Interactive comment on “ Quantification of organic carbon sampling artifacts in US non-urban and urban networks ”

Although there is not an absolute definition of “urban” and “non-urban” sites, we followed the definition found in 40 CFR Part 50 (National Ambient Air Quality Standards for Particulate Matter; Final Rule. U.S. EPA, 2006) and documented in the U.S. EPA Guidance for Network Design and Optimum Site Exposure for PM2.5 and PM10 (U.S. EPA, 1997). Specifically, these terms were used to contrast the non-urban and urban sampling sites located in the IMPROVE network and STN/CSN, respectively. Out of

1 Introduction PM 2.5 and PM 10 (particulate matter with aerodynamic diameters <2.5 and 10 µm, respectively) sampling onto quartz-fiber filters is accompanied by positive (e.g., adsorption of organic vapors) and negative (e.g., volatilization of organic aerosols after sample Figures

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Full Screen / Esc Printer-friendly Version Interactive Discussion (Robinson et al., 2007).By the time urban emissions transport to non-urban and remote atmospheres, many SVOCs could have been scavenged or converted to more stable compounds.Three hypotheses are examined using data from the three networks: H1: The OC sampling artifact represented by bQF or QBQ depends on sampling protocol and differs among ambient networks.
H2: Sampling artifact and SVOC content are lower at non-urban sites than at urban sites.
H3: Artifact-free carbon concentrations can be better estimated by the difference between measured PM 2.5 and the weighted sum of elemental and ionic measurements (Frank, 2006) than by direct carbon measurements.

