Topic 1 – Sampling Issues Related to Organic Speciation of PM and SVOC

Abstract

1a. How are SVOC and PM-associated OC defined?

i. Theoretical Definitions

SVOC

Borrowing from Van Vaeck et al (1984) and others (for example, Finlayson-Pitts and Pitts, 2000, p 412) we define semi-volatile organic compounds (SVOCs) as organic compounds that show significant gas and particulate concentrations in the atmosphere. Pankow (1993) generalized this by including other surfaces. He observed that SVOCs can have non-negligible fractions of their environmental masses in both the atmosphere and partitioned to surfaces such as soil, plants, building materials and sampling media. SVOCs have vapor pressures between about 10-4 or 10-5 and 10-11 atm (100.1 and 10-6 Pa; 10-1 and 10-8 Torr) over the ambient temperature range.

SVOCs are ‘sticky’ or multi-phasic; they partition their mass between the gas phase and any surfaces that afford a degree of sorption, such as fine particles in air. The degree of gas-particle partitioning affects the transport, deposition, and atmospheric fate of these compounds, since SVOCs, once airborne, can deposit onto vegetative surfaces, windows, carpets, soils, or even the human body. Compounds with higher vapor pressures are present primarily in the gas phase, whereas low volatility compounds are found on or within particles.

Nearly all classes of organic compounds contain semi-volatiles: alkanes, PAHs, PCBs, PCDDs, PBDEs, nitro-aromatics, terpenes, acids, carbonyls, and lipids, to name a few. SVOCs enter the atmosphere by direct emission, frequently as byproducts of incomplete combustion. Polar SVOCs are also produced by oxidation of precursor unsaturated compounds, and they can be incorporated into PM as secondary organic aerosols (SOA). Precursor organics are emitted from transportation, industrial and biogenic sources. Also, many of the potentially carcinogenic organic compounds found in the atmosphere are semi-volatile.

PM-associated OC

Roughly half the mass of urban fine particles in the US can be attributed to carbonaceous components. About a third of this is elemental or black carbon, and carbonate-containing compounds have negligible contributions. Here we define PM-associated organic carbon as the complex mixture of organic compounds that are incorporated into airborne particles by direct emission, abrasion, condensation and surface reactions. The term PM-associated OC may be useful when considering the influence of the complex mixture of particle-associated organic compounds as a whole, for example, when investigating aerosol properties like organic film thickness, hydrophilicity, and optical absorption.

Relationships between SVOCs and PM-associated OC

SVOCs and PM-associated OC are related through the partitioning of SVOCs onto PM. However, less than half of PM-associated OC is semi-volatile under temperate conditions. The partition coefficient Kp is the most commonly used parameter for describing gas/particle partitioning, primarily because of its log-linear relationship to compound vapor pressure, poL. Although a compound’s vapor pressure at the temperature of interest has the greatest influence on partitioning, the interaction between compound structure and the sorptive medium plays an important role (e.g., compound size and polarity vs. adsorptive affinity or absorptive capacity). Plots of log Kp vs. log poL can provide information on the nature of the partitioning and may indicate whether sampling artifacts have impacted the measurement. Gas/filter partitioning coefficients can also be used to improve sampler and field study design (Mader et al., 2001).

Using Pankow’s nomenclature (Chapter 3 of Lane, ed. 1999), the equilibrium partitioning of a semi-volatile compound to an environmental surface S can be represented most simply by

G + S = P (1)

where G and P represent the gas and particulate phases of the SVOC. Since the gas and particulate phase concentrations are usually collected on an adsorbent and filter, respectively, their concentrations have been conveniently represented by A and F in the literature. If the sorbing surface is total suspended particulate matter, its concentration can be represented by TSP. At equilibrium the gas/particle partitioning constant Kp for adsorption of the SVOC i onto the solid surface of a particle surface can be expressed as

Kp = Fi / (Ai*TSP) (2) Adsorption to a solid surface
Pankow (1987) showed that Langmuir adsorption theory predicts that Kp (at constant temperature) is inversely proportional to the vapor pressure of i. If i is a solid, the sub-cooled liquid vapor pressure poL is used. For compounds of the same class, with similar enthalpies of desorption and vaporization among the members, plots of log Kp versus log poL for will be linear slope of -1.

SVOCs can also absorb into liquid particles such as environmental tobacco smoke or liquid (organic and/or water) films on particles with solid cores. Gas/particle partitioning of SVOCs in urban areas is better explained as absorption than adsorption. The absorptive partitioning of SVOC i into a liquid organic layer on a particle is like a gas dissolving in a liquid (Finlayson-Pitts and Pitts, 2000, p 417), and the measured partitioning coefficient takes the same form as equation 2, for adsorption.

Kp = Fi, om /(Ai*TSP) (3) Absorption into a liquid film or droplet

Fi, om represents the particle- associated concentration of i in air as measured from a filter, with explicit recognition that i has dissolved in liquid organic material, om, on the particle. For absorption into liquid films on particles, Pankow (1994) showed that Kp is proportional to the weight fraction of om to TSP. Kp is inversely proportional to the product of poL and the activity coefficient of i in the liquid phase. If the activity coefficient does not vary much across members of a class of SVOCs, plots of Kp vs log poL will have a slope of -1 for both adsorption and absorption. Goss and Schwarzenbach (1998) showed that slope deviations from -1 do not necessarily indicate non-equilibrium conditions, and they may be used to identify types of sorbate/sorbent interactions and characterize sorption processes.

ii. Operational Definitions

As Turpin et al. (2000) point out, the term SVOC is usually operationally defined, or undefined. Because the typical meaning of SVOC is rooted in sampling strategy, they prefer to use the term ‘condensable’ for airborne compounds that are found in both the gas an particle phases. At present, measured concentrations of PM-associated OC depend on both the sampling and analytical methods. The total carbon content of PM-associated OC can be determined by thermal analysis, but it is not yet possible to determine the total organic carbon content of real-world SVOC.

