Environmental Pollution 206 (2015) 113e121
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Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Uncertainties in observational data on organic aerosol: An annual perspective of sampling artifacts in Beijing, China Yuan Cheng a, Ke-bin He a, b, c, * a
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing, China c Collaborative Innovation Center for Regional Environmental Quality, Beijing, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2015 Received in revised form 8 June 2015 Accepted 12 June 2015 Available online xxx
Current understanding of organic aerosol (OA) is challenged by the large gap between simulation results and observational data. Based on six campaigns conducted in a representative mega city in China, this study provided an annual perspective of the uncertainties in observational OA data caused by sampling artifacts. Our results suggest that for the commonly-used sampling approach that involves collection of particles on a bare quartz filter, the positive artifact could result in a 20e40 % overestimation of OA concentrations. Based on an evaluation framework that includes four criteria, an activated carbon denuder was demonstrated to be able to effectively eliminate the positive artifact with a long useful time of at least one month, and hence it was recommended to be a good choice for routine measurement of carbonaceous aerosol. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Particulate matter Organic aerosol Semivolatile Sampling artifact Denuder
1. Introduction Organic aerosol (OA), a ubiquitous and abundant component in ambient fine particulate matter (PM2.5), exerts important effects on the environment and on climate in particular (Laskin et al., 2015). Despite considerable progress over the past few decades, current understanding of OA is still far from being complete, as indicated by re et al., the limited ability to identify OA on a molecular level (Nozie 2015) and the large gap between simulated and observed OA concentrations (Hallquist et al., 2009). Among topics that are of great scientific concern in the area of OA, here we focus on sampling, the first step to get OA concentration through observation. Typically, filter-based sampling of OA is biased by two types of artifacts. One is the positive artifact due to the adsorption of volatile organic compounds (VOCs) by the generally used quartz fiber filter, while the other one is the negative artifact caused by the volatilization of organic carbon (OC) from the particles already collected on the filter. It has long been recognized that the positive and negative sampling artifacts could introduce substantial
* Corresponding author. State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China. E-mail addresses: [email protected] (Y. Cheng), [email protected]. cn (K.-b. He). http://dx.doi.org/10.1016/j.envpol.2015.06.012 0269-7491/© 2015 Elsevier Ltd. All rights reserved.
uncertainties to observational OA results (Turpin et al., 2000). However, sampling of OA is still frequently accomplished by a bare quartz filter, which is especially the case for the studies conducted in China (e.g., Zheng et al., 2005; Cao et al., 2007; Zhang et al., 2012; Zhao et al., 2013; Zheng et al., 2015). Two common approaches have been developed to account for the sampling artifacts. In one approach, a backup quartz filter is placed downstream of either the primary quartz filter or a Teflon filter that collected in parallel with the primary quartz filter. It is then assumed that OC measured on the backup quartz filter is primarily due to VOCs and equals the positive artifact. In the other approach, a denuder that is capable of adsorbing VOCs is placed upstream of the primary quartz filter to eliminate the positive artifact, and meanwhile a backup filter is placed behind the primary quartz filter to account for the negative artifact. Based on these two types of sampling approaches, here we present an annual perspective of the sampling artifacts of organic aerosol in Beijing, China. To our knowledge, this study is the first one in China that systematically investigates the uncertainties in observational OC data caused by the sampling artifacts. Results from this study are expected to be useful for bridging the gap between simulated and observed OA concentrations, and useful for the choice of sampling techniques in future studies.
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2. Methods 2.1. Field sampling A total of 95 sets of 24 h averaged ambient PM2.5 samples were collected at an urban site (Tsinghua University) in Beijing during the spring (from 1 March to 15 April; N ¼ 45) and autumn (from 27 September to 16 November; N ¼ 50) of 2010. Tsinghua University (40.00 N, 116.32 E) is located in the urban area of Beijing. There is a main road with heavy traffic (i.e., the 4th Ring Road) about 1 km south of the campus, whereas there are no major industrial sources nearby. A Spiral Ambient Speciation Sampler (SASS; Met One Instruments, OR, USA), which has five separate channels operated through a common pump, was used for the sampling. Configuration of the SASS sampler is shown in Fig. 1. Briefly, a quartzequartz filter pack was used in channel 1 and 2, while a Teflonequartz filter pack was used in channel 4 and 5. Channel 1 and 5 also included an activated carbon monolith (ACM) denuder upstream of respective filter packs. Channel 3 was the breakthrough channel (also termed the dynamic blank channel), in which a quartz filter, an ACM denuder and another quartz filter were placed in series. The ACM denuder (Met One Instruments, OR, USA) was 20 mm long and 38 mm in diameter, with about 1000, 1 mm 1 mm channels. A new denuder was used for each campaign. The quartz filters (8 in 10 in, 2500 QAT-UP; Pall Corporation, NY, USA) were first cut into punches with a diameter of 47 mm and then baked at 550 C in air for 24 h before use. A total of 18 and 19 quartz filters were kept as blank during the spring and autumn campaigns, respectively. The Teflon filters (R2PJ047, with a diameter of 47 mm) were also from Pall and were used as received from the manufacturer.
580 C) corresponding to four carbon fractions (namely OC1eOC4), whereas there are three temperature plateaus (580, 740, and 840 C) in the oxidizing mode defining another three carbon fractions (EC1eEC3). The next temperature step is not initiated until the carbon signal reaches the baseline or stays constant for a certain time. The transmittance charring correction was used to separate OC and EC such that EC is defined as the carbon evolving after the filter transmittance signal (monitored at a wavelength of 632 nm) returns to its initial value. All of the OC and EC results (mgC/m3) reported in this article have been corrected by the filter blank concentrations. Refer to Section 3.1 for OC levels of the blank filters, whereas no blank EC was detected in the study.
2.3. Additional data Besides the spring-2010 and autumn-2010 campaigns, the SASS sampler was operated at the same sampling site during the winter (between 9 January and 12 February; N ¼ 29), spring (from 9 April to 9 May; N ¼ 30) and summer (from 20 June to 20 July; N ¼ 30) of 2009 (Cheng et al., 2010) and during the summer (from 25 June to 1 August; N ¼ 37) of 2010 (Cheng et al., 2012). Configurations of the SASS sampler during these two previous studies are summarized in Table 1. Results from the spring-2010 and autumn-2010 campaigns and from the four additional campaigns described by Cheng et al. (2010, 2012) are combined to provide an annual perspective of the sampling artifacts of organic aerosol in Beijing (with a total of 221 sets of samples).
3. Results and discussion 2.2. Sample analysis 3.1. Denuder evaluation The quartz filters were analyzed for OC and EC (elemental carbon) using a DRI carbon analyzer (Model 2001; Atmoslytic Inc., CA, USA). The temperature protocol implemented was IMPROVE-A (IMPROVE refers to the Interagency Monitoring of Protected Visual Environments network), which heats the sample in an inert (i.e., He) and oxidizing (i.e., He/O2) atmosphere sequentially. The inert mode has four temperature plateaus (140, 280, 480, and
When using a carbon denuder to remove VOCs, additional artifacts may be introduced to the measurement of carbonaceous aerosol (e.g., particle loss and off-gassing of particulate OC within the denuder), indicating the importance of denuder evaluation. Here we use a framework that includes four criteria to evaluate the ACM denuder used in the present study.
Fig. 1. Configuration of the SASS sampler during the spring-2010 and autumn-2010 campaigns. Denuder is a device that can remove gas-phase components by diffusion to an adsorbent surface but allows nearly all of the particles to pass through. Based on the targeted components, denuders can be classified into different types, such as carbon denuders that adsorb volatile organic compounds, oxidant denuders that remove reactive gases, and nitric acid denuders that adsorb gaseous nitric acid.
