIF denuder, SVOC determined by modified Sunset monitor. c Values reported in μg/m3, determined by dual-oven Sunset monitor. b

separate gaseous organics not removed by the denuder (the first two steps, up to 250 °C) and particulate SVOC (the third step, up to 450 °C). A dual-oven Sunset carbon monitor was designed and field tested in Riverside, CA (Grover et al., 2008b). In that instrument, gaseous organics were removed by a CIF denuder, while particles were collected on a quartz filter placed in the first oven and were thermal-optically analyzed for the determination of NVOC, evaporated particulate OC was adsorbed by the CIG filter placed in the second oven and was analyzed using the same procedure as the modified Sunset SVOC monitor. The concentrations of carbonaceous components determined by the denuder-based semi-continuous equipment are summarized in Table 3, and integrated results are given for comparison. 5. Implications for measurement of carbonaceous aerosols

3.5. Results from XAD denuder-based sampler 5.1. Origin of QBQ-OC and QBT-OC The 8-channel XAD-coated annual denuder described by Fan et al. (2003) has been validated in Toronto (Fan et al., 2004a) and Vancouver (Fan et al., 2004b), Canada. NVOC and SVOC were collected by a quartz and XAD-Q filter respectively; the quartz filter was analyzed by TOT with a temperature program described by Sharma et al. (2002), the XAD-Q filter was analyzed by a two step thermal program in an He atmosphere to separate gaseous organics not removed by the denuder (up to 200 °C) and particulate SVOC (up to 350 °C). The sampling duration was carefully selected to avoid breakthrough. The XAD-coated honeycomb denuder developed by Mader et al. (2001) has been validated in Pasadena, CA, and in an airborne measurement program during the ACE-Asia (Mader et al., 2002). Sequential quartz filters were placed downstream of the denuder, and were analyzed by TOT with a temperature program described by Mader et al. (2001). The denuder efficiency of removing gaseous organics that could be adsorbed by a quartz filter is demonstrated, by running a breakthrough configuration in parallel with the monitoring configuration for the whole sampling period. 4. Semi-continuous measurement of SVOC Semi-continuous equipment for the measuring of carbonaceous aerosols, such as the Ambient Carbon Particulate Monitor (Series 5400, Rupprecht & Patashnick) and the Sunset Laboratory Carbon Aerosol Monitor, are also prone to the positive and negative artifacts. The CIF denuder has been placed upstream of the OC/EC analyzer to eliminate the positive artifact encountered by the R & P 5400 (Matsumoto et al., 2003) and by the Sunset monitor (Bae et al., 2004; Arhami et al., 2006; Polidori et al., 2006; Grover et al., 2006b). A modified Sunset monitor for the semi-continuous measurement of SVOC was described by Grover et al. (2005, 2008a), in which after removal of gaseous organics by a CIF denuder supplied by Sunset Laboratory, particles were collected by a quartz filter immediately before the entrance to the OC/EC analyzer; evaporated particulate OC passed into the filter collection region of the analyzer where it was adsorbed by a CIG filter. The CIG filter was then analyzed by thermal evolution, which was done in a three step temperature program in an He atmosphere to

A backup quartz filter, either behind a front quartz filter (QBQ) or in a parallel port behind a Teflon filter (QBT), has long been used to correct the sampling artifact encountered by a bare quartz filter. But the origin of the organics collected by the backup filter (backup-OC), whether from gaseous organics passing through the front filter or from evaporated particulate OC, has been debated for years (Turpin et al., 2000). The application of an organic denuder provides a direct approach to quantitatively evaluate the origin of the backup-OC. Three collocated samplers should be included for this purpose: one denuder-based sampler containing sequential quartz filters and operated in monitoring configuration to measure the NVOC and SVOC; another denuder-based sampler operated in breakthrough configuration to measure the breakthrough concentrations; and a filter pack sampler containing a backup quartz filter to determine the concentration of backup-OC (QBQOC or QBT-OC). It is assumed that (1) both gaseous organics passing through the front filter (positive artifact-contributed) and evaporated particulate OC (negative artifact-contributed) contribute to backup-OC; (2) the SVOC, corrected with the breakthrough concentration, provides an upper limit of negative artifact-contributed backup-OC, since the evaporation of particulate OC is enhanced by the removal of gaseous organics; (3) finally, the positive artifact-contributed backup-OC is estimated as the difference between backup-OC and SVOC. 5.1.1. Positive artifact-contributed OC dominant According to some studies, positive artifact-contributed OC dominates the backup-OC. The results, based on these three assumptions, obtained with different organic denuders are summarized in Table 4. Less than 11% of backup-OC (QBQ-OC) was caused by the negative artifact, with the exception of the results reported by Chow et al. (2006), in which the negative artifact accounted for 20–55%. The relatively high contribution of negative artifact reported by Chow et al. (2006) may be caused by two factors: (1) the concentrations of SVOC were not corrected by the breakthrough concentration, leading to overestimation of QBQ-OC contributed by the negative artifact; and (2) backup-OC was underestimated by QBQ-OC, which is illustrated by the large difference between QBQ-OC and QBT-OC, as shown in Table 4. Though QBT may encounter