Operational definitions of SVOC and PM-associated OC will be illustrated for several sampling methods. Differentiation of PM-associated OC from EC (BC) will also be described briefly.

1b. What are the potential bias or problems associated with different PM and SVOC sampling techniques?

Dr. Douglas Lane will discuss how characterization of particle-associated organic species can be affected by the sampler geometry. Collected PM should be representative of the PM composition at the time of sampling, and not influenced by volatilization of SVOCs from the collected particles or sorption of SVOCs to the particles or sampling media. From the perspective of particle composition, these processes lead to negative and positive sampling artifacts, respectively, and they will be illustrated for samplers that collect particles onto filters upstream of sorbents for SVOC. Multi-channel annular diffusion denuders are being used to minimize positive PM artifacts. They have also been incorporated upstream of filters in the filter/sorbent geometry for determination of the gas/particle partitioning coefficients of source and atmospheric SVOCs. Data from recent field measurements using the Integrated Organic Gas and Particle Sampler (IOGAPS) will be presented to demonstrate the uses and limitations of denuder-based samplers. The application of denuders in smog chamber studies will be illustrated with gas/particle partitioning of the products of reactions of PAH with OH. The importance of understanding the limitations of air samplers and the proper selection of a sampler for a particular sampling objective will be emphasized.

1c. Advances in sampling and analysis of SVOC

Dr. John Volckens will 1) highlight recent advances in sampling and analysis of SVOCs, 2) discuss techniques to interpret artifact-biased data, and 3) recommend strategies for future research based on needs of the community.

As mentioned in section 1a, the ratio Kp is the most commonly used parameter for describing gas-particle partitioning, primarily because of its log-linear relationship to compound vapor pressure, poL. Gas-particle partitioning ratios (Kp) are defined at equilibrium, when the rates of mass transfer between phases are equal and at steady state. However, the conditions governing SVOC equilibrium are easily disrupted, especially when trying to measure gas-particle phase distributions in air. However, care must be taken with the use of Kp because this ratio is easily corrupted by even minute artifacts. Such sampling artifacts are widely known but difficult to prevent, predict, or account for after the fact (Volckens and Leith, 2003).

Plots of log Kp vs. log poL can provide information on the nature of the partitioning and may indicate whether sampling artifacts have impacted the measurement. Furthermore, most time-integrated sampling techniques (i.e. filter) cannot provide a true representation of ‘average Kp’. A need exists to develop improved sampling techniques with less perturbation of semi-volatile equilibrium, shorter sampling periods, and lower limits of detection.

References:

B. Finlayson-Pitts and Pitts (2000). Chemistry of the Upper and Lower Atmosphere: Theory Experiments and Applications, Academic Press, San Diego, 969 pp; generally relevant: Chapter 9, Particles in the Troposphere, p. 349-435; especially relevant: Section 9.D, p 412-423.

K.-U. Goss and R.P. Schwarzenbach (1998). Gas/solid and gas/liquid partitioning of organic compounds: Critical evaluation of the interpretation of equilibrium constants, Environ. Sci. Technol., 32, 2025-2032.

M.C. Jacobson, H.-C. Hanson, K.J. Noone and R.J. Charlson (2000). Organic atmospheric aerosols: review and state of the science, Reviews of Geophysics, 38, 267-294.

D.A. Lane, ed. (1999). Gas and Particle Phase Measurements of Atmospheric Organic Compounds, Vol. 2 of Advances in Environmental, Industrial and Process Control Technologies, Gordon and Breach, Amsterdam, 402 pp; especially relevant, Chapter 3, Pankow, Fundamentals and Mechanisms of Gas/Particle Partitioning in the Atmosphere; Chapter 6, Mc Dow, Sampling Artifact Errors in Gas/Particle Partitioning Measurements, Chapter 11, Gundel and Lane, Sorbent-Coated Diffusion Denuders for Direct Measurement of Gas/Particle Partitioning by Semi-Volatile Organic Compounds.

B.T. Mader, R.C. Flagan and J.H. Seinfeld (2001). Sampling atmospheric carbonaceous aerosols using a particle trap impactor/denuder sampler, Environ. Sci. Technol., 35, 4857-4867.

J.F. Pankow (1987). Review and comparative analysis of the theories on partitioning between the gas and aerosol particulate phases in the atmosphere, Atmos. Environ., 21, 2275-2283.

J.F. Pankow (1993). A simple box model for the annual cycle of partitioning of semi-volatile organic compounds between the atmosphere and the earth’s surface, Atmos. Environ., 27A, 1139-1152.

J.F. Pankow (1994). An absorption model of gas/particle partitioning of organic compounds in the atmosphere, Atmos. Environ., 28, 185-188.

B.J. Turpin, P. Saxena and E. Andrews (2000) Measuring and simulating particulate organics in the atmosphere: problems and prospects, Atmos. Environ., 34, 2983-3013.

L. Van Vaeck, K. Van Cauwenberghe and J. Janssens (1984). The gas-particle distribution of organic aerosol constituents: Measurement of the volatilization arteface in Hi-Vol cascade impactor sampling, Atmos. Environ., 18, 417-430.

J. Volckens and D. Leith (2003). Comparison of methods for measuring gas-particle partitioning of semi-volatile compounds, Atmos. Environ., 37, 3177-3188.