Methods
As shown in Table 1, seven different filter samplers are used among the three networks with flow rates ranging from 6.7-22.8liter per minute (L/min).The largest variability is in STN/CSN, which uses five types of samplers, varying from single channel (e.g., STN/CSN used 47 mm Whatman QMA filters (Clifton, NJ), which contain a 5% borosilicate binder prior to 2007.These filters may differ in: 1) capacity and affinity for VOC and gaseous SVOC adsorption and desorption, and 2) the rate to reach saturation or equilibrium between gaseous SVOC and particulate OC.The effects of these differences cannot be determined from the available data.Deposit areas range from 3.53 cm 2 (IM-PROVE) to 11.78 cm 2 (R&P 2025) and face velocities range from 9.5 cm/s (MetOne) to 107.2 cm/s (IMPROVE).The different filter holder configurations (e.g., single/tandem filter packs vs. magazine [R&P 2025, with a stack of 16 filter cassettes]) and materials (e.g., polycarbonate, aluminum, or Teflon-coated) might also affect levels of sampling artifact (Watson and Chow, 2009).
Quartz-fiber filters are treated at 900 • C for three to four hours and submitted for acceptance testing prior to deployment.After this treatment, average blank levels are 0.15±0.15µg OC or total carbon (TC=OC+EC)/cm 2 and 0±0.02 µg EC/cm 2 for Pallflex quartz-fiber filters, and 0.10±0.10µg OC/cm 2 and 0±0.01 µg EC/cm 2 for Whatman QMA quartz-fiber filters.Approximately 2-3% of laboratory blanks are maintained for each network.The acceptance criteria are ≤2.0,1.5, and 0.5 µg/cm 2 for TC, OC, and EC, respectively, in the IMPROVE and SEARCH networks, and <1 µg/cm 2 for TC in STN/CSN.STN/CSN collects 3% trip blanks (i.e., tbQF), which are loaded into filter holders and accompany the sampled filters to and from each sampling site.Trip blanks are intended to assess contamination during shipping and are not installed in the sampler or exposed to ambient air.
Field blanks (e.g., dynamic blanks), accompany sample shipments and are placed in the sampler along with the sampled filters (Chow, 1995).The only difference between samples and bQF is that air is not drawn through bQF.The bQF fraction of total sample number varies by tenfold among the networks: ∼2% of sample filters for IMPROVE, ∼10% for STN/CSN sites and SEARCH, and ∼25% for Texas non-trends CSN.The passive period for bQF is 1-15 min for STN/CSN and SEARCH, and ∼7 d for IMPROVE and Texas non-trends CSN sites.Since the bQF fraction of all samples is only 2-10% of the total number of samples, average OC bQF concentrations are used to correct the sampled values with the standard deviation of the average representing the blank precision.Outliers are identified (i.e., values >3 or 4 times the standard deviation) and excluded from the averages and standard deviations.
QBQs are obtained from six IMPROVE and all eight of the SEARCH sites (Watson et al., 2009).Both networks collect QBQ every-third-day with the exception of daily sampling at two SEARCH sites (i.e., Jefferson Street, Atlanta, GA and Birmingham, AL; see Fig. 1).Ten percent of SEARCH QBQ are randomly selected for analysis.Without preceding organic denuders, the IMPROVE OC QBQ represents a combination of positive and negative OC artifacts.SEARCH corrects the organic sampling artifact by calculating the quarterly mean concentrations for the QBQ and bQF and attributing them to negative and positive artifacts, respectively.OC bQF is multiplied by two to account for passive adsorption on both QF and QBQ.Thus, where: OC QF =Quartz-fiber front filter OC, OC QBQ =Quartz-fiber behind quartz-fiber filter OC, OC bQF =field blank OC from the quartz-fiber front filter.Collocated IMPROVE-STN/CSN samples are acquired from three urban vs. nonurban paired sites (see Fig. 1; Seattle and Mt.Rainier, WA; Phoenix and Tonto National Monument, AZ; and Washington, DC and Dolly Sods Wilderness, WV).In addition, collocated measurements are available from the urban Fresno, CA (Watson et al., 2000) and the non-urban Big Bend, TX (Chow et al., 2004b)  Richards, 2002) while the IMPROVE and SEARCH networks followed the IMPROVE thermal/optical reflectance (TOR) protocol (Chow et al., 1993(Chow et al., , 2001(Chow et al., , 2004a(Chow et al., , 2005(Chow et al., , 2007)).Since blank and backup filter EC levels are expected to be negligible, the analysis protocols should return equivalent TC and OC results.As noted in the footnote to Table 1, a new STN/CSN carbon sampling and analysis protocol have been fully implemented as of October 2009 to be consistent with the IMPROVE network.acpd-9-27359-2009-supplement.pdf) and in more detailed reports (Chow et al., 2008b;Watson et al., 2008).OC bQF at some sampling locations statistically differ from the network mean, though the small number of bQF at some sites may not represent the true distribution of OC bQF levels over the two-year sampling period.eral hours (Subramanian et al., 2004;Turpin et al., 1994) Figure 4 shows little difference between urban and non-urban IMPROVE OC bQF , but with 24% higher OC bQF for non-urban SEARCH sites.Average OC QBQ for the SEARCH samples was ∼25% higher at urban (1.51±1.50µg/cm 2 ) than at non-urban (1.18± 0.98 µg/cm 2 ) sites (Fig. 5).The urban increment for OC QBQ is mostly in the OC1 fraction, which is 146% higher at urban compared with non-urban sites (0.51±0.84 vs. 0.21±0.35µg/cm 2 ).OC2 is ∼11% higher (0.42±0.37 vs. 0.38±0.48µg/cm 2 ) at the urban sites, while the other carbon fraction levels are similar.These results are consistent with hypothesis H2, indicating more organic adsorption at urban locations.Average OC QBQ levels from the six non-urban IMPROVE sites (3.1±0.8 µg/cm 2 ) are 2.6 times higher than OC QBQ from the four non-urban SEARCH sites (1.18±0.98 µg/cm 2 ), consistent with the denuder removing large amounts of adsorbable organic vapors.

Blank and backup filter levels
Blank TC areal densities in Fig. 6 show that STN/CSN tbQF TC (i.e., TC tbQF ) are similar for urban and non-urban sites, but they differ among samplers, consistent with two-year average tbQF levels in densities at the Seattle and Mount Rainier sites are 0.53±0.19and 0.67±0.12µg/cm 2 , respectively, lower than the 0.84-1.12µg/cm 2 found at sites using the Andersen RAAS or MetOne SASS samplers.TC bQF and TC tbQF levels are similar, with a few bQF levels higher than those of tbQF.These blanks were not always acquired together.
For the collocated IMPROVE vs. STN/CSN comparison, IMPROVE TC bQF is most consistent among the four urban sites (Seattle, Phoenix, Washington, DC, and Fresno), ranging from 2.5-2.7 µg/cm 2 , with lower areal densities measured at two non-urban sites: Mount Rainier (1.4±0.4 µg/cm 2 ) and Tonto Monument (2.0±1.1 µg/cm 2 ).Collocated STN/CSN TC bQF are 40-75% lower than IMPROVE, with larger variability, ranging from 0.66±0.42(Mount Rainier using URG MASS) to 1.44±0.48µg/cm 2 (Big Bend using R&P 2025 sequential FRM).This is consistent with hypothesis H1 that longer passive deposition periods result in higher field blank levels.The number of blanks is insufficient to evaluate seasonal variability for individual sites.Site-averaged non-blank corrected ambient TC concentrations (µg/m 3 ) at each IMPROVE-STN/CSN collocated site are within ±30-50% of each other.STN/CSN site-averaged TC areal densities (µg/cm 2 ) are 9-20% of those for collocated IM-PROVE samples.TC bQF to TC QF ratios are larger for non-urban than for urban sites due to the lower ambient TC QF levels.For a given site, TCbQF to TCQF ratios are ∼2 to 4 times higher for STN/CSN than IMPROVE samples.The actual difference could be larger, if the STN/CSN sampler underestimates OC bQF adsorption due to the short passive exposure period.

