Evidence for Positive and Negative
Organic Sampling Artifacts
John G. Watson (john.watson@dri.edu)
Judith C. Chow
L.-W. Antony Chen
Desert Research Institute, Reno, NV
Presented at:
IMPROVE–CSN Carbon PM Monitoring Workshop
University of California, Davis
January 22, 2008
Definition 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 Teflonmembrane filter OM, which is used to
determine PM mass concentration.
Atmospheric Organic Volatility
Categories Span a Continuum
PL0 at 20 ºC
Volatile
H2O: 17.54
10-1 Torr
Fluorene: 1.9  10-3
Semi-Volatile
(SVOC)
Benzo(e)pyrene: 4.3  10-8
Non-Volatile
HULIS, WSOC
10-8 Torr
Organic Sampling Artifacts
Particle (P)
• Positive sampling
artifact: SVOC is
volatilized “before”
captureby filters
Gas Molecule
Quartz- or other filter material
• Negative sampling
artifact: SVOC is
volatilized “after”
captured by filters
Backup fiber
CIG Absorbent
• Particle and gas are in a dynamic
equilibrium!
CIG: Charcoal-impregnated glass-fiber filter
Early 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 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):593600.
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)
Filter-FilterAdsorbent (FFA)
Denuder-FilterAdsorbent or -Filter
(DFA or DFF)
Electrostatic precipitator
(EA)
QF
A
QF QBQ
A
D
QF
E
A or F
A
Several Methods to Compensate for
Positive 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
Field Blanks
MORA
PUSO
DOSO
FRES
WASH
SHEN
YOSE
HANC
TONT
CHIR
PHOE
OKEF
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
Analyzed 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
Vary by Season
100
90
Number of Sites
80
IMPROVE Blank Filter (bQF) Loading
1/1/2005 - 12/31/2006
Summer
Winter
70
60
50
40
30
20
10
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
Total Carbon Concentration (µg/filter)
Blank Levels Don’t Depend on Average
Carbon Levels(1/05 – 12/06)
200
EC2
160
EC1
140
OC4
120
100
80
Active Sampling
180
OC3
OC2
OC1
blk TC (BLKTC)
60
40
20
H
A
V
W O1
H
P
H A1
A
L
D E1
EN
N A1
O
C
M A1
EL
A
M X
O
Z
C I1
R
L
U A1
LB
H E1
O
O
LY VX
B
R R1
ED
C W1
A
N
H Y1
EC
B A1
O
A
SE P1
N
C EX
EB
B L1
A
N
SA D 1
W
TH E1
B
IN A1
G
SH A 1
E
C N1
O
G
FO O1
P
EV E1
ER
FL X
A
T
EL 1
H LI1
EG
SA L1
M
JO A1
SH
A 1
G
C TI1
A
B
C A1
A
D DI1
O
U
G
B 1
R
IG
SI 1
K
E
PI 1
TT
FR 1
E
M S1
O
N
T1
0
Sampling Sites
Passive
Deposition
Carbon Concentration (g/filter)
EC3
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.)
Blank OC Levels Don’t Show a Spatial
Pattern
Spring
Fall
(March – May)
(September through November)
*Blanks Acquired between 01/05 and 05/06
Summer
Winter
(June – August)
(December through February)
100
IMPROVE
90
80
Number of Sites
IMPROVE Field Blanks
Stay Longer than Those of
Other Networks
(181 Sites)
70
60
50
40
30
20
(1/1/2005 – 12/31/2006)
10
0
0
0.5
1
1.5
2
2.5
3
200
STN/CSN
~every 7 days (once per
week)
Number of Sites
160
IMPROVE
5
120
100
80
60
40
20
0
0
Varies (~1-15 minutes) with
exceptions (~5-7 days)
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Average Organic Carbon Field Blank Concentration
(µg/cm2)
5
SEARCH
Number of Sites
Varies (~1-15 minutes)
4.5
(239 Sites)
140
4
SEARCH
4
STN/CSN
180
Blank Deposition Period
3.5
Average Organic Carbon Field Blank Concentration
(µg/cm2)
(8 Sites)
3
2
1
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Average Organic Carbon Field Blank Concentration
2
(µg/cm )
5
2
1
0
10/16/2001
1/16/2002
4/16/2002
7/16/2002
10/16/2002
1/16/2003
4/16/2003
7/16/2003
MORA1
3
10/16/2003
IMP_bQF
STN_FB
STN_TB
4
2
1
0
10/16/2001
2
Blank Carbon (g/cm )
2
PUSO1
3
2
STN_TB: STN/CSN
trip blanks
IMP_bQF
STN_FB
STN_TB
4
Blank Carbon (g/cm )
STN_FB: STN/CSN
field blanks
Blank Carbon (g/cm )
IMP_bQF: IMPROVE
field blanks
Blank Carbon (g/cm 2)
IMPROVE Field Blank Carbon is
Higher than that for STN/CSN
1/16/2002
4/16/2002
7/16/2002
10/16/2002
1/16/2003
4/16/2003
7/16/2003
IMP_bQF
STN_FB
STN_TB
4
3
10/16/2003
PHOE1
2
1
0
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
IMP_bQF