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Table 1 Configurations of the SASS sampler in Cheng et al. (2010, 2012). In the present study, all of the four campaigns listed below were used to evaluate the activated carbon denuder and investigate the positive sampling artifact, whereas only the winter-2009 and summer-2009 campaigns were used to evaluate the backup quartz filter approach. Campaigns
SASS configuration a
Winter-2009 Spring-2009a Summer-2009a Summer-2010b
Same as Fig. 1 except that sequential quartz filters were used in channel 3. Only channel 1e3 were operated, with backup CIG filter.c Same as Fig. 1 except that sequential quartz filters were used in channel 3.d Only channel 1e4 were operated, with backup Nylon filter in channel 1 and 3.
a
Cheng et al. (2010). Cheng et al. (2012). c CIG refers to the activated carbon impregnated glass fiber. d The breakthrough channel (i.e., channel 3) was not operated during the sampling period, instead, was run for 6 days after the sampling was finished. b
(1) Denuder breakthrough. Denuder breakthrough (also referred to as dynamic blank) is defined as the amount of gaseous organics that are not removed by the denuder and subsequently adsorbed by the filter downstream. To determine breakthrough, particles need to be removed before passing through the denuder. In this study, breakthrough was measured by the denuded quartz filter in channel 3 (Fig. 1). During the whole measurement period (i.e., from the winter of 2009 to the autumn of 2010), breakthrough OC averaged 0.36 ± 0.13 mg C/cm2, comparable with filter blank OC (averaging 0.39 ± 0.12 mg C/cm2; a total of 81 filters were kept
as blank during the six campaigns) (Fig. 2 (a) and (b)). Thus, the ACM denuder used in the present study is able to completely remove the gaseous organics that could be adsorbed by a quartz filter. In addition, as mentioned in the Method section, a same denuder was used during each campaign which lasted 1e2 months, indicating a long useful time of the ACM denuder in a heavily-polluted environment such as Beijing. (2) Particle loss within the denuder. This potential artifact can be evaluated by comparing EC concentrations measured by the denuded (ECDQ) and bare (ECBQ) quartz filters. ECBQ and ECDQ
Fig. 2. OC loadings of filter blank, dynamic blank (i.e., breakthrough), and denuded QBQ during (a) the spring-2010 and autumn-2010 campaigns, and (b) the whole measurement period (i.e., from the winter of 2009 to the autumn of 2010). Also shown is the comparison of ECBQ and ECDQ during (c) the spring-2010 and autumn-2010 campaigns, and (d) the whole measurement period. In (a) and (b), the boundary of the box closest to the horizontal axis indicates the 25th percentile, the open circle within the box marks the median, and the boundary of the box farthest from the horizontal axis indicates the 75th percentile, whiskers above and below the box indicate the maximum and minimum, respectively; similar hereinafter. The dashed line in (c) and (d) indicates the linear regression result with K as slope (intercept is set as zero).
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correlated strongly with a slope of quite close to 1.0 during the whole measurement period (Fig. 2 (c) and (d)). In addition, results from the summer-2010 campaign showed that sulfate concentrations measured by the denuded and bare quartz filters also agreed well (Cheng et al., 2012). Thus, particle loss in the ACM denuder should be negligible in terms of mass concentration. (3) Shedding of the denuder material. This potential artifact is caused by the activated carbon particles that have become loose from the denuder, and will cause an overestimation of ECDQ. It can be qualitatively identified by the visual examination of tiny black particles on the denuded front filters (Subramanian et al., 2004; Viana et al., 2006), and can be more reliably evaluated by the EC loading of the denuded quartz filter in the breakthrough channel. No breakthrough EC was detected throughout the whole measurement period, indicating that the ACM denuder did not exhibit any shedding. (4) Off-gassing of particulate OC within the denuder. This potential artifact is due to the removal of gaseous organics, which results in a gas-phase-poor environment in the denuder and consequently shifts the gaseparticle equilibrium of semivolatile organic compounds towards the gasphase. It tends to be more significant with increasing residence time of particles in the denuder. In some early studies, a residence time of 0.2 or 0.3 s was used as the threshold value to minimize this potential artifact (e.g., Mader et al., 2001; Swartz et al., 2003). But more recently, the evaporation time scale of ambient organic aerosol was found to be on the order of minutes to tens of minutes (Vaden et al., 2011; Saleh et al., 2013). Results from a field campaign in Sacramento, CA showed that when gas-phase organics were removed, it took approximately 200 min for ambient organic aerosol to evaporate 20% of their mass, while negligible evaporation occurred within 2 min (Vaden et al., 2011). In this study, the residence time was 0.18 s at the operating flow rate of the SASS sampler, indicating that off-gassing of particulate OC within the ACM denuder should be insignificant. Our evaluation results demonstrate that the ACM denuder can effectively remove the positive sampling artifact of organic aerosol and meanwhile does not bias the measurement of non-volatile species. Other advantages of the ACM denuder include: (1) Small in size. The ACM denuder used in the present study was only 20 mm long and 38 mm in diameter, much smaller than the Sunset denuder (the default operating flow rate is 8 L/min; Sunset laboratory Inc., OR, USA) which consists of 15 strips of 32 mm (1.25 in) 203 mm (8 in) activated carbon impregnated cellulose fiber filter (CIF) that are separated at the long edges by 2 mm (Sunset Laboratory, 2004), and also much smaller than the URG eight-channel XAD denuders (URG Corporation, NC, USA; XAD refers to polystyrenedivinylbenzene resin). The eight-channel XAD denuder is typically 600 mm in length and 52 mm in outer diameter for the Integrated Organic Gas and Particle Sampler (IOGAPS, with a flow rate of 16.7 L/min; Peters et al., 2000; Fan et al., 2003), whereas the denuder length is usually reduced to 285 mm in the case of the Versatile Air Pollution Sampler (VAPS, the flow rate through the denuder is 15 L/min; Lewtas et al., 2001). In addition to these commercially available ones, several types of carbon denuders have also been built by different research groups, such as the CIF denuder designed by D. J. Eatough and co-workers (Eatough et al., 2003), and the annular diffusion denuder that using activated carbon as
the sorbent (Mikuska et al., 2003). All of these laboratorymade denuders are much larger in size compared to the ACM denuder used in this study. For example, the CIF denuder (operated at ~32 L/min) is composed of 17 strips of 45 mm 580 mm CIF filter that are separated by 2 mm (Eatough et al., 2003), while the annular diffusion denuder (one channel with an annuli spacing of 1.5 mm; operated at 17 L/min) is 430 mm long and 63 mm in outer diameter (Mikuska et al., 2003). The much smaller size of the ACM denuder used in the present study is partly due to its substantially higher specific surface area (~3.5 mm1) compared to the other types of denuders. The specific surface area is only ~0.9 mm1 for the Sunset denuder and that described by Eatough et al. (2003), and is as low as ~0.1 mm1 for that built by Mikuska et al. (2003). It should be mentioned that ACM denuder is also available in other configurations. For example, an ACM denuder with a specific surface area of ~1.4 mm1 was used during the Pittsburgh Air Quality Study (Subramanian et al., 2004). (2) Easy to use. The XAD denuder requires complex pretreatment (e.g., coating followed by conditioning) and maintenance (e.g., extraction by organic solvents after each sampling period and re-coating after several uses) procedures (Lewtas et al., 2001), whereas the ACM denuder does not. (3) Capacity to minimize the reaction artifact of organic aerosol. The reaction artifact refers to the on-filter degradation of organic compounds such as particulate polycyclic aromatic hydrocarbons during sampling. It is caused by the reactive trace gases such as ozone and nitrogen dioxide. The reaction artifact is typically reduced by means of an oxidant denuder, in which commonly used sorbents include potassium iodide (KI; Mikuska et al., 2003), potassium nitrite (KNO2; Tsapakis and Stephanou, 2003), sodium sulfite (Na2SO3; Mikuska et al., 2003), and manganese dioxide (MnO2; Mikuska et al., 2003; Liu et al., 2006). Besides these sorbents, it has been demonstrated that activated carbon (Mikuska et al., 2003; Schauer et al., 2003) as well as XAD (Goriaux et al., 2006) can also remove the reactive trace gases with an efficiency of close to 100%. Thus, the ACM denuder, which is made of active carbon, is expected to be able to minimize the reaction artifact, although its efficiency to remove the reactive trace gases is not measured in this study. (4) Capacity to adsorb gaseous nitric acid (HNO3). Similar to organic aerosol, measurement of particulate nitrate, a dominant inorganic component in PM2.5, is also susceptible to sampling artifacts (especially the volatilization of nitrate from the already collected particles, i.e., the negative artifact). Sampling method of particulate nitrate has been studied since late 1980s and has been relatively well addressed (US EPA, 2004). A common approach to account for the sampling artifacts of nitrate is to use a HNO3 denuder in combination with a TefloneNylon filter pack. This technique has been implemented in the PM2.5 chemical speciation monitoring networks such as the PM2.5 National Chemical Speciation Network (CSN) and the IMPROVE network established by the U.S. Environmental Protection Agency (EPA) (Solomon et al., 2014), and the National Air Pollution Surveillance (NAPS) network in Canada (DabekZlotorzynska et al., 2011). The HNO3 denuders usually use sodium carbonate (Na2CO3) or magnesium oxide (MgO) as the sorbent. During the summer-2010 campaign, the ACM denuder was demonstrated to be able to remove gaseous HNO3 with an efficiency of 100% and a useful time of at least one month (Cheng et al., 2012).