Regression method
A regression method similar to that of White and Macias be interpreted as the difference in organic sampling artifacts.A robust perpendicular least squares regression method (Dutter and Huber, 1981) is used to minimize biases caused by a few outliers and to account for the presence of errors in both variables.Using Phoenix data as an example, Fig. 7 shows a positive STN/CSN TC sampling artifact of 1.65 µg/m 3 or 1.34 µg/cm 2 (using MetOne SASS sampling volume and deposit area) relative to the IMPROVE sampler.Reversing the independent and dependent variables in Fig. 7 does not change the conclusion when using the robust regression.
Figure 8 shows that the regression intercepts are positive for each season at the eight sites, consistent with lower flow rates for the STN/CSN samples.For five of the eight sites, the intercept is largest during summer, ranging from 0.22-2.03µg/m 3 .
It is highest during spring at the Mount Rainier and Tonto sites, and highest during fall at the Fresno site.The intercepts in Table 5 represent the average of four seasons.The largest two intercepts are found at the Phoenix (1.34 µg/cm 2 ) and Big Bend (1.29 µg/cm 2 ) sites using the MetOne SASS and R&P 2025 samplers, respectively, while the lowest two are found at the Seattle (0.24 µg/cm 2 ) and Mount Rainier (0.50 µg/cm 2 ) sites using the URG MASS samplers.
Based on the sample volume/deposit area for each sampler type (Table 1), the relationship between STN/CSN and IMPROVE sampling artifacts (i.e., TC STN vs. TC IMP in µg/m 3 ) can be expressed as: where the intercept, TC STN art in µg/m 3 , represents the additional artifact in TC STN relative to TC IMP .Regression statistics are summarized in Table 5.Table 6 shows that STN/CSN TC bQF is 11-34% lower than TC STN art at all sites except for the non-urban Tonto and Dolly Sod sites.Calculated STN/CSN TC bQF (0.87 µg/cm 2 ) is the same as TC STN art at the Tonto site.This may be due to the low TC bQF levels (0.3 and 0.4 µg/cm 2 ; see Fig. 6).The Dolly Sods site exhibits low correlations (r=0.7) between IMPROVE and STN (Andersen RAAS) samples.

Organic mass estimated by the SANDWICH method
Teflon-membrane filters are inert and their tendency to adsorb organic vapors is expected to be low.These filters would have a minimal positive OC artifact, although their negative organic artifact might be larger than that of quartz-fiber filters.The SAND-WICH (Sulfate, Adjusted Nitrate, Derived Water, and Inferred Carbonaceous Material) method (Frank, 2006) assumes that all of the unaccounted PM 2.5 mass measured on a Teflon-membrane filter (i.e., when weighted sums of elements and ions are subtracted) can be associated with the carbonaceous component.The OC or organic carbon mass (OCM) estimated from the SANDWICH method can be compared with those measured from quartz-fiber filters using different artifact correction methods.
The SANDWICH method was applied to 716 collocated filter pairs taken at four urban (i.Fresno site were not available.Retained NO − 3 was calculated using the daily average temperature and relative humidity during the sampling period; and particle-bound water was calculated using the Aerosol Inorganics Model (AIM) as described by Frank (2006).
OCM concentrations from the SANDWICH method are converted to measured OC using a multiplier that accounts for unmeasured hydrogen, oxygen, and other elements in the organic compounds (El Zanan et al., 2005;Turpin and Lim, 2001;White and Roberts, 1977): where: X =unmeasured element multiplier (assumed to be 1.4 for fresh and 1.8 for aged aerosol), OC=measured particulate organic carbon.
For IMPROVE samples, average OCM concentrations are 3.99±2.96µg/m 3 , 4.40±3.45µg/m 3 , 3.00±3.16µg/m 3 , and 6.73±3.56µg/m 3 at the Seattle, Phoenix, Washington, DC, and Fresno sites, respectively (Table 7).Better agreement with measured OC was found for a multiplier of 1.4 rather than 1.8 for all but the Fresno site.Agreement between OC×1.4 and OCM was 95%, 100%, 123%, and 71% at the Seattle, Phoenix, Washington DC, and Fresno sites, respectively.For STN/CSN samples, agreement was 90% (URG MASS) at the Seattle site, 79% (Andersen RAAS) at the Washington, DC site, and 123% and 88% (both using MetOne SASS) at the Phoenix and Fresno sites, respectively.To assess whether low, mid-range, or high concentration samples exhibit differences, Table 7 compares estimated 10th, 50th, and 90th percentiles, respectively.The percent differences between the average and median (50% of total) are similar (within±25%) for the sites using IMPROVE samples for OC multipliers of 1.4 or 1.8.At low concentrations (the 10th percentile), OCM by the SANDWICH method is 217-279% higher than measured OCM concentrations at the Washington, DC site.Using MetOne SASS, Introduction

