Post on 17-Dec-2015
Evidence for Positive and Negative Evidence for Positive and Negative Organic Sampling ArtifactsOrganic Sampling Artifacts
John G. Watson (john.watson@dri.edu)
Judith C. ChowL.-W. Antony Chen
Desert Research Institute, Reno, NV
Presented at:IMPROVE–CSN Carbon PM Monitoring Workshop
University of California, DavisJanuary 22, 2008
Definition of Organic Sampling ArtifactDefinition of Organic Sampling Artifact
• Fundamental: the difference between filter-based organic matter (OM) and “actual” OM in the atmosphere.
• Practical: the difference between filter-based OM and Teflon-membrane filter OM, which is used to determine PM mass concentration.
Atmospheric Organic Volatility Atmospheric Organic Volatility Categories Span a ContinuumCategories Span a Continuum
PL0 at 20 ºC
10-1 Torr
10-8 Torr
H2O: 17.54
Benzo(e)pyrene: 4.3 10-8
Fluorene: 1.9 10-3
Volatile
Semi-Volatile
(SVOC)
Non-Volatile HULIS, WSOC
Organic Sampling ArtifactsOrganic Sampling Artifacts
• Positive sampling artifact: SVOC is volatilized “before” captureby filters
• Negative sampling artifact: SVOC is volatilized “after” captured by filters
Quartz- or other filter material
Backup fiber CIG Absorbent
Particle (P)
• Particle and gas are in a dynamic equilibrium!
Gas Molecule
CIG: Charcoal-impregnated glass-fiber filter
Early Reports of Negative ArtifactEarly Reports of Negative Artifact• Commins, B.T. (1962). Interim report on the study of techniques for determination of
polycyclic aromatic hydrocarbons in air. Report No. Monograph 9. Prepared by National Cancer Institute.
• Lee, F.S.; Pierson, W.R.; and Ezike, J. (1980). The problem of PAH degradation during filter collection of airborne particulates - An evaluation of several commonly used filter media. In Polynuclear Aromatic Hydrocarbons: The Fourth International Symposium on Analysis, Chemistry and Biology, A. Bjorseth and A.J. Dennis, Eds. Battelle Press, Columbus, OH, pp. 543-563.
• Schwartz, G.P.; Daisey, J.M.; and Lioy, P.J. (1981). Effect of sampling duration on the concentration of particulate organics collected on glass fiber filters. J. Am. Ind. Hyg. Assoc., 42:258-263.
• Galasyn, J.F.; Hornig, J.F.; and Soderberg, R.H. (1984). The loss of PAH from quartz fiber high volume filters. J. Air Poll. Control Assoc., 34(1):57-59.
• van Vaeck, L.; van Cauwenberghe, K.; and Janssens, J. (1984). The gas-particle distribution of organic aerosol constituents: measurements of the volatilisation artifact in Hi-Vol cascade impactor sampling. Atmos. Environ., 18:417-430.
• Coutant, R.W.; Brown, L.L.; Chuang, J.C.; Riggin, R.M.; and Levis, R.G. (1988). Phase distribution and artifact formation in ambient air sampling for polynuclear aromatic hydrocarbons. Atmos. Environ., 22:403-409.
• Eatough, D.J.; Sedar, B.; Lewis, L.; Hansen, L.D.; Lewis, E.A.; and Farber, R.J. (1989). Determination of semivolatile organic compounds in particles in the Grand Canyon area. Aerosol Sci. Technol., 10:438-449.
Early Reports of Positive ArtifactEarly Reports of Positive Artifact• Cadle, S.H.; Groblicki, P.J.; and Mulawa, P.A. (1983). Problems in the
sampling and analysis of carbon particulate. Atmos. Environ., 17(3):593-600.
• McDow, S.R. (1986). The effects of sampling procedures on organic aerosol measurement. Ph.D. Dissertation, Oregon Graduate Center, Beaverton, OR.