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Fig. 3. Ratios of alternative OC estimations to OCDQ during different seasons in Beijing. The values around (either above or below) each box are the average of the alternative-toreference ratios, while the red and blue fonts indicate an overestimation and underestimation, respectively. Alternative estimations of OC concentration include OCBQ, OCBQ e OCQBQ, OCBQ e OCQBT, and OCBQ e (OCQBT)*. (OCQBT)* indicates OCQBT contributed by gaseous organics. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
In summary, the ACM denuder can be used for multiple purposes (e.g., to eliminate the sampling artifacts of organic and inorganic aerosols and to minimize the reaction artifact of organic aerosol); moreover, its small size and simple operation procedure make it quite suitable for routine measurement of PM2.5.
backup filter in several previous studies (e.g., Eatough et al., 2003; Fan et al., 2003). However, it seems that these sorbent-impregnated filters could vary considerably in terms of thermal stability (e.g.,
3.2. The negative sampling artifact When operating a carbon denuder in combination with a filter pack, OC measured by the front and backup filters are typically referred to as non-volatile OC and semi-volatile OC (resulting from the negative sampling artifact), respectively. As shown in Fig. 2(a) and (b), OC loadings of the denuded backup quartz filter in channel 1 (OCD-QBQ, with an average of 0.39 ± 0.10 mg C/cm2 during the whole measurement period of this study) were comparable with filter blank OC (averaging 0.39 ± 0.12 mg C/cm2), indicating that particulate OC volatilized from the front quartz filter (i.e., DQ) is not retained by the backup one either. To achieve a higher collection efficiency for the volatilized OC, a highly adsorbent filter, such as CIF, activated carbon impregnated glass fiber filter (CIG) and the XAD impregnated quartz filter (XAD-Q), was used as the denuded
Fig. 4. Apportionment of OC measured by the bare quartz filter (OCBQ) to particulate OC and the positive sampling artifact. Particulate OC is estimated by OCDQ (i.e., OC measured by the denuded front quartz filter), whereas the positive sampling artifact is calculated as the difference between OCBQ and OCDQ. The percentiles above each bar indicate the contributions of the positive sampling artifact.
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channel 3 in Fig. 1) should not be necessary in routine measurement of carbonaceous aerosol, given that a denuded backup quartz filter (i.e., D-QBQ) is included. 3.3. The positive sampling artifact
Fig. 5. Temporal variations of the OCDQ concentration (solid circles) and the OCBQ to OCDQ ratio (open circles) during the autumn-2010 campaign (a), and the dependence of the OCBQ to OCDQ ratio on OCDQ during the whole measurement period (i.e., from the winter of 2009 to the autumn of 2010). The dashed line in (b) indicates an OCBQ to OCDQ ratio of 1.0.
degradation temperature of the sorbent), which is a substantial concern in thermal analysis. For example, the peak analytical temperature (in an inert atmosphere) used for the CIG filter was quite different among previous studies (varying from 300 to 450 C; Cheng et al., 2010 and references therein), which is expected to have a pronounced influence on the amount of semi-volatile OC measured. Therefore, it remains a difficult challenge to address the negative sampling artifact of organic aerosol. Although OCD-QBQ cannot provide a robust estimation of the negative sampling artifact, we suggest that it can be used as an indicator for the denuder efficiency. Based on our measurement practice in Beijing, OCD-QBQ was comparable with filter blank OC as long as the ACM denuder could effectively remove the positive sampling artifact. Therefore, a parallel breakthrough channel (i.e.,
Fig. 6. BQ to DQ ratio of OC and EC fractions during different seasons in Beijing. The EC3 carbon fraction was zero for all of the BQ and DQ filters measured in this study; hence it is not shown here. The dashed line indicates a BQ to DQ ratio of 1.0.
Throughout the whole measurement period of this study, OC concentrations measured by the bare quartz filter in channel 2 (OCBQ) were substantially higher than the corresponding results from the denuded front filter in channel 1 (OCDQ), indicating significant positive artifact. The 2009-winter campaign exhibited the lowest OCBQ to OCDQ ratios (with an average of 1.19 ± 0.23) and the lowest contribution of positive sampling artifact to OCBQ (~10%), whereas the highest OCBQ to OCDQ ratios (averaging 1.38 ± 0.30) and the highest contribution of positive sampling artifact (~23%) were observed during the 2009-summer campaign (Figs. 3 and 4). In addition, the OCBQ to OCDQ ratio was found to exhibit a decreasing trend with the increase of OCDQ and eventually approach an asymptotic value of 1.0 once OCDQ exceeded 25 mg C/ m3 (Fig. 5). For example, during the autumn-2010 campaign, the OCBQ to OCDQ ratios averaged 1.60 ± 0.19 and 1.07 ± 0.05 when OCDQ were below 5 (N ¼ 10) and above 25 (N ¼ 11) mgC/m3, respectively (Fig. 5(a)). Fig. 6 compares OC and EC fractions measured by the bare (i.e., BQ) and denuded (i.e., DQ) quartz filters. It is with expectation that the OC1 fraction showed the largest discrepancy between BQ and DQ, regardless of the measurement seasons. Besides, the BQ to DQ ratios of the OC2 and OC3 fractions were much higher in summer compared to the other seasons, indicating that for summer BQ samples, the adsorbed gaseous organics could survive to a much higher temperature in the inert mode of thermal-optical analysis. This phenomenon might be associated with two factors. The first one is that the adsorbed gaseous organics encountered in summer are more likely to pyrolyze during the inert mode of thermaloptical analysis, while the second one is that the oxidation of the adsorbed gaseous organics, which can lead to the formation of oxidant products that have a higher thermal stability compared to the parent compounds (Goldfarb and Suuberg, 2008), is more significant during summer. 3.4. Evaluation of the backup quartz filter approach When sampling without a carbon denuder, OC measured by a backup quartz filter (OCbackup, usually specified as OCQBQ and OCQBT, respectively, when the front filter is quartz and Teflon) is frequently used to estimate the positive sampling artifact, giving rise to the backup quartz filter approach. This approach is being used in several (6 in 2006 and 12 in 2012) IMPROVE sites in the U.S.
Fig. 7. Average concentrations of OCDQ and the alternative OC estimations, including OCBQ, OCBQ e OCQBQ, OCBQ e OCQBT, and OCBQ e (OCQBT)*, during different seasons in Beijing. (OCQBT)* indicates OCQBT contributed by gaseous organics.
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Fig. 8. Apportionment of OCQBT (i.e., OC measured by the backup quartz filter in channel 4) to the adsorbed gaseous organics (i.e., gaseous OCQBT) and the volatilized particulate OC (i.e., particulate OCQBT) during the autumn-2010 campaign. OCQBQ (i.e., OC measured by the backup quartz filter in channel 2) is also shown for comparison.
(Solomon et al., 2014) and all of the NAPS sites in Canada (DabekZlotorzynska et al., 2011). However, the backup quartz approach has long been debated because: (i) OCbackup might include both VOCs and particulate OC lost from the front filter, and (ii) OCbackup might not equal the positive sampling artifact even if OCbackup is constituted only by gaseous organics. In this section, OCQBQ and OCQBT will be evaluated as estimations of the positive sampling artifact, using OCDQ as the reference value of OC concentration. As mentioned in Section 3.2, negligible OC was measured by the
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denuded backup filter in channel 1 (D-QBQ), indicating that volatilized particulate OC does not contribute to OCQBQ. However, OCQBQ underestimated the positive sampling artifact throughout the measurement period of this study, resulting in an overestimation of OC by OCBQ e OCQBQ (Figs. 3 and 7). The ratios of OCBQ e OCQBQ to OCDQ averaged 1.13 ± 0.24, 1.18 ± 0.23, 1.11 ± 0.08 and 1.10 ± 0.17 during the winter-2009, summer-2009, spring-2010 and autumn2010 campaigns, respectively. Unlike D-QBQ, considerable OC was detected on the denuded backup filter in channel 5 (D-QBT), indicating that volatilized particulate OC indeed contributes to OCQBT. OC measured by D-QBT (OCD-QBT) provides an estimation of OCQBT contributed by volatilized particulate OC (particulate OCQBT), and hence OCQBT contributed by gaseous organics (gaseous OCQBT) can be calculated as the difference between OCQBT and OCD-QBT. Fig. 8 presents the apportionment results of OCQBT during the autumn-2010 campaign. As shown in Figs. 3 and 7, OCQBT overestimated the positive sampling artifact throughout the measurement period of this study, resulting in an underestimation of OC by OCBQ e OCQBT. The ratios of OCBQ e OCQBT to OCDQ averaged 0.97 ± 0.14, 0.89 ± 0.09, 0.94 ± 0.06 and 0.91 ± 0.07 during the winter-2009, summer-2009, spring-2010 and autumn-2010 campaigns, respectively. Figs. 3 and 7 also indicate that when using the gaseous OCQBT to account for the positive sampling artifact, the agreement between the corrected OCBQ and OCDQ was significantly improved, although the gaseous OCQBT still frequently overestimated the positive artifact. The comparison of OCBQ e gaseous OCQBT and OCDQ suggests that a particle free quartz filter could adsorb more VOCs than a loaded one, presumably
Fig. 9. Ratios of alternative OC estimations to OCDQ versus OCDQ concentrations during the autumn-2010 campaign. Alternative estimations of OC concentration include OCBQ, OCBQ e OCQBQ, OCBQ e OCQBT, and OCBQ e (OCQBT)*. (OCQBT)* indicates OCQBT contributed by gaseous organics. A most noticeable feature of this figure is that compared to the ratios of OCBQ e OCQBQ to OCDQ, the OCBQ e OCQBT to OCDQ ratios depend less apparently on the OCDQ concentrations. Results from the other seasons show the same trend (see Supplementary material for details).
