Gas/particle partitioning of polycyclic aromatic hydrocarbons in coastal atmosphere of the north Yellow Sea, China

Samples of gas- and particle-phase polycyclic aromatic hydrocarbons (PAHs) were collected at three sampling stations (Xiaomai Island, Laohutan, and Zhangzi Island) in the north Yellow Sea, China during November 2008 and September 2009 to study their atmospheric transport potential and the gas/particle distributions. The composition of PAHs was dominated by gaseous compounds. The percentages of the particle-phase PAHs to the total concentrations were found to be higher during the heating period than the non-heating period. The ratios of naphthalene and acenaphthene to phenanthrene, chrysene and dibenzo(a,h)anthracene showed an increasing trend from Xiaomai Island to Zhangzi Island, which can be called as the local atmospheric distillation of PAHs. Gas/particle partitioning coefficients (K _p) and their relationship with the sub-cooled liquid vapor pressures (pº_L) of PAHs were investigated. The regressions of logK _p versus logpº_L gave significant correlations for all samples of the three sites with r ² values in the range 0.56–0.66 (p?<?0.01). Both Junge–Pankow adsorption model and octanol–air partition coefficient absorption model tended to underestimate the sorption for most PAHs, but the absorption model appeared to be more suitable for predicting the particle fraction of PAHs than the Junge–Pankow model.     

Gas/solid partitioning of semivolatile organic compounds (SOCs) to air filters. 2. Partitioning of polychlorinated dibenzodioxins,polychlorinated dibenzofurans,and polycyclic aromatic hydrocarbons to quartz fiber filters

Gas/particle distributions of atmospheric semi-volatile organic compounds (SOCs) are often measured using filter/sorbent samplers. Unfortunately, the adsorption of gaseous SOCs onto a filter can cause positive biases in the measured particle-phase concentrations, and negative biases in the measured gas-phase concentrations. When quartz fiber filters (QFFs) are used, surface-area-normalized gas/quartz partition coefficient (K_p,s, m³ m⁻²) values will be useful when estimating the magnitudes of these errors. Gas/QFF K_p,s values have been reported in the literature only for polycyclic aromatic hydrocarbons (PAHs) and n-alkanes. Gas/QFF K_p,s values were measured here for a series of polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), and also for a range of PAHs. Within each of the three individual compound classes, plots of log K_p,s vs. log p_L^o (sub-cooled liquid vapor pressure) were found to be linear with slopes of approximately −1. At relative humidity (RH)=25%, the pooled log K_p,s data at 25°C for the three compound classes were correlated with log p_L^o nearly as well (r²=0.95) as were the data for the individual compound classes (r²≈0.97). In general, the K_p,s values for the PAHs and PCDD/PCDFs studied were found to be about a factor of 2 larger for partitioning to clean QFFs at RH=25% than for TMFs at RH=21–52%. Backup QFFs used in filter/sorbent sampling in a suburban area yielded K_p,s values for PAHs at RH=37% that were significantly lower than for clean QFFs at the same RH. (This may have been the result of the adsorption of ambient organic compounds that at least partially blocked the direct adsorption of the SOCs to the QFF surface). Therefore, when QFFs are used to separate atmospheric gas- and particle-phase SOCs, corrections for compound-dependent gas adsorption artifacts for QFFs may need to be carried out using K_p,s values that were obtained with ambient backup QFFs.     

Determination of octanol-air partition coefficients and supercooled liquid vapor pressures of organochlorine pesticides

Octanol-air partition coefficients (K_OA) and supercooled liquid vapor pressures (P_L) of nine organochlorine pesticides (OCPs) including p,p′-DDE, p,p′-DDD, o,p′-DDT, o,p′-DDE, o,p′-DDD, α-HCH, β-HCH, γ-HCH, δ-HCH were determined as functions of temperature using a gas chromatographic retention time method. Among them, the K_OA of o,p′-DDE and o,p′-DDD and the P_L of o,p′-DDE, o,p′-DDD, β-HCH and δ-HCH were determined for the first time. The determined K_OA and P_L values of investigated compounds at 25°C ranged from 3.14 × 10⁷ (α-HCH) to 3.76×10⁹ (p,p′-DDD), and 8.95×10^{? 4} Pa (p,p′-DDD) to 1.08×10^{? 1} Pa (α-HCH), respectively. The K_OA and P_L data were compared with published data. The K_OA values of o,p′-DDT at 25°C were 3.23×10⁹, higher than o,p′-DDE (1.02×10⁹) and o,p′-DDD (2.01×10⁹), indicating o,p′-DDT were more preferred to partition in soil compared with the metabolites. The K_OA values were lower and P_L values were higher for o,p′-DDE and o,p′-DDD, compared with their p,p′-isomeric counterparts, leading to a potential difference in behavior and fate of these isomers. The discrepancies among chemicals are obvious, which reflected in the increasing K_OA and decreasing P_L values in order of α-HCH, γ-HCH, β-HCH, δ-HCH, o,p′-DDE, p,p′-DDE, o,p′-DDD, o,p′-DDT, p,p′-DDD. For each compound, the LogK_OA decreased linearly with reciprocal absolute temperature, while LogP_L had a significant positive correlation with the inverse absolute temperature. The present study suggested that the method of gas chromatographic retention time was appropriate to measure the K_OA and P_L of a number of OCPs.     

