Carbon Artifact on Version 2 IMPROVE Samplers

 

Author:  Robert Eldred, CNL, University of California, Davis (raeldred@ucdavis.edu)

Date:  February 8, 2002

 

Summary:  Significant changes were made in the IMPROVE cassettes in the new version of the sampler installed in the year 2000.  The Version 2 cassettes sampler are made of an inert plastic, Delrin®, and the filters sit on stainless steel screens.  The Version 1 cassettes were  polycarbonate and sat on cellulose drain discs over polycarbonate support grids. 

 

·        Elemental Carbon:  The elemental carbon artifact for Version 2 cassettes is now similar to a laboratory blank.  This shows that the elemental carbon artifact observed in the past was produced by contact with the Version 1 cassette. 

 

·        Organic Carbon:  The organic carbon artifact does not change significantly with the shift to new cassettes.  This confirms the earlier findings that the organic carbon artifact is not associated with the cassette material. 

 

Types of  blanks:  Lab blanks have never been placed in the cassette.  Field blanks are loaded into cassettes and sent on a predetermined schedule to all sites.  Secondary filters are mounted in tandem with the primary filter at four to six sites.  They are identical to field blanks except air is drawn through that has had the particles removed by the primary filter. 

 

Past Studies:  The appendix summarizes tests conducted in 1995, primarily to examine the organic artifact.  The results were consistent with the hypothesis that organic artifact is acquired by the quartz filter by contact with the atmosphere; this reaches a saturation limit after a few days.  All filters reach this saturation value during the time in the field.  Secondary filters are similar to field blanks because both reach the saturation value.  The secondary filters measure neither the organic gases in the incident air stream nor any organic gas that may volatilize from the primary filter.  The median of all secondary filters is therefore a reasonable estimate of the positive organic artifact.  We have no information on the organic gas that may volatilize from the primary filter (negative artifact). 

 

For elemental carbon, the secondary filters were also similar to field blanks.  This indicates that no elemental artifact is added from the air stream.  The few samples that were exposed to the atmosphere but not placed in cassettes showed no elemental carbon.  We did not think the data were robust enough to conclude how the quartz filter obtained the elemental carbon artifact. 

 

Current Results:  Table 1 presents the medians and standard deviations for field blanks and secondary filters collected since March 1995.  For the Version 1 cassettes, there were 1068 field blanks and 1882 secondary filters collected between March 1995 and August 2000.  For the Version 2 cassettes, there were 347 field blanks and 694 secondary filters collected between January 2000 and August 2001. 

 

Medians are used in IMPROVE instead of means because of potential inaccuracies due to outliers; with medians there is no reason to remove outliers.  The standard deviation, like the mean, also can be inaccurate when there are outliers.  A safer statistic is an equivalent standard deviation based on percentiles.  For a normal distribution, 68% of the cases are within 1 standard deviation.  This is maintained by the following definition of percentile standard deviation:

percentile standard deviation = (84th percentile – 16th percentile) / 2

Note that most values in the table are near the normal standard deviation.  However, they do differ for a few cases with very large normal standard deviations. 

 

Figures  1 shows the median values for the eight fractions, while Figure 2 shows the median values for total OC and LAC (EC).    

 

Table 1.  Field blanks and secondary filters collected between March 1995 and August 2001.  The units are µg per filter; to convert to ng/m3, multiply each value by 30.5. 
p.s.d. = percentile standard deviation = (84th percentile – 16th percentile) / 2

 

Version 1

O1

O2

O3

O4

OP

E1

E2

E3

 

OC

LAC

median

field blanks

4.9

3.7

4.1

1.1

0.0

0.3

1.1

0.1

 

14.1

1.5

median

secondary filters

2.7

2.7

3.7

1.1

0.0

0.3

0.9

0.0

 

10.6

1.2

 

 

 

 

 

 

 

 

 

 

 

 

 

st dev

field blanks

8.0

3.9

1.5

0.7

0.5

0.7

0.9

0.3

 

12.3

1.4

st dev

secondary filters

2.2

2.0

1.7

0.7

0.5

0.5

0.7

0.3

 

5.2

1.2

 

 

 

 

 

 

 

 

 

 

 

 

 

p.s.d

field blanks

5.3

2.7

1.4

0.6

0.0

0.4

0.9

0.3

 

