UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TRIANGLE PARK NC 27711

June 14, 2012 

 OFFICE OF 

 AIR QUALITY PLANNING 

 AND STANDARDS 

MEMORANDUM

SUBJECT: Recommendations to Users of CSN and IMPROVE Speciation Data Regarding 

 Sampling Artifact Correction for PM2.5 Organic Carbon 

FROM: Neil Frank, Air Quality Analysis Group, EPA (C304-04) /s/ 

TO: PM NAAQS Review Docket EPA-HQ-OAR-2007-0492

As part of the review of the national ambient air quality standards (NAAQS) for particulate 

matter (PM) completed in 1997, EPA established a PM2.5 Chemical Speciation Network (CSN) 

to conduct routine speciation monitoring, primarily in urban areas across the United States 

(U.S.). Similar to the CSN, the Interagency Monitoring of PROtected Visual Environments 

(IMPROVE) program, a cooperative measurement effort managed by a steering committee 

composed of representatives from federal, regional, and state organizations, also conducts 

routine PM2.5 speciation monitoring, primarily in non-urban areas across the U.S.1 

For a number of years, EPA and IMPROVE Steering Committee representatives have been 

working together to ensure uniformity between the measurement and analytical procedures used 

in the CSN and IMPROVE programs. 

In 2011, a committee was formed by the IMPROVE Steering Committee to review the existing 

procedures used by the IMPROVE program to adjust PM2.5 organic carbon (OC) measurements 

for sampling artifact and to identify appropriate procedures that might be adopted for the U.S. 

EPA’s CSN data to ensure comparability and consistency with the similar data collected by 

IMPROVE. The committee consists of representatives of EPA, National Park Service, Desert 

Research Institute, Cooperative Institute for Research in the Atmosphere (CIRA) and University 

of California-Davis. IMPROVE publicly reports its OC data with an adjustment for sampling 

artifact; while the EPA currently reports it OC data without such an adjustment. The attached 

document, issued in January 2012, provides the Committee’s rationale and recommendations for 

those adjustments. 

 

1

 The IMPROVE Steering Committee includes representatives from EPA, the National Oceanic and Atmospheric 

Administration, the Fish and Wildlife Service, the Bureau of Land Management, the National Park Service, the U.S. 

Forest Service, National Park Service, the National Association of Clean Air Agencies, Northeast States for 

Coordinated Air Use Management, Western States Air Resources Council, Mid-Atlantic Regional Air Management 

Association, and the State of Arizona. 
Prior to the Committee’s recommendations being issued, EPA reflected on the ongoing carbon 

measurement work of the IMPROVE program when conducting analyses as part of its ongoing 

review of the PM NAAQS. Specifically, in considering the adequacy of the current secondary 

(welfare-related) PM standards to provide protection against PM-related visibility impairment as 

well as alternative standards that were appropriate to consider based upon the available scientific 

information, the EPA staff made a nominal adjustment to the CSN’s measured OC based on 

sampler-specific network-wide field blanks (US EPA, 2011, Chapter 4, Appendix G).2

 

For the aforementioned policy assessment, a nominal value of 0.4 μg/m3 was used to adjust OC 

measurements which were derived with EPA’s IMPROVE-like monitoring method in the CSN. 

However, the attached technical committee report describes lower adjustment values based on 

monthly median backup filters or field blanks. Those would result in adjustments of ~0.32 μg/m3

and 0.12 μg/ m3

, respectively, and the latter approach is tentatively recommended to adjust the 

data from both networks. Assuming a multiplier of 1.4 to convert adjusted OC into estimated 

extinction, the use of the two adjustment approaches both result in slightly higher estimate of 

organic mass (+0.1 to + 0.4 ug/m3

) and estimated extinction (+ 0.4 to + 1.6 Mm-1). Nevertheless, 

the latter change in visibility for locations with higher calculated extinction is extremely small 

for either approach when the results are expressed in deciview (dv) units. If the total aerosolbased extinction using the IMPROVE algorithm is 160 Mm-1 (28.3 dv), the change in calculated 

extinction using the alternative backup filter or field blank derived OC adjustment both result in 

a value of 28.4 dv (+ 0.1 dv). Thus, the tentatively recommended field blank approach is 

reasonable for the purposes of PM-related visibility assessment. 

Attachment 

 

2 US EPA (2011). Policy Assessment for the Review of the Particulate Matter National Ambient 

Air Quality Standards. Office of Air Quality Planning and Standards. U.S. Environmental 

Protection Agency, Research Triangle Park, NC. EPA 452/R-11-003. April 2011. Available: 

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pa.html. 
Rationale and Recommendations for Sampling Artifact Correction for 

PM2.5 Organic Carbon 

Ann M. Dillner1, Mark Green2

Bret Schichtel3

, Bill Malm4, Joann Rice5, Neil Frank5, 

Judy Chow2, John Watson2, Warren White1, Marc Pitchford2

January 2012 

1

 University of California, Davis 2 Desert Research Institute, 3 US Park Service, 4 Cooperative Institute for Research in the 

Atmosphere – CIRA, 5

 US Environmental Protection Agency. 

