Environ. Sci. Technol. 2007, 41, 1272-1278
Alternative Approaches for Modeling Gas-Particle Partitioning of Semivolatile Organic Chemicals: Model Development and Comparison CHRISTIAN W. GO 2 TZ,† M A R T I N S C H E R I N G E R , * ,† MATTHEW MACLEOD,† CHRISTINE M. ROTH,‡ AND KONRAD HUNGERBU 2 HLER† Safety and Environmental Technology Group, Institute for Chemical and Bioengineering, Swiss Federal Institute of Technology, ETH Zurich, Switzerland, and Department of Environmental Health, Harvard School of Public Health, Boston, Massachusetts 02215
We present a novel model of gas-particle partitioning based on polyparameter linear free energy relationships (ppLFERs) that is capable of representing a broad range of aerosol properties. We apply the model to semivolatile organic chemicals including PCBs, DDT, and polar pesticides, and compare it to a widely adopted model based on the octanol-air partition coefficient (KOA). For nonpolar chemicals and cases where sorption to aerosols is dominated by absorption into organic matter, the two models are highly correlated and both are appropriate. Significant differences between the models are found for (a) polar chemicals and (b) aerosols with low organic matter content. The explicit description of polar interactions in the ppLFER approach implies stronger interactions between chemicals and aerosols than the KOA-based model, which describes polar interactions only implicitly and to a limited extent. Practical application of the ppLFER-based model to a wide range of chemicals is currently limited by data gaps in measured Abraham solvation parameters and uncertainties in estimation methods.
Introduction Semivolatile organic compounds (SOCs) are simultaneously present in the gas phase and associated with aerosol particles in the atmosphere (1). Removal of chemicals from the atmosphere via deposition or degradation is strongly influenced by the distribution between these two phases and the extent of gas-particle partitioning is a key factor determining long-range transport potential and overall persistence of SOCs (2-4). Models of gas-particle partitioning are a crucial part of multimedia chemical fate and transport models which seek to describe the long-term fate of SOCs in the environment. The distribution of chemicals between the gas phase and aerosol particles is commonly quantified by KP (m3 air/µg aerosol) * Corresponding author e-mail:
[email protected]; fax: +41 (0) 44 632 11 89. † Swiss Federal Institute of Technology. ‡ Harvard School of Public Health (now BMG Engineering AG, Switzerland). 1272
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Kp ) Cp/CA
(1)
where CP is the particle-associated concentration (mol/µg aerosol) and CA is the gas-phase concentration (mol/m3 air). KP can also be expressed in a dimensionless form (KP*) by multiplication with the total suspended particle concentration (TSP, µg aerosol/m3 air)
Kp* ) Cp‚TSP/CA ) Kp‚TSP
(2)
KP* is the ratio of moles of chemical on aerosol particles to moles in the gas phase in the same volume of air. Contemporary multimedia models use single-parameter linear free energy relationships (spLFERs) to describe sorption to aerosol particles (2, 3, 5, 6). Generally, spLFERs relate the gas-particle partition coefficient to the subcooled liquid vapor pressure, pL*, or the octanol-air partition coefficient, KOA, of the chemical. These relationships have two important limitations. First, they are only valid within the compound class for which they were developed (7). Most spLFERs are based on sorption data for nonpolar chemicals such as PCBs and PAHs, for which total sorption is assumed to be dominated by absorption into organic matter (OM) (8). Other possible chemical-sorbate interactions are not explicitly considered. Second, variability in composition and properties of aerosol particles are virtually ignored in the spLFER approach. In most cases the aerosol is described only in terms of OM content. An alternative conceptual model for sorption to aerosol particles has been developed by Goss and co-workers (7, 9-11) based on the pioneering work of Abraham and coworkers (12). They advocate describing gas-particle partitioning with polyparameter linear free energy-relationships (ppLFERs). A ppLFER describing the gas-particle partition coefficient has the general form
log Kp ) w‚W + x‚X + y‚Y + ... + C
(3)
