Gas-phase Semivolatile Organic Compounds Research Articles

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

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[Kf/(Kp⋅Cp,a)] 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 (Ts,s∗) and the chemical Vp 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.

The effect of dry and wet deposition of condensable vapors on secondary organic aerosols concentrations over the continental US

Abstract. The effect of dry and wet deposition of semi-volatile organic compounds (SVOCs) in the gas phase on the concentrations of secondary organic aerosol (SOA) is reassessed using recently derived water solubility information. The water solubility of SVOCs was implemented as a function of their volatility distribution within the WRF-Chem regional chemistry transport model, and simulations were carried out over the continental United States for the year 2010. Results show that including dry and wet removal of gas-phase SVOCs reduces annual average surface concentrations of anthropogenic and biogenic SOA by 48 and 63% respectively over the continental US. Dry deposition of gas-phase SVOCs is found to be more effective than wet deposition in reducing SOA concentrations (−40 vs. −8% for anthropogenics, and −52 vs. −11% for biogenics). Reductions for biogenic SOA are found to be higher due to the higher water solubility of biogenic SVOCs. The majority of the total mass of SVOC + SOA is actually deposited via the gas phase (61% for anthropogenics and 76% for biogenics). Results are sensitive to assumptions made in the dry deposition scheme, but gas-phase deposition of SVOCs remains crucial even under conservative estimates. Considering reactivity of gas-phase SVOCs in the dry deposition scheme was found to be negligible. Further sensitivity studies where we reduce the volatility of organic matter show that consideration of gas-phase SVOC removal still reduces average SOA concentrations by 31% on average. We consider this a lower bound for the effect of gas-phase SVOC removal on SOA concentrations. A saturation effect is observed for Henry's law constants above 108 M atm−1, suggesting an upper bound of reductions in surface level SOA concentrations by 60% through removal of gas-phase SVOCs. Other models that do not consider dry and wet removal of gas-phase SVOCs would hence overestimate SOA concentrations by roughly 50%. Assumptions about the water solubility of SVOCs made in some current modeling systems (H* = H* (CH3COOH); H* = 105 M atm−1; H* = H* (HNO3)) still lead to an overestimation of 35%/25%/10% compared to our best estimate.

Daily mortality and PM2.5 sources in Denver estimated using only particle-phase or total semi-volatile organic compounds (SVOCs)

Background: Source apportionment approaches have provided insights into chemical classes of fine particle matter (PM2.5) responsible for adverse health effects. The Denver Aerosol Sources and Health (DASH) study used the Positive Matrix Factorization (PMF) to identify chemical classes of PM2.5 in the Denver area. Aim: To compare estimated health effects of PM2.5 components with those of chemical classes identified by PMF based on particle-phase or total semi-volatile organic compounds (SVOCs). Methods: Daily measurements of four PM2.5 chemical components (elemental and organic carbon (EC and OC), sulfate, and nitrate), as well as a large number of organic compounds, were obtained over a 32 month period at a single monitoring station located in populated residential area in Denver. PMF factors were estimated based on particle-phase SVOCs only and total SVOCs adding estimated gas-phase SVOCs. The two approaches resulted in eight (Inorganic Ion, n alkane, summer odd n alkane, light n alkane polycyclic aromatic hydrocarbon (PAH), medium alkane alkanoic acid, PAH, winter methoxyphenol, and EC sterane) and seven (n alkane, light SVOC, PAH, summer ECOC, EC sterane, sulfate, and nitrate) factors, respectively. Daily death counts of cardiovascular and respiratory diseases were obtained from the Denver death certificate records. We used generalized additive models to estimate relative risks of mortality at lags 0 to 3 for each of PM2.5 components and PMF factors adjusting for daily humidity and temperature, long-term temporal trend, and day of week. Results: None of four PM2.5 components was associated with daily mortality. With the total SVOC source factors, the PAH factor was associated with cardiovascular mortality at lag 0; with both PMF approaches, n alkanes were associated with respiratory mortality at lag 3. Conclusions: Use of source apportionment allowed us to identify PM2.5 mortality associations that were not found using only single PM2.5 components.

Analysis of the Dynamic Interaction Between SVOCs and Airborne Particles

A proper quantitative understanding of the dynamic interaction between gas-phase semivolatile organic compounds (SVOCs) and airborne particles is important for human exposure assessment and risk evaluation. Questions regarding how to properly address gas/particle interactions have introduced uncertainty when predicting SVOC concentrations and assessing exposures to these compounds. In this study, we have developed a dimensionless description for the dynamic interaction between SVOCs and organic particles. A better criterion to judge whether the internal resistance (diffusion in and out of aerosols) is negligible compared with the external one (from bulk air to aerosol surfaces) is presented. The analysis is applicable regardless of the phase state of particles (either liquid or amorphous semisolid/solid). It is found that for both porous and nonporous particles, the internal resistance can be reasonably neglected for particles with diameters between 0.01 and 10 μm if the particulate organic matter is in t...