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Full OCM by the SANDWICH method is also twofold higher at low concentrations for the Phoenix site; but the agreement is reasonable (87-117%) for high concentration samples (90th percentile) at this and other STN/CSN sites.

Deviations from hypotheses
Findings from this study are used to address the three hypotheses (H1 to H3): H1: The OC sampling artifact represented by bQF or QBQ depends on sampling protocol and differs among ambient networks.
This hypothesis is valid based on observations.The IMPROVE, STN/CSN, and SEARCH networks use different sampling configurations, flow rates, filter material, and filter sizes.For bQF, which accompany sample filters to the field and are intended to emulate their passive deposition and adsorption, only the IMPROVE network provides an adequate (∼7 d) passive exposure period for blank subtraction.artifact by ∼20-30% (assuming IMPROVE bQF fully represents the artifact), but the number of bQF available for comparison were limited.QBQ stay in the field for more than 24 h and experience active sampling.With a similar level of sampling artifact in areal density (µg/cm 2 ), STN/CSN and SEARCH TC (or OC) concentration (µg/m 3 ) would be more influenced than those of IMPROVE due to smaller sampling volumes and larger filter sizes.Average OC QBQ concentration is 0.33±0.09µg/m 3 for IMPROVE and 0.35±0.15µg/m 3 for SEARCH (with proceeding denuder).This demonstrates appreciable negative sampling artifact.The negative artifact could have been enhanced by the preceding organic denuder equipped in the Particle Composition Monitor (PCM3).
H2: Sampling artifact and SVOC content are lower at non-urban sites than at urban sites.
Comparisons between urban and non-urban sites in the SEARCH network are consistent with this hypothesis, but they are not sufficient to prove it.Average OC QBQ was ∼25% higher at urban sites, (1.51±1.50µg/cm 2 ) than non-urban sites (1.18±0.98 µg/cm 2 ) in the SEARCH network.The increments between the urban and non-urban sites were ∼146% for OC1 and 11% for OC2.The majority of this low temperature OC is gaseous VOCs.During the IMPROVE-STN/CSN comparisons, TC bQF were not always lower at non-urban than urban sites, though this could be due to the extent of VOC saturation.The contrast between urban and non-urban sites can only provide indirect indication of aging effect since the degree of aging is not certain.H3: Artifact-free carbon concentrations can be better estimated by the difference between measured PM 2.5 and the weighted sum of elemental and ionic measurements (Frank, 2006) than by direct carbon measurements.
This hypothesis is invalid based on observations.The SANDWICH approach is based on PM 2.5 mass closure, but many species are not measured on Teflon-membrane filters, including carbon, NO ent collection/retention efficiencies of Teflon-membrane, quartz-fiber, and nylonmembrane filters with respect to these species have not been quantified.In addition, the mass of water and unidentified species may generate more uncertainties.All these contribute to mass closure uncertainties.Even if organic carbon mass (OCM) can be calculated from the SANDWICH method, this study shows that variation in OCM concentration due to the choice of OC multiplier (e.g., 1.4 or 1.8) is comparable to the magnitude of the organic sampling artifact (5-30% of OCM).It is difficult to determine whether the excess OCM mass, if any, is due to sampling artifact or the correction coefficient used to convert OC to OCM.The SANDWICH method did not work well for samples with low concentrations, for which the calculated and measured OC ratio exceeded 200% (e.g., Washington, DC).
Even though the SANDWICH method did not provide a better representation of OC or OC artifact, it is a useful tool to estimate OC when carbon measurements are not available.