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Finally, a proper measurement of semi-volatile OC (i.e., the negative artifact) requires filter materials with not only a high adsorption capacity (to collect semi-volatile OC) but also a high and reproducible thermal stability (to quantify semi-volatile OC). However, little work has been done to evaluate highly adsorbent filters (especially their thermal stability), indicating more efforts are necessary. Acknowledgments This work was supported by the National Natural Science Foundation of China (21307067 and 21190054). The first author was also supported by the International Postdoctoral Exchange Fellowship Program. Fig. 10. R2 values derived from the linear regression of alternative OC estimations on OCDQ during different seasons in Beijing. Intercept is set as zero for the regression analyses. Alternative estimations of OC concentration include OCBQ, OCBQ e OCQBQ, OCBQ e OCQBT, and OCBQ e (OCQBT)*. (OCQBT)* indicates OCQBT contributed by gaseous organics. It is noticed that R2 values are lower during the summer-2009 campaign compared to the other seasons. This is because in the summer of 2009 when the OC concentrations are relatively low, the discrepancies between alternative OC estimations and OCDQ depend more strongly on the OCDQ concentrations (see Supplementary material for details).
because the adsorption capacity of the loaded filter is reduced by the collected particles, and/or there is a lager pressure drop across the loaded filter which is unfavorable for adsorption (the pressure drop across a sampling filter provides a driving force for the loss of adsorbed gaseous organics as well as semivolatile components in the particle phase). Finally, compared to the ratios of OCBQ e OCQBQ to OCDQ, the OCBQ e OCQBT to OCDQ ratios not only exhibited smaller variations (Fig. 3) but also depended less apparently on the OCDQ concentrations (Fig. 9). In addition, the linear regressions of OCBQ e OCQBT on OCDQ resulted in higher R2 values than the regressions of OCBQ e OCQBQ on OCDQ (Fig. 10). Compared to OCQBQ therefore, OCQBT provides a more consistent estimation of the positive sampling artifact. 4. Conclusions and implications Based on six campaigns conducted in Beijing, China, this study provided an annual perspective of the uncertainties in observational OA data caused by sampling artifacts. To our knowledge, this study is the first of its kind in China. Our results suggest that for the bare-quartz-filter sampling approach which is commonly used in China, the positive artifact could result in an overestimation of OA concentrations by 20e40 %. An activated carbon monolith denuder was demonstrated to be capable of effectively eliminating the positive artifact with a long useful time (at least one month in a heavily-polluted environment such as Beijing). It was also shown that the denuder did not bias the measurement of non-volatile species (e.g., EC and sulfate). Moreover, the denuder was small in size and easy to use, making it a good choice for routine measurement of carbonaceous aerosol. The backup quartz filter approach was evaluated as an alternative method to account for the positive artifact. Although OCQBT provided a more consistent estimation of the positive artifact compared to OCQBQ, OCQBT and OCQBQ usually overestimated and underestimated the positive artifact, respectively. Increasing the sampling flow rate or the sampling duration can also, to some extent, reduce the influence of positive artifact, but meanwhile, might enhance the negative artifact which remains a measurement challenge.
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2015.06.012. References Cao, J.J., Lee, S.C., Chow, J.C., Watson, J.G., Ho, K.F., Zhang, R.J., Jin, Z.D., Shen, Z.X., Chen, G.C., Kang, Y.M., Zou, S.C., Zhang, L.Z., Qi, S.H., Dai, M.H., Cheng, Y., Hu, K., 2007. Spatial and seasonal distributions of carbonaceous aerosols over China. J. Geophys. Res. 112, D22S11. http://dx.doi.org/10.1029/2006JD008205. Cheng, Y., Duan, F.K., He, K.B., Du, Z.Y., Zheng, M., Ma, Y.L., 2012. Sampling artifacts of organic and inorganic aerosol: implications for the speciation measurement of particulate matter. Atmos. Environ. 55, 229e233. Cheng, Y., He, K.B., Duan, F.K., Zheng, M., Ma, Y.L., Tan, J.H., Du, Z.Y., 2010. Improved measurement of carbonaceous aerosol: evaluation of the sampling artifacts and inter-comparison of the thermal-optical analysis methods. Atmos. Chem. Phys. 10, 8533e8548. Dabek-Zlotorzynska, E., Dann, T.F., Martinelango, P.K., Celo, V., Brook, J.R., Mathieu, D., Ding, L.Y., Austin, C.C., 2011. Canadian National Air Pollution Surveillance (NAPS) PM2.5 speciation program: methodology and PM2.5 chemical composition for the years 2003e2008. Atmos. Environ. 45, 673e686. Eatough, D.J., Long, R.W., Modey, W.K., Eatough, N.L., 2003. Semi-volatile secondary organic aerosol in urban atmospheres: meeting a measurement challenge. Atmos. Environ. 37, 1277e1292. Fan, X.H., Brook, J.R., Mabury, S.A., 2003. Sampling atmospheric carbonaceous aerosols using an integrated organic gas and particulate sampler. Environ. Sci. Technol. 37, 3145e3151. Goldfarb, J.L., Suuberg, E.M., 2008. Vapor pressures and thermodynamics of oxygencontaining polycyclic aromatic hydrocarbons measured using Knudsen effusion. Environ. Toxicol. Chem. 27, 1244e1249. Goriaux, M., Jourdain, B., Temime, B., Besombes, J.L., Marchand, N., Albinet, A., LeozGarziandia, E., Wortham, H., 2006. Field comparison of particulate PAH measurements using a low-flow denuder device and conventional sampling systems. Environ. Sci. Technol. 40, 6398e6404. Hallquist, M., Wenger, J.C., Baltensperger, U., Rudich, Y., Simpson, D., Claeys, M., Dommen, J., Donahue, N.M., George, C., Goldstein, A.H., Hamilton, J.F., Herrmann, H., Hoffmann, T., Iinuma, Y., Jang, M., Jenkin, M.E., Jimenez, J.L., Kiendler-Scharr, A., Maenhaut, W., McFiggans, G., Mentel, ThF., Monod, A., vo ^t, A.S.H., Seinfeld, J.H., Surratt, J.D., Szmigielski, R., Wildt, J., 2009. The Pre formation, properties and impact of secondary organic aerosol: current and emerging issues. Atmos. Chem. Phys. 9, 5155e5236. Laskin, A., Laskin, J., Nizkorodov, S.A., 2015. Chemistry of atmospheric brown carbon. Chem. Rev. 115, 4335e4382. Lewtas, J.L., Pang, Y.B., Booth, D., Reimer, S., Eatough, D.J., Gundel, L.A., 2001. Comparison of sampling methods for semi-volatile organic carbon associated with PM2.5. Aerosol Sci. Technol. 34, 9e22. Liu, Y., Sklorz, M., Schnelle-Kreis, J., Orasche, J., Ferge, T., Kettrup, A., Zimmermann, R., 2006. Oxidant denuder sampling for analysis of polycyclic aromatic hydrocarbons and their oxygenated derivates in ambient aerosol: evaluation of sampling artefact. Chemosphere 62, 1889e1898. Mader, B.T., Flagan, R.C., Seinfeld, J.H., 2001. Sampling atmospheric carbonaceous aerosols using a particle trap impactor/denuder sampler. Environ. Sci. Technol. 35, 4857e4867. , A., Stĕp Mikuska, P., Ve cera, Z., Broskovi cova an, M., Chi, X., Maenhaut, W., 2003. Development of a diffusion denuder for the elimination of sampling artifacts for carbonaceous aerosols. J. Aerosol Sci. 34, S761eS762. re, B., Kalberer, M., Claeys, M., Allan, J., D'Anna, B., Decesari, S., Finessi, E., Nozie Glasius, M., Grgi c, I., Hamilton, J.F., Hoffmann, T., Iinuma, Y., Jaoui, M., Kahnt, A., Kampf, C.J., Kourtchev, I., Maenhaut, W., Marsden, N., Saarikoski, S., SchnelleKreis, J., Surratt, J.D., Szidat, S., Szmigielski, R., Wisthaler, A., 2015. The molecular identification of organic compounds in the atmosphere: state of the art and challenges. Chem. Rev. 115, 3919e3983.
