Evidence for Positive and Negative Organic Sampling Artifacts

1. 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

2. 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 Teflon-membrane filter OM, which is used to determine PM mass concentration.

3. 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 10-8 Torr Non-Volatile HULIS, WSOC

4. Organic Sampling Artifacts Particle (P) • Positive sampling artifact: SVOC is volatilized “before” captureby filters • Negative sampling artifact: SVOC is volatilized “after” captured by filters Gas Molecule Quartz- or other filter material Backup fiber CIG Absorbent • Particle and gas are in a dynamic equilibrium! CIG: Charcoal-impregnated glass-fiber filter

5. 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.

6. 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):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.

7. Filter-Adsorbent (FA) QF A • Filter-Filter-Adsorbent (FFA) QBQ QF A D E QF A or F A • Electrostatic precipitator (EA) • Operational Definitions of Particulate OC • FParticulate OC = Total – APositive or Negative Sampling Artifact • Denuder-Filter-Adsorbent or -Filter (DFA or DFF)

8. 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

9. IMPROVE Acquires Backup Filters and Field Blanks • 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.

10. 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).

11. Blank Levels Vary by Season

12. Blank Levels Don’t Depend on Average Carbon Levels(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.)

13. Spring (March – May) Summer (June – August) Fall (September through November) Winter (December through February) Blank OC Levels Don’t Show a Spatial Pattern *Blanks Acquired between 01/05 and 05/06

14. IMPROVE Field Blanks Stay Longer than Those of Other Networks(1/1/2005 – 12/31/2006) IMPROVE STN/CSN SEARCH

15. IMPROVE Field Blank Carbon is Higher than that for STN/CSN IMP_bQF: IMPROVE field blanks STN_FB: STN/CSN field blanks STN_TB: STN/CSN trip blanks

16. IMPROVE Field Blank TC is Higher than STN/CSN (continued) IMP_bQF: IMPROVE field blanks STN_FB: STN/CSN field blanks STN_TB: STN/CSN trip blanks

17. But STN/CSN OC Artifact Correction is Higher than IMPROVE due to Lower Flow Rates and Larger Filter Area (Intercept method) STN = a(IMPROVE) + b

18. More OC on Blanks is in Low Temperature OC Fractions, but there is also Blank OC at High Temperatures Fractions up to OC4 can be found on blank filters

19. 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.

20. IMPROVE Blank OC and Backup OC Agree in Winter, but Not in Summer QBQ QBQ bQF bQF QBQ bQF QBQ and bQF OC agree well in winter, but more OC is found on QBQ in summer! (Adapted from Warren White 2007)

21. IMPROVE Negative Artifact is Small 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).

22. A Conceptual Model • 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)

23. Key Question: • Is the difference between QBQ and bQF OC due to positive or negative sampling artifact? QBQ-bQF QF Excess OC on the backup filter (with respect to the field blank) correlated well with ambient PM filter mass loading (from Jay Turner, 2006)

24. Organic artifact may be estimated by slicing the bottom half of the quartz-fiber filter • Procedure: • Analyze a whole punch • Acquire another punch from the same filter and weight the whole punch • Slice the punch and weight each of the two halves • Analyze both halves for carbon concentration • Estimate sampling artifact by scaling carbon measured on the bottom-half filter to the whole filter • Filter slicer

25. Similar OC between bottom half of QF and QBQ Pattern of Sliced Filter Carbon Loading (I) QF QBQ Original QF analysis QFtop or QBQtop QFbott or QBQbott

26. Higher OC in bottom half of QF than QBQ Pattern of Sliced Filter Carbon Loading (II) QF QBQ Original QF analysis QFtop or QBQtop QFbott or QBQbott

27. 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.

28. 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.