• Fung, K.K. (1988). Artifacts in the sampling of ambient organic aerosols, S. Hochheiser and R.K.M. Jayanty, Eds. Air Pollution Control Association, Pittsburgh, PA, pp. 369-376.
• Watson, J.G.; Chow, J.C.; Richards, L.W.; Andersen, S.R.; Houck, J.E.; and Dietrich, D.L. (1988). The 1987-88 Metro Denver Brown Cloud Air Pollution Study, Volume II: Measurements. Report No. 8810.1F2. Prepared for Greater Denver Chamber of Commerce, Denver, CO, by Desert Research Institute, Reno, NV.
Operational Definitions of Particulate OC FParticulate OC = Total – APositive or Negative Sampling Artifact
Filter-Adsorbent (FA) AQF
A or FQFDDenuder-Filter-Adsorbent or -Filter (DFA or DFF)
A E
Electrostatic precipitator (EA)
Filter-Filter-Adsorbent (FFA)
QBQQF A
Several Methods to Compensate for Several Methods to Compensate for Positive ArtifactPositive Artifact
• Do nothing and assume it is zero
• Denude organic gases before sampling and assume it is zero
• Subtract the quartz lab blank
• Subtract the quartz field blank
• Subtract the back half of the filter
• Subtract the quartz backup behind quartz
• Subtract the quartz backup behind Teflon
• Calculate the intercept of OC vs. mass as mass approaches zero (Solomon’s method)
• Subtract weighted ions and elements from mass, assume remainder is carbon. Excess measured carbon is positive artifact (Frank’s SANDWICH)
• Subtract low temperature fractions
IMPROVE Acquires Backup Filters and IMPROVE Acquires Backup Filters and Field BlanksField Blanks
MORA
YOSE
HANC
CHIR
SHEN
OKEF
MORA
YOSE
HANC
CHIR
SHEN
OKEF
FRES
PUSO
PHOE
TONT
WASH
DOSO
BIBE
MORA
YOSE
HANC
CHIR
SHEN
OKEF
MORA
YOSE
HANC
CHIR
SHEN
OKEF
FRES
PUSO
PHOE
TONT
WASH
DOSO
BIBE
• The six circled sites are locations where backup filters are acquired ~6% of the time
• The eight square sites are collocated IMPROVE and STN/CSN sites.
IMPROVE has a Large Number of IMPROVE has a Large Number of Analyzed Blanks and Backup FiltersAnalyzed Blanks and Backup Filters
Between 1/1/2005 and 12/31/2006:• 44,016 samples from the IMPROVE network were
analyzed for OC and EC following the IMPROVE_A protocol
• 959 (2.2% of the total) field blanks were collected at 187 sites (including six collocated sites).
• 1,406 backup filters (i.e., QBQ) were acquired at six sites (i.e., MORA, YOSE, HANC, CHIR, SHEN, and OKEF).
Blank Levels Blank Levels Vary by Season Vary by Season
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Total Carbon Concentration (µg/filter)
Nu
mbe
r o
f Site
s
SummerWinter
IMPROVE Blank Filter (bQF) Loading 1/1/2005 - 12/31/2006
Blank Levels Don’t Depend on Average Blank Levels Don’t Depend on Average Carbon LevelsCarbon Levels(1/05 – 12/06)(1/05 – 12/06)
Averaged blank TC (bQF) compared with concurrent averaged front filter carbon loading in the IMPROVE network. (Only 77 sites with data from > 5 blanks are included.)
