The BOSS and Big Boss Diffusion Denuder Samplers

Diffusion denuder sampling systems for the determination of total fine particulate organic material have been developed at Brigham Young University.

The objectives which guided the development of these sampling systems were:

1. The sampling system should have a flow rate sufficient to enable measurement of low concentrations of particulate carbonaceous material and to allow the detailed chemical characterization of particulate organic material, e.g. flow rates of from 30 to 300 L min_i were considered desirable.

2. The sampler should have a diffusion denuder capable of removing all gas phase semi-volatile organic compounds which are in equilibria with compounds in the particulate phase in the atmosphere.

3. The diffusion denuder of the sampler should be effective in removing all gas phase compounds which can be adsorbed by a quartz filter or by collected particles during sampling.

4. The capacity of the diffusion denuder for the removal of gas phase organic compounds should be high enough that samples can be collected at the target flow rates over sampling periods of several days to weeks.

5. Particle losses during the passage of sampled air through the diffusion denuder should be small.

6. The sampler after the diffusion denuder should collect both particles and any semi-volatile organic material lost from particles during sampling with high efficiency.

7. The collection materials used in the sampler should be compatible both with the determination of total carbonaceous material and with the detailed chemical characterization of particulate organic material.

The BOSS (BYU Organic Sampling System) requires two different samplers as shown schematically in Figure 2:

1. A charcoal impregnated filter (CIF), multi-channel, parallel plate diffusion denuder followed by a filter pack containing quartz and CIF filters. The denuder removes gas phase organic compounds. The quartz filter after the denuder collects fine ( <2.5 |im) particles. The organic compounds collected by the CIF sorbent filter in this sampler are semi-volatile organic compounds lost from the particles during sampling and a small fraction (about 5%) of the gas phase organic material not collected by the diffusion denuder.

2. A quartz filter followed by a CIF diffusion denuder and a CIF sorption filter. The quartz filter collects particles and any gas phase organic compounds which can be absorbed by quartz, both those ori-

Figure 2 Schematic of the BOSS. Non-volatile particulate carbonaceous material is determined from analysis of T1;1. Semi-volatile carbonaceous material lost from particles is determined from analysis of CIF^, corrected for the denuder inefficiency determined from analysis of CIF21.

ginally in the gas phase and those lost from the particles during sampling. The denuder then removes gas phase compounds passing the quartz filter. Any gas phase compounds not removed by the denuder are then collected by the CIF sorbent filter. This system is used to determine independently the gas phase organic compounds not collected by the denuder to correct the data obtained with the CIF filter of Sampler 1.

The various 47 mm diameter filters of the BOSS are contained in Teflon filter packs (University Research Glass, Model 2000-30F) with the filter packs holding the quartz Rlter in Sampler 2, Figure 2, being modi-Red so that the outlet is identical to the inlet to allow for convenient connection to the diffusion denuder (University Research Glass, Model 2000-30FB). The diffusion denuder is based on a design originally reported by Fitz (1990). Each denuder is comprised of 17 (4.5 x 58 cm) strips of Schleicher and Schuell charcoal impregnated Rlter paper which are separated at the long edges by 2-mm rods. The multi-parallel plate array of filter strips is contained within a (5 x 5 cm) square aluminium tube. The entire assembly is nominally 90 cm in length to accommodate 58 cm sorbent filter strips and two nominally 15 cm long flow straightening sections ahead of and behind the denuding section. The multi-channel diffusion denuder is designed to have acceptable efRciency for the removal of gas phase organic material in the denuder, negligible loss of particles to the denuder during sampling, and high capacity for the collection of gas phase organic material. The total capacity of the CIF multichannel denuder has not been directly measured. However, no degradation of the efRciency of the denuder for the collection of gas phase organic compounds was seen during continuous operation at 40 Lmin-1 for over two months or for sampling at 180Lmin~1 for continuous periods equivalent to seven and fourteen days in the Los Angeles Basin, for ten days in the Mohave Desert, or for twelve days at Research Triangle Park, NC.

The CIF (Schleicher and Schuell, Inc.) strips in the diffusion denuder are used as received from the manufacturer. The 47 mm CIF (Schleicher and Schuell, Inc., No. 508) filters are cleaned with dich-loromethane and dried at 200°C before use. Alternately, a 47 mm Carbon EMPORE (3M) filter may be used. The Carbon EMPORE Rlters may be used as received from the manufacturer, however, flow through these filters is limited to about 7Lmin"i. The 47 mm quartz filters (Pallflex, 2500 QAT-UP) are pretreated by firing at 800°C for four hours prior to sample collection. The flow through the two samplers of the BOSS, Figure 2, is controlled at about 40 L min"i. A version of the BOSS using a shortened denuder (27 cm CIF strips) with a flow of from 4 to 20 L min~i has also been described. The CIF or Carbon EMPORE filters may also be replaced with an XAD sorbent bed. The XAD (Rohn & Haas) is cleaned by first sonicating 10 times with CH3OH to remove very Rne particles and then Soxhlet extracting for 24 hours sequentially with CH3OH, CH2Cl2 and C2H5OC2H5.

