Investigation of a Systematic Offset in the Measurement of Organic Carbon With a Semicontinuous Analyzer

By Offenberg, John H; Lewandowski, Michael; Edney, Edward O; Kleindienst, Tadeusz E; Jaoui, Mohammed

ABSTRACT

Organic carbon (OC) was measured semicontinuously in laboratory experiments of steady-state secondary organic aerosol formed by hydrocarbon + nitrogen oxide irradiations. Examination of the mass of carbon measured on the filter for various sample volumes reveals a systematic offset that is not observed when performing an instrumental blank. These findings suggest that simple subtraction of instrumental blanks determined as the standard analysis without sample collection (i.e., by cycling the pump and valves yet filtering zero liters of air followed by routine chemical analysis) from measured concentrations may be inadequate. This may be especially true for samples collected through the filtration of small air volumes wherein the influence of the systematic offset is greatest. All of the experiments show that filtering a larger volume of air minimizes the influence of contributions from the systematic offset. Application of these results to measurements of ambient concentrations of carbonaceous aerosol suggests a need for collection of sufficient carbon mass to minimize the relative influence of the offset signal.

INTRODUCTION

Time-resolved measurements of elemental carbon (EC) and organic carbon (OC) in the atmosphere have been reported using an instrument based on the thermal-optical carbon technique. The particulate carbon measurements have been based on the in situ carbon analyzer developed by Turpin et al.1 Versions of this instrument have been used to measure atmospheric concentrations of particulate OC in many locations, including Los Angeles, CA,2-5 Atlanta, GA,6 St. Louis, MO,7 Pittsburgh, PA,8-10 and over the Pacific Ocean and Sea of Japan.11

An evaluation of the impact of the blank value using a semicontinuous instrument for EC-OC analysis is important because of the low flow rates (typically ~8 L min^sup -1^) inherent in the instrument operation and short sampling times desired for time- resolved measurements (typically 1-2 hr). Various methods have been used to determine blank values associated with the semicontinuous thermal-optical analyzer. Lim and Turpin6 report the measurement of instrument blanks by conducting the standard analysis in the absence of sample collection (instrument blank) by cycling the pump and valves yet filtering zero liters of air followed by routine chemical analysis. In another study, Bae et al.7 report comparison measurements between the semicontinuous EC-OC analyzer and an offline measurement. In those comparisons, 24-hr averages of OC concentrations reported by the semicontinuous EC-OC instrument were higher by ~1 g carbon (C) m^sup -3^. For 0.5 m^sup 3^ samples, this is 0.5 g C for each sample. For a 60-min sample collected at 8 L min^sup -1^ at a typical ambient OC concentration of 4 gm^sup -3/^, an offset of 0.5 g C would represent 25% of the apparent concentration.

To study the potential for a systematic offset in the semicontinuous carbon analysis under controlled conditions, a photochemical reaction chamber operated such that laboratory- generated secondary organic aerosol (SOA) was maintained at a constant value over long periods of time. In these experiments, the volume of air that was filtered was systematically varied. Linear relationships between the mass of carbon measured and the volume of air sampled show nonzero intercepts for clean air, as well as for photochemically generated SOA. These intercepts indicate a systematic offset that may adversely impact measured concentrations if corrections are not made, especially when small masses of carbon are collected for analysis.

EXPERIMENTAL WORK

Particulate carbon concentrations were measured using a semicontinuous EC-OC analyzer developed by Sunset Laboratory (Carbon Aerosol Analysis Field Instrument). Because there is no EC generated in or produced by the chamber, the OC is equivalent to the total carbon measured by the analyzer. Samples were collected at 8 L min^sup -1^ onto a quartz filter (Tissuquartz 2500QAT-UP; Pall Corp.) located within the oven assembly. The total volume of air collected during sampling ranged from 0.008 to 3.84 m^sup 3^. Samples were analyzed for OC using oven temperatures and gas flows as given by the National Institute for Occupational Safety and Health analysis protocol.12 Upstream of the instrument inlet, a parallel plate denuder (Sunset Laboratory), consisting of fifteen 21.6-cm-long by 3-cmwide carbon impregnated cellulose strips (β- SAFE Schleicher & Schuell, Inc.) suspended 1.5 mm apart, was used to remove gas-phase organic compounds from the airstream.

Experiments were conducted with both clean air (without aerosol) and steady-state organic aerosol produced by irradiating hydrocarbon mixtures in a smog chamber. The clean air supply was generated with an AADCO, model 737 (AADCO, Inc.) pure air generator and was filtered with an M30 type C, 0.01-m coalescing filter (Wilkerson Corp.). Total hydrocarbon concentration in the purified air was <50 ppb C.13 Typical particle mass concentrations in the air were <0.1 gm^sup -3^ as determined with a scanning mobility particle sizer (SMPS model 3071A; TSI, Inc.) assuming unit density. Steady-state SOA was photochemically produced from several hydrocarbon nitrogen oxide (NO^sub x^) mixtures. Irradiations were performed in a 14.5- m^sup 3^ Teflon-lined, stainless steel chamber with UV lights.14 The reaction chamber was operated in a steady-state flow mode with an average volumetric residence time of 6 hr. The steady-state aerosol was continuously monitored with the SMPS.