0
20
40
60
80
100
120
140
160
180
200
HAVO1
WHPA1
HALE1
DENA1
NOCA1
MELAX
MOZI1
CRLA1
ULBE1
HOOVX
LYBR1
REDW1
CANY1
HECA1
BOAP1
SENEX
CEBL1
BAND1
SAWE1
THBA1
INGA1
SHEN1
COGO1
FOPE1
EVERX
FLAT1
ELLI1
HEGL1
SAMA1
JOSH1
AGTI1
CABA1
CADI1
DOUG1
BRIG1
SIKE1
PITT1
FRES1
MONT1
Sampling Sites
Car
bo
n C
on
cen
trat
ion
( g
/fil
ter)
EC3
EC2
EC1
OC4
OC3
OC2
OC1
blk TC (BLKTC)
Act
ive
Sam
pli
ng
Pas
sive
D
epo
siti
on
Blank OC Levels Don’t Show a Spatial Blank OC Levels Don’t Show a Spatial PatternPattern
*Blanks Acquired between 01/05 and 05/06
Spring (March – May)
Summer (June – August)
Fall (September through November)
Winter (December through February)
IMPROVE Field Blanks IMPROVE Field Blanks Stay Longer than Those of Stay Longer than Those of
Other NetworksOther Networks (1/1/2005 – 12/31/2006)(1/1/2005 – 12/31/2006)
IMPROVE
STN/CSN
SEARCH
Blank Deposition Period
~every 7 days (once per week)
Varies (~1-15 minutes) with exceptions (~5-7 days)
Varies (~1-15 minutes)
0
10
20
30
40
50
60
70
80
90
100
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Average Organic Carbon Field Blank Concentration(µg/cm2)
Num
ber
of S
ites
IMPROVE(181 Sites)
0
20
40
60
80
100
120
140
160
180
200
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Average Organic Carbon Field Blank Concentration
(µg/cm2)
Nu
mb
er
of
Sit
es
STN/CSN(239 Sites)
0
1
2
3
4
5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Average Organic Carbon Field Blank Concentration
(µg/cm2)
Nu
mb
er o
f S
ites
SEARCH(8 Sites)
IMPROVE Field Blank Carbon is IMPROVE Field Blank Carbon is Higher than that for STN/CSNHigher than that for STN/CSN
IMP_bQF: IMPROVE field blanks
STN_FB: STN/CSN field
blanks
STN_TB: STN/CSN trip blanks
PUSO1
0
1
2
3
4
10/16/2001 1/16/2002 4/16/2002 7/16/2002 10/16/2002 1/16/2003 4/16/2003 7/16/2003 10/16/2003
Bla
nk
Car
bo
n ( g
/cm
2) IMP_bQF
STN_FBSTN_TB
MORA1
0
1
2
3
4
10/16/2001 1/16/2002 4/16/2002 7/16/2002 10/16/2002 1/16/2003 4/16/2003 7/16/2003 10/16/2003
Bla
nk
Car
bo
n ( g
/cm
2 ) IMP_bQFSTN_FBSTN_TB
PHOE1
0
1
2
3
4
10/16/2001 1/16/2002 4/16/2002 7/16/2002 10/16/2002 1/16/2003 4/16/2003 7/16/2003 10/16/2003
Bla
nk
Car
bo
n ( g
/cm
2 ) IMP_bQFSTN_FBSTN_TB
TONT1
0
1
2
3
4
10/16/2001 1/16/2002 4/16/2002 7/16/2002 10/16/2002 1/16/2003 4/16/2003 7/16/2003 10/16/2003
Bla
nk
Car
bo
n ( g
/cm
2 ) IMP_bQFSTN_FBSTN_TB
IMPROVE Field Blank TC is Higher IMPROVE Field Blank TC is Higher than STN/CSN than STN/CSN (continued)(continued)
IMP_bQF: IMPROVE field blanks
STN_FB: STN/CSN field
blanks
STN_TB: STN/CSN trip blanks
WASH1
0
1
2
3
4
10/16/2001 1/16/2002 4/16/2002 7/16/2002 10/16/2002 1/16/2003 4/16/2003 7/16/2003 10/16/2003
Bla
nk
Car
bo
n ( g
/cm
