3. Results and discussion
3.1 Building structure and sampling regime
Fig. 1 depicts the spatial situation on the third floor below the water
reservoir (drip floor) from which the boiler feed water was dispensed to
railway steam engines. The circular room exhibits staircases from the
second floor and to the fourth floor containing the reservoir tank for
the water supply.
Figure 1: Layout of the third floor, PAH sources and sampling
spots.
This building displays a complex air flow situation and was designed for
a constant air exchange with the outside via an open lantern. The air
exchange rate is unknown to its extend and within the building there was
never any sensing of air movement by the authors. The maximum indoor
temperature on the drip floor follows largely the outside temperature
and ranged in 2019 between 5 °C in January and 30 °C in early July. It
was decided to measure the airborne PAH concentration near the wall
(Fig. 1, position P) as this would be the area basically used by museum
visitors.
The PAH source is the original jute fabric soaked with a bituminous
material wrapped around the steel tubes that promoted groundwater to the
reservoir. This material and also the cardboard display a noticeable
tar-like scent and releases PAHs contained in the soaked fabric by
diffusion into the room and eventually to the outside. Table 1 comprises
the content of the 16 most often quantified PAH congeners in the jute
fabric. The PAH congener pattern is typical for hard coal
tar22,23 with phenanthrene, fluoranthene and pyrene as
most abundant congeners and with phenanthrene dominating over
anthracene, benzo[b ]fluoranthene over
benzo[k ]fluoranthene and indeno[1,2,3-cd] pyrene
over dibenz[ah ]anthracene. The levels of naphthalene,
acenaphthylene, and acenaphthene are probably largely reduced since
production due to their higher volatility. The levels of airborne PAHs
are influenced by complex processes including diffusion and evaporation
from the partly more than 115 years old source material, and the
interaction with the inorganic and organic construction materials such
as concrete, brick, iron and wood, as well as the omnipresent dust
particles.
Table 1 : Content of PAH congeners in the jute fabric (means and
standard deviations, n=3) and respective vapor pressures at 25 °C
Therefore, in order to estimate the actual airborne PAH as basis for a
risk assessment for visitors cannot be done using documented vapor
pressure data for PAHs (Table 1) and diffusion models but requires
measurement data. Since the building was not easily accessible during
the restoration period and never meant to be accessed by visitors the
technical execution of pump sampling to arrive at a PAH related risk
assessment on the drip floor with the equipment described in section 2.3
was rather tedious. Thus, the situation was used to investigate if SBSE
could offer an easily accessible complement to indoor risk assessment
regarding harmful (semi)volatile substances. Therefore, it was firstly
tested if there was any significant PAH extraction via SBSE devices and
what time frame would be needed. A preliminary exposure of SBSE devices
on the drip floor over up seven days revealed detectable amounts of all
16 PAH congeners already after one day of exposure. This encouraged two
SBSE sampling campaigns, each displaying exposure periods of one day
intervals up to five or seven days, respectively. The campaigns were run
temporally apart (July 2019 and September 2019) such that the outside
temperature influence could be assessed. On each measurement day three
SBSE devices were sampled to enable estimation of analytical
repeatability. In addition to the sampling on the wall (Fig. 1, position
S1) SBSE devices were placed along a cross section of the room (Fig. 1,
position S3) in order to assess the dependence of PAH content from the
distance of the source (Fig. 1, positions A2). Likewise, pump sampling
followed by quantification of airborne PAH levels was performed next to
the site of SBSE sampling (Fig. 1, position P) but temporally apart.
Again, measurements were performed with three replicates each on
different days in order to evaluate repeatability of and temperature
influence on determined airborne PAH concentrations.