Y. Cheng et al. / Environment International 35 (2009) 674–681

679

Table 4 Positive artifact- and negative artifact-contributed backup OC SVOC

Breakthrough QBQ-OC SVOCa

Reference Positive Negative artifactartifactcontributed contributed

0.29 0.24 0.04 0.06 −0.08 0.00 0.00 0.00 0.00 0.00 0.05 1.3 1.4 1.6 0.25 0.50 0.31

– – – – – 0.00 0.00 0.00 0.00 0.00 0.00 0.9 1.2 1.4 – – –

0.29 0.24 0.04 0.06 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.4 0.2 0.2 0.25 0.50 0.31

a b c d

2.73 4.10 0.50 0.83 0.16 2.59 1.57 0.95 2.51 1.33 1.36 3.5 (7.9)d 9.2 (15.1) 6.1 (11.6) 1.28 (2.10) 0.91 (1.84) 0.91 (1.75)

2.44 3.86 0.46 0.77 0.16 2.59 1.57 0.95 2.51 1.33 1.31 3.1 (7.5) 9.0 (14.9) 5.9 (11.4) 1.03 (1.95) 0.41 (1.34) 0.60 (1.44)

Fitz (1990)b Ding et al. (2002a)c Maenhasut et al. (2004)b Viana et al. (2006a,b)b Mader et al. (2001)c

Olson and Norris (2005)b

Chow et al. (2006)b

Uncorrected SVOC is used, when breakthrough concentration is not available. Values reported in μg/m3. Values reported in μg C/m3. QBT-OC in parentheses.

more OC evaporated from the front filter, it can't explain the large difference (if the particle collection filter is quartz, some of the evaporated OC would be re-adsorbed by the filter; if the filter is Teflon, all of the evaporated OC would pass through). So the only possible explanation is that the backup quartz filter (QBQ) adsorbed less gaseous organics than the front one, because the front quartz filter was not saturated and continuingly depleted the gaseous organics reaching the backup filter. This would be the case especially for low-volume (Olson and Norris, 2005) or short-duration sampling conditions (Kirchstetter et al., 2001). As shown in Table 4, less than 5% of the backup-OC was from evaporated particulate OC, if the only results considered are the SVOC corrected by breakthrough, and QBT-OC is used to estimate the backup-OC. It should be pointed out that, these results do not mean that the negative artifacts do not exist. As shown in Tables 2 and 4, though little or no SVOC was detected by the backup quartz filter in a denuder-based sampler, a substantial amount was collected by CIF, CIG, and XAD-Q filters, indicating that the evaporated OC has a much higher affinity to an adsorbent filter. Results from some of the studies operating collocated denuder-based sampler and filter pack sampler containing sequential quartz filter are summarized in Fig. 4. As shown in Fig. 4, BQ-OC is continuous higher than DQ-OC, indicating the adsorption of gaseous organics by the bare quartz filter; (Q-QBQ)-OC and DQ-OC are generally comparable, indicating that the backup quartz filter is capable of estimating the positive artifact.

Fig. 4. Comparison of DQ-OC determined by different denuder-based samplers and BQOC (a), (Q-QBQ)-OC (b). Data from Ding et al. (2002a), Viana et al. (2006a,b), Cabada et al. (2004), Pang et al. (2002a), and Mader et al. (2001).

the positive artifact is taken into consideration, the measured OC concentration determined using a bare quartz (BQ) filter is:
mp mg OCbare quartz μg C=m3 = + V V

5.1.2. Negative artifact-contributed OC dominant Backup-OC has been demonstrated to be dominated by negative artifact-contributed OC by some other studies. As reported by Cui et al. (1997), comparable OC was determined by backup quartz filter in a denuder-based sampler (1.1 μg C/m3) and a collocated filter pack sampler (1.0 μg C/m3), indicating all the backup-OC was from the re-adsorption of evaporated particulate organic carbon; and equivalent OC (1.8 μg C/m3) was measured by the bare quartz filter and the quartz filter downstream the BOSS denuder, indicating that adsorption of the gaseous organics by the bare quartz filter is neglectable. It is also shown in another study conducted at the Canyonlands National Park, Utah that the OC collected by the backup quartz filter was unchanged after removal of the gaseous organics by a diffusion denuder (Eatough et al., 1993, 1996).