STN_FB
STN_TB
4
TONT1
3
2
1
0
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
2
2
Blank Carbon ( g/cm )
STN_TB: STN/CSN
trip blanks
2
STN_FB: STN/CSN
field blanks
Blank Carbon ( g/cm )
IMP_bQF: IMPROVE
field blanks
Blank Carbon ( g/cm )
2
Blank Carbon ( g/cm )
IMPROVE Field Blank TC is Higher
than STN/CSN (continued)
IMP_bQF
STN_FB
STN_TB
4
WASH1
3
2
1
0
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
IMP_bQF
STN_FB
STN_TB
4
DOSO1
3
2
1
0
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
IMP_bQF
STN_FB
STN_TB
4
BIBE1
3
2
1
0
1/1/2005
4/1/2005
7/1/2005
10/1/2005
1/1/2006
4/1/2006
7/1/2006
10/1/2006
IMP_bQF
STN_FB
STN_TB
4
FRES1
3
2
1
0
1/1/2005
4/1/2005
7/1/2005
10/1/2005
1/1/2006
4/1/2006
7/1/2006
10/1/2006
But STN/CSN OC Artifact Correction is Higher
than IMPROVE due to Lower Flow Rates and
Larger Filter Area (Intercept method)
3
STN-IMP TC Intercept (g/m )
2.5
2.0
1.5
All
Spring
Summer
Fall
Winter
1.0
0.5
STN = a(IMPROVE) + b
Big Bend
NP, TX
Fresno, CA
Dolly Sodds
Wldrns, WV
Washington,
DC
Tonto Natnl
Mon, AZ
Phoenix, AZ
Mt. Rainier,
WA
Seattle, WA
0.0
More OC on Blanks is in Low Temperature
OC Fractions, but there is also Blank OC at
High Temperatures
3.0
IMPROVE Urban
IMPROVE Rural
STN/CSN Urban
SEARCH Non-Urban
SEARCH Urban
2
Concentration (µg/cm )
2.5
2.0
1.5
1.0
0.5
0.0
TC
OC
EC
OC1
OC2
OC3
Carbon Fractions
Fractions up to OC4 can be
found on blank filters
OC4
EC1
EC2
EC3
Implications
• 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
Winter, but Not in Summer
18
CS
QBQ
OC
FB
bQF
16
14
Se
rie
s3
12
10
8
6
4
4
bQF
FB
3
Se
rie
s3
2.5
2
1.5
1
0.5
0
1/1/02
2
0
1/1/02
1/1/03
1/1/04
1/1/05
1/1/06
QBQ and bQF OC agree
well in winter, but more OC
is found on QBQ in
summer!
(Adapted from Warren White 2007)
QBQ
CS
O1
3.5
1/1/03
1/1/04
1/1/05
1/1/06
10
carbon on backup filter or field blank, ug
carbon on backup filter or field blank, ug
20
carbon on backup filter or field blank, ug
4.5
9
CS
QBQ
O3
bQF
FB
8
7
Se
rie
s3
6
5
4
3
2
1
0
1/1/02
1/1/03
1/1/04
1/1/05
1/1/06
IMPROVE Negative Artifact is Small
Average across 163 IMPROVE sites; QBQ is only available at six
sites.
35
Average Intercept OC
Average QBQ
Average bQF
OC Concentration (mg/filter)
30
25
20
15
10
5
0
Spring
Summer
Fall
Winter
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 Model
• Teflon filter is not subject
to positive 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)
pSVOC (volatilized)
140
OC Concentration (mg/filter)
• More volatilization on
Teflon filters, resulting in
a higher negative
sampling artifact
160
pSVOC (retained)
120
pOC
100
VOC and gSVOC (adsorption)
80
60
40
20
0
(20)
(40)
Teflon
Quartz
Summer
Teflon
Quartz
Winter
Key Question:
• Is the difference between QBQ and bQF OC due to positive
or negative sampling artifact?
40
OC
35
30
QBQ-bQF
CS - FB
25
20
15
10
5
0
-5
-10
1
10
100
1000
10000
QF
CP
Excess OC on the backup filter (with respect to the field blank) correlated
well with ambient PM filter mass loading (from Jay Turner, 2006)
Organic artifact may be estimated by
slicing the bottom half of the quartzfiber filter 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
•Filter slicer
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 QBQ
Pattern of Sliced Filter Carbon Loading (I)
SHEN1 2005/1/13 (Q89488)
Carbon Loading (ug)
70
QF
60
50
QBQ
40
30
Original QF analysis
20
10
QFtop or QBQtop
0
0
1
3
4
CHIR1 2005/4/7(Q94596)
25
Carbon Loading (ug)
2
Slice Mass (mg)
20
15
10
5
0
0
1
2
Slice Mass (mg)
3
4
QFbott or QBQbott
Higher OC in bottom half of QF than QBQ
Pattern of Sliced Filter Carbon Loading (II)
SHEN1 5/17/2005 (Q93898)
Carbon Loading (ug)
60
QF
Front Filter
Backup Filter
50
40
QBQ
30
Original QF analysis
20
10
0
0
1
2
Slice Mass (mg)
3
4
QFbott or QBQbott
YOSE1 2006/2/16 (R14098)
Carbon Loading (ug)
30
25
20
15
10
5
0
0
1
2
Slice Mass (mg)
QFtop or QBQtop
3
4
Conclusions
• 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 (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.