1997 Progress Report: A Study of the Gas/Particle Partitioning of Chlorinated Dibenzodioxins (CDDs) and Chlorinated Dibenzofurans (CDFs) to Ambient and Model Aerosol MaterialsEPA Grant Number: R825376
Title: A Study of the Gas/Particle Partitioning of Chlorinated Dibenzodioxins (CDDs) and Chlorinated Dibenzofurans (CDFs) to Ambient and Model Aerosol Materials
Investigators: Pankow, James F.
Institution: Oregon Graduate Institute of Science & Technology
EPA Project Officer: Shapiro, Paul
Project Period: October 1, 1996 through September 30, 1999
Project Period Covered by this Report: October 1, 1996 through September 30, 1997
Project Amount: $466,448
RFA: Exploratory Research - Air Engineering (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Land and Waste Management , Air , Engineering and Environmental Chemistry
Objective:Construct a Linear Free Energy Relationship to predict the gas/particle partitioning behavior of the polychlorinated dibenzodioxins and dibenzofurans to ambient aerosols.
A two stage air sampler to conduct our controlled field experiments has been designed and is currently under construction . Work has commenced in two areas:
The first is developing a method to control the temperature and relative humidity of the ambient air. Our method will use a modified environmental chamber to condition the air before it enters the second stage of the air sampler. In most cases the air will only need to be heated and our calculations indicate this can be achieved with a rather small, portable environmental chamber. We are in the process of acquiring such an environmental chamber.
The second area of work has focused on the generation and sampling of the gas and particle phase PCDD/Fs. This stage of the air sampler has been constructed and is currently under pilot testing. A gas phase concentration of a compound is obtained by passing a portion of the main air flow through the generator cartridges and then mixing this air back into the main air flow. The gas phase concentration of the compounds can be controlled by adjusting the flowrate of air through each set of generator cartridges. The main air flow containing the PCDD/Fs is then passed first through a filter containing particles and then through a series of PUFs. In our preliminary experiments we have used a series of PAHs instead of PCDD/F so as to reduce the risk to our graduate students of exposure to the highly toxic PCDD/Fs. The PAHs were chosen over a similar range of vapor pressures as the PCDD/F of interest and therefore both classes of compounds should behave similarly in our generator cartridges and result in similar gas phase concentrations. We have packed 4 sets of generator cartridges containing 2 PAHs per set. The PAHs chosen encompass a 3.5 order of magnitude range in vapor pressure. The gas phase concentration of the compounds is determined by passing the total airflow downstream of a glass fiber filter through a series of polyurethane foam (PUF) plugs. The gas phase concentration was measured every three hours by exchanging the PUFs. Front and backup PUF are used to asses breakthrough. The compounds of interest are extracted from the PUF by rinsing the PUF with methylene chloride. The mass of PAH in the extracts are quantified by GC/MS and deuterated surrogate standards are used to assess recoveries. The gas phase concentration is the mass of compound measured divided by the volume of air passed through the PUF.
Ambient particles have been collected on Quartz Fiber filters using a standard Hi-Vol air sampler. The particle loading is determined by weighing the filter before and after sampling. A punch from the loaded filter was placed into a filter holder contained in the second stage of our CFE air sampler and then equilibrated with gas phase PAHs. The time to reach gas/particle equilibrium is determined as the point where the gas phase concentration of PAHs measured downstream of the filter is constant as a function of time. At the end of the experiment the filter containing the particle phase is Soxhlet extracted. The polar compounds and water were removed from the extracts using a short cleanup column. The mass of PAH in the extracts are quantified by GC/MS and deuterated surrogate standards are used to assess recoveries. Gas/Particle partition coefficients, Kp (m3/g) are calculated as a ratio of the concentration of chemical in the particle phase (ng/g) to the concentration of chemical in the gas phase (ng/m3). The Kp value of a compound is known to be function of the compound subcooled liquid vapor pressure. To obtain our desired predictive relationship we have plotted our Kp (m3/g) values as a function of the compounds subcooled liquid vapor pressure, pL (torr). In our pilot studies we have determined the Kp values of a series of PAHs partitioning to an ambient suburban Portland aerosol. To ensure the system is operating correctly our plots of log Kp vs. log pL for the PAHs can be compared to those determined by previous researchers. Our results are provided in the next section.
