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).