2 ) IMP_bQFSTN_FBSTN_TB
DOSO1
0
1
2
3
4
10/16/2001 1/16/2002 4/16/2002 7/16/2002 10/16/2002 1/16/2003 4/16/2003 7/16/2003 10/16/2003
Bla
nk
Car
bo
n ( g
/cm
2 ) IMP_bQFSTN_FB
STN_TB
BIBE1
0
1
2
3
4
1/1/2005 4/1/2005 7/1/2005 10/1/2005 1/1/2006 4/1/2006 7/1/2006 10/1/2006
Bla
nk
Car
bo
n ( g
/cm
2 ) IMP_bQFSTN_FB
STN_TB
FRES1
0
1
2
3
4
1/1/2005 4/1/2005 7/1/2005 10/1/2005 1/1/2006 4/1/2006 7/1/2006 10/1/2006
Bla
nk
Car
bo
n ( g
/cm
2 ) IMP_bQFSTN_FB
STN_TB
But STN/CSN OC Artifact Correction is Higher But STN/CSN OC Artifact Correction is Higher than IMPROVE due to Lower Flow Rates and than IMPROVE due to Lower Flow Rates and
Larger Filter Area (Intercept method)Larger Filter Area (Intercept method)
0.0
0.5
1.0
1.5
2.0
2.5
Sea
ttle
, W
A
Mt.
Rai
nier
,W
A
Pho
enix
, A
Z
Ton
to N
atnl
Mon
, A
Z
Was
hing
ton,
DC
Dol
ly S
odds
Wld
rns,
WV
Fre
sno,
CA
Big
Ben
dN
P,
TX
ST
N-I
MP
TC
Int
erce
pt ( g
/m3 )
AllSpringSummer FallWinter
STN = a(IMPROVE) + b
0.0
0.5
1.0
1.5
2.0
2.5
3.0
TC OC EC OC1 OC2 OC3 OC4 EC1 EC2 EC3
Carbon Fractions
Co
nce
ntra
tion
(µg
/cm
2 )
IMPROVE Urban
IMPROVE Rural
STN/CSN Urban
SEARCH Non-Urban
SEARCH Urban
Fractions up to OC4 can be found on blank filters
More OC on Blanks is in Low Temperature More OC on Blanks is in Low Temperature OC Fractions, but there is also Blank OC at OC Fractions, but there is also Blank OC at
High TemperaturesHigh Temperatures
ImplicationsImplications
• Blank filter does not reach equilibrium with organic gases within a few minutes of atmospheric exposure (i.e., STN/CSN approach).
• At most ambient conditions, the blank filter is close to saturation with VOCs after the equilibrium is attained
• The equilibrium/saturation may depend on ambient temperature.
IMPROVE Blank OC and Backup OC Agree in IMPROVE Blank OC and Backup OC Agree in Winter, but Not in SummerWinter, but Not in Summer
0
2
4
6
8
10
12
14
16
18
20
1/1/02 1/1/03 1/1/04 1/1/05 1/1/06
carb
on o
n ba
ckup
filt
er o
r fie
ld b
lank
, ug
CS
FB
Series3
OC
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
1/1/02 1/1/03 1/1/04 1/1/05 1/1/06
carb
on o
n ba
ckup
filt
er o
r fie
ld b
lank
, u
g CS
FB
Series3
O1
0
1
2
3
4
5
6
7
8
9
10
1/1/02 1/1/03 1/1/04 1/1/05 1/1/06
carb
on o
n ba
ckup
filt
er o
r fie
ld b
lank
, ug
CS
FB
Series3
O3
QBQ and bQF OC agree well in winter, but more OC is found on QBQ in summer!
(Adapted from Warren White 2007)
QBQ
bQF
QBQ
bQF
QBQ
bQF
IMPROVE Negative Artifact is SmallIMPROVE Negative Artifact is Small
0
5
10
15
20
25
30
35
Spring Summer Fall Winter
OC
Co
nce
ntr
atio
n (m g
/fil
ter)
Average Intercept OCAverage QBQAverage bQF
Average across 163 IMPROVE sites; QBQ is only available at six sites.
