Evaluation of deposition samplers for polycyclic aromatic hydrocarbon monitoring

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are an important environmental concern due to their carcinogenic and toxic properties. Moreover, several PAHs have been classified into mutagenic compounds. The sources of PAHs are classified into anthropogenic and natural sources such as incomplete combustion of fossil fuel, biomass burning, industrial boilers, and forest fires etc. Emitted PAHs can distribute both gaseous phase and particulate phase. As a large amount of PAHs emitted to the atmosphere is deposited to the land or sea, atmospheric deposition of PAHs is a significant phenomenon. There have been many studies of atmospheric deposition of PAHs, and several kinds of deposition samplers have been used. However, the results with different deposition samplers cannot be directly compared, because their performance was not fully compared or calibrated. Besides, the deposition fluxes and deposition velocities measured in the previous studies showed large variations even though the same samplers were used. Therefore, a through evaluation of different types of deposition samplers are required to clearly understand the process of atmospheric deposition of PAHs. In this study, various information on widely used deposition samplers (basic theories, structures, and advantages/disadvantages) was collected. Then, the performance of four types of deposition samplers was evaluated. In addition, a high volume air sampler was used to investigate the ambient levels of PAHs and deposition velocities. The samplers were deployed on the roof of the engineering building #2 in the Ulsan National Institute of Science and Technology (UNIST) from May 2013 to October 2013. Four types of deposition samplers (dry deposition sampler (DDP), velcro deposition sampler, resin deposition sampler, and bulk deposition sampler) were used. Particulate and gaseous PAHs were indivisually collected by a high volume air sampler once a week. The target compounds in this study were thirteen US-EPA priority PAHs except naphthalene, acenaphthene, and acynaphthylene. After sample extraction and clean up using silica gel columns, a gas chromatograph/mass spectrometer (GC/MS) was used for PAH qualification and quantification. In order to further interpretate the source-recepor relationship of PAHs, houly data of criteria air pollutants (CO, SO2, NO2, O3, and PM10) measured nearby UNIST was acquired from the Ulsan Institute of Health and Environment (UIHE). The ranges of SO2 were 1.3–9.9 ppb (Mean: 3.8 ppb) and 3.4–15.0 ppb (Mean: 6.8 ppb) in Samnam and Mugeo air pollution monitoring stations. The SO2 level showed seasonal variations related with fossil fuel consumption and wind direction. The level of SO2 in Mugeo was higher than that of Samnam because Mugeo is more influenced by vehicles and has more urban characteristics. The ranges of NO2 were 6.7–21.9 ppb (Mean: 12.6 ppb) in Samnam and 4.7–43.1 ppb (Mean: 24.0 ppb) in Mugeo. The ranges of O3 were 14.1–63.9 ppb (Mean: 34.0 ppb) in Samnam and 13.5–60.5 ppb (Mean: 27.4 ppb) in Mugeo. The ranges of PM10 were 15.1–57.1 μg/m3 (Mean: 30.9 μg/m3) in Samnam and 14.6–74.8 μg/m3 (Mean: 38.4 μg/m3) in Mugeo. The sampling site at UNIST was confirmed to be seasonally influenced by air pollution sources in Samnam and Mugeo according to geographical positions and major wind directions. The levels of gaseous PAHs were 1.10–7.02 ng/m3 (Mean: 4.23 ng/m3), and those of particulate PAHs were 0.85–2.82 ng/m3 (Mean: 1.67 ng/m3). Namely, the total PAH concentrations were 1.95–9.84 ng/m3 (Mean: 5.91 ng/m3) during the sampling period. The variation of PAH concentrations was not large, but they increased in spring and fall. The sampling site in this study is located in a rural area. Therefore, it was assumed that the sampling site was affected by criteria air pollutants and PAHs emitted from urban sources. Deposition fluxes and velocities of PAHs were calculated based on the amount of deposited PAHs and ambient air concentrations. The ranges of deposition fluxes of PAHs collected by DDP, velcro, resin, and bulk samplers were 4.55–15.13 μg/m2/d (Mean: 8.89 μg/m2/d), 13.14–30.92 μg/m2/d (Mean: 22.04 μg/m2/d), 7.72–55.41 μg/m2/d (Mean: 28.43 μg/m2/d), and 32.72–49.44 μg/m2/d (Mean: 40.13 μg/m2/d), respectively. This result indicates that deposition fluxes derived from different types of deposition samplers do not coincide and they should not be directly compared. DDP could not collect high molecular weight-PAHs mostly associated with fine particles. Therefore, the performance of the velcro sampler as a dry deposition sampler was better than that of DDP. As a bulk (dry/wet deposition) sampler, the performance of the Resin and bulk samplers was similar. On the basis of this result, further studies were suggested to improve the calculation of deposition fluxes and velocities.