Polycyclic aromatic hydrocarbons in air on small spatial and temporal scales – II. Mass size distributions and gas-particle partitioning

Polycyclic aromatic hydrocarbons (PAHs) were measured together with inorganic air pollutants at two urban sites and one rural background site in the Banja Luka area, Bosnia and Hercegovina, during 72 h in July 2008 using a high time resolution (5 samples per day) with the aim to study gas-particle partitioning, aerosol mass size distributions and to explore the potential of a higher time resolution (4 h-sampling).In the particulate phase the mass median diameters of the PAHs were found almost exclusively in the accumulation mode (0.1–1.0 μm of size). These were larger for semivolatile PAHs than for non-volatile PAHs. Gas-particle partitioning of semivolatile PAHs was strongly influenced by temperature. The results suggest that the Junge–Pankow model is inadequate to explain the inter-species variation and another process must be significant for phase partitioning which is less temperature sensitive than adsorption. Care should be taken when interpreting slopes m of plots of the type log K_p = m log p_L⁰ + b based on 24 h means, as these are found sensitive to the time averaging, i.e. tend to be higher than when based on 12 h-mean samples.     

Seasonal concentrations and partitioning of PAHs in a suburban site of Bursa, Turkey

Effects of space heating on atmospheric concentrations and gas-particle partitioning of PAHs were investigated in a suburban site of Bursa, Turkey. The average concentrations of ∑₁₅-PAHs in heating season samples were approximately 10 times higher than those of non-heating season samples. The plot of log K_p versus log P_L⁰ for all the data set gave significantly different slopes. The slope for the heating season (−0.75) samples was steeper than the one for the non-heating season (−0.64) samples. It was also observed that partitioning of PAHs for the non-heating season samples showed different characteristics depending on air parcel trajectories. Generally steeper slopes were obtained for the air parcels traveled across the Black sea and arrived to the sampling site from northern sector. On the other hand, the variations in slopes according to air mass origin were insignificant for the heating season samples. Local contributions from space heating are the reason for this observation.     

Models for gas/particle partitioning,transformation and air/water surface exchange of PCBs and PCDD/Fs in CMAQ

We have added the capability to simulate polychlorinated biphenyls (PCBs) and polychlorinated dibenzo [p] dioxins and polychlorinated dibenzo-furans (PCDD/Fs) to the Community Multiscale Air Quality (CMAQ) modeling system, thus taking advantage of the latter's capability to simulate atmospheric advection, diffusion, gas-phase chemistry, cloud/precipitation, and aerosol processes. The modifications reported here include the addition to the CMAQ system of two gas/particle partitioning models options: the Junge–Pankow adsorption model and the K_OA absorption model, as well as chemical transformations and atmosphere/water surface exchange processes for these semi-volatile organics. Simulations for the purpose of model testing and validation were conducted for the years 2000 and 2002 on a domain covering most of North America. Both partitioning models give reasonable results when compared with available measurements. The model predictions of deposition and air concentrations also agree well with measurements. The modeling results also indicate that the long-range transport is important and anthropogenic emissions of PCBs and PCDD/Fs are dominant although surface exchange of PCBs may be important for some clean locations.     