Y. Cheng, K.-b. He / Environmental Pollution 206 (2015) 113e121 Peters, A.J., Lane, D.A., Gundel, L.A., Northcott, G.L., Jones, K.C., 2000. A comparison of high volume and diffusion denuder samplers for measuring semivolatile organic compounds in the atmosphere. Environ. Sci. Technol. 34, 5001e5006. Saleh, R., Donahue, N.M., Robinson, A.L., 2013. Time scales for gas-particle partitioning equilibration of secondary organic aerosol formed from alpha-pinene ozonolysis. Environ. Sci. Technol. 47, 5588e5594. €schl, U., 2003. Polycyclic aromatic hydrocarbons in urban Schauer, C., Niessner, R., Po air particulate matter: decadal and seasonal trends, chemical degradation, and sampling artifacts. Environ. Sci. Technol. 37, 2861e2868. Solomon, P.A., Crumpler, D., Flanagan, J.B., Jayanty, R.K.M., Rickman, E.E., McDade, C.E., 2014. U.S. national PM2.5 chemical speciation monitoring networkseCSN and IMPROVE: description of networks. J. Air Waste Manag. Assoc. 64, 1410e1438. Subramanian, R., Khlystov, A.Y., Cabada, J.C., Robinson, A.L., 2004. Positive and negative artifacts in particulate organic carbon measurement with denuded and undenuded sampler configurations. Aerosol Sci. Technol. 38, 27e48. Sunset Laboratory, 2004. Semi-continuous OCEC Carbon Aerosol Analyzer: a Guide to Running and Maintaining the Sunset Laboratory Semi-continuous OCEC Analyzer. Oregon, Portland. Available at: www.epa.gov/ttnamti1/files/ambient/ pm25/spec/Sunset_Manual.pdf. Swartz, E., Stockburger, L., Vallero, D.A., 2003. Polycyclic aromatic hydrocarbons and other semivolatile organic compounds collected in New York city in response to the events of 9/11. Environ. Sci. Technol. 37, 3537e3546. Tsapakis, M., Stephanou, E.G., 2003. Collection of gas and particle semi-volatile organic compounds: use of an oxidant denuder to minimize polycyclic aromatic hydrocarbons degradation during high-volume air sampling. Atmos. Environ. 37, 4935e4944.
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Turpin, B.J., Saxena, P., Andrews, E., 2000. Measuring and simulating particulate organics in the atmosphere: problems and prospects. Atmos. Environ. 34, 2983e3013. US EPA, 2004. Air Quality Criteria for Particulate Matter. Research Triangle Park, North Carolina, EPA/600/P-99/002aF. neka, J., Shrivastava, M., Zelenyuk, A., 2011. Evaporation Vaden, T.D., Imre, D., Bera kinetics and phase of laboratory and ambient secondary organic aerosol. Proc. Natl. Acad. Sci. 108, 2190e2195. Viana, M., Chi, X., Maenhaut, W., Cafmeyer, J., Querol, X., Alastuey, A., Mikuska, P., Ve cera, Z., 2006. Influence of sampling artefacts on measured PM, OC, and EC levels in carbonaceous aerosols in an urban area. Aerosol Sci. Technol. 40, 107e117. Zhang, X.Y., Wang, Y.Q., Niu, T., Zhang, X.C., Gong, S.L., Zhang, Y.M., Sun, J.Y., 2012. Atmospheric aerosol compositions in China: spatial/temporal variability, chemical signature, regional haze distribution and comparisons with global aerosols. Atmos. Chem. Phys. 12, 779e799. Zhao, X.J., Zhao, P.S., Xu, J., Meng, W., Pu, W.W., Dong, F., He, D., Shi, Q.F., 2013. Analysis of a winter regional haze event and its formation mechanism in the North China Plain. Atmos. Chem. Phys. 13, 5685e5696. Zheng, G.J., Duan, F.K., Su, H., Ma, Y.L., Cheng, Y., Zheng, B., Zhang, Q., Huang, T., € schl, U., Cheng, Y.F., He, K.B., 2015. Exploring the severe Kimoto, T., Chang, D., Po winter haze in Beijing: the impact of synoptic weather, regional transport and heterogeneous reactions. Atmos. Chem. Phys. 15, 2969e2983. Zheng, M., Salmon, L.G., Schauer, J.J., Zeng, L.M., Kiang, C.S., Zhang, Y.H., Cass, G.R., 2005. Seasonal trends in PM2.5 source contributions in Beijing, China. Atmos. Environ. 39, 3967e3976.
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Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Uncertainties in observational data on organic aerosol: An annual perspective of sampling artifacts in Beijing, China Yuan Cheng a, Ke-bin He a, b, c, * a
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing, China c Collaborative Innovation Center for Regional Environmental Quality, Beijing, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2015 Received in revised form 8 June 2015 Accepted 12 June 2015 Available online xxx
Current understanding of organic aerosol (OA) is challenged by the large gap between simulation results and observational data. Based on six campaigns conducted in a representative mega city in China, this study provided an annual perspective of the uncertainties in observational OA data caused by sampling artifacts. Our results suggest that for the commonly-used sampling approach that involves collection of particles on a bare quartz filter, the positive artifact could result in a 20e40 % overestimation of OA concentrations. Based on an evaluation framework that includes four criteria, an activated carbon denuder was demonstrated to be able to effectively eliminate the positive artifact with a long useful time of at least one month, and hence it was recommended to be a good choice for routine measurement of carbonaceous aerosol. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Particulate matter Organic aerosol Semivolatile Sampling artifact Denuder
1. Introduction Organic aerosol (OA), a ubiquitous and abundant component in ambient fine particulate matter (PM2.5), exerts important effects on the environment and on climate in particular (Laskin et al., 2015). Despite considerable progress over the past few decades, current understanding of OA is still far from being complete, as indicated by re et al., the limited ability to identify OA on a molecular level (Nozie 2015) and the large gap between simulated and observed OA concentrations (Hallquist et al., 2009). Among topics that are of great scientific concern in the area of OA, here we focus on sampling, the first step to get OA concentration through observation. Typically, filter-based sampling of OA is biased by two types of artifacts. One is the positive artifact due to the adsorption of volatile organic compounds (VOCs) by the generally used quartz fiber filter, while the other one is the negative artifact caused by the volatilization of organic carbon (OC) from the particles already collected on the filter. It has long been recognized that the positive and negative sampling artifacts could introduce substantial
* Corresponding author. State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China. E-mail addresses: [email protected] (Y. Cheng), [email protected]. cn (K.-b. He). http://dx.doi.org/10.1016/j.envpol.2015.06.012 0269-7491/© 2015 Elsevier Ltd. All rights reserved.
uncertainties to observational OA results (Turpin et al., 2000). However, sampling of OA is still frequently accomplished by a bare quartz filter, which is especially the case for the studies conducted in China (e.g., Zheng et al., 2005; Cao et al., 2007; Zhang et al., 2012; Zhao et al., 2013; Zheng et al., 2015). Two common approaches have been developed to account for the sampling artifacts. In one approach, a backup quartz filter is placed downstream of either the primary quartz filter or a Teflon filter that collected in parallel with the primary quartz filter. It is then assumed that OC measured on the backup quartz filter is primarily due to VOCs and equals the positive artifact. In the other approach, a denuder that is capable of adsorbing VOCs is placed upstream of the primary quartz filter to eliminate the positive artifact, and meanwhile a backup filter is placed behind the primary quartz filter to account for the negative artifact. Based on these two types of sampling approaches, here we present an annual perspective of the sampling artifacts of organic aerosol in Beijing, China. To our knowledge, this study is the first one in China that systematically investigates the uncertainties in observational OC data caused by the sampling artifacts. Results from this study are expected to be useful for bridging the gap between simulated and observed OA concentrations, and useful for the choice of sampling techniques in future studies.