Precursor gas concentrations used in these experiments are listed in Table 1. In some of the experiments, SO^sub 2^ was injected to the mixtures to assess the impact of acid aerosols on the instrument operation. Gas concentrations were measured in an inlet manifold and in the reaction chamber during the irradiation. To aid aerosol formation, ammonium sulfate seed aerosol was introduced into the inlet at a concentration of ~0.05 g m^sup -3^. The relative humidity in the chamber was dynamically maintained at 30%. The reaction mixture generated robust SOA concentrations ranging from 20 to 100 g m^sup -3^. Once the steady-state conditions were reached, less than a 10% deviation occurred over a 24-hr period. Sampling using the Sunset Laboratories EC-OC instrument was then performed on this organic aerosol.

RESULTS

Carbon masses are determined for a series of sampling times between 1 min (0.008 m^sup 3^) and 8 hr (3.84 m^sup 3^). The volume can then be plotted against the measured carbon mass, and, in the absence of a systematic offset, the measured mass of carbon will approach zero as the volume approaches zero. The measured mass of carbon in clean air plotted against sample volume is presented in Figure 1 (open triangles). The observed relationship from these data is given by the equation TC = 0.13 (V) + 1.06 (R^sup 2^ = 0.42; n = 27; P < 0.05), where TC is the total carbon mass loading on the filter (micrograms), and V is the sample volume (cubic meters). The intercept from this equation (1.06 0.10 g C), is greater than the instrumental blank as determined by cycling the pump and valves, yet filtering zero liters of air followed by routine chemical analysis, which averaged 0.72 0.10 g C. The intercept indicates that there is a systematic offset that adds 1.06 g C regardless of the volume of air filtered. The slope of this relationship suggests that the concentration of particulate carbon in the clean air is 0.13 g C m^sup -3^. However, when measurements include the influence of both the slope and intercept, the apparent concentration ranges from 24.3 9 g C m^sup -3^ for a 0.04-m^sup 3^ sample to 0.4 0.1 g C m^sup - 3^ for a 3.84-m^sup 3^ sample. The influence of the systematic offset for clean air resulted in an increase in the apparent carbon concentration calculated by the instrument software as 3-to 187- fold.

Such an offset can be influential when a small volume of air containing a low concentration of carbon is filtered. For example, when a 0.24-m^sup 3^ sample (30 min at 8 L/min) of air containing 2 g C m^sup -3^ is filtered, 0.48 g of particulate carbon is collected on the filter. In this case, a 1-g C systematic offset would contribute twice the mass of carbon to the measurement. The instrumental signal would be greatly increased, with 67% of the signal coming from the offset.

In experiments using aerosol formed from the photochemical reactions, the aerosol in the chamber is brought to a constant mass concentration before measurements are made with the carbon analyzer. These measurements show intercepts substantially greater than those from the clean air measurement. In a steady-state α-pinene + NO^sub X^ irradiation (2 ppm C +0.25 ppm; τ = 6 hr), measured total carbon masses were found to be linearly related to the sample volume, as seen in Figure 1. The intercept of this relationship (1.24 0.23 g C) is almost twice as large as the triplicate instrumental blanks. Likewise, the mass of carbon on the filter for a follow-up experiment modified by the addition of 245 ppbV SO^sub 2^ results i\n an intercept of a linear correlation of 1.37 0.31 g C. Similarly, in a photo-oxidation experiment using an α- pinene/isoprene/NO^sub x^ system, the intercept of the linear best fit was 1.55 0.37 g C. Equations for these linear fits are given in Figure 1.

To examine a more extreme case, measurements were also made using an α-pinene/isoprene/toluene/NO^sub X^ mixture in the presence of SO^sub 2^. In the case, the intercept was found to be 5.34 0.67 g C. Yet, in all of the investigations, as the volume of air is increased, the influence of the systematic offset to the resulting signal becomes less significant. For large volumes of air, a sufficiently large carbon mass is collected to render the systematic offset negligible.

DISCUSSION

Once the systematic offset has been quantified, a measurement can be corrected for systematic offset by subtracting the mass of carbon in the systematic offset from the measured mass of carbon in the sample. The possibility that the offset is changing during a set of measurements may increase the uncertainty in the time-resolved carbon measurement. Moreover, this correction is in addition to potential impacts of the conventional positive or negative sampling artifacts.