Gas- and particle-phase distribution of polycyclic aromatic hydrocarbons in two-stroke, 50-cm3 moped emissions

Gas- and particle-phase polycyclic aromatic hydrocarbons (PAHs) concentrations evaluated in the exhaust of 10 two-stroke, 50-cm³ mopeds belonging to three different levels of emission legislation (EURO-0, EURO-1 and EURO-2) were used to assess the prevalent mechanism driving the gas/particle partitioning of PAHs in moped exhaust. Sampling was performed on a dynamometer bench both during the “cold-start” and the “hot” phases of the ECE-47 driving cycle. Gas and particulate phase PAHs were collected on polyurethane foam (PUF) plugs and 47-mm Pallflex T60A20 filters, respectively, under isokinetic conditions by using sampling probes inserted into the dilution tunnel of a Constant Volume Sampling – Critical Flow Venturi (CVS–CFV) system.The results show that semi-volatile PAHs were predominantly partitioned to the particle phase. The soluble organic fraction (SOF) of the collected particulates ranged between 72 and 98%. Measured total suspended particulate matter normalized partition coefficients (K_p) were predicted within a factor of 3–5 by assuming absorption into the organic fraction according to a model developed by Harner and Bidleman [Harner, T., Bidleman, T.F., 1998. Octanol–air partition coefficient for describing particle/gas partitioning of aromatic compounds in urban air. Environmental Science & Technology 32, 1494–1502.]. This suggests that the gas/particle partitioning in moped exhaust is mainly driven by the high fraction of organic matter of the emitted particles and that absorption could be the main partitioning mechanism of PAHs.     

Semivolatile PAH and n-alkane gas/particle partitioning using the dual model: up-to-date coefficients and comparison with experimental data

The gas/particle partitioning coefficient K _p , of a semivolatile compound is a key parameter for its atmospheric fate. The most complete method of predicting K _p for polycyclic aromatic hydrocarbons (PAHs) is offered by the dual model, as it describes both the adsorption on soot and absorption into organic matter processes. However, experimental and model data exist almost exclusively for PAHs. In order to bridge this gap, experimental data on the phase partitioning of both PAHs and n-alkanes were collected at an urban and a remote site. Moreover, all the necessary parameters (e.g., octanol–air and soot–air partitioning coefficients) for the dual model have been collected and updated or (if missing) estimated for the first time. The results point out that both absorption and adsorption seem to contribute to the partitioning of PAHs and n-alkanes. However, it seems that the dual model always underestimates the particle sorption not only for PAHs but also for n-alkanes.     

Octanol–air partition coefficients of polybrominated biphenyls

The octanol–air partition coefficients (K_OA) for PBB15, PBB26, PBB31, PBB49, PBB103 and PBB153 were determined as a function of temperature using a gas chromatographic retention time technique with 1,1,1-trichloro-2,2-bis (4-chlorophenyl) ethane (p,p′-DDT) as a reference substance. The internal energies of phase change from octanol to air (Δ_OAU) were calculated for the six compounds and were in the range from 74 to 116 kJ mol⁻¹. Simple regression equations of log K_OA versus relative retention times (RRTs) on gas chromatography (GC), and log K_OA versus molecular connectivity indexes (MCI) were obtained, for which the correlation coefficients (r²) were greater than 0.985 at 283.15 K and 298.15 K. Thus the K_OA values of the remaining PBBs can be predicted by using their RRTs and MCI according to these relationships.     

Further studies on the uptake of persistent organic pollutants (POPs) by polyurethane foam disk passive air samplers

Passive air samplers (PAS) can be used to monitor semi-volatile organic compounds in the atmosphere. Polyurethane foam (PUF) disks are a popular sampling medium because they have a high retention capacity for such compounds. This paper reports a highly time-resolved uptake study, to derive uptake rate data under field conditions, and investigate the effects of using different foam densities on the uptake rate. PUF disks were deployed alongside an active sampler, for periods of up to 12 weeks. The uptake rates were measured for a range of gas- and particle-bound persistent organic pollutants (polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs)), of different properties, to explore whether gas–particle partitioning affected uptake rate. Uptake rates for two different densities of foam (0.021 and 0.035 g cm^?3) were not statistically significantly different from each other. Uptake rates of light PCBs averaged ～6.5 m³ day^?1, somewhat higher than in previous studies; higher wind speeds and lower temperatures in this study are the likely reason for this difference. The study showed: i) the uptake rate of the compound with lowest K_OA considered in this study (PCB-28/31) declined in the later weeks, indicating an approach to equilibrium; ii) uptake rates of lighter BDEs and heavier PCBs (compounds of intermediate K_OA in this study) remain similar throughout the study period, indicating that they are not approaching equilibrium during the 12-week-study; iii) uptake rates were typically: ～8 m³ day^?1 for PCB-52; ～9.5 m³ day^?1 for PCB-95; ～11 m³ day^?1 for BDE-28 and ～2 m³ day^?1 BDE-99. The latter compound has an important particle-bound component and this lowers the sampling rate compared to predicted uptake rates for compounds which are in the gas phase only. It is shown that knowledge of gas–particle partitioning is needed to correct for this effect, and to improve predicted uptake rates.     