Background 

Organic sampling artifacts occur during PM2.5 sampling in the U.S. long-term urban Chemical 

Speciation Network (CSN) and non-urban Interagency Monitoring of PROtected Visual 

Environments (IMPROVE) networks. Organic sampling artifacts are interferences in the 

collection and quantification of particulate organic carbon (OC) due to adsorption and desorption 

of volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) on to the 

quartz-fiber filter substrate. Estimates of organic sampling artifacts and their uncertainties are 

needed to estimate the mass contribution of particulate carbon in the calculation of aerosol-based 

light extinction with the IMPROVE algorithms in urban or rural areas (Pitchford et al., 2007). In 

addition, estimates of sampling artifacts are needed to understand compliance with the massbased PM2.5 National Ambient Air Quality Standard (NAAQS) and to determine long-term 

trends and chemical mass closure. 

Organic vapor adsorption can occur during active sampling (with air drawn through the filter) 

and passive exposure (zero face velocity) (Chow et al., 2010; Watson et al., 2009). These vapors 

leave the sample during thermal analyses (Chow et al., 2007; 2011) and are interpreted as part of 

the measured particulate OC. This positive (additive) OC artifact yields higher values for PM2.5

OC samples than exist in ambient air. SVOCs can be initially collected as particles, but portions 

can evaporate during sampling because of increases in temperature or decreases of their gasphase concentrations in the sampled air, causing the particles to leave the filter and become part 

of the gas stream (Mader and Pankow, 2001). Evaporated SVOCs from sampled particles yield 

negative OC artifacts which tend to be multiplicative and could be reported as part of the 

particulate mass to represent ambient concentrations. A large number of investigations have been 

performed to address organic sampling artifacts based on: passive field blanks (FB), backup 

quartz-fiber filters (QBQ) placed behind the front quartz-fiber filter (QF), pre-filter organic 

denuders, and OC to PM2.5 mass regression analyses, but the results are not consistent among the 

different studies. 

CSN and IMPROVE Sampling, Analysis and Artifact Corrections 

Efforts have been made since 2007 to obtain consistency of PM2.5 carbon sampling and analyses 

between CSN and the IMPROVE network. As of October 2009, the URG3000N sampler (URG 

Corporation, Chapel Hill, NC; modified IMPROVE carbon sampler) has been operating at all 

CSN sites. The sampler uses 25 mm Pallflex® TissuquartzTM filters at a nominal flow rate of 22 

L/min for OC and elemental carbon (EC) detection following the IMPROVE_A thermal/optical 
reflectance (TOR) protocol (Chow et al., 2007). This approach is consistent with procedures 

applied in the IMPROVE network. 

Currently, carbon data is not artifact-corrected for CSN. For the IMPROVE network, monthly 

median OC from the quartz-fiber backup filter (OCQBQ) is subtracted from all front filters in the 

network to correct positive organic sampling artifact. This approach assumes negative sampling 

artifacts are negligible. No artifact correction is made for EC. 

For CSN, FB and QBQ filters were collected at a frequency of 20 percent at all sites during 

2007–2010 and then reduced to a 10 percent frequency as of January 2011. For IMPROVE, 

QBQ were collected at six sites (1988 to August 2008), which increased to 12 sites with 13 sets 

of blanks (including 3 urban sites and one collocated sampler in Phoenix, AZ) from September 

2008 onward. FB were collected at ~2–5 percent frequency at all sites from 1988 to 2008, and 

switched to the 12 QBQ sites after August 2008. Analysis of OCFB and OCQBQ for CSN and 

IMPROVE showed that: 

• For CSN, median OCFB (0.12 μg/m3

) is about 60 percent lower than median OCQBQ (0.32 

μg/m3

), accounting for 6.7 percent and 17.9 percent of OCQF concentrations (1.79 μg/m3, 

n=5043) during 2008–2010. 

• For IMPROVE during 2005-2006, the median OCFB (0.24 μg/m3) was ~12 percent lower 

than OCQBQ (0.28 μg/m3) accounting for 24 percent and 26 percent of OCQF concentrations 

(1.03 μg/m3, n=41588) respectively. From September, 2008 through December 2010, the 

median OCFB (0.13 μg/m3

) was 43 percent lower than OCQBQ (0.22 μg/m3) accounting for 16 

percent and 28 percent of OCQF concentrations (0.81 μg/m3, n=47526) respectively. 

• OCFB levels are independent of front quartz-fiber filter OC, for both the CSN and the 

IMPROVE networks. 

• OCFB levels are lower than OCQBQ so they represent a lower bound on the additive artifact. 

• OCQBQ increase with increasing OCQF concentrations at some CSN and at all 12 QBQ 

IMPROVE sites and therefore overestimate the additive artifact. 

• OCFB are more consistent between sites and by season or day than OCQBQ..