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2. Methods 2.1. Field sampling A total of 95 sets of 24 h averaged ambient PM2.5 samples were collected at an urban site (Tsinghua University) in Beijing during the spring (from 1 March to 15 April; N ¼ 45) and autumn (from 27 September to 16 November; N ¼ 50) of 2010. Tsinghua University (40.00 N, 116.32 E) is located in the urban area of Beijing. There is a main road with heavy traffic (i.e., the 4th Ring Road) about 1 km south of the campus, whereas there are no major industrial sources nearby. A Spiral Ambient Speciation Sampler (SASS; Met One Instruments, OR, USA), which has five separate channels operated through a common pump, was used for the sampling. Configuration of the SASS sampler is shown in Fig. 1. Briefly, a quartzequartz filter pack was used in channel 1 and 2, while a Teflonequartz filter pack was used in channel 4 and 5. Channel 1 and 5 also included an activated carbon monolith (ACM) denuder upstream of respective filter packs. Channel 3 was the breakthrough channel (also termed the dynamic blank channel), in which a quartz filter, an ACM denuder and another quartz filter were placed in series. The ACM denuder (Met One Instruments, OR, USA) was 20 mm long and 38 mm in diameter, with about 1000, 1 mm 1 mm channels. A new denuder was used for each campaign. The quartz filters (8 in 10 in, 2500 QAT-UP; Pall Corporation, NY, USA) were first cut into punches with a diameter of 47 mm and then baked at 550 C in air for 24 h before use. A total of 18 and 19 quartz filters were kept as blank during the spring and autumn campaigns, respectively. The Teflon filters (R2PJ047, with a diameter of 47 mm) were also from Pall and were used as received from the manufacturer.
580 C) corresponding to four carbon fractions (namely OC1eOC4), whereas there are three temperature plateaus (580, 740, and 840 C) in the oxidizing mode defining another three carbon fractions (EC1eEC3). The next temperature step is not initiated until the carbon signal reaches the baseline or stays constant for a certain time. The transmittance charring correction was used to separate OC and EC such that EC is defined as the carbon evolving after the filter transmittance signal (monitored at a wavelength of 632 nm) returns to its initial value. All of the OC and EC results (mgC/m3) reported in this article have been corrected by the filter blank concentrations. Refer to Section 3.1 for OC levels of the blank filters, whereas no blank EC was detected in the study.
2.3. Additional data Besides the spring-2010 and autumn-2010 campaigns, the SASS sampler was operated at the same sampling site during the winter (between 9 January and 12 February; N ¼ 29), spring (from 9 April to 9 May; N ¼ 30) and summer (from 20 June to 20 July; N ¼ 30) of 2009 (Cheng et al., 2010) and during the summer (from 25 June to 1 August; N ¼ 37) of 2010 (Cheng et al., 2012). Configurations of the SASS sampler during these two previous studies are summarized in Table 1. Results from the spring-2010 and autumn-2010 campaigns and from the four additional campaigns described by Cheng et al. (2010, 2012) are combined to provide an annual perspective of the sampling artifacts of organic aerosol in Beijing (with a total of 221 sets of samples).
3. Results and discussion 2.2. Sample analysis 3.1. Denuder evaluation The quartz filters were analyzed for OC and EC (elemental carbon) using a DRI carbon analyzer (Model 2001; Atmoslytic Inc., CA, USA). The temperature protocol implemented was IMPROVE-A (IMPROVE refers to the Interagency Monitoring of Protected Visual Environments network), which heats the sample in an inert (i.e., He) and oxidizing (i.e., He/O2) atmosphere sequentially. The inert mode has four temperature plateaus (140, 280, 480, and
When using a carbon denuder to remove VOCs, additional artifacts may be introduced to the measurement of carbonaceous aerosol (e.g., particle loss and off-gassing of particulate OC within the denuder), indicating the importance of denuder evaluation. Here we use a framework that includes four criteria to evaluate the ACM denuder used in the present study.
Fig. 1. Configuration of the SASS sampler during the spring-2010 and autumn-2010 campaigns. Denuder is a device that can remove gas-phase components by diffusion to an adsorbent surface but allows nearly all of the particles to pass through. Based on the targeted components, denuders can be classified into different types, such as carbon denuders that adsorb volatile organic compounds, oxidant denuders that remove reactive gases, and nitric acid denuders that adsorb gaseous nitric acid.
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Table 1 Configurations of the SASS sampler in Cheng et al. (2010, 2012). In the present study, all of the four campaigns listed below were used to evaluate the activated carbon denuder and investigate the positive sampling artifact, whereas only the winter-2009 and summer-2009 campaigns were used to evaluate the backup quartz filter approach. Campaigns
SASS configuration a
Winter-2009 Spring-2009a Summer-2009a Summer-2010b
Same as Fig. 1 except that sequential quartz filters were used in channel 3. Only channel 1e3 were operated, with backup CIG filter.c Same as Fig. 1 except that sequential quartz filters were used in channel 3.d Only channel 1e4 were operated, with backup Nylon filter in channel 1 and 3.
a
Cheng et al. (2010). Cheng et al. (2012). c CIG refers to the activated carbon impregnated glass fiber. d The breakthrough channel (i.e., channel 3) was not operated during the sampling period, instead, was run for 6 days after the sampling was finished. b
(1) Denuder breakthrough. Denuder breakthrough (also referred to as dynamic blank) is defined as the amount of gaseous organics that are not removed by the denuder and subsequently adsorbed by the filter downstream. To determine breakthrough, particles need to be removed before passing through the denuder. In this study, breakthrough was measured by the denuded quartz filter in channel 3 (Fig. 1). During the whole measurement period (i.e., from the winter of 2009 to the autumn of 2010), breakthrough OC averaged 0.36 ± 0.13 mg C/cm2, comparable with filter blank OC (averaging 0.39 ± 0.12 mg C/cm2; a total of 81 filters were kept
as blank during the six campaigns) (Fig. 2 (a) and (b)). Thus, the ACM denuder used in the present study is able to completely remove the gaseous organics that could be adsorbed by a quartz filter. In addition, as mentioned in the Method section, a same denuder was used during each campaign which lasted 1e2 months, indicating a long useful time of the ACM denuder in a heavily-polluted environment such as Beijing. (2) Particle loss within the denuder. This potential artifact can be evaluated by comparing EC concentrations measured by the denuded (ECDQ) and bare (ECBQ) quartz filters. ECBQ and ECDQ
Fig. 2. OC loadings of filter blank, dynamic blank (i.e., breakthrough), and denuded QBQ during (a) the spring-2010 and autumn-2010 campaigns, and (b) the whole measurement period (i.e., from the winter of 2009 to the autumn of 2010). Also shown is the comparison of ECBQ and ECDQ during (c) the spring-2010 and autumn-2010 campaigns, and (d) the whole measurement period. In (a) and (b), the boundary of the box closest to the horizontal axis indicates the 25th percentile, the open circle within the box marks the median, and the boundary of the box farthest from the horizontal axis indicates the 75th percentile, whiskers above and below the box indicate the maximum and minimum, respectively; similar hereinafter. The dashed line in (c) and (d) indicates the linear regression result with K as slope (intercept is set as zero).