Measurement of octanol–air partition coefficients using solid-phase microextraction (SPME)—application to hydroxy alkyl nitrates

Multifunctional organic compounds are thought to constitute a major component of the organic matter found in atmospheric particles. Their partitioning into the organic matter depends on their structure, their chemical properties and the properties of the absorbing matrix. It was recently shown that octanol is a suitable surrogate for organic particles and the octanol–air partition coefficient (K_OA) was suggested as a useful tool for estimating the partitioning of organic compounds into atmospheric particles that contain high organic mass fractions. In this paper, we present a new and simple technique for the determination of K_OA using solid phase microextraction (SPME) relative to a known Henry's law constant. We apply the technique for the determination of K_OA of β-, γ- and δ-C₃–C₅ hydroxy alkyl nitrates. The temperature dependence of K_OA for some of the compounds is also measured. It is shown that the solubility constants of these compounds are higher in octanol than in water and that the solubility in octanol increases with the length of the hydrophobic chain and with increasing distance between the hydroxy and the nitrooxy groups. Partition coefficients between the gas and particulate phase (K_p) are calculated using the determined K_OA values and their atmospheric implications are discussed.     

Assessing the importance of ab- and adsorption to the gas-particle partitioning of PCDD/Fs

The gas-particle partitioning of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) was examined (i) by re-interpreting results from controlled field experiments and (ii) in diurnal samples taken in the New Jersey (NJ), New York (NY) City region. In the controlled field experiments, aerosol-laden filters were exposed to elevated concentrations of PCDD/Fs. Gas-particle partitioning coefficients, K_p's, were significantly correlated with octanol–air partitioning coefficients, K_oa's. The regression of all individual datapoints resulted in the following best fit (r²=0.74, n=87): log K_p,meas=1.00(±0.13) log(10⁻¹²f_omK_oa/ρ_oct)−0.15(±0.48). We interpret this as showing that the ability of organic matter (OM) to absorb PCDD/Fs is generally well described by the octanol–air partitioning model (f_omK_oa). At the NJ land-based sites, samples were taken and analyzed for organic carbon (OC) and elemental carbon (EC), gaseous and particulate PCDD/Fs. K_p's were significantly correlated with the f_omK_oa approach. Adsorption to the filter and possibly to black carbon (BC), which was estimated based on EC measurements, could have contributed to the observed K_p values. Gas-particle predictions based on BC adsorption and OM absorption, with K_p=f_omK_oa/(10¹²ρ_oct)+f_BCK_BC–air/(10¹²ρ_BC) resulted in K_p predictions that were close to measured values. Adsorption to the filter might have been the major reason for elevated K_p's: The one NJ site with highest K_p's was most prone to the filter adsorption sampling artefact because of relatively low sampling volumes and concentrations of total suspended particulates. In addition, while adsorption to BC would result in better prediction of K_p values, no influence of f_BC or f_BC/f_om ratios was seen, suggesting that it was of lesser importance in our sample set.     

Hydroxypropyl-β-cyclodextrin as non-exhaustive extractant for organochlorine pesticides and polychlorinated biphenyls in muck soil

Hydroxypropyl-β-cyclodextrin (HPCD) was used as a non-exhaustive extractant for organochlorine pesticides (OCs) and polychlorinated biphenyls (PCBs) in muck soil. An optimized extraction method was developed which involved using a HPCD to soil mass ratio of 5.8 with a single extraction period of 20 h. An aging experiment was performed by spiking a muck soil with ¹³C-labeled OCs and non-labeled PCBs. The soil was extracted with the optimized HPCD method and Soxhlet apparatus with dichloromethane over 550 d periodically. The HPCD extractability of the spiked OCs was greater than of the native OCs. A decreased in HPCD extractability was observed for the spiked OCs after 550 d of aging and their extractability approached those of the natives. The partition coefficient between HPCD and soil (log K_CD-Soil) was negatively correlated with the octanol-water partition coefficient (log K_OW) with r² = 0.67 and p < 0.05.     

