3.2 Stir bar sorptive extraction
Fig. 2 comprises the results of both SBSE sampling campaigns for selected PAH congeners. All measurement data can be found in supplementary Tables S4 and S5. On each sampling day three replicates were taken. Congeners displaying up to three rings (naphthalene to anthracene, with the exception of acenaphthylene) and pyrene are observed well above 100 ng absolute (= ng per SBSE device) after 24 h while further congeners with four rings (chrysene and benzo[a ]anthracene) and all congeners with five rings display respective amounts below 20 ng absolute. These significant differences in absolute masses on the SBSE devices is in accordance with the respective vapor pressures (Table 1) that are > 10-4 Pa for the three-membered ring congeners and pyrene and below 10-5 for the other investigated congeners24,25.
The repeatability of PAH determinations by SBSE as estimated from the three devices analysed for each measuring day ranges mostly between 5 and 10% (for standard deviations see supplemental Tables S4 and S5) and is regarded satisfactory for this purpose.
Figure 2: Absolute PAH content on SBSE devices after different exposition times (wall; see Fig. 1, position S1), means and standard deviations (n=3). 1st campaign: 29 July - 3 August; 2nd campaign: 23 – 30 September.
While during the first campaign sorbed naphthalene and acenaphthylene levels did not change significantly after the first 24 h the levels of the other depicted congeners increased over the sampled period. This effect tended to increase with molecular weight from acenaphthene to pyrene while on the last day the amount of these higher molecular weight congeners dropped significantly. In contrast to this, the second campaign displayed this increasing tendency without the decrease of the higher molecular weight congeners but here an initial increase of the naphthalene content between day 1 and day 2 was followed by significant decrease on the last three days. This behavior cannot be comprehensively explained only on basis of the temperature courses during the campaigns. During the first campaign the room temperature dropped from 25 °C on the first day to 20 °C on the last day, while in the second campaign started at 16 °C and ended at 14 °C. For a tentative reasoning it should be noted that naphthalene is transported to a significant degree in the gas phase while the other congeners display a tendency increasing with molecular weight to be transported bound to particles26.
It appears that influence factors with opposite effect affected the absolute amount and pattern of PAH congeners on the SBSE devices. While in the first campaign sorption of higher molecular weight congeners tended to accelerate in the first days the decrease on the last day is best explained with strong air exchange on the last, coolest day of the campaign. This loss tended to increase with increasing molecular weight and decreasing vapor pressure and is consistent with the concept of a blow-off – possibly by a gust-like event – that partly removed particles from the SBSE devices. As a consequence, the absolute amount of predominantly particle bound congeners (e.g. fluoranthene and pyrene) would be reduced while congeners displaying a significant vaporisation and thus a largely constant airborne concentration would be less affect as observed in case of naphthalene and acenaphthene.
In the second campaign with cooler temperatures nearly all congener contents increased over the days, those with lower vapor pressure initially slower than in first campaign. Naphthalene stands out though its content also increased on the devices in the first days consistent with cooler temperatures that might have affected the desorption from the source and the diffusion through the room. The content decrease observed only for naphthalene in the last days of the second campaign cannot be explained with any certainty on basis of the available information. It might be due to re-volatilisation as a consequence of air exchange and cooler temperature resulting in less desorption from the source that decreased the naphthalene air concentration without in this case significantly affecting the particles bound to the devices.
Fig. 3 shows that the PAH distribution did not differ significantly across the room between source and wall (Fig. 1, positions S3). For measurement data in detail see supplementary Table S6. This observation is supported by the results obtained from SBSE devices that were placed close to the PAH source (Fig. 1, position S2) during both campaigns. The three devices each were sampled after 24 h in the first campaign and after 48 h in the second campaign. The results depicted in detail in supplementary Figure S1 and supplementary tables S4 and S5 reveal no significant differences of sampled PAH congener masses at positions S1 and S2 (Fig. 1).
Figure 3: Absolute PAH content on SBSE devices exposed for 24 h (11 – 12 September) on the floor with different distances from the PAH source (see Fig. 1, position S3), means and standard deviations (n=3).