An examination of the impact of correcting carbon masses for systematic offsets as a function of sample volume shows that the influence of the offset is greatest at the smallest sample volumes. The ratio of uncorrected to corrected carbon concentration decreases with increasing sample integration volume. In the data reported here, the ratio of uncorrected carbon concentration to corrected concentration is greater than unity for sampling periods <0.24 m^sup 3^ (30 min at 8 L min^sup -1^). The effect of correcting for the systematic offset in these laboratory experiments is negligible for sample volumes >0.48 m^sup 3^.

For these laboratory-generated organic aerosols, uncorrected OC concentrations with collected volumes >0.24 m^sup 3^ have resulted in values within 10% of those when the systematic offset was subtracted. For successively larger sample volumes, the relative importance of this correction becomes smaller, suggesting that it may simply be more practical to collect ≥ 0.48 m^sup 3^ of air. This would minimize the potential contribution of a systematic offset without requiring correction. This would effectively eliminate the need to collect samples over a range of sample integration periods to determine the value of the systematic offset that would need to be subtracted from all of the measured filter loadings before the calculation of concentrations.

Table 2 shows the volume of air that would be needed to reduce the contribution of the systematic offset to <10% of the measured signal. Thus, with a larger offset, more air would need to be collected to minimize the influence of systematic offset. Conversely, at higher particulate carbon concentrations, the volume of air collected can be smaller, and the offset would still remain a minor proportion of the total signal. This indicates that collecting larger sample volumes might be a practical approach to minimize the influence of the systematic offset. It is important to note that longer sample integration times are not particularly confounding for steadystate chamber experiments.14-16 However, ambient measurements are complicated by the time scales of meteorological and chemical dynamics. Likewise, nonsteady-state chamber operations may also be complicated by the intrinsic time scales of the photochemical reactions being investigated.

An alternate approach described by Lim et al.11 uses a filter and a parallel plate carbon strip denuder placed ahead of the sample inlet. The resulting measurement was defined as a dynamic blank. For ambient aerosols, Lim et al.11 reported dynamic blank values that increased with sampling volume on a given day and also varied from one day to another. As measured, this dynamic blank includes the systematic offset described here, as well as any volume-dependent potential positive and negative sampling artifacts. As such, the systematic offset described above needs to be quantified to understand the magnitudes of positive and negative artifacts.

For one particular instrument, the magnitude of the systematic offset may not be constant, because the value of the offset changed with differing photochemical systems investigated. This study does not address the possible variability between instruments, which should be fully explored. Furthermore, the source of the carbon measured as the offset remains unclear, although contamination of tubing and valves may be a contributing factor. The source of the offset may also be related to carbon strip denuder in line upstream of the instrument. However, incomplete capture of gases by the denuder is not expected to contribute to this offset. Denuder breakthrough, proportional to the volume collected, would be seen as an increase in the slope of this same plot and not as a change in the intercept.

Without accounting for this systematic offset, reported concentrations measured by collecting one volume of air may not be comparable to that measured at another. This may be especially troublesome when protocols involve short duration sampling or when sample volumes are most dissimilar. As shown above, samples collected by collecting differing sample volumes can result in large differences even when using identical thermal optical analysis methods.

ACKNOWLEDGMENTS

This work has been funded wholly or in part by U.S. Environmental Protection Agency under contract 68-D-00-206 to Alion Technology, Inc. It has been subjected to agency review and approved for publication. Mention of trade names or commercial products does not constitute an endorsement or recommendation for use.

IMPLICATIONS

The laboratory experiments presented here indicate a systematic offset that may be influential in the measurement of OC in a semicontinuous instrument. This offset may have the greatest impact on measured concentrations collected through the filtration of small volumes of air.

REFERENCES

1. Turpin, B.J.; Cary, R.A.; Huntzicker, J.J. An in Situ, Time- Resolved Analyzer for Aerosol Organic and Elemental Carbon; Aerosol Sci. Technol. 1990, 12, 161-171.

2. Turpin, B.J.; Huntzicker, J.J.; Larson, S.M.; Cass, G.R. Los Angeles Summer Midday Particulate Carbon: Primary and Secondary Aerosol; Environ. Sci. Technol. 1991, 25, 10, 1788-1793.

3. Turpin, B.J.; Huntzicker, J.J.; Hering, S.V. Investigation of Organic Aerosol Sampling Artifacts in the Los Angeles Basin; Atmos. Environ. 1994, 28, 3061-3071.

4. Turpin, B.J.; Huntzicker, J.J. Identification of Secondary Organic Aerosol Episodes and Quantification of Primary and Secondary Organic Aerosol Concentrations during SCAQS; Atmos. Environ. 1995, 29, 3527-3544.

