Deposition of suspended fine particulate matter in a library
© Smolik et al.; licensee Chemistry Central Ltd. 2013
Received: 13 December 2012
Accepted: 28 February 2013
Published: 3 April 2013
To analyse deposition of fine particulate matter (PM) on book surfaces we put twelve bunches of cellulose filters on a free shelf of the National Library in Prague, exposed them for three, six, nine, and twelve months to indoor air and analysed them after each period by Scanning Electron Microscopy (SEM) and Ion Chromatography (IC). Results showed that fine particles were deposited predominantly on the surface of the top filter but partly also on the surfaces of inner filters. It indicates fine particles penetrated between filters. The penetration and deposition of particles was also modelled as Brownian diffusion between two parallel filters. The model prediction demonstrated that fine particles penetrate between filters, with the depth of penetration limited by parallel diffusional deposition on filter surfaces. This is in qualitative agreement with SEM and IC investigations. The results show that beside the top part fine PM can deposit onto all available surfaces of books.
Airborne particles deposited on cultural heritage artefacts have many negative effects. Beside soiling and abrasion of surfaces particles can also cause material deterioration by chemical reactions. Ultrafine atmospheric particles, penetrating indoors from the outdoor environment, contain soot and organic matter from traffic that are hygroscopic and effective for transport of acids. Fine particles consist of secondary organic matter and ammonium sulfate, ammonium nitrate, and sometimes sulfuric acid. Coarse particles, formed predominantly by resuspended dust contain crustal elements and in the indoor environment sometimes alkaline particles emitted from concrete structures [1, 2].
There are several studies reporting particle deposition measurements in museums and other buildings with works of art or historical artefacts [3–13]. The deposition has been determined using vertical and horizontal collection plates or filters, attached to the walls or horizontal surfaces with subsequent analysis of amount and chemical composition of deposit by various techniques. The results, though somewhat diverse, have several common features: a) the amount of deposit on the horizontal surfaces is larger than on vertical surfaces, b) the composition of deposit corresponds to the composition of particulate matter (PM) suspended in the indoor air, and c) particles deposited on vertical surfaces are formed frequently by soot, organic matter and ammonium sulphate. The rate of submicron particle and sulfate deposition onto a vertical surfaces measured inside five Southern California Museums were in a good agreement with theoretical prediction .
Dry deposition is considered to occur by a combination of Brownian and eddy diffusion and gravitational settling [15, 16] where prevailing deposition mechanism depends on the particle size. Coarse particles are deposited on upward-facing surfaces by gravitational settling and fine particles predominantly by diffusion on surfaces of any orientation. In principle the submicron particles can penetrate by diffusion also between books and even into the gaps between pages and thus can be deposited on the inner surfaces of books. To test this hypothesis we examined deposition of particles on paper filters located on the free shelf of the library.
The deposition experiments were performed during one year comprehensive study of indoor air quality in the Baroque Library Hall of the National Library in Prague, that included measurements of gaseous pollutants, size resolved particle number concentrations, and size resolved chemical composition of indoor PM. The results showed that the average concentrations of size fraction 100 nm - 1 μm of the indoor PM was of the order 103 particles/cm3. The IC analyses revealed that the major water-soluble norganic component of this fraction was ammonium sulfate with maximum concentration centred at about 300 nm .
Bunches were exposed for three, six, nine, and twelve months and examined after each period by Scanning Electron Microscopy (SEM) and Ion Chromatography (IC).
Particle penetration and deposition
As can be seen the concentration of particles rapidly decreases with increasing distance from the edge due to fast deposition by axial diffusion. To estimate the deposition we calculated amount and mass of particles (assuming spherical particles of unit density) deposited on inner surface during one year of exposition. (i.e. J.t). The results showed that particle penetration and deposition depends on particle size and width of the gap, with the depth of the penetration limited by parallel diffusional deposition on filter surfaces. Smaller particles penetrated faster and deeper resulting in higher particle number concentration of deposited particles but higher mass was transported by larger particles.
Scanning electron microscopy
Results from the parallel study  showed that ammonium sulfate formed up to 60% of mass of water-soluble part of indoor submicron PM. Since ammonium sulfate is stable compound, it is considered as a tracer of ambient fine aerosols that penetrate indoors [19, 20]. In this study sulfate was used as a marker for deposited particles.
Exposed filters were analysed after each period by Ion Chromatography (IC). Since the bunches were not exactly planar but slightly convex (see Figure 2) the downward-facing surface of the last filter could be exposed as well. Thus, we analysed the top (first), internal (second) and bottom (tenth) filter from two bunches after each period. The analyses were provided using the setup by Watrex Ltd., Czech Rep., with columns Watrex IC Anion II 10 μm 150 × 3 mm for anions and Alltech universal cation 7 μm 100 × 4.6 mm for cations. The conductivity detector used in this setup was a SHODEX CD-5. The samples were extracted by 10 ml of ultrapure water with conductivity 0.08 μSm− 1 (Ultrapur, Watrex Ltd.) for 0.5 h using ultrasonic bath and 1 h using a shaker. The filtered extracts were then analysed for anions and the next day for cations. The sample solutions for cation analysis were stored in the fridge prior the analysis.
To test if the indoor submicron particles can penetrate by diffusion into gaps between books or even between pages of books, we exposed twelve bunches of Whatman filters to the indoor air fixed in open Petri dishes on a free shelf of the library. Bunches were exposed for three, six, nine, and twelve months and analysed after each period by Scanning Electron Microscopy (SEM) and Ion Chromatography (IC). The penetration and deposition of particles has been modelled assuming Brownian diffusion between two parallel discs, The simple model showed that the particle penetration and deposition depends on particle size and width of the gap, with the depth of penetration limited by parallel diffusional deposition on filter surfaces. Smaller particles penetrate faster and deeper resulting in higher particle number concentration of deposited particles, but higher mass is transported by larger particles. The results of modelling have been qualitatively confirmed by Ion Chromatography using sulfate as marker for deposited particles. The results show that beside the top part fine PM can deposit onto all available surfaces of books.
This work is a part of research project No. DF11P01OVV020 supported by the Ministry of Culture of the Czech Republic.
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