Conclusions
There is no simple way to correct for sampling artifacts using current measurements.
With the newly implemented STN/CSN carbon measurements (US EPA, 2006), using the modified IMPROVE Module C sampler (i.e., URG 3000N sampler), sampling artifacts will be reduced via a higher flow rate (e.g., 22.8 L/min instead of 6.7 L/min) and a smaller deposit area (3.53 cm 2 instead of 11.76 cm 2 ).In addition, bQF will remain in the sampler for the same period as QF and QBQ samples at all STN/CSN sites.For each network, blank corrections should be made and uncertainties propagated, even though the reported OC is under-corrected for adsorbed organic vapors due to inadequate passive deposition period for field blanks.Each network should acquire bQFs and QBQs at the same frequency and passive deposit duration (e.g., once a month on an every-sixth-day sampling schedule; expose field blanks for a minimum of three days).More research, perhaps through controlled experiments, is warranted on:   (Eldred et al., 1990).Module A collects PM 2.5 through an Air and Industrial Hygiene Laboratory (AIHL) cyclone (22.8 L/min) followed by a 25 mm Pall Teflon-membrane filter analyzed for mass by gravimetry and for elements by X-ray fluorescence (XRF).Module B collects PM 2.5 through a sodium carbonate denuder (Ashbaugh et al., 2004) followed by an AIHL cyclone, and followed by a 25 mm Nylon-membrane filter analyzed for nitrate (NO   Table 1.Continued.
b Sampler Type, continued Channel 2 contains a sodium carbonate-coated annular denuder followed by a citric acid-coated annular denuder and a 47 mm Nylasorb Nylon-membrane filter for total NH + 4 and total NO − 3 by AC and IC, respectively.Channel 3 samples through a URG PM 10 cyclone, followed by an activated carbon honeycomb denuder to remove carbon vapors, then through a WINS PM 2.5 impactor onto a 37 mm Pall quartz-fiber filter followed by a backup quartz-fiber filter for OC and EC by the IMPROVE A TOR protocol (Chow et al., 2007).c All inlets are made of anodized aluminum.d RTI uses 11.76 cm 2 for quartz-fiber filters and 11.70 cm 2 exposed area for Teflon-membrane filters for the STN/CSN sites.
e DRI uses 11.78 cm 2 for quartz-fiber and Teflon-membrane exposed area for TCEQ non-trends CSN sites.f Whatman QMA filters were switched to Pallflex Tissuquartz (Ann Arbor, MI) quartz-fiber filters as of May 2007.g Field blank is in inlet and outlet of the 16 filter cassette magazines for as long as 5-7 d depending on the sampling frequency, but is in sampling position (without air being drawn through it) for only a few seconds.h QF=quartz-fiber front filter only, QBQ=quartz-fiber behind quartz-fiber filter, with the backup quartz-fiber used to estimate adsorbed organic vapors.i Based on the assumption of once per week site visits.j Field blanks usually in samplers for ∼1-15 min, but in some cases for as long as ∼5-7 d. k Laboratory blanks are selected from each batch of 100 unexposed filters and submitted for acceptance testing.l Trip blanks accompany batches of shipped filters but are not removed from their storage containers.m Field blanks accompany batches of shipped filters, but are removed from storage containers and left exposed to passive sampling.Only the IMPROVE network exposes field blanks for the same length of times as the sampled filters.Introduction