OC = a(Mass) + b
*If volatilization (negative artifact) is negligible, we expect to see the average Intercept OC agree with QBQ or bQF OC (representing the positive sampling artifact).
A Conceptual ModelA Conceptual Model
(40)
(20)
0
20
40
60
80
100
120
140
160
Teflon Quartz Teflon Quartz
Summer Winter
OC
Con
cent
ratio
n (
m g/fi
lter)
pSVOC (volatilized)
pSVOC (retained)
pOC
VOC and gSVOC (adsorption)
• Teflon filter is not subject to positive sampling artifact
• More volatilization on Teflon filters, resulting in a higher negative sampling artifact
• Cannot rule out the volatilization from quartz-fiber filters
• Volatilization is stronger in summer than in winter
• The volatilized OC is not always recaptured by the backup filter (same for positive sampling artifact)
Key Question:Key Question:• Is the difference between QBQ and bQF OC due to positive
or negative sampling artifact?
Excess OC on the backup filter (with respect to the field blank) correlated well with ambient PM filter mass loading (from Jay Turner, 2006)
-10
-5
0
5
10
15
20
25
30
35
40
1 10 100 1000 10000
CP
CS
- F
BOC
QB
Q-b
QF
QF
Organic artifact may be estimated by Organic artifact may be estimated by slicing the bottom half of the quartz-slicing the bottom half of the quartz-
fiber filterfiber filter
•Filter slicer
Procedure:
1. Analyze a whole punch
2. Acquire another punch from the same filter and weight the whole punch
3. Slice the punch and weight each of the two halves
4. Analyze both halves for carbon concentration
5. Estimate sampling artifact by scaling carbon measured on the bottom-half filter to the whole filter
Similar OC between bottom half of QF and QBQSimilar OC between bottom half of QF and QBQ
SHEN1 2005/1/13 (Q89488)
0
10
20
30
40
50
60
70
0 1 2 3 4Slice Mass (mg)
Car
bon
Load
ing
(ug)
CHIR1 2005/4/7(Q94596)
0
5
10
15
20
25
0 1 2 3 4
Slice Mass (mg)
Car
bon
Load
ing
(ug)
Pattern of Sliced Filter Carbon Loading (I)
QF
QBQ
Original QF analysis
QFtop or QBQtop
QFbott or QBQbott
SHEN1 5/17/2005 (Q93898)
0
10
20
30
40
50
60
0 1 2 3 4Slice Mass (mg)
Car
bon
Load
ing
(ug) Front Filter
Backup Filter
YOSE1 2006/2/16 (R14098)
0
5
10
15
20
25
30
0 1 2 3 4Slice Mass (mg)
Car
bon
Load
ing
(ug)
Higher OC in bottom half of QF than QBQHigher OC in bottom half of QF than QBQ
Pattern of Sliced Filter Carbon Loading (II)
QF
QBQ
QFtop or QBQtop
Original QF analysis
QFbott or QBQbott
ConclusionsConclusions
• Blank levels are higher in summer, lower in winter, but have no consistent spatial pattern.
• Blank filter artifact contains high temperature OC (i.e., OC4 at 580 °C), suggesting changes in thermo/chemical properties of VOCs after adsorption.
• Short (a few minutes) blank filter exposure in CSN/STN and the SEARCH network underestimates actual positive OC artifact.
Conclusions Conclusions (continued)(continued)
• In rural areas and during winter, backup filters (QBQ) resemble blank filters (bQF) with respect to carbon loading, possibly due to less SVOC.
• Negative artifact may be more for Teflon than for quartz filters (especially in summer).
• OC artifact on the bottom-half of sliced filter (QFbott) are similar to or higher than backup filter (QBQ), and appear to differ by environment.