Each multiplicative group on the right side of the equation describes an interaction between the chemical and the sorbent such as van-der-Waals interactions or hydrogen bonding. The multiplicative groups are composed of a term representing the chemical’s ability to participate in an interaction (W, X, Y, ...) and the sorbent’s ability to participate in the interactions (w, x, y, ...). To calculate KP for a chemical, one needs Abraham solvation parameters for the chemical (Achemical ) {W, X, Y, ...}), a set of complementary sorbent parameters (Asorbent ) {w, x, y, ...}), and the constant, C. In principle, the ppLFER approach can describe any surface adsorption or bulk phase absorption interactions that are possible between gas-phase molecules and aerosols. In laboratory and field studies, ppLFERs have been developed to describe the distribution of volatile organic compounds between the gas phase and various surfaces and bulk phases (7, 10, 11, 13-18). In these studies ppLFERs for sorbing phases have been developed by empirically determining the parameters for the sorbent (Asorbent) from measured sorbentair partition coefficients for chemicals with known solvation parameters (Achemical). Here we develop a general model of gas-particle partitioning that combines existing ppLFERs for sorption to fine aerosols and components of coarse aerosols into a model that is applicable to aerosols with defined size distribution and composition. The new model is applicable to organic non-ionic polar and nonpolar chemicals and can describe a wide range of aerosol properties. Our primary goal is to 10.1021/es060583y CCC: $37.00
2007 American Chemical Society Published on Web 01/19/2007
TABLE 1. Mass Concentrations (g/m3 Air) and Surface Area Concentrations (m2/m3 Air) of Sorption-Relevant Components for Seven Generic Aerosol Scenarios (OM, Organic Matter; EC, Elemental Carbon)
references:
TSP or surface concn. composition
coarse fraction
cOM cEC cseasalt cmineral cOM
fine fraction rel. humidity
urban aerosols
rural continental aerosols
remote continental aerosols
marine aerosols
desert aerosols
arctic aerosols
free tropospheric aerosols
(2,20)
(1,2,20)
(1,2,20)
(20,28)
(20)
(20,30)
(2,20)
(19,25)
(19,25,26)
(19,25)
(28,29)
(27)
(30)
(19)
g/m3 m2/m3 m2/m3 m2/m3 g/m3
4.5E-06 2.1E-05 1.8E-06 7.5E-06 1.5E-05
5.0E-07 6.1E-06 3.1E-06 2.5E-06 1.0E-06
1.0E-07 1.5E-07 7.3E-08 5.5E-08 6.0E-07
2.0E-08 1.0E-08 5.0E-07 7.5E-08 2.0E-07
1.0E-09 1.0E-08 1.0E-08 6.0E-06 4.0E-08
1.0E-08 8.7E-08 8.6E-08 1.0E-08 4.0E-08
5.0E-09 4.0E-08 2.0E-08 1.0E-08 2.0E-08
%
60
60
60
80
40
40
60
compare and contrast the spLFER and ppLFER models by (i) comparing the sorption models for a reference aerosol to identify compound classes where KP is sensitive to which model is used, and (ii) investigating the variability of KP among different aerosols. Our second goal is to evaluate the suitability of the ppLFER gas-particle partitioning model for use in multimedia chemical fate models. These models are typically applied to screen chemicals for environmental fate characteristics, and are used in decision-oriented hazard and risk assessments. Based on our model results and uncertainty analysis we define the limits of applicability of currently used spLFERs, and identify situations where the two types of models produce qualitatively different descriptions of gasparticle partitioning.
Methods and Model Description General ppLFER Gas-Particle Partitioning Model. The gasparticle partitioning model developed here is designed to be flexible enough to describe sorption to a wide range of aerosols with different compositions. The model combines information about the aerosol composition with Asorbent parameters that have been empirically determined for individual components of the aerosols. The composition and mechanisms of formation of aerosol particles varies with particle size (19, 20). Therefore, the gasparticle partitioning model distinguishes a fine and a coarse fraction. The fine fraction is defined as PM2.5 (diameter 1 in the arctic and desert scenarios. Adsorption to minerals and salt is negligible for PCBs and DDTs in all scenarios except for the desert scenario. In the desert scenario, adsorption to mineral surfaces contributes between 51% and 93% to KP*. Adsorption to EC is important in the arctic scenario, accounting for 66% to 92% of KP*. In all other scenarios, KP* is dominated by absorption into OM. For the polar pesticides (Figure 3B), the ratio of KP* (ppLFER) to KP* (KOA-based) depends much more strongly
on the aerosol scenario. Low OM aerosols (desert, arctic, or marine environments) show the strongest differences in KP* between the two sorption models. In particular, the desert aerosol scenario shows for all investigated polar pesticides significant differences between the two models. In extreme cases, the ppLFER model estimates KP* values up to 5 orders of magnitude higher than the KOA-based model. The two highest outlying points in all scenarios are acetochlor and metoxuron, for which property data are suspect, see above. In all aerosol scenarios except the arctic scenario, the KP* values of polar pesticides calculated with the ppLFER model are dominated by adsorption to mineral surfaces with contributions from 28% (carbaryl in the free tropospheric aerosol scenario) to 100% (all polar pesticides in the desert scenario). In the arctic scenario, adsorption to EC is similar to adsorption to mineral surfaces and contributes between 14% (aldicarb) and 79% (metolachlor) to KP*. Parameter Uncertainty. One important limitation to the application of the ppLFER approach is the