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Full   a Carbon analysis follows the IMPROVE A thermal/optical reflectance (TOR) protocol (Chow et al., 2007) for teh IMPROVE and SEARCH networks and the STN thermal/optical transmittance (TOT) protocol (Chu et al., 2004;Peterson and Richards, 2002) for STN/CSN.a Carbon analysis follows the IMPROVE A thermal/optical reflectance (TOR) protocol (Chow et al., 2007) for the IMPROVE network.b Carbon analysis follows the STN thermal/optical transmittance (TOT) protocol (Chu et al., 2004) MASS (Chapel Hill, NC) and Rupprecht & Patashnick (R&P; now Thermo Scientific) Partisol-Plus Model 2025 Sequential Federal Reference Method (FRM) sampler (Franklin, MA)) to five parallel channels (e.g., MetOne Spiral Aerosol Speciation Sampler (SASS; Grants Pass, OR)).STN sites were originally required to use one of three samplers (i.e., URG MASS, MetOne SASS, or the Andersen RAAS).In 2005, about 75% of the STN/CSN sites used 6.7 L/min MetOne SASS samplers.The Texas Commission on Environmental Quality (TCEQ) uses the R&P 2025 to collect PM 2.5 at non-trends CSN sites.The IMPROVE and SEARCH networks use 25 mm and 37 mm diameter Pallflex ® Tissuquartz ™ (Pall Life sciences, Ann Arbor, MI) quartz-fiber filters, respectively, while (1989)  is used to evaluate the relative sampling artifact between the collocated samples.If the collocated IMPROVE and STN/CSN samples measure the same TC, a linear regression of collocated data pairs should yield a slope of 1.0, an intercept of 0, and a correlation of 1.0, within experimental precision.A statistically significant positive or negative intercept at TC=0 can e., Seattle, WA; Phoenix, AZ; Washington, DC; and Fresno, CA) sites from 28 April 2001 to 29 December 2004.The number of sample pairs varied from 27 at the Fresno Supersite to 354 at the Seattle site.The total carbonaceous mass (TCM) is calculated by subtracting NO − an estimate for water (H 2 O), and crustal components from the measured PM 2.5 mass.The calculated OCM is derived by subtracting measured EC from TCM: TCM = PM 2.5 −(SO 2− 4 + Retained NO − 3 + NH + 4 + H 2 O + Crustal Material + Blank) (3) OCM = TCM − EC (4) where: Crustal Material = 3.73 × Si + 1.63 × Ca + 2.42 × Fe + 1.94 × Ti (5) Blank = 0.3 − 1.5µg/m 3 for STN/CSN; 0 for IMPROVE All IMPROVE data have been blank-subtracted (in µg/m 3).For STN/CSN, a nominal OC bQF value of 0.3-1.5 µg/m 3 is used for carbon blank subtraction(Frank, 2006), which varies by sampler type.This interval overlaps with the OC bQF of 0.66±0.94µg/m 3 at the Seattle, Phoenix, and Washington, DC, sites; OC bQF for the Figures The limited exposure times (1-15 min) in the STN/CSN and SEARCH networks are of insufficient duration to represent passive adsorption on the sampled filter.Based on both the network averages and collocated-site comparisons, IMPROVE TC bQF (or OC bQF ) ranges from 2.0-2.5 µg/cm 2 , while STN/CSN and SEARCH field blanks are below or close to 1 µg/cm 2 .STN/CSN field and trip blank TC and OC concentrations are within ±5% for site averages.Among the five STN/CSN samplers, URG MASS reports the lowest OC bQF levels.Using non-blank corrected TC from collocated IMPROVE-STN/CSN samplers shows lower STN/CSN than IMPROVE areal densities (µg/cm 2 ) at the same site.Without blank correction, sampling artifacts for STN/CSN samplers in µg/m 3 could be 5-11 times higher than those in IMPROVE, depending on the sampler type.When corrected with respective field blanks, STN/CSN TC concentrations are still higher at most sites, suggesting that STN/CSN field blanks could under-represent the organic Figures ion chromatography (IC).Module C collects PM 2.5 through an AIHL cyclone followed by a 25 mm Pallflex ® Tissuquartz ™ quartz-fiber filter for OC and EC by the IMPROVE A thermal/optical reflectance (TOR) protocol.Module D collects PM 10 through a louvered PM 10 inlet at 16.7 L/min followed by a 25 mm Pall Teflon-membrane filter for mass by gravimetry.SASS (Spiral Aerosol Speciation Sampler, Met One, Grants Pass, OR): Spiral centrifugal impaction inlets were originally used on this sampler (thus the name), but excessive re-entrainment from impaction surfaces caused these to be replaced with sharp-cut cyclones(Watson and Chow, 2009).The Super SASS can contain up to eight parallel channels, but the STN/CSN configuration uses three channels of a five channel version, each channel containing one 47 mm filter with a 6.7 L/min flow rate.For STN/CSN, Channel 1 contains a Whatman Teflon-membrane filter for mass by gravimetry and elements by XRF, Channel 2 can be used for a field blank, Channel 3 includes a magnesium oxide-coated aluminum (Al) honeycomb after the cyclone and followed by a Nylasorb Nylon-membrane filter for water-soluble anions (i.e., NO − 3 and SO 2− 4 ) and cations (i.e., ammonium [NH + 4 ] and water-soluble sodium [Na + ] and potassium [K + ]) by IC, Channel 4 contains a Whatman QMA quartz-fiber filter for OC and EC by the STN thermal/optical transmission (TOT) protocol (Peterson and Richards, 2002), and Channel 5 is available for field blanks or special study samples.

