Modeling and analysis of sampling artifacts in measurements of gas-particle partitioning of semivolatile organic contaminants using filter-sorbent samplers

Highlights

• A mechanistic model to characterize air sampling process was developed and validated.

• Positive sampling bias were observed for almost all the target SVOCs.

• Correlations and plots provided can be used to estimate sampling bias for various SVOCs.

• Particle penetration may result in underestimation of the gas/particle partition coefficient.

• The option of backup filters must be considered carefully in field measurements.

Abstract

Measurements of gas/particle partition coefficients for semivolatile organic compounds (SVOCs) using filter-sorbent samplers can be biased if a fraction of gas-phase mass is measured erroneously as particle-phase due to sorption of SVOC gases to the filter, or, if a fraction of particle-phase mass is measured erroneously as gas-phase due to penetration of particles into the sorbent. A fundamental mechanistic model to characterize the air sampling process with filter-sorbent samplers for SVOCs was developed and partially validated. The potential sampling artifacts associated with measurements of gas-particle partitioning were examined for 19 SVOCs. Positive sampling bias (i.e., overestimation of gas/particle partition coefficients) was observed for almost all the SVOCs. For certain compounds, the measured partition coefficient was several orders of magnitude greater than the presumed value. It was found that the sampling artifacts can be ignored when the value of $\log \left[K_f/\left(K_p\cdot C_{p,a}\right)\right]$ is less than 7. By normalizing the model, general factors that influence the sampling artifacts were investigated. Correlations were obtained between the dimensionless time required for the gas-phase SVOCs within the filter to reach steady state $\left(T_{s,s}^{\ast }\right)$ and the chemical V_p values, which can be used to estimate appropriate sampling time. The potential errors between measured and actual gas/particle partition coefficients of SVOCs as a function of sampling velocity and time were calculated and plotted for a range of SVOCs (vapor pressures: 10⁻⁸ ∼ 10⁻³ Pa). These plots were useful in identifying bias from the sampling in previously-completed field measurements. Penetration of particles into the sorbent may result in significant underestimation of the partition coefficient for particles in the size range between 10 nm and 2 μm. For most of the selected compounds, backup filters can be used to correct artifacts effectively. However, for some compounds with very low vapor pressure, the artifacts remained or became even larger than they were without the backup filter. Thus, the option of backup filters must be considered carefully in field measurements of the gas/particle partitioning of SVOCs. The results of this work will allow researchers to predict potential artifacts associated with SVOC gas/particle partitioning as functions of compounds, the concentration of particles, the distribution of particle sizes, sampling velocity, and sampling time.

Introduction

Semivolatile organic compounds (SVOCs) are a class of chemical compounds that typically have vapor pressure (V_p) values between 10⁻⁹ and 10 Pa (Weschler and Nazaroff, 2008). Many SVOCs, such as polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenylethers (PBDEs), polychlorinated biphenyls (PCBs), and phthalates, are persistent organic pollutants that are ubiquitous in ambient and indoor environments. Exposure to some of the SVOCs may result in adverse health effects, including allergic reactions (Bornehag et al., 2004, Jaakkola and Knight, 2008), irreversible changes in the development of the human reproductive tract (Albert and Jégou, 2014, Latini et al., 2006, Saillenfait et al., 2013, Su et al., 2014 and Wolff et al., 2014), neurodevelopmental and behavioral disorders (Le Cann et al., 2011 and Wang et al., 2015), immunotoxicity (Birnbaum, 1994 and Van den Berg et al., 2006), and even cancer (Boström et al., 2002, Denissenko et al., 1996 and Lewtas, 2007).

Inhalation, dermal absorption, and nondietary ingestion of settled dust have been identified as the major exposure pathways to SVOCs (Allen et al., 2007, Frederiksen et al., 2009, Harrad et al., 2006, Meeker et al., 2009, Weschler and Nazaroff, 2010, Weschler and Nazaroff, 2014, Wilson et al., 2009, Xu et al., 2009 and Xu et al., 2010). A number of field campaigns have been conducted to measure air concentrations of SVOCs in indoor (Blanchard et al., 2014, Kolarik et al., 2008, Trabue et al., 2008, Wang et al., 2014 and Xu et al., 2014) and outdoor environments (Albinet et al., 2007, Chao et al., 2003, He and Balasubramanian, 2010, MacLeod et al., 2007 and Xie et al., 2014b). The fate and transport of SVOCs, as well as human exposures to them, can be influenced significantly by gas/particle partitioning. Particle-phase SVOCs are expected to account for a great fraction of the air concentrations due to their low V_p, particularly for compounds with high molecular weight (Wang et al., 2014, Weschler and Nazaroff, 2008 and Weschler et al., 2008). Recent studies have shown that inhalation of particle-phase phthalates is significant because such exposure is capable of creating high local concentrations in airways at the particle deposition site and potentially causing bronchial obstruction (Jaakkola and Knight, 2008, Oie et al., 1997 and Pankow, 2001). Both the location and the efficiency of the deposition of SVOCs in the respiratory tract are dependent strongly on gas/particle partitioning. Furthermore, because the kinetics of gas-particle sorption/desorption are sufficiently rapid (Benning et al., 2013, Odum et al., 1994 and Weschler and Nazaroff, 2008 and Liu et al., 2013), in indoor environments, airborne particles might be important carriers that accelerate the transport of SVOCs from their original sources to other indoor locations through deposition, resuspension and air advection, although particle-phase SVOCs typically have lower characteristic transport distances than the gas-phase in outdoor environments. Therefore, to understand the fate and transport of SVOCs, accurate estimations of the partition coefficients between SVOC gases and particles are necessary (Hung et al., 2013, Melymuk et al., 2014).