Accomplishments and Research Results:
Our first experiments have been concerned with the generation and sampling of gas phase and particle phase PAHs. In Figure 1 the gas phase concentration of a series of PAHs are plotted as a function of the volume of air sampled downstream of a clean glass fiber filter. As shown in Figure 1 we are able to generate a nearly constant concentration of PAH in the gas phase. The gas phase concentration increases slightly from the initial value since it must first breakthrough the glass fiber filter. After breakthrough, the average relative standard deviation of our gas phase PAHs concentration is 15%.
Regarding the QA/QC of our gas phase PAH generation and sampling method, the recovery of PAHs from our PUFs ranged from 67 to 110% averaging 86%. Furthermore breakthrough of the most volatile PAH, phenanthrene, from the front to backup PUF was only 1-2%. The mass of all PAHs in blank PUFs were less than 1% of that in the front PUFs.
Ambient suburban Portland aerosols were equilibrated with PAHs using our system. The gas phase PAH and particle phase concentrations were determined as before. The recovery of surrogate standards from the particles was 140%. The concentration of PAHs on the backup glass fiber filter was on average 5% that on the front, particle loaded quartz fiber filter. Gas/Particle partition coefficients for the PAHs were determined from the gas phase and particle phase PAH concentrations. In Figure 2 the log Kp value of each PAH was plotted against the log of the compounds subcooled liquid vapor pressure. The linear free energy relationship shown in figure 2 can be used to predict the Kp value of other PAHs knowing only that compounds subcooled liquid vapor pressure.
The slope and intercept of a linear regression of a plot of log Kp vs. log pL can be compared to previous studies to confirm the integrity of our method, this is shown in Table 1. Theory predicts that at equilibrium for a given class of organic compounds the slope of a plot of log Kp vs. log pL should be -1. Our results are not significantly different from this value. Generally, partitioning to suburban Portland aerosols was a factor of ten weaker than to urban Portland aerosols. Our values were in the range of that reported by Yamasaki et al. for Osaka, Japan. Gas phase adsorption to our backup glass fiber filter was insignificant compared to sorption to the front loaded filter, confirming the absence of any sampling artifacts.
Table 1. Comparison regression coefficients.
|Suburban Particulate Matter, Portland||-1.11||-8.09||This study|
|Urban Particles, Portland||-0.84||-6.54||Ligoki and Pankow|
|Urban Particles, Portland||-0.94||-6.80||Hart and Pankow|
|Urban Particles, Japan||-0.98||-7.87||Yamasaki et al.|
|Glass Fiber Filter
|Quartz Fiber Filter
|-1.00||-9.88||Luo and Pankow|
|Air/Water Interface||-0.91||-10.06||Luo and Pankow|
In the next three months we plan on combining the air conditioning stage of the sampler to the chemical generation/collection stage of the sampler. We will run controlled field experiments (CFEs) in Portland, Oregon continuing to use the PAHs as the target compounds. Upon satisfactory completion of these pilot tests we will begin running CFEs in Portland which include the PCDD/Fs. By the summer of 1998 we are planning to run CFEs in Denver, Chicago and in a rural location.
Supplemental Keywords: organohalogen, fate, transport
Financial: Expenditures have lagged slightly behind budget because of the
availability of in-house materials such as air sampler materials and PAH standards. In addition time has been spent training a highly qualified student. In the next few months very expensive dioxin standards will be required and we anticipate that expenditures will
quickly fall in line as proposed. No significant budget deviations have been identified to date and we anticipate being able to accomplish the proposal objectives in accordance with the original budget.