Transcript Minimizing Charring in OC/EC Analysis by Thermal Method

Charring Minimization in Thermal
Analysis of Aerosol Carbon
Jian Zhen Yu and Qianfeng Li
Department of Chemistry
Hong Kong University of Science & Technology
EC/OC workshop, Durango, CO
March 2003
Thermal conditions that maximize OC removal /
minimize charring of OC
Low-temperature oxidation (Cadle et al., 1983;
Cachier et al., 1989)
Flash heating (Tanner et al., 1982)
Determination of the temperature upper-limit
for low-temperature oxidation
Criterion: EC remains intact.
We need EC materials that are free of
OC and representative of aerosol EC .
Thermal Creation of EC-Only Test Samples from
Atmospheric Aerosols
Thermal conditions for creation of EC-only (mixture of
PEC and native aerosol EC) samples
Carrier gas
Temperature (oC) Time (sec)
He
He-1
He-2
50
600
800
Purging online
60
300
Successful creation of EC-only samples is evidenced by:
In He atmosphere at temperatures as high as 800oC
FID signal remains at baseline.
No discernable increase in filter laser transmittance is observed.
Thermal evolution of an EC-only sample in 2% O2/He
900
800
Hong Kong sample
600
Temperature
o
Temperature ( C)
700
Transmittance
500
400
300
200
FID response
100
0
0
200
400
600
Time (sec)
800
1000
1200
Percentage of EC Evolved as a Function of
Oxidation Temperature
100
Qingdao-1 (China)
Qingdao-2 (China)
Nanjing-1 (China)
Nanjing-2 (China)
Hong Kong-1
Hong Kong-2
Hong Kong-3
Hong Kong-4
Korea-1
Korea-2
Percentage EC Evolved (%)
80
60
40
20
0
0
100
200
300
400
500
o
Temperature ( C)
350 oC
600
700
800
Earlier Work Using a Low-temperature
Oxidation Step in thermal analysis:
 Optimal temperature for OC/EC: 300-350 oC, (Dod et al.,
1978; Ellis et al., 1984; Ohta and Okita, 1984).
 2-step method for OC/EC (Cachier et. al., Tellus, 1989, 41B, 379).
340 oC, pure O2 , precombustion 2 hours. Pure graphite
was intact.
Flash Heating
Kinetics Competition:
Vaporization versus Decomposition
Ref: Buehler R.J., et.al. J.Am. Chem.Soc. “Proton Transfer Mass
Spectrometry of Peptides. A Rapid Heating Technique for Underivatized
Peptides Containing Arginine”, 1974, 96, 3990
Thermal Methods Used for Comparison
IMPROVE
He
NIOSH
Optimized-1
Optimized-2
200s, 350 oC
200s, 350 oC
10s, purging
Purging online (10s)
O2/He
He-1
180 s, 120 oC
60 s, 250 oC
10s, purging
He-2
180 s, 250 oC
60 s , 500 oC
60 s, 500 oC
He-3
180 s, 450 oC
60 s , 650 oC
60 s, 650 oC
He-4
180 s, 550 oC
90 s, 850 oC
90 s, 850 oC
200 s, 850 oC
He-5
Purging, 60 s
O2/He-1
240 s, 550 oC
30 s, 650 oC
30 s, 650 oC
30 s, 650 oC
O2/He-2
210 s, 700 oC
30 s, 750 oC
30 s, 750 oC
30 s, 750 oC
O2/He-3
210 s, 800 oC
60 s, 850 oC
60 s, 850 oC
60 s, 850 oC
120 s, 940 oC
120 s, 940 oC
120 s, 940 oC
O2/He-4
Combination of low-temperature oxidation and flash heating
forms least charring from water-soluble aerosol OC
50%
PEC/WSOC
40%
IMPROVE
30%
NIOSH
Optimized-1
20%
Optimized-2
10%
0%
TW
H
K
K
H
U
R
K
H
K
M
na
hi
C
2
JN
na
hi
1C
JN
na
hi
C
D
Q
K
H
-2
ST
U
K
H
-1
ST
U
e
os
cr
Su
50%
40%
IMPROVE
PEC/TC
NIOSH
30%
Optimized-1
Optimized-2
20%
10%
0%
ST
ST
3
4
2
1
ST
6
ST
ST
D
U
Q
U
U
U
U
-5
ST
U
-3
an
os
K
-7
ST
U
-1
an
os
K
-2
an
os
K
low
C loading
high
70%
60%
IMPROVE
NIOSH
PEC/OC
50%
Optimized-1
Optimized-2
40%
30%
20%
10%
0%
ST
ST
3
4
2
1
ST
6
ST
ST
D
U
Q
U
U
U
U
-5
ST
U
-3
an
os
K
-7
ST
U
-1
an
os
K
-2
an
os
K
low
C loading
high
Transmittance Increase before the OCEC split
in the He/O2 step
Smaller transmittance increase is expected with lower amount of charring.
Deviation of EC measurements by non-optimal
methods from the optimized method
20%
15%
D EC/TC
IMPROVE
NIOSH
10%
Optimized-1
5%
0%
ST
ST
3
4
2
1
ST
6
ST
ST
D
U
Q
U
U
U
U
-5
ST
U
-3
an
os
K
-7
ST
U
-1
an
os
K
-2
an
os
K
-5%