9.1

1.4

p.s.d

secondary filters

1.7

1.5

1.5

0.7

0.2

0.4

0.8

0.3

 

4.5

1.2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Version 2

O1

O2

O3

O4

OP

E1

E2

E3

 

OC

LAC

median

field blanks

1.9

2.3

3.5

0.6

0.0

0.0

0.0

0.0

 

8.4

0.0

median

secondary filters

1.9

2.5

4.3

1.0

0.0

0.0

0.2

0.0

 

10.4

0.2

 

 

 

 

 

 

 

 

 

 

 

 

 

st dev

field blanks

1.1

0.7

1.1

0.5

0.1

0.2

0.2

0.1

 

2.6

0.4

st dev

secondary filters

1.5

1.9

2.4

0.9

0.9

0.6

0.6

0.1

 

5.7

0.6

 

 

 

 

 

 

 

 

 

 

 

 

 

p.s.d

field blanks

1.0

0.7

1.0

0.4

0.0

0.2

0.2

0.0

 

2.5

0.3

p.s.d

secondary filters

1.3

1.2

2.1

0.8

0.1

0.3

0.4

0.0

 

4.7

0.5

 

 

 

 

 

 

 

 

 

 

 

 

 

mean

lab blanks

0.1

0.3

1.4

0.2

0.0

0.0

0.1

0.0

 

1.8

0.1

 

Conclusions for Organic Carbon

  1. Field blanks and secondary filters are not significantly different.  This corroborates earlier conclusions that no additional organic artifact is added during sampling. 
  2. The OC fractions and total OC for the secondary filters do not change from Version 1 to 2.  This corroborates earlier conclusions that the organic artifact is not associated with the cassette.
  3. The medians and standard deviations of low temperature organic fractions in the Version 1 field blanks were much larger than corresponding values in Version 1 secondary filters, Version 2 field blanks, and Version 2 secondary filters.  The new protocols for field blanks have removed a minor problem.  Note that the carbon artifacts are estimated from secondary filters rather than field blanks. 


 

Conclusions for Light-Absorbing (Elemental) Carbon

  1. The Version 2 secondary filters and field blanks are not significantly greater than lab blanks.  The first conclusion is that the Version 2 cassettes add no major EC artifact.  The second conclusion is that there is no EC artifact from contact with the atmosphere.
  2. The Version 1 cassettes have a much larger EC artifact than the Version 2 cassettes.  The conclusion is that the Version 1 EC artifact was associated with the plastic of the old cassette.  
  3. Field blanks and secondary filters for a given cassette type are not significantly different.  This corroborates earlier conclusions that no additional elemental carbon artifact is added during sampling.   

 

Figure 1.  Median quartz blank filters for the 8 fractions.

 

Figure 2.  Median quartz blank filters for total OC and LAC (EC).


 

Appendix

 

1995 LABORATORY TESTS ON QUARTZ ARTIFACT

 

                                                                   OC tot                                        EC tot

network

1992-93 all field blanks                         11.0 ± 2.9                                 1.9 ± 1.3

1992-93 all secondaries                       10.9 ± 4.7                                 1.7 ± 1.2

 

time in cassette test (1 each)

1 week                                                       12.4                                         2.1

2 weeks                                                     11.3                                         0.6

3 weeks                                                     16.1                                         5.1

4 weeks                                                     11.1                                         1.8

5 weeks                                                     14.5                                         0.0

average                                               13.1 ± 2.2                                 1.9 ± 2.0 

 

1 week tests                                        test 1          test 2                      test 1            test 2

refrigerated Petri                         1.6 ± 0.5                                    0.0

closed Petri                                  2.0 ± 0.9      2.9 ± 1.3               0.0                0.0

in&out cassette, then Petri          1.7 ± 0.6                                    0.0

cassette, drain disk                   10.4 ± 2.2      8.9 ± 2.1               0.3 ± 0.5      0.1 ± 0.1

cassette, no drain disk                9.7 ± 1.6                                    0.5 ± 0.5       

cassette in bag                                                 8.3 ± 0.3                                    0.0

loose in bag with cassette                               7.6 ± 0.4                                    0.0

loose in bag, no cassette                                 6.7 ± 0.6                                    0.0

in open Petri                                                    11.1 ± 0.7                                    0.1 ± 0.1