A preliminary investigation of sorbent-impregnated filters (SIFs) as an alternative to polyurethane foam (PUF) for sampling gas-phase semivolatile organic compounds in air

Filters impregnated with XAD-4™ resin were used in a small series of high-volume air samples to compare their collection of gas-phase semivolatile toxic substances (organochlorine pesticides, OCs, and polycyclic aromatic hydrocarbons, PAHs) with that achieved by polyurethane foam (PUF). The advantages of the use of such sorbent-impregnated filters (SIFs) include a reduction in size which leads to numerous benefits. The latter include simplified sample handling, shipping and storage, and the potential for a decrease in solvent requirements for pre-cleaning and extraction. Furthermore, such SIFs could be used to measure combined particle/gas concentrations of target compounds. Gas concentrations derived from the SIFs in a filter-SIF–SIF–PUF configuration agreed well with values derived from the PUF plugs in a comparison filter-PUF configuration. The collection efficiency of a single SIF was ∼80% on average. As such, these SIFs are viewed as a promising alternative to PUF and further, more extensive study of their performance characteristics appears to be warranted.

Measurement of emissions from air pollution sources. 3. C1-C29 organic compounds from fireplace combustion of wood.

Organic compound emission rates for volatile organic compounds (VOC), gas-phase semivolatile organic compounds, and particle-phase organic compounds are measured from residential fireplace combustion of wood. Firewood from a conifer tree (pine) and from two deciduous trees (oak and eucalyptus) is burned to determine organic compound emissions profiles for each wood type including the distribution of the alkanes, alkenes, aromatics, polycyclic aromatic hydrocarbons (PAH), phenol and substituted phenols, guaiacol and substituted guaiacol, syringol and substituted syringols, carbonyls, alkanoic acids, resin acids, and levoglucosan. Levoglucosan is the major constituent in the fine particulate emissions from all three wood types, contributing 18-30% of the fine particulate organic compound emissions. Guaiacol (2-methoxyphenol), and guaiacols with additional substituents at position 4 on the molecule, and resin acids are emitted in significant quantities from pine wood combustion. Syringol (2,6-dimethoxyphenol) and syringols with additional substituents at position 4 on the molecule are emitted in large amounts from oak and eucalyptus firewood combustion, but these compounds are not detected in the emissions from pine wood combustion. Syringol and most of the substituted syringols are found to be semivolatile compounds that are present in both the gas and particle phases, but two substituted syringols that have not been previously quantified in wood smoke emissions, propionylsyringol and butyrylsyringol, are found exclusively in the particle phase and can be used to help trace hardwood smoke particles in the atmosphere. Benzene, ethene, and acetylene are often used as tracers for motor vehicle exhaust in the urban atmosphere. The contribution of wood smoke to the ambient concentrations of benzene, ethene, and acetylene could lead to an overestimate of the contribution of motor vehicle tailpipe exhaust to atmospheric VOC concentrations.

Nicotine as a Marker for Environmental Tobacco Smoke: Implications of Sorption on Indoor Surface Materials

ABSTRACT Recently developed models and data describing the interactions of gas-phase semi-volatile organic compounds with indoor surfaces are employed to examine the effects of sorption on nicotine's suitability as an environmental tobacco smoke (ETS) marker. Using parameters from our studies of nicotine sorption on carpet, painted wallboard, and stainless steel and previously published data on ETS particle deposition, the dynamic behavior of nicotine was modeled in two different indoor environments: a house and a stainless steel chamber. The results show that apparently contradictory observations of nicotine's behavior in indoor air can be understood by considering the effects of sorption under different experimental conditions. In indoor environments in which smoking has occurred regularly for an extended period, the sorbed mass of nicotine is very large relative to the mass emitted by a single cigarette. The importance of nicotine adsorption relative to ventilation as a gas-phase removal mechanism is reduced. Where smoking occurs less regularly or the indoor surfaces are cleaned prior to smoking (as in a laboratory chamber), nicotine deposition is more significant. Nicotine concentrations closely track the levels of other ETS constituents in environments with habitual smoking if the data are averaged over a period significantly longer than the period between cigarette combustion episodes. However, nicotine is not a suitable tracer for predicting ETS exposures at fine time scales or in settings where smoking occurs infrequently and irregularly.

Dynamics of gaseous semivolatile organic compounds in a terrestrial ecosystem—effects of diurnal and seasonal climate variations

The dynamics of gas-phase semivolatile organic compounds (SOCs) were examined in a forested bog in northern Minnesota. A strong diurnal variation in the gas-phase concentration was observed for over 100 compounds including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and other organochlorines. This diurnal variation, whereby afternoon concentrations exceeded early morning levels by factors of 2–10, is a likely result of volatilization and deposition of SOCs to and from local plant surfaces. The variation cannot be explained by air turbulence, wind direction, photoreactivity, or plant stomatal activity. Gas-phase SOC concentrations ([SOC g]) are strongly correlated with both local temperature and relative humidity. The correlation between [SOC g] and temperature was examined separately from the correlation between [SOC g] and relative humidity (RH). Only during the winter sampling period, when snow covered the forest surface, was relative humidity not a significant predictor of [SOC g]. For sampling periods when relative humidity was significant, the following relationship was found: In [ SOC g ] = a + m T + c RH , where T is the ambient temperature (K) at ∼1 m, a, m, and c are constants that vary with compound and season. This study suggests that surface adsorption processes dominate atmospheric SOC cycling, rather than absorptive or stomatal partitioning processes.