The efRciency of removal of gas phase organic compounds by the CIF denuder (or by an annular denuder configuration) is described by eqn [1]:

where Co and C are the concentrations of organic compounds entering and exiting a section of the denuder, respectively, Dj is the diffusion coefRcient of the gas phase organic compound(s) at the experimental conditions, L and W are the length and effective width of the denuder section, F is the flow and d is the space between the denuder surfaces. A plot of the log of the amount collected in equal length sections of a denuder versus the distance from the start of the denuder through the section should be linear with a slope of —22.5 DjW/4Fd. The expected deposition gradient was observed for organic material collected by a CIF based denuder containing two parallel sheets of the charcoal impregnated Rlter material. The slope of the line describing the deposition pattern for the collection of ambient gas phase organic compounds gives an average diffusion coefRcient for the collected gases of 0.052 $ 0.008 cm2 s"i. This diffusion coefRcient gives a calculated effective average molecular weight of 160 + 25. This average molecular weight is consistent with the majority of the organic material which has been shown to be collected by the diffusion denuder. The deposition pattern was also consistent with the measured efRciency of the CIF denuder for the removal of gas phase organic compounds.

The importance of the particulate organic compounds which have not been identified in past studies where particles are collected on a Rlter will be dependent on the chemical composition and the size distribution of the particulate organic compounds, both those lost from the particles during sampling and those remaining on the particles after sampling. A high-volume, multi-component diffusion denuder sampling system (BIG BOSS) for the determination of the size distribution and chemical composition of Rne particulate organic compounds using diffusion denuder sampling technology has been developed and tested.

The BIG BOSS uses a variety of size selective virtual impactor inlets to control the particle size of the particles introduced to the diffusion denuder sampler. The inlet system is a modiRcation of a high-volume, multi-jet virtual impactor. The nominal total flow through all systems of the BIG BOSS is 0.9 m3 min~i inlet flow. This flow is divided among four systems, each with a coarse particle minor flow stream and a fine particle major flow stream. Two of the four systems have an inlet cut of 2.5 | m. The other two systems are designed to operate with an inlet cut of 0.8 and 0.4 |im (see Tang, 1994).

The PC-BOSS Denuder Sampler

The combination of the technology used in the previously described BIG BOSS sampling system and the

Figure 3 Schematic of the PC-BOSS. The composition of fine particulate matter is determined from analysis of the two filter packs after the denuder. The efficiency and losses of the fine particle concentrator is determined by comparison of sulfate on Q2 with that on Q, or T-!.

Harvard particle concentrator results in the Particle Concentrator-Brigham Young University Organic Sample System (PC-BOSS) shown schematically in Figure 3. The system has been optimized to meet the following criteria: (1) removal of at least 75% of the gas phase material before the sampled aerosol is passed through the diffusion denuder, (2) efficiency, >99% for the removal of SO2, HNO3 and gas phase semi-volatile organic material, (3) determination of particle mass, carbonaceous material and nitrate with a diffusion denuder sampler, (4) operation on less than 20 amps of 110 V power.

The inlet to the sampler is a Bendix cyclone with a particle cut of 2.3 |im aerodynamic diameter at an inlet flow of 150Lmin"i. Following the inlet, 20 L min~i is diverted to a filter pack to provide data for calculating the efficiency of and losses in the PC-BOSS particle concentrator. The remaining flow enters the virtual impactor particle concentrator. The design and evaluation of the particle concentrator has been previously described. The particle concentrator separates most of the gas phase material into the major flow and leaves particles larger than the cut point (about 0.1 |im) along with a significantly reduced fraction of the gas phase material in the minor flow. The performance of the particle concentrator for collection of ambient samples with the PC-BOSS was evaluated as a function of the minor to major flow ratio, and the distance between the accelerator and receiver slits of the virtual impactor. The optimum design uses a single particle concentrator with a 9.5 cm long slit and a distance between the accelerator and receiver slits 1.5 times the slit width of 0.32 mm. The minor flow (25% of the total 150 L min-1 flow) containing concentrated particles enters the BOSS diffusion denuder. The denuder is followed by two parallel filter packs (Figure 3). The filter pack containing a 47 mm quartz filter (Pallflex, prefired) followed by a 47 mm charcoal impregnated filter is used to determine fine particulate carbonaceous material, including semi-volatile organic material lost from the particles during sampling. The second filter pack contains 47 mm Teflon (Gelman Zefluor) and nylon (Gelman Nylasorb) filters to determine mass, sulfate and nitrate, including any nitrate lost from particles during sampling.