R&P 2300 (
Rupprecht & Patashnick [now Thermo Scientific]  Model 2300; Franklin, MA): Twelve channels are available that can be programmed to be operated in parallel or sequentially.The non-trends CSN sites in Texas use four parallel channels with 47 mm diameter filters.Channel 1 contains a Whatman Teflon-membrane filter with 16.7 L/min for mass by gravimetry and elements by XRF, Channel 2 contains an additional Teflon-membrane filter for anion and cation analyses by IC, Channel 3 contains a quartz-fiber filter, with an optional quartz-fiber backup filter, at 10 L/min for OC and EC by the IMPROVE A TOR protocol, and Channel 4 contains a sodium carbonate-coated honeycomb denuder followed by a Nylasorb Nylon 10 L/min for total NO − 3 by IC.R&P 2025 (Rupprecht & Patashnick [now Thermo Scientific] Model 2025; Franklin, MA): Contains two parallel modules operated in a sequential mode using 47 mm diameter filters at 16.7 L/min.Filters are stored in a 16 cassette magazine.Both modules are preceded by a louvered PM 10 inlet followed by a sharp cut cyclone PM 2.5 inlet.Module 1 contains a Whatman Teflon-membrane filter for mass by gravimetry, elements by XRF, and cations and anions by ion chromatography.Module 2 contains a quartz-fiber filter for OC and EC by the IMPROVE A TOR protocol.PCM3 (Particle Composition Monitor, Aerosol Research Associates, Plano, TX;Edgerton et al., 2005): Uses three parallel channels operated at 16.7 L/min with a URG PM 10 cyclone followed by a PM 2.5 WINS impactor.Solenoid valves behind the filter packs allow up to four sample sets to be acquired sequentially.Channel 1 contains sodium carbonate-coated annular denuder followed by a citric acid-coated annular denuder, then followed by three-stage 47 mm filter packs including a Whatman Teflon-membrane filter for mass by gravimetry and elements by XRF, followed by a Nylasorb Nylon-membrane filter for volatilized NO − 3 by IC, then followed by a citric acid-impregnated filter for volatilized NH Fig 1.

Fig. 2 .
Fig. 2. Field blank organic carbon (OC bQF ) concentration density (µg/cm 2 ) for: (a) 181 IM-PROVE sites, (b) 239 STN/CSN sites, and (c) 8 SEARCH sites for the period from 1 January 2005 to 31 December 2006 (each bar represents the concentration sector less than or equal to the assigned value).

Fig. 5 .
Fig. 5. Comparison of quartz-fiber backup filter (QBQ) carbon fractions between the urban and non-urban sites in the IMPROVE and SEARCH networks for the period from 1 January 2005 to 31 December 2006.Carbon fractions follow the IMPROVE A thermal/optical reflectance (TOR) protocol(Chow et al., 2007).

Fig. 6 .
Fig. 6.Time series of IMPROVE and STN/CSN blank total carbon (TC) concentrations at eight collocated sites from

Fig. 7 .
Fig. 7. Linear regression of non-blank corrected STN/CSN TC vs. IMPROVE TC acquired from the Phoenix, AZ site (PHOE1).The non-zero intercept indicates the sampling artifacts between STN/CSN and the IMPROVE samplers.

Table 3
compares average bQF levels for TC, OC, and EC in terms of areal density (µg/cm 2 ) and ambient concentration equivalents (µg/m 3 , based on exposed filter areas and 24 h sample volumes for each instrument).EC values are at or near detection limits, indicating that passive PM deposition is negligible.As expected, TC and OC are nearly the same and can be used interchangeably.Average bQF levels for individual sampling sites and the number of bQF acquired for IMPROVE and STN/CSN are available as supplemental information (Tables S1-S4 http://www.atmos-chem-phys-discuss.net/9/27359/2009/