Estimating the vapor pressures of multi-functional oxygen-containing organic compounds using group contribution methods

A UNIFAC-based method for estimating the vapor pressure (p_L^o) values of oxygen-containing compounds of intermediate-to-low volatility has been developed as an aid in modeling the formation and behavior of organic aerosols. This UNIFAC-p_L^o method was constructed using a set of 76 compounds with experimentally determined p_L^o values. The compounds chosen are of intermediate-to-low volatility and contain multiple oxygen-containing functionalities. For test and development purposes, the 76 compounds were divided into a basis set of 43 compounds used to generate the coefficients required in the UNIFAC-p_L^o method and a second set of 33 compounds that was used to test the coefficients generated using the basis set. Both the basis and test sets contained compounds that possessed similar structures and functionalities. For the 33 compounds in the test set, on average UNIFAC-p_L^o predicted the p_L^o values to within a factor of 2 over the temperature range 290–320 K. Furthermore, the UNIFAC-p_L^o method did not show any correlation in prediction error with p_L^o so that it was equally likely to underpredict as overpredict p_L^o regardless of volatility. For comparison, three other vapor pressure estimation methods were applied to the test set of compounds. On average, these other methods all predicted the test set p_L^o values to within a factor of 3 over the temperature range 290–320 K. In contrast to the UNIFAC-p_L^o method, the prediction errors from the methods were found to be correlated with p_L^o so that the other methods overpredicted p_L^o as volatility decreased.     

Uptake of polychlorinated biphenyls and organochlorine pesticides from soil and air into radishes (Raphanus sativus)

Uptake of organochlorine pesticides and polychlorinated biphenyls from soil and air into radishes was measured at a heavily contaminated field site. The highest contaminant concentrations were found for DDT and its metabolites, and for β-hexachlorocyclohexane. Bioconcentration factor (BCF, defined as a ratio between the contaminant concentration in the plant tissue and concentration in soil) was determined for roots, edible bulbs and shoots. Root BCF values were constant and not correlated to log K_OW. A negative correlation between BCF and log K_OW was found for edible bulbs. Shoot BCF values were rather constant and varied between 0.01 and 0.22. Resuspended soil particles may facilitate the transport of chemicals from soil to shoots. Elevated POP concentrations found in shoots of radishes grown in the control plot support the hypothesis that the uptake from air was more significant for shoots than the one from soil. The uptake of POPs from air was within the range of theoretical values predicted from log K_OA.     

Gas/particle partitioning of polychlorinated dibenzo-p-dioxins and dibenzofurans in atmosphere; evaluation of predicting models

The gas/particle partitioning of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) was measured at three sites for a year in order to monitor the variation of PCDD/Fs levels and describe their partitioning. The air concentrations of PCDD/Fs ranged from 71 to 1161 fg I-TEQ/m³ and large changes in these levels did not correlate with seasonal changes during this study. Different homolog patterns were observed in the gas/particle phase. High chlorinated dioxin/furans dominated the particle phase while low chlorinated dioxin/furans dominated the gas phase. The high correlation coefficient between log [(PCDD/Fs_vap)(TSP)/(PCDD/Fs_pat)] and 1/T was observed in lower chlorinated dioxin/furans unlike in OCDD/F. The slope of homolog ranged from −0.410 to −1.025 and that of 2,3,7,8-substituted isomers ranged from −0.379 to −0.772 in plots of the log partition coefficient (K_p) versus the log subcooled vapor pressure (P_L°).The octanol/air partition coefficient (K_oa)-based model of PCDD/Fs is more compatible with experimental data than those of the Junge–Pankow model that tends to overestimate results, even though both models include some level of uncertainty. However, both models can underestimate the particle phase of PCDD/Fs, especially when the ambient air temperature is extremely low in winter.     

Modeling PAH uptake by vegetation from the air using field measurements

We examined PAH uptake by Norway spruce needles following the emergence of new buds in spring 2004–June 2005. Atmospheric PAH concentrations (gaseous phase and particle-bound) were monitored during this period, and PAH concentrations from these three environmental media were then used to calculate deposition and transfer velocities. Benzo(a)pyrene was found almost exclusively associated to particles and thus was used to determine a particle-bound deposition velocity of 10.8 m h^?1. PAHs present in both compartments had net gaseous transfer velocities ranging from negligible values to 75.6 m h^?1 and correlated significantly with log K_OA. The loss velocities thereafter calculated were found to be higher for more volatile PAHs. Using the calculated average atmospheric PAH concentrations and deposition velocities, it was thus possible to model PAH uptake by vegetation through time. We demonstrate that this approach can be used to determine deposition velocities without the use of a surrogate surface. In considering both particulate-bound and gaseous deposition processes this model can be used not only to study air–foliage exchange of semi-volatile organic compounds, but also to illustrate the relative contribution of gaseous deposition and particulate-bound deposition in the overall atmospheric vegetation uptake of semi-volatile organic compounds.