The IOVPS and IOGAPS Denuder Samplers

Researchers at Lawrence Berkeley Laboratories have developed an annular denuder sampling system, the Integrated Organic Vapour/Particle Sampler (IOVPS) with an XAD-IV based diffusion denuder for the measurement of SVOC. This diffusion denuder sampler is similar in design and operation to the BOSS systems described above. The IOVPS is shown schematically in Figure 4. An advantage of the IOVPS sampler is that the gas phase material collected by the denuder can be easily recovered for organic compound chemical characterization and quantitation. Current disadvantages of the sampler are the total carbonaceous material is not determinable in the denuder or post-filter XAD sorbent beds (Figure 4) and the capacity of the denuder limits the length of time over which the denuder may be used from hours to days.

The denuder of the IOVPS system is prepared by adhering very fine mesh XAD to a glass multi-annular denuder surface. The adhesion of the finely ground XAD to the sandblasted glass is strong enough that the coating is resistant to removal by handling, solvent washing and air sampling. Quantitation of gas phase organic compounds removed by the IOVPS denuder is accomplished by extraction with a suitable solvent and analysis by GC or GC/MS. The collection efficiency of these denuders for various gas phase organic compounds has been shown to be close to that predicted by eqn [1]. A 5-channel denuder with 1 mm spacing in the annulus and a coating length of 38 cm has been used for most applications of the IOVPS denuder.

The capacity of the IOVPS XAD based denuder is dependent on two factors: (1) the capacity of the XAD surface for a given compound and (2) the time required to elute a dilute concentration of a given gas down the XAD column length. The dominant factor appears to be the movement of collected gas phase material down the XAD column. As a result, studies using the IOVPS denuder have generally been limited to chamber studies where the sampling period is short or to ambient studies where the sample collection occurred only over a few hours. By increasing the length and surface area of the denuder (including using parallel denuders) prototype systems have been developed by Lawrence Livermore Laboratory and the Atmospheric Environment Service of Environment Canada (IOGAPS, Integrated Organic Gas and Particle Sampler) which are capable of sample collection for up to 48 hours. Comparisons of results obtained from 24 hour IOGAPS and sequential 4 hour IOVPS data where the annulus width of the IOGAPS was 1.5-3.0 mm with a residence time of 2.6 s indicated there was about 10% breakthrough of naphthalene in the IOGAPS. A redesign with an annulus width of 1.0-1.4 mm is expected to eliminate this problem.

Particle losses to the wall of the IOVPS denuder has been evaluated in several studies. The results are essentially identical to those reported above for the BOSS and BIGBOSS samplers. With face velocities of around 20 cm s _i through the denuder, losses are less than 2%. At higher face velocities of 35 to 50 cm s_i, the losses increase to about 5-7%. These losses are comparable to that seen for conventional annular denuders.

Other Diffusion Denuder and Related Samplers

Diffusion denuder sampling techniques have also been developed and used by several other investigators to determine fine particulate organic material. The focus of these studies has been on the determination of specific organic compounds. Krieger and Hites have used short sections of capillary gas chromato-graphic columns as a diffusion denuder and determined concentrations of gas and particulate phase polychlorinated biphenyl (PCB) and polyaromatic hydrocarbon (PAH) compounds. Coutant et al. have described the development of a circular multi-channel diffusion denuder for the study of PAH in ambient air. However, results on field studies using the sampling system have not yet been published. The

Figure 4 Schematic of the IOVAPS (from Gundel, 1999). The denuders contain XAD as the gas phase organic sorbent. Non-volatile particulate carbonaceous material is determined from analysis of the filters in either of the filter packs. Semi-volatile carbonaceous material lost from particles is determined from analysis of the denuder d3.

Figure 4 Schematic of the IOVAPS (from Gundel, 1999). The denuders contain XAD as the gas phase organic sorbent. Non-volatile particulate carbonaceous material is determined from analysis of the filters in either of the filter packs. Semi-volatile carbonaceous material lost from particles is determined from analysis of the denuder d3.

Atmospheric Environment Service of Environment Canada has been involved since 1984 in the development and use of a diffusion denuder sampler for the determination of PCBs and chlorinated hydrocarbons. The instrument uses a silicone gum/Tenax-coated, multi-tube, annular, diffusion denuder to remove the target organic compounds. Turpin et al. have developed a sampling system which corrects for the loss of semi-volatile organic compounds during sampling by removal of most of the gas phase material from the particles in a diffusion separator sampling system. The system has been evaluated for the collection of PAH. All of the systems which have been described by other research groups collect samples at a flow rate of a few L min-1. One advantage of the use of the diffusion denuder sampling systems described above is that the attainable high flow rate, 200 L min~i, allows for more collected material and a wider range of analyses on the collected samples.

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