• There is a decreasing trend in median OCF, OCFB and OCQBQ from 2005 to 2010 for the 

IMPROVE sites. Median OCF was 1.07 μg/m3

 in 2005 and 0.80 μg/m3 in 2010. OCQBQ was 

0.29 μg/m3

 in 2005 and 0.22 μg/m3 in 2010. OCFB was 0.25 μg/m3 in 2005, and 0.12 μg/m3

in 2010. 

• A sharp decrease in IMPROVE OCFB was found during August 2008 as network FB 

collection is switched from a random selection of 2–5 percent of the sites to the 12 QBQ 

sites. Data analysis shows that changing the sites of collection for the FB is not likely the 

cause of the decrease. 

Recommendation 

Based on the above preliminary analyses, we tentatively recommend applying a monthly 

network median OCFB for artifact correction for the CSN and IMPROVE networks, based on 

each networks OCFB data. The use of OCFB appears to adequately address the additive artifact. A 

data adjustment for the multiplicative artifact can be incorporated into the multiplicative 
conversion of particulate organic carbon into particulate organic mass. For the original 

IMPROVE algorithm, this adjustment uses a 1.4 multiplier. The revised IMPROVE algorithm 

uses 1.8 (Pitchford et al, 2007). For its urban locations, CSN uses a 1.4 multiplier. 

Additional Measurements and Analyses 

It is recognized that these proposed artifact corrections utilizing a monthly median OCFB may 

change upon additional field experiments and data analyses. The following questions will be 

addressed to better understand the behavior of the OCFB in IMPROVE and CSN: 

• Have there been changes in the physical properties of the quartz-fiber filters used by both 

CSN and IMPROVE (manufacturer and product purchased has not changed during the time 

periods addressed in this report) that caused changes in the OCFB, particularly the sharp 

decrease observed in 2008? Tests to address this question include (but are not limited to): 

o Weigh archived laboratory quartz-fiber filter blanks from different manufacturer lots 

used in IMPROVE (2005-2010) and CSN (2008-2010) as an indicator of the reactive 

surface area available for organic vapor adsorption. 

o Perform optical microscopy and possibly other analyses on the archived filters to 

evaluate if physical properties have changed. 

o Determine if there is a relationship between mass or other properties of the filter and 

measured OCFB. 

• Did operational changes (changes from single to double quartz filter blanks, changes in 

sampler operations) in the IMPROVE network cause the sharp decrease in QF values 2008? 

Tests to address this question include (but are not limited to): 

o Single field blanks and double field blanks will be collected in parallel in the 

IMPROVE network and evaluated for differences. 

o Samples collected using the old sampler protocol and with the new sampler protocol 

will be collected in parallel and evaluated for differences. 

o Data from IMPROVE sites with FBs will be evaluated just prior to and just after the 

change in sampler protocol. 

References 

 

Chow, J.C.; Watson, J.G.; Chen, L.-W.A.; Chang, M.C.O.; Robinson, N.F.; Trimble, D.L.; Kohl, S.D. (2007). The 

IMPROVE_A temperature protocol for thermal/optical carbon analysis: Maintaining consistency with a longterm database. J. Air Waste Manage. Assoc., 57(9): 1014-1023. 

http://pubs.awma.org/gsearch/journal/2007/9/10.3155-1047-3289.57.9.1014.pdf. 

Chow, J.C.; Watson, J.G.; Chen, L.W.A.; Rice, J.; Frank, N.H. (2010). Quantification of PM2.5 organic carbon 

sampling artifacts in US networks. Atmos. Chem. Phys., 10(12): 5223-5239. http://www.atmos-chemphys.net/10/5223/2010/acp-10-5223-2010.pdf. 

Chow, J.C.; Watson, J.G.; Robles, J.; Wang, X.L.; Chen, L.-W.A.; Trimble, D.L.; Kohl, S.D.; Tropp, R.J.; Fung, 

K.K. (2011). Quality assurance and quality control for thermal/optical analysis of aerosol samples for organic 

and elemental carbon. Anal. Bioanal. Chem., 401(10): online. DOI 10.1007/s00216-011-5103-3. 

http://www.springerlink.com/content/a60122x13p757421/fulltext.pdf. 

Mader, B.T.; Pankow, J.F. (2001). Gas/solid partitioning of semivolatile organic compounds (SOCs) to air filters - 3. 

An analysis of gas adsorption artifacts in measurements of atmospheric SOCs and organic carbon (OC) when 

using Teflon membrane filters and quartz fiber filters. Environ. Sci. Technol., 35(17): 3422-3432. 

Pitchford M; Maim W; Schichtel B; Kumar N; Lowenthal D; Hand J (2007). Revised algorithm for 

estimating light extinction from IMPROVE particle speciation data. J Air Waste Manag Assoc, 57: 

1326-36. 098066
Watson, J.G.; Chow, J.C.; Chen, L.-W.A.; Frank, N.H. (2009). Methods to assess carbonaceous aerosol sampling 

artifacts for IMPROVE and other long-term networks. J. Air Waste Manage. Assoc., 59(8): 898-911. 

http://pubs.awma.org/gsearch/journal/2009/8/10.3155-1047-3289.59.8.898.pdf.