where mp (μg C) is the mass of particulate OC collected, mg (μg C) is the mass of gaseous OC adsorbed and V is the volume of air sampled. Then, mp/V and mg/V could be considered as the “true value” of the OC concentration and the positive artifact, respectively. Once the quartz filter is in equilibrium with gaseous organics, or it is saturated, mg (μg C) would not increase with the sampling volume. Then, the positive artifact decreases with the increasing sampling volume. As reported by Pang et al. (2002d), the positive artifacts were 2.3 and 1.3 μg C/m3, for the samplers running at 15 L/min and 240 L/min respectively, accounting for 25% and 16% of the total carbon determined.

5.2. High-volume sampling

5.3. Rethinking denuder efficiency

High-volume sampling is an alternative approach to the backup quartz (QBQ or QBT) method to minimize the positive artifact. If only

Given that particulate OC evaporated from front filter could be collected by the backup adsorbent filter, the determination of gas phase OC

ð3Þ

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in Eq. (1) is questionable. The purpose of the organic denuder is to ensure that only nonvolatile particulate OC is retained by the front quartz filter and all of the OC collected by the backup adsorbent filter is from evaporated particulate OC. So the denuder efficiency is essentially the efficiency of removing both the gaseous organics that could be adsorbed by quartz filter and the gaseous organics passing through the quartz that could be subsequently adsorbed by the backup adsorbent filter. Thus, denuder efficiency is more properly estimated as

denuder efficiency = 100k−

breakthrough OC ðBQ ‐OC  NVOCÞ + ðABQ ‐OC  SVOCÞ ð4Þ

where ABQ-OC is the amount of OC measured by the backup adsorbent filter in a filter pack sampler, including both gaseous organic passing through the front filter and the evaporated particulate OC, the same as the gas phase OC in Eq. (1). (BQ-OC–NVOC) is the amount of OC adsorbed by a bare quartz filter, an estimation of the capacity of the denuder to remove gaseous organics sensitive to the quartz filter. (ABQ-OC–SVOC) is an estimation of the capacity of the denuder to remove gaseous organics sensitive to the adsorbent filter, using SVOC to correct for the adsorption of evaporated particulate OC. The efficiency determined by Eqs. (4) and (1) are 98.08% and 98.23%, 40% and 45%, for the BOSS denuder and XAD denuder, calculated from corresponding data reported by Lewtas et al. (2001). 6. Summary (1) An organic denuder in combination with an adsorbent backup filter is an effective approach to eliminate the positive and negative artifacts encountered by the measurement of carbonaceous aerosols. SVOC and the difference between BQ-OC and DQ-OC provide an estimation of negative and positive artifacts, respectively. (2) An organic denuder with high efficiency to remove gaseous organics is the precondition for the determination of SVOC. The type of adsorbent backup filter has a substantial influence on the SVOC collected. (3) Activated carbon-based denuders not only have a higher efficiency and a longer useful lifetime compared with XADbased denuders but also more convenient for use, hence are more suitable for routine monitoring. Activated carbon impregnated filters (CIF or CIG) have a higher collection efficiency than XAD-Q filters and are commercial available, thus are good adsorbents for the collection of SVOC. As a result, a sampling system containing activated carbon-based denuder and CIF or CIG as backup adsorbent filter is capable of routine measurement of semivolatile organic carbon. (4) The backup-OC is shown to be dominated by either gaseous organics passing through the front filter or the evaporated particulate organic carbon, based on the denuder-based method. Acknowledgments This work was supported by the National Natural Science Foundation of China through grant number 20625722 to Prof. He Kebin. The authors would like to acknowledge visiting scholar Charles N. Freed for his valuable help. Appendix A. Different definitions of organic carbon and elemental carbon POC SVOC NVOC

Particulate Organic Carbon, the sum of SVOC and NVOC OC determined by the backup filter in an organic denuderbased sampler OC determined by the front quartz filter in an organic denuderbased sampler

OC determined by a bare quartz filter OC determined by quartz filter downstream of an organic denuder QBQ-OC OC determined by the backup quartz filter behind a quartz filter QBT-OC OC determined by the backup quartz filter behind a Teflon filter ABQ-OC OC determined by the backup adsorbent filter in a filter pack sampler BQ-EC EC determined by a bare quartz filter DQ-EC EC determined by quartz filter downstream of an organic denuder.

BQ-OC DQ-OC

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