Carbon Measurements and Adjustments

Measurement of organics by IMPROVE & STN networks, Use of blank data to correct carbon concentration measurements, Planned changes by EPA,Implications for continuity in longitudinal analyses

Neil Frank USEPA/OAQPS

For presentation at Air Quality Data in Health Effects Research

Nov 30 – Dec 1, 2006

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Carbon Measurements by STN and IMPROVEFactors that affect estimated ambient carbon concentrations

• Analytical protocol (thermal optical technique) – STN: NIOSH-type TOT method, OC/EC and 4 OC sub-fractions– IMPROVE: DRI TOR method, OC/EC, 5 OC and 3 EC sub-fractions

• Collection device (field samplers)– Quartz filters without denuders– Different flow rates, face velocity and different artifacts

• Sampler location (rural vs urban and intra-urban)• Data Reporting (& adjustments)

– STN (2001- )• AQS: without blank or other adjustments, field blanks after 2003• Air Explorer web site: with adjustments

– http://www.epa.gov/airexplorer/

– IMPROVE (1990- )• VIEWS web site: with adjustments

– http://vista.cira.colostate.edu/views/

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Carbonaceous concentrations from measured C can be a very uncertain calculation

Many sources of error beyond analytical uncertainty

• Blank correction, b, to adjust for sampling artifacts– STN network uses different samplers & without backup filters

• Avg value derived from field blanks provides a crude estimate– can result in negative daily values

• Adjustments vary among 4 different EPA urban speciation samplers (Most are MetOne)

• Large field blank variability may not justify site or seasonal adjustments• These network values are in same ballpark as intercept of regressing OC vs. PM2.5

• The above values are used to adjust OC measurement data on Air Explorer and in EPA analyses

– IMPROVE: • OC artifact adjustments are relatively small (on average, ~0.3-0.5 ug/m3)

– Sampler has smaller filter, higher flow rate and uses backup filters• Blank adjusted using monthly network median backup filter by carbon fraction

ug C/ filter ug C/m3

MetOne (SASS) 9.6 14.8 1.5Anderson (RAAS) 10.4 13.5 1.3R&P 2300 14.4 16.1 1.1URG (MASS) 24 7.7 0.3

Network Average Total Carbon on STN Blank Filters* 24-hr sample

Volume(m3)STN sampler*Network-wide values from 2001-02 data as reported by RTI

Recent investigations reveal changes after 2003 (see next slide)

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sampler_type=Anderson RASS

filte

r bl

ank,

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l car

bon

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/m3)

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2001 2002 2003 2004 2005 2006

Sampler sites 2001 2002 2003 2004 2005Anderson RASS 22 1.19 1.50 1.03 0.90 0.99METONE SASS 140 1.53 1.63 1.21 1.01 1.09URG MAAS 5 0.29 0.34 0.25 0.26 0.43

Trends in STN field blank levels should be consideredSince 2002, STN field and trip blanks values for Met One (SASS) and Anderson (RAAS) samplers have decreased 33% and values for URG (MASS) samplers have increased 25%. Reasons for the changes are not known at this time.

Preliminary Average Total Carbon Blank by Year, ug/m3 (subset of sites with long-term data)

Met One Field & Trip Blanks

(Data from RTI and AQS.)

Most prevalent

Most like IMPROVE flowrate

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Carbon Mass is Different than Measured CarbonTotal Carbonaceous Mass (TCM) is generally defined as k*(OC-b)+EC

• Mass multiplier, k, varies with aerosol type and mix– 1.6 ± 0.2 (urban) – 2.1 ± 0.2 (rural) [Weighted average needed for mixed urban/regional aerosol]– 2.4 ± 0.2 (smoke)– Turpin's above estimates (2001) are based on limited OC speciation data– 1.4 has been used for IMPROVE and STN. New IMPROVE algorithm uses 1.8

• Uncertainty in k is ~ + 33% (relative to 1.8) • Multiplier may also depend on

– Analytical protocol • More OC with STN than IMPROVE and there is no extra mass assigned to “EC”

– Selection of blank adjustment, “b,” and any unaccounted for concentration related sampling artifacts

• Other sources of uncertainty: Retained carbon mass on FRM Teflon– Volatile or other OC may pass thru Teflon