5. Arhami, Kuhn, Fione, Delfino, Sioutas. Effects of Sampling Artifacts and Operating Parameters on the Performance of a Semicontinuous Particulate EC/OC Analyzer; Environ. Sci. Technol. 2006, 40, 945-954.

6. Lim, H.J.; Turpin, B.J. Origins of Primary and Secondary Organic Aerosol in Atlanta: Results’ of Time-Resolved Measurements during the Atlanta Supersite Experiment; Environ. Sci. Technol. 2002, 36, 4489-4496.

7. Bae, M.S.; Schauer, J.J.; DeMinter, J.T.; Turner, J.R.; Smith, D.; Cary, R.A. Validation of a Semicontinuous Instrument for EC and OC Using a Thermal-Optical Method; Atmos. Environ. 2004, 38, 2885- 2893.

8. Subramanian, R.; Khlystov, A.Y.; Cabada, J.C.; Robinson, A.L. Positive and Negative Artifacts in Particulate OC Measurements with Denuded and Undenuded Sampler Configurations; Aerosol Sci. Technol. 2004, 38, 27-48.

9. Cabada, J.C.; Pandis, S.N.; Subramanian, R.; Robinson, A.L.; Polidori, A.; Turpin, B. Estimating the Secondary Organic Aerosol Contribution to PM2.5 Using the EC Tracer Method; Aerosol Sci. Technol. 2004, 38, 140-155.

10. Polidori, A.; Turpin, B.J.; Lim, H.J.; Cabada, J.C.; Subramanian R.; Pandis, S.N.; Robinson, A.L. Local and Regional Secondary Organic Aerosol: Insights from a Year of Semicontinuous Carbon Measurements at Pittsburgh; Aerosol Sci. Technol. 2006, 40, 861-872.

11. Lim, H. J.; Turpin, B.J.; Russell, L.M.; Bates, S. Organic and EC Measurements during ACE-Asia Suggest a Longer Atmospheric Lifetime for EC; Environ. Sci. Technol. 2003, 37, 14, 3055-3061.

12. Birch, M.E.; Cary, R.A. EC-Based Method for Monitoring Occupational Exposures to Particulate Diesel Exhaust; Aerosol Sci. Technol. 1996, 25, 221-241.

13. Kleindienst, T.K.; Smith, D.F.; Li, W.; Edney, E.O.; Driscoll, D.J.; Speer, R.E.; Weathers W.S. Secondary Organic Aerosol Formation from the Oxidation of Aromatic Hydrocarbons in the Presence of Dry Submicron Ammonium Sulfate Aerosol; Atmos. Environ. 1999, 33, 3669-3681.

14. Edney, E.O.; Kleindienst, T.E.; Jaoui, M.; Lewandowski, M.; Offenberg, J.H.; Wangc, W.; Claeys, M. Formation of 2-Methyl Tetrols and 2-Methylglyceric Acid in Secondary Organic Aerosol from Laboratory Irradiated Isoprene/NO^sub x^/SO^sub 2^/Air Mixtures and Their Detection in Ambient PM2.5 Samples collected in the Eastern United States; Atmos. Environ. 2005, 39, 5281-5289.

15. Jaoui, M.; Kleindienst, T.K.; Lewandowski, M.; Offenberg J.H.; Endey, E.O. Identification and Quantification of Aerosol Polar Oxygenated Compounds Bearing Carboxylic or Hydroxyl Groups. 2. Organic Tracer Compounds from Monoterpenes; Environ. Sci. Technol. 2005, 39, 5661-5673.

16. Offenberg J.H.; Kleindienst, T.K.; Jaoui, M.; Lewandowski, M.; Endey E.O. Thermal Properties of Secondary Organic Aerosols; Geophys. Res. Lett. 2006, 33, L03816.

John H. Offenberg, Michael Lewandowski, Edward O. Edney, and Tadeusz E. Kleindienst

U.S. Environmental Protection Agency, National Exposure Research Laboratory, Human Exposure Atmospheric Sciences Division, Research Triangle Park, NC

Mohammed Jaoui

Alion Science and Technology, Research Triangle Park, NC

About the Authors

John Offenberg, Michael L\ewandowski, Edward Edney, and Tadeusz Kleindienst are research scientists at U.S. Environmental Protection Agency, National Exposure Research Laboratory. Mohammed Jaoui is a research scientist with Alion Science and Technology. Please address correspondence to: John Offenberg, U.S. Environmental Protection Agency, National Exposure Research Laboratory, Human Exposure Atmospheric Sciences Division, 109 T.W. Alexander Drive/MD D205-03, Research Triangle Park, NC 27711; phone: +1-919-541-2915; fax: +1- 919-541-1153; e-mail: offenberg.john@epa.gov.

Copyright Air and Waste Management Association May 2007

(c) 2007 Journal of the Air & Waste Management Association. Provided by ProQuest Information and Learning. All rights Reserved.