Measurement of fine particulate matter nonvolatile and semi-volatile organic material with the Sunset Laboratory Carbon Aerosol Monitor.

ABSTRACT

Semi-volatile organic material (SVOM) in fine particles is not reliably measured with conventional semicontinuous carbon monitors because SVOM is lost from the collection media during sample collection. We have modified a Sunset Laboratory Carbon Aerosol Monitor to allow for the determination of SVOM. In a conventional Sunset monitor, gas-phase organic compounds are removed in the sampled airstream by a diffusion denuder employing charcoal-impregnated cellulose filter (CIF) surfaces. Subsequently, particles are collected on a quartz filter and the instrument then determines both the organic carbon and elemental carbon fractions of the aerosol using a thermal/optical method. However, some of the SVOM is lost from the filter during collection, and therefore is not determined. Because the interfering gas-phase organic compounds are removed before aerosol collection, the SVOM can be determined by filtering the particles at the instrument inlet and then replacing the quartz filter in the monitor with a charcoal-impregnated glass fiber filter (CIG), which retains the SVOM lost from particles collected on the inlet filter. The resulting collected SVOM is then determined in the analysis step by measurement of the carbonaceous material thermally evolved from the CIG filter. This concept was tested during field studies in February 2003 in Lindon, UT, and in July 2003 in Rubidoux, CA. The results obtained were validated by comparison with Particle Concentrator-Brigham Young University Organic Sampling System (PC-BOSS) results. The sum of nonvolatile organic material determined with a conventional Sunset monitor and SVOM determined with the modified Sunset monitor agree with the PC-BOSS results. Linear regression analysis of total carbon concentrations determined by the PC-BOSS and the Sunset resulted in a zero-intercept slope of 0.99 [+ or -] 0.02 ([R.sup.2] = 0.92) and a precision of [sigma] = [+ or -]1.5 [micro]g C/[m.sup.3] (8%).

INTRODUCTION

Exposure to fine particulate matter ([PM.sub.2.5]) has been implicated as a contributor to adverse human health effects including increases in cardiovascular and cardiopulmonary disease leading to elevated human mortality and morbidity. (1-3) Carbonaceous material is a major component of urban fine particulate material. However, a significant portion of atmospheric carbonaceous material is often semi-volatile, which tends to be lost from the collected particles during sample collection. (4) The SVOM may be lost during sampling because of the pressure drop across the filter, changes in the concentrations of gas-phase compounds in equilibrium with the particulate SVOM, and changes in ambient temperature. Diffusion denuder samplers (4-7) have been developed that allow for the determination of this fraction of the fine particulate carbonaceous material. Some of these diffusion denuder samplers were developed to measure specific organic compounds. (6,7) The Particle Concentrator-Brigham Young University Organic Sampling System (PC-BOSS) (4) and Real-Time Ambient Mass Sampler (RAMS) (5) were developed to measure total SVOM. The application of these samplers to the study of atmospheric chemistry in urban environments has shown that a substantial fraction of the fine particulate matter in these environments is SVOM. (8-11) Furthermore, these studies have shown that the majority of the SVOM is secondary. (4,11) Single filter samplers such as the [PM.sub.2.5] Federal Reference Method sampler (11) and semicontinuous monitors such as the R & P Carbon monitor (12) or the Sunset Laboratory Carbon Aerosol Monitor, do not reliably sample and measure SVOM. Because the SVOM may be important with respect to human cardiovascular health effects, (13) the development of reliable procedures for both the integrated and semicontinuous monitoring of this material is important. We have previously reported on a simple modification of the conventional U.S. Environmental Protection Agency (EPA) Speciation Sampler with a diffusion denuder, (14) which allows for the routine determination of fine particulate SVOM in an integrated sampler. We report here on a modification of the Sunset Laboratory Carbon Aerosol Monitor that allows for the semicontinuous determination of this material.