• FRM has higher face velocity than many STN samplers– Water [20-24% of measured PM2.5 water is assigned to C]– Therefore, other adjustments are needed to represent retained FRM mass

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Inferred FRM Carbon using "SANDWICH“Sulfate, Adjusted Nitrate, Derived Water, Inferred Carbonaceous mass Hybrid

material balance approach

• Total Carbonaceous Mass by material balance: TCMmb = PM2.5 - { [SO4] + [NO3FRM] +[NH4] +[water] + [Crustal] + [FRM blank] }

• Other PM2.5 constituents eg. salt, could also be considered• Water and reduced FRM NO3 are estimated by models.• OCMmb = [TCMmb] – [EC]

• TCMmb explicitly accounts for positive and negative sampling artifacts, OC hydration, and mass multipliers for carbonaceous material retained on FRM Teflon.

• TCMmb is upper estimate for TCM and is subject to error in non-C components.

• This approach has been used by EPA for regulatory modeling• SANDWICHed PM2.5 (2001-05) representing what may have been

measured by the FRM are available on Air Explorer

Reference: Frank, N. Retained Nitrate, Hydrated Sulfates, and Carbonaceous Mass in Federal Reference Method Fine Particulate Matter for Six Eastern U.S. Cities, J. Air & Waste Manage. Assoc. 56: 500–511 (2006)

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SE

PLOT tcm tcm14 ytcm ytcm14

TC

M,

ug/m

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2002 2003 2004 2005 2006

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PLOT tcm tcm14 ytcm ytcm14

TC

M,

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2002 2003 2004 2005 2006

IMW

PLOT tcm tcm14 ytcm ytcm14

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2002 2003 2004 2005 2006

TCMmb tracks 1.4*(OC-b) reasonably wellDisplays similar seasonal pattern but different trends for some regions

Regional quarterly averages and annual trend lines are shown

TCMmb

TCM1.4(old blank)

Industrial MidWest, 11 sites Northeast, 10 sites

Southeast, 11 sites

TCMmb

TCM1.4(old blank)

TCM1.4(old blank)

TCMmb

For these comparisons, blank values “b”, were derived from the RTI report

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NE

PLOT tcm tcm14 tcm14_newytcm ytcm14 ytcm14_new

TC

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ug/m

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2002 2003 2004 2005 2006

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PLOT tcm tcm14 tcm14_newytcm ytcm14 ytcm14_new

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ug/m

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IMW

PLOT tcm tcm14 tcm14_newytcm ytcm14 ytcm14_new

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2002 2003 2004 2005 2006

• Regional average values of TCMmb agree well with 1.4*(OC-b) and year-specific b values - 3rd quarter deviation in SE may reflect SOA (need higher “k”)

• The successful choice of k (e.g. 1.4) to represent retained FRM mass (on Teflon) may depend on the selected value for “b”.• The value for “k” may also depend on the particular thermal analytical technique and OC-EC split. •Recognize that carbon adjustments are uncertain and multiple approaches may be needed

Better agreement with year-specific field blanks TCMmb can help validate choice of “fudge” factors (k and b) and can

provide continuity for longitudinal studies

TCMmb

Industrial MidWest, 11 sites

TCM1.4(yrly blank)

Northeast, 10 sites

Southeast, 11 sites

TCMmb

TCMmb

TCM1.4(yrly blank)

TCM1.4(yrly blank)

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Next Steps• New STN carbon protocol

– For national consistency in ambient carbon monitoring, EPA is switching the STN carbon protocol to the IMPROVE method. STN’s new IMPROVE-like samplers (22 Lpm flow rate) to be phased in over three years will also likely have less carbon sampling artifact than current STN data from SASS and RAAS samplers.

– Health study and NA cities are in phase I implementation• Development of new procedures to adjust carbon

measurements for sampling artifacts– EPA has funded Chow and Watson to recommend procedures to

estimate sampling artifact for the new urban samplers (e.g. using backup filters) and to develop appropriate factors for estimating historical STN OCM and EC consistent with the future measurements.

– EPA has funded Warren White to define total uncertainty (incl. blanks)– Findings are expected Spring 2007

• Carbon trends during transition period– EPA will be using TCMmb to describe trends until appropriate carbon

adjustments and comparability can be established between the old STN and new urban-IMPROVE carbon protocols.