Table 3 .
Using the URG MASS sampler, TC tbQF areal Figures

Table 1 .
duration for filter saturation of adsorbed gases; 2) dependence of adsorbed gas saturation on particle composition, temperature, relative humidity, and sampling face velocity; 3) evaporation rates of semi-volatile organic compounds during sampling; and 4) source-specific tests (e.g., diesel, gasoline, and wood smoke).Figures Techniques and Applications, edited by: Willeke, K. and Baron, P., accepted, 2009.Watson, J. G., Chow, J. C., Chen, L.-W. A., and Frank, N. H.: Methods to assess carbonaceous aerosol sampling artifacts for IMPROVE and other long-term networks, J. Air Waste Manage., 59(8), 898-911, 2009.White, W. H. and Roberts, P. T.: On the nature and origins of visibility-reducing aerosols in the Introduction Sampling protocols for carbon in the IMPROVE, STN/CSN, and SEARCH networks from 1 January 2005 to 31 December 2006.
pair: urban Gulfport (GLF) in Gulfport and non-urban Oak Grove (OAK) near Hattiesburg; Alabama pair: urban Birmingham (BHM) in North Birmingham and non-urban Centreville (CTR) south of Tuscaloosa; Georgia pair: urban Jefferson Street (JST) in Atlanta and non-urban Yorkville (YRK) northwest of Atlanta; and Florida pair: urban Pensacola (PNS) in Pensacola and suburban outlying field (OLF) northwest of Pensacola).Sampling every third day except for daily at the BHM and JST sites.b Sampler Type IMPROVE (New units manufactured by special order from URG, Inc. (Chapel Hill, NC)): Four parallel filter modules, each with up to four sequential sample sets

Table 1 .
Watson and Chow, 2002)bient Air Sampler, Andersen [now Thermo Scientific] Model 25-400; Franklin, MA, no longer manufactured;Watson and Chow, 2002): Contains four parallel channels with two 2.5 µm AIHL cyclones; all filters are 47 mm in diameter.In the STN/CSN configuration, only three channels are used: Channel 1 contains a Whatman QMA quartz-fiber filter at 7.3 L/min for OC and EC by the STN TOT protocol, Channel 2 contains a Whatman Teflon-membrane filter at 16.7 L/min for mass by gravimetry and elements by XRF, Channel 3 is empty, but can be used for blanks or replicates at a flow of 16.7 L/min, and Channel 4 contains a magnesium oxidecoated denuder followed by a Whatman Nylasorb Nylon-membrane filter at a flow rate of 7.3 L/min for total nitrate by IC.URG MASS (URG, Chapel Hill, NC): Uses two parallel modules with 47 mm filters operating at 16.7 L/min.Module 1 includes a louvered PM 10 inlet followed by a PM 2.5 WINS impactor, a magnesium oxide-coated denuder, and a stacked filter pack with a Whatman Teflon-membrane filter on top for mass by gravimetry and elements by XRF followed by a Pall Nylasorb Nylon-membrane backup filter for anions and cations by IC.Module 2 contains a louvered PM 10 inlet followed by a WINS PM 2.5 impactor, which includes a Whatman QMA quartz-fiber filter for OC and EC by the STN TOT protocol.
b Sampler Type, continued RAAS (

Table 2 .
Collocated IMPROVE and STN/CSN PM 2.5 speciation data from 16 October 2001 to 31 December 2006.
a See Table1 forsampler specifications.b The completed 2005 and 2006 data would provide 308 sample pairs and 49 field blanks.c Big Bend NP (BIBE) is a CSN site.Introduction

Table 3 .
Comparison of average field blank (bQF), trip blank (tbQF), and backup (QBQ) filter carbon levels (±standard deviation) among the IMPROVE, STN/CSN, and SEARCH networks for the period from 1 January 2005 to 31 December 2006.
b 253 if counting 14 sites where sampler type changed between 1 January 2005 and 31 December 2006.
c Data is not available.Introduction

Table 4 .
Average blank TC concentrations for the eight collocated IMPROVE-STN/CSN sites.
for STN/CSN.Introduction

Table 5 .
Robust regression statistics of non-blank corrected STN/CSN TC versus IMPROVE TC for data from the eight collocated sites.

Table 6 .
Comparison between estimated and measured sampling artifact for the eight collocated IMPROVE/STN sites.
a IMPROVE field blanks b STN/CSN field blanks c Estimated STN/CSN artifact=conversion factor (

Table 5 )
×IMP TC bQF +areal density intercept (Table5), assuming that IMP TC bQF fully explains the sampling artifact of the IMPROVE network.d measured STN/CSN TC bQF −calculated TC STN art calculated TC STN art

Table 7 .
Estimates of organic carbon mass (OCM) based on the SANDWICH method for the four collocated IMPROVE-STN/CSN sites.