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correlated strongly with a slope of quite close to 1.0 during the whole measurement period (Fig. 2 (c) and (d)). In addition, results from the summer-2010 campaign showed that sulfate concentrations measured by the denuded and bare quartz filters also agreed well (Cheng et al., 2012). Thus, particle loss in the ACM denuder should be negligible in terms of mass concentration. (3) Shedding of the denuder material. This potential artifact is caused by the activated carbon particles that have become loose from the denuder, and will cause an overestimation of ECDQ. It can be qualitatively identified by the visual examination of tiny black particles on the denuded front filters (Subramanian et al., 2004; Viana et al., 2006), and can be more reliably evaluated by the EC loading of the denuded quartz filter in the breakthrough channel. No breakthrough EC was detected throughout the whole measurement period, indicating that the ACM denuder did not exhibit any shedding. (4) Off-gassing of particulate OC within the denuder. This potential artifact is due to the removal of gaseous organics, which results in a gas-phase-poor environment in the denuder and consequently shifts the gaseparticle equilibrium of semivolatile organic compounds towards the gasphase. It tends to be more significant with increasing residence time of particles in the denuder. In some early studies, a residence time of 0.2 or 0.3 s was used as the threshold value to minimize this potential artifact (e.g., Mader et al., 2001; Swartz et al., 2003). But more recently, the evaporation time scale of ambient organic aerosol was found to be on the order of minutes to tens of minutes (Vaden et al., 2011; Saleh et al., 2013). Results from a field campaign in Sacramento, CA showed that when gas-phase organics were removed, it took approximately 200 min for ambient organic aerosol to evaporate 20% of their mass, while negligible evaporation occurred within 2 min (Vaden et al., 2011). In this study, the residence time was 0.18 s at the operating flow rate of the SASS sampler, indicating that off-gassing of particulate OC within the ACM denuder should be insignificant. Our evaluation results demonstrate that the ACM denuder can effectively remove the positive sampling artifact of organic aerosol and meanwhile does not bias the measurement of non-volatile species. Other advantages of the ACM denuder include: (1) Small in size. The ACM denuder used in the present study was only 20 mm long and 38 mm in diameter, much smaller than the Sunset denuder (the default operating flow rate is 8 L/min; Sunset laboratory Inc., OR, USA) which consists of 15 strips of 32 mm (1.25 in) 203 mm (8 in) activated carbon impregnated cellulose fiber filter (CIF) that are separated at the long edges by 2 mm (Sunset Laboratory, 2004), and also much smaller than the URG eight-channel XAD denuders (URG Corporation, NC, USA; XAD refers to polystyrenedivinylbenzene resin). The eight-channel XAD denuder is typically 600 mm in length and 52 mm in outer diameter for the Integrated Organic Gas and Particle Sampler (IOGAPS, with a flow rate of 16.7 L/min; Peters et al., 2000; Fan et al., 2003), whereas the denuder length is usually reduced to 285 mm in the case of the Versatile Air Pollution Sampler (VAPS, the flow rate through the denuder is 15 L/min; Lewtas et al., 2001). In addition to these commercially available ones, several types of carbon denuders have also been built by different research groups, such as the CIF denuder designed by D. J. Eatough and co-workers (Eatough et al., 2003), and the annular diffusion denuder that using activated carbon as
the sorbent (Mikuska et al., 2003). All of these laboratorymade denuders are much larger in size compared to the ACM denuder used in this study. For example, the CIF denuder (operated at ~32 L/min) is composed of 17 strips of 45 mm 580 mm CIF filter that are separated by 2 mm (Eatough et al., 2003), while the annular diffusion denuder (one channel with an annuli spacing of 1.5 mm; operated at 17 L/min) is 430 mm long and 63 mm in outer diameter (Mikuska et al., 2003). The much smaller size of the ACM denuder used in the present study is partly due to its substantially higher specific surface area (~3.5 mm1) compared to the other types of denuders. The specific surface area is only ~0.9 mm1 for the Sunset denuder and that described by Eatough et al. (2003), and is as low as ~0.1 mm1 for that built by Mikuska et al. (2003). It should be mentioned that ACM denuder is also available in other configurations. For example, an ACM denuder with a specific surface area of ~1.4 mm1 was used during the Pittsburgh Air Quality Study (Subramanian et al., 2004). (2) Easy to use. The XAD denuder requires complex pretreatment (e.g., coating followed by conditioning) and maintenance (e.g., extraction by organic solvents after each sampling period and re-coating after several uses) procedures (Lewtas et al., 2001), whereas the ACM denuder does not. (3) Capacity to minimize the reaction artifact of organic aerosol. The reaction artifact refers to the on-filter degradation of organic compounds such as particulate polycyclic aromatic hydrocarbons during sampling. It is caused by the reactive trace gases such as ozone and nitrogen dioxide. The reaction artifact is typically reduced by means of an oxidant denuder, in which commonly used sorbents include potassium iodide (KI; Mikuska et al., 2003), potassium nitrite (KNO2; Tsapakis and Stephanou, 2003), sodium sulfite (Na2SO3; Mikuska et al., 2003), and manganese dioxide (MnO2; Mikuska et al., 2003; Liu et al., 2006). Besides these sorbents, it has been demonstrated that activated carbon (Mikuska et al., 2003; Schauer et al., 2003) as well as XAD (Goriaux et al., 2006) can also remove the reactive trace gases with an efficiency of close to 100%. Thus, the ACM denuder, which is made of active carbon, is expected to be able to minimize the reaction artifact, although its efficiency to remove the reactive trace gases is not measured in this study. (4) Capacity to adsorb gaseous nitric acid (HNO3). Similar to organic aerosol, measurement of particulate nitrate, a dominant inorganic component in PM2.5, is also susceptible to sampling artifacts (especially the volatilization of nitrate from the already collected particles, i.e., the negative artifact). Sampling method of particulate nitrate has been studied since late 1980s and has been relatively well addressed (US EPA, 2004). A common approach to account for the sampling artifacts of nitrate is to use a HNO3 denuder in combination with a TefloneNylon filter pack. This technique has been implemented in the PM2.5 chemical speciation monitoring networks such as the PM2.5 National Chemical Speciation Network (CSN) and the IMPROVE network established by the U.S. Environmental Protection Agency (EPA) (Solomon et al., 2014), and the National Air Pollution Surveillance (NAPS) network in Canada (DabekZlotorzynska et al., 2011). The HNO3 denuders usually use sodium carbonate (Na2CO3) or magnesium oxide (MgO) as the sorbent. During the summer-2010 campaign, the ACM denuder was demonstrated to be able to remove gaseous HNO3 with an efficiency of 100% and a useful time of at least one month (Cheng et al., 2012).
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Fig. 3. Ratios of alternative OC estimations to OCDQ during different seasons in Beijing. The values around (either above or below) each box are the average of the alternative-toreference ratios, while the red and blue fonts indicate an overestimation and underestimation, respectively. Alternative estimations of OC concentration include OCBQ, OCBQ e OCQBQ, OCBQ e OCQBT, and OCBQ e (OCQBT)*. (OCQBT)* indicates OCQBT contributed by gaseous organics. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
In summary, the ACM denuder can be used for multiple purposes (e.g., to eliminate the sampling artifacts of organic and inorganic aerosols and to minimize the reaction artifact of organic aerosol); moreover, its small size and simple operation procedure make it quite suitable for routine measurement of PM2.5.
backup filter in several previous studies (e.g., Eatough et al., 2003; Fan et al., 2003). However, it seems that these sorbent-impregnated filters could vary considerably in terms of thermal stability (e.g.,
3.2. The negative sampling artifact When operating a carbon denuder in combination with a filter pack, OC measured by the front and backup filters are typically referred to as non-volatile OC and semi-volatile OC (resulting from the negative sampling artifact), respectively. As shown in Fig. 2(a) and (b), OC loadings of the denuded backup quartz filter in channel 1 (OCD-QBQ, with an average of 0.39 ± 0.10 mg C/cm2 during the whole measurement period of this study) were comparable with filter blank OC (averaging 0.39 ± 0.12 mg C/cm2), indicating that particulate OC volatilized from the front quartz filter (i.e., DQ) is not retained by the backup one either. To achieve a higher collection efficiency for the volatilized OC, a highly adsorbent filter, such as CIF, activated carbon impregnated glass fiber filter (CIG) and the XAD impregnated quartz filter (XAD-Q), was used as the denuded
Fig. 4. Apportionment of OC measured by the bare quartz filter (OCBQ) to particulate OC and the positive sampling artifact. Particulate OC is estimated by OCDQ (i.e., OC measured by the denuded front quartz filter), whereas the positive sampling artifact is calculated as the difference between OCBQ and OCDQ. The percentiles above each bar indicate the contributions of the positive sampling artifact.
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channel 3 in Fig. 1) should not be necessary in routine measurement of carbonaceous aerosol, given that a denuded backup quartz filter (i.e., D-QBQ) is included. 3.3. The positive sampling artifact
Fig. 5. Temporal variations of the OCDQ concentration (solid circles) and the OCBQ to OCDQ ratio (open circles) during the autumn-2010 campaign (a), and the dependence of the OCBQ to OCDQ ratio on OCDQ during the whole measurement period (i.e., from the winter of 2009 to the autumn of 2010). The dashed line in (b) indicates an OCBQ to OCDQ ratio of 1.0.
degradation temperature of the sorbent), which is a substantial concern in thermal analysis. For example, the peak analytical temperature (in an inert atmosphere) used for the CIG filter was quite different among previous studies (varying from 300 to 450 C; Cheng et al., 2010 and references therein), which is expected to have a pronounced influence on the amount of semi-volatile OC measured. Therefore, it remains a difficult challenge to address the negative sampling artifact of organic aerosol. Although OCD-QBQ cannot provide a robust estimation of the negative sampling artifact, we suggest that it can be used as an indicator for the denuder efficiency. Based on our measurement practice in Beijing, OCD-QBQ was comparable with filter blank OC as long as the ACM denuder could effectively remove the positive sampling artifact. Therefore, a parallel breakthrough channel (i.e.,
Fig. 6. BQ to DQ ratio of OC and EC fractions during different seasons in Beijing. The EC3 carbon fraction was zero for all of the BQ and DQ filters measured in this study; hence it is not shown here. The dashed line indicates a BQ to DQ ratio of 1.0.