EXPERIMENTAL PROCEDURES

Sunset Laboratory Carbon Aerosol Field Instrument

The Sunset instrument is a semicontinuous, carbon aerosol analysis monitor. The inlet is a 2.5-[micro]m sharp-cut cyclone (R & P) with a total flow of 16 L/min; 8 L/min of the flow goes to the carbon monitor, and the remaining flow is directed to the modified Sunset instrument described in the next section. After the flow split, the sampled air passes through a parallel plate charcoal-impregnated filter denuder similar to that described for the Brigham Young University real-time ambient mass sampler (5,8) and supplied by Sunset Laboratory with the instrument. This denuder is intended to remove gas-phase organic compounds that can be adsorbed by a quartz filter, thus eliminating any positive quartz filter artifact for the data obtained with the monitor. (12,14,15) The particles in the sampled airstream are then collected on a 12.3-mm diameter quartz filter for a controlled time period (45 min in the study reported here). Sample collection is then interrupted and the sample analyzed, using a thermal/optical transmittance (TOT) volatilization method comparable to the National Institute for Occupational Safety and Health Method 5040. (16) The TOT method consists of a two-stage analysis. Initially organic carbon (OC) concentrations are determined by heating the filter in a pure helium atmosphere to temperatures of 250, 500, 650 and 850 [degrees]C. A 98% helium, 2% oxygen atmosphere is used in the second stage and heated to temperatures of 650, 750 and 850 [degrees]C to determine elemental carbon (EC) concentrations. On the basis of the laser transmission during the analysis, a correction is made because of carbon pyrolysis that occurs. (17) Carbon thermally evolved from the filter is converted to carbon dioxide (C[O.sub.2]) in a manganese oxide catalyst and detected by a nondispersive infrared detector (NDIR). The temperature stabilized NDIR system has its own reduced instruction set computer processor for enhanced sensitivity, linearity, and long term stability. Reduced analysis times maximize online sample collection. The enhanced time-resolution capability of this new instrument is made possible by a rapid thermal total carbon measurement and a laser-based absorbance technique to measure EC. The data analysis step is followed with a calibration step for each analysis. The data obtained by this procedure are illustrated in Figure 1A. After the 15-min analysis and purge step, sample collection is again initiated for the next 45-min period.

Sunset Monitor Modified to Measure Fine Particulate Semi-Volatile Organic Compound Lost from Particles during Sample Collection

The Sunset monitor was modified to allow for the determination of SVOC lost from particles during the 45-min sample collection period. The modified instrument sampled the second of the two split flow lines after the sharp-cut cyclone inlet. A diffusion denuder, identical to that used in the unmodified instrument, removed gas-phase material with an expected efficiency based on past studies of better than 99%. (14) After the removal of the gas-phase material, the particles were removed from the sampled airstream immediately before the entrance to the Sunset monitor using a pre-fired (800 [degrees]C) 47-mm quartz filter in a MACE in-line, Teflon filter holder. The particle-free air (with any SVOC lost from the particles during sample collection) passed into the filter collection region of the Sunset monitor. The quartz filter normally used in the unmodified instrument was preceded by a charcoal-impregnated glass fiber filter (CIG, Schleicher and Schuell). The quartz filter was kept after the CIG to provide additional structural support for the CIG filter. Any SVOC lost from particles collected on the inlet quartz filter were collected with high efficiency (<95%) by this CIG filter. (4,15,18) At the end of the 45-min sample period, the SVOC collected on the CIG were analyzed by thermal evolution. This analysis was done in a three-step temperature program in a He atmosphere. Previous experience with the analysis of SVOC collected on CIG filters (4) has indicated that only lower molecular weight gas-phase compounds are not removed by the diffusion denuder and that these organic compounds that break through the denuder are evolved from the CIG at temperatures below 200 [degrees]C. In contrast, the SVOC lost from particles are removed from the CIG at temperatures from approximately 250-350 [degrees]C. The first of the three temperature steps in the analysis of the SVOC involved heating to 190 [degrees]C to remove and measure the denuder breakthrough material. The second temperature step to 220 [degrees]C was to insure that the breakthrough gas-phase organic compounds had been cleanly removed. The final temperature step to 450 [degrees]C was to remove and measure the SVOC lost from the particles collected on the inlet quartz filter. This was followed by a calibration step for each analysis. The data obtained by this procedure are illustrated in Figure 1B. After the 15-min analysis and purge steps, sample collection was again initiated for the next 45-min period.

[FIGURE 1 OMITTED]

The PC-BOSS Sampler