Several field measurements were conducted to determine the gas/particle partitioning for SVOCs (Cincinelli et al., 2014, Saral et al., 2015, Wang et al., 2014 and Xie et al., 2014a, Xie et al., 2014b). In those studies, filter-sorbent samplers were used extensively, in which air was pulled through a filter to collect the particle-phase SVOCs and then followed by a sorbent to collect the gas fraction. This technique has been one of the most popular methods of sampling SVOCs over the past 40 years because of its simplicity and its ability to sample large volumes of air (Galarneau and Bidleman, 2006, Melymuk et al., 2014). Although measurements of the total airborne concentrations of SVOCs usually were acceptable, sampling artifacts were identified in the determination of gas/particle partitioning (Ahrens et al., 2011, Melymuk et al., 2014). When gas-phase molecules not being fully trapped by the sorbent, breakthrough happens and results in overestimation of the particle/gas partition coefficient. This positive bias may occur due to saturated sorbents or desorption of compounds from sorbents and is affected by the type and geometry of sampling medium, target compound properties and concentrations, and sampling volume, flow rate, temperature, and humidity (Melymuk et al., 2014). Breakthrough has been investigated extensively in previous studies (Harper, 1993, Peters et al., 2000, Martin et al., 2002 and Galarneau and Harner et al., 2006). In contrast, gas-phase SVOCs may sorb strongly to the filter, thereby increasing the mass of SVOCs on the filter. Mader and Pankow (2000) conducted systematic experiments and found that artifacts could be orders of magnitude for certain SVOC compounds due to their adsorption on the filter (Mader and Pankow, 2000, Mader and Pankow, 2001a). Arp et al. (2007) studied the equilibrium sorption of SVOCs on fiber filters and derived linear free energy relationships for predicting the sorption isotherms. Corrections for the gas/filter adsorption artifacts were made by using a backup filter and subtracting the mass of SVOCs found on the backup filter from the total amount found on the front filter. However, the assumption that the sorption of the SVOCs on the front and backup filters is equal is not valid if the gas/filter sorption equilibrium has not been reached on either of the filters (Mader and Pankow, 2001b). In addition, particles may pass through the filter and subsequently be captured by the gas-phase sorbent material. However, there is limited information on how particles are distributed within the sampler and how the penetration contributes to uncertainty in the gas/particle partitioning of SVOCs.

Theoretical analyses have been conducted to estimate the sampling artifacts associated with various sampling parameters. McDow (1999) presented a basic mass balance model to analyze the adsorption artifacts with various sampling volumes and types of filters. Galarneau and Bidleman (2006) used a simple mathematic model to study the bias due to temperature variations over the sampling period. Mader and Pankow (2001b) examined the partitioning of SVOCs to two types of filters and predicted the magnitude of the compound-dependent gas adsorption artifacts. However, the process of air samples passing through filter-sorbent samplers has not been fully characterized. Typically, some important processes, such as the diffusion of gas-phase SVOCs within the filter, the distribution of particles and particle-phase SVOCs in the filter, and the extent of penetration of particles, have been ignored even though they may have significant impacts on the estimation of sampling artifacts.

The aim of this study was to analyze the sampling artifacts associated with measurements of gas-particle partitioning of SVOCs using filter-sorbent samplers. The specific objectives were to: 1) develop and validate a fundamental mechanistic model to characterize the air sampling process with filter-sorbent samplers for SVOCs; 2) examine the potential sampling artifacts of gas/particle partitioning for a range of SVOC contaminants; and 3) normalize the model and investigate the general factors that influence sampling artifacts and the effectiveness of backup filters. The results of this work can be used by researchers to predict the sampling artifacts in measurements of SVOC gas/particle partitioning as a function of compounds, sampling velocity, sampling duration, and the concentration and size distribution of particles.