Throughout the whole measurement period of this study, OC concentrations measured by the bare quartz filter in channel 2 (OCBQ) were substantially higher than the corresponding results from the denuded front filter in channel 1 (OCDQ), indicating significant positive artifact. The 2009-winter campaign exhibited the lowest OCBQ to OCDQ ratios (with an average of 1.19 ± 0.23) and the lowest contribution of positive sampling artifact to OCBQ (~10%), whereas the highest OCBQ to OCDQ ratios (averaging 1.38 ± 0.30) and the highest contribution of positive sampling artifact (~23%) were observed during the 2009-summer campaign (Figs. 3 and 4). In addition, the OCBQ to OCDQ ratio was found to exhibit a decreasing trend with the increase of OCDQ and eventually approach an asymptotic value of 1.0 once OCDQ exceeded 25 mg C/ m3 (Fig. 5). For example, during the autumn-2010 campaign, the OCBQ to OCDQ ratios averaged 1.60 ± 0.19 and 1.07 ± 0.05 when OCDQ were below 5 (N ¼ 10) and above 25 (N ¼ 11) mgC/m3, respectively (Fig. 5(a)). Fig. 6 compares OC and EC fractions measured by the bare (i.e., BQ) and denuded (i.e., DQ) quartz filters. It is with expectation that the OC1 fraction showed the largest discrepancy between BQ and DQ, regardless of the measurement seasons. Besides, the BQ to DQ ratios of the OC2 and OC3 fractions were much higher in summer compared to the other seasons, indicating that for summer BQ samples, the adsorbed gaseous organics could survive to a much higher temperature in the inert mode of thermal-optical analysis. This phenomenon might be associated with two factors. The first one is that the adsorbed gaseous organics encountered in summer are more likely to pyrolyze during the inert mode of thermaloptical analysis, while the second one is that the oxidation of the adsorbed gaseous organics, which can lead to the formation of oxidant products that have a higher thermal stability compared to the parent compounds (Goldfarb and Suuberg, 2008), is more significant during summer. 3.4. Evaluation of the backup quartz filter approach When sampling without a carbon denuder, OC measured by a backup quartz filter (OCbackup, usually specified as OCQBQ and OCQBT, respectively, when the front filter is quartz and Teflon) is frequently used to estimate the positive sampling artifact, giving rise to the backup quartz filter approach. This approach is being used in several (6 in 2006 and 12 in 2012) IMPROVE sites in the U.S.
Fig. 7. Average concentrations of OCDQ and the alternative OC estimations, including OCBQ, OCBQ e OCQBQ, OCBQ e OCQBT, and OCBQ e (OCQBT)*, during different seasons in Beijing. (OCQBT)* indicates OCQBT contributed by gaseous organics.
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Fig. 8. Apportionment of OCQBT (i.e., OC measured by the backup quartz filter in channel 4) to the adsorbed gaseous organics (i.e., gaseous OCQBT) and the volatilized particulate OC (i.e., particulate OCQBT) during the autumn-2010 campaign. OCQBQ (i.e., OC measured by the backup quartz filter in channel 2) is also shown for comparison.
(Solomon et al., 2014) and all of the NAPS sites in Canada (DabekZlotorzynska et al., 2011). However, the backup quartz approach has long been debated because: (i) OCbackup might include both VOCs and particulate OC lost from the front filter, and (ii) OCbackup might not equal the positive sampling artifact even if OCbackup is constituted only by gaseous organics. In this section, OCQBQ and OCQBT will be evaluated as estimations of the positive sampling artifact, using OCDQ as the reference value of OC concentration. As mentioned in Section 3.2, negligible OC was measured by the
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denuded backup filter in channel 1 (D-QBQ), indicating that volatilized particulate OC does not contribute to OCQBQ. However, OCQBQ underestimated the positive sampling artifact throughout the measurement period of this study, resulting in an overestimation of OC by OCBQ e OCQBQ (Figs. 3 and 7). The ratios of OCBQ e OCQBQ to OCDQ averaged 1.13 ± 0.24, 1.18 ± 0.23, 1.11 ± 0.08 and 1.10 ± 0.17 during the winter-2009, summer-2009, spring-2010 and autumn2010 campaigns, respectively. Unlike D-QBQ, considerable OC was detected on the denuded backup filter in channel 5 (D-QBT), indicating that volatilized particulate OC indeed contributes to OCQBT. OC measured by D-QBT (OCD-QBT) provides an estimation of OCQBT contributed by volatilized particulate OC (particulate OCQBT), and hence OCQBT contributed by gaseous organics (gaseous OCQBT) can be calculated as the difference between OCQBT and OCD-QBT. Fig. 8 presents the apportionment results of OCQBT during the autumn-2010 campaign. As shown in Figs. 3 and 7, OCQBT overestimated the positive sampling artifact throughout the measurement period of this study, resulting in an underestimation of OC by OCBQ e OCQBT. The ratios of OCBQ e OCQBT to OCDQ averaged 0.97 ± 0.14, 0.89 ± 0.09, 0.94 ± 0.06 and 0.91 ± 0.07 during the winter-2009, summer-2009, spring-2010 and autumn-2010 campaigns, respectively. Figs. 3 and 7 also indicate that when using the gaseous OCQBT to account for the positive sampling artifact, the agreement between the corrected OCBQ and OCDQ was significantly improved, although the gaseous OCQBT still frequently overestimated the positive artifact. The comparison of OCBQ e gaseous OCQBT and OCDQ suggests that a particle free quartz filter could adsorb more VOCs than a loaded one, presumably
Fig. 9. Ratios of alternative OC estimations to OCDQ versus OCDQ concentrations during the autumn-2010 campaign. Alternative estimations of OC concentration include OCBQ, OCBQ e OCQBQ, OCBQ e OCQBT, and OCBQ e (OCQBT)*. (OCQBT)* indicates OCQBT contributed by gaseous organics. A most noticeable feature of this figure is that compared to the ratios of OCBQ e OCQBQ to OCDQ, the OCBQ e OCQBT to OCDQ ratios depend less apparently on the OCDQ concentrations. Results from the other seasons show the same trend (see Supplementary material for details).
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Finally, a proper measurement of semi-volatile OC (i.e., the negative artifact) requires filter materials with not only a high adsorption capacity (to collect semi-volatile OC) but also a high and reproducible thermal stability (to quantify semi-volatile OC). However, little work has been done to evaluate highly adsorbent filters (especially their thermal stability), indicating more efforts are necessary. Acknowledgments This work was supported by the National Natural Science Foundation of China (21307067 and 21190054). The first author was also supported by the International Postdoctoral Exchange Fellowship Program. Fig. 10. R2 values derived from the linear regression of alternative OC estimations on OCDQ during different seasons in Beijing. Intercept is set as zero for the regression analyses. Alternative estimations of OC concentration include OCBQ, OCBQ e OCQBQ, OCBQ e OCQBT, and OCBQ e (OCQBT)*. (OCQBT)* indicates OCQBT contributed by gaseous organics. It is noticed that R2 values are lower during the summer-2009 campaign compared to the other seasons. This is because in the summer of 2009 when the OC concentrations are relatively low, the discrepancies between alternative OC estimations and OCDQ depend more strongly on the OCDQ concentrations (see Supplementary material for details).
because the adsorption capacity of the loaded filter is reduced by the collected particles, and/or there is a lager pressure drop across the loaded filter which is unfavorable for adsorption (the pressure drop across a sampling filter provides a driving force for the loss of adsorbed gaseous organics as well as semivolatile components in the particle phase). Finally, compared to the ratios of OCBQ e OCQBQ to OCDQ, the OCBQ e OCQBT to OCDQ ratios not only exhibited smaller variations (Fig. 3) but also depended less apparently on the OCDQ concentrations (Fig. 9). In addition, the linear regressions of OCBQ e OCQBT on OCDQ resulted in higher R2 values than the regressions of OCBQ e OCQBQ on OCDQ (Fig. 10). Compared to OCQBQ therefore, OCQBT provides a more consistent estimation of the positive sampling artifact. 4. Conclusions and implications Based on six campaigns conducted in Beijing, China, this study provided an annual perspective of the uncertainties in observational OA data caused by sampling artifacts. To our knowledge, this study is the first of its kind in China. Our results suggest that for the bare-quartz-filter sampling approach which is commonly used in China, the positive artifact could result in an overestimation of OA concentrations by 20e40 %. An activated carbon monolith denuder was demonstrated to be capable of effectively eliminating the positive artifact with a long useful time (at least one month in a heavily-polluted environment such as Beijing). It was also shown that the denuder did not bias the measurement of non-volatile species (e.g., EC and sulfate). Moreover, the denuder was small in size and easy to use, making it a good choice for routine measurement of carbonaceous aerosol. The backup quartz filter approach was evaluated as an alternative method to account for the positive artifact. Although OCQBT provided a more consistent estimation of the positive artifact compared to OCQBQ, OCQBT and OCQBQ usually overestimated and underestimated the positive artifact, respectively. Increasing the sampling flow rate or the sampling duration can also, to some extent, reduce the influence of positive artifact, but meanwhile, might enhance the negative artifact which remains a measurement challenge.
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