limited availability of ∑R and ∑β parameters. There are only a few studies providing measured ∑R and ∑β values, and the estimation program ABSOLV (38) often yields high uncertainty ranges for SOCs. In some cases, estimated values are significantly different from experimental data. For the polar pesticides investigated, ∑β is the most influential parameter. If ∑β is overestimated, KP* is also overestimated. Atrazine, for example, has a calculated ∑β value of 1.41 from ABSOLV and an experimental value of 0.96 (38). This discrepancy leads to a difference in KP* of a factor of 200 in the urban aerosol scenario and a factor of 400 in the desert aerosol scenario. More experimental data are needed for ∑R and ∑β values of SOCs to expand the applicability of the ppLFER approach. Another uncertainty is related to the Asorbent parameters describing the aerosol components. Laboratory measurements for individual components may differ from actual values for aerosols in the field. Measurements of Asorbent parameters for field aerosols will reduce this uncertainty. Recommendations for Describing Gas-Particle Partitioning in Multimedia Models. When the total sorptive capacity of aerosols is dominated by OM, sorption of nonpolar chemicals is described in a consistent fashion by both models. The systematic deviation in KP* of a factor of 10 between the two models cannot be ascribed to one of the models. We suggest that the KOA-based model (eq 7) overestimates KP* by assuming a value of 1 for MO/MOM. We recommend a value of 0.26 for MO/MOM, which results from assuming a MW of aerosol OM of 500 g/mol (36). Generally, for nonpolar chemicals, both approaches are appropriate for multimedia models including aerosols with a sorptive capacity dominated by OM (urban/rural regional models, unit world models, generic global multimedia models). The application of KOA-based models to nonpolar chemicals in spatially resolved global multimedia models or regional desert or arctic models is conceptually questionable, because the dominating mineral or EC surfaces are not represented by the octanol phase. However, with the parameterization used here, the discrepancies between the models are generally still within the uncertainty ranges of KP*. For polar chemicals, our analysis demonstrates that some polar interactions are implicitly represented in KOA-based models. Interestingly, for polar chemicals sorbed to aerosols whose sorptive capacity is dominated by OM, the KOA-based model, which was developed for nonpolar chemicals, estimates KP values that are in good agreement with the ppLFER approach. KOA-based models are therefore suitable for environmental fate models containing aerosols with a sorptive capacity dominated by OM (regional models of urban or continental areas, unit world models, generic global multimedia models). However, applying a KOA-based model to polar chemicals may underestimate polar interactions and
thus KP*. Therefore, we suggest for polar chemicals that the ppLFER approach is adopted whenever solvation parameters are available. For desert or arctic aerosols, the discrepancies in KP* between the two sorption models can be up to several orders of magnitude. KOA-based models, which do not consider mineral and EC surfaces, are not appropriate for these kinds of aerosols. Therefore, sorption to aerosols in atmospheric or multimedia models including arctic or desert regions should be modeled with the ppLFER approach. In summary, the ppLFER approach for modeling gasparticle partitioning is conceptually satisfying, and holds considerable potential. However its broad application to chemicals from a wide variety of classes is limited by lack of experimental data for solvation parameters and limitations of estimation methods. A comprehensive evaluation of the ppLFER approach is not currently possible due to a lack of KP data for polar substances. However, based on our model comparison and underlying theory, it is clear that if absorption into organic matter dominates total sorption, both spLFERs based on KOA and the ppLFER approach are appropriate for modeling nonpolar and polar semivolatile organic chemicals. Under environmental conditions in which organic matter does not dominate the aerosol’s sorptive capacity, such as very low organic matter content of the aerosol, very low relative humidity (nonpolar chemicals), and low to average relative humidity (polar chemicals), KOAbased models are not appropriate and a ppLFER-based model should be preferred.
Acknowledgments We thank Urs Baltensperger for support concerning aerosol properties, Kai-Uwe Goss for support concerning ppLFERs, and Michael H. Abraham for discussion of solvation parameters.
Supporting Information Available Information about measured partitioning data, aerosol components, solvation parameters, and uncertainty analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review March 13, 2006. Revised manuscript received November 21, 2006. Accepted December 4, 2006. ES060583Y
