Trouble in Storage? Understanding the dynamics of airborne organic acids in storage buildings and its consequences for the air quality, energy use, and preservation of heritage collections

Formic acid and acetic acid are among the most critical indoor air pollutants known to cause damage to heritage collections. This Ph.D. project examined the route of organic acids in storage buildings from emission sources, such as the collections themselves, to the removal through air exchange with ambient, air filtration in mechanical ventilation systems, and deposition onto interior surfaces. The aim was to establish the fate of formic acid and acetic acid in storage buildings and to determine control methods for the protection of heritage collections from corrosive organic acids.

The area-specific emission rates of formic acid and acetic acid from six naturally aged wood and paper samples and from a wood packaging material used to transport and storage collections were measured at laboratory conditions. The emission rate ranged from 10 to 300 µg m -2 h -1 at indoor room conditions (23ºC, 50% RH). The study also demonstrated that a decrease in temperature from normal room conditions to 10ºC reduced the emission rate of organic acids 2-4 times from wood and paper, whereas a decrease in the RH from 50% to 20% reduced the emission rate with a factor 2 or more. A similar organic acid behaviour was observed in unoccupied storage buildings where the decrease in temperature from summer to winter led to a reduction in the concentration of organic acids in air.

The spatial distribution in temperature, moisture and organic acids were measured in one storage building belonging to the Royal Library in Denmark (with a mechanical ventilation system), and in two rooms in the shared storage facility at the Centre for Preservation of Cultural Heritage in Vejle (with semi-passive climate control). The spatial temperature and moisture distribution were almost uniform except for a weak vertical temperature gradient causing a RH gradient opposite to that of temperature. The spatial organic acid distribution showed areas in the storage building with semi-passive climate control where incomplete mixing caused a local accumulation of organic acids. However, the survey demonstrated that both storage buildings provided acceptable temperature and moisture distributions without problematic microclimates, and with an acceptable air quality performance irrespective of the ventilation form.

An intervention study was additionally carried out in the two buildings, in order to measure the removal efficiency of two commercially available activated carbon filters in situ. The organic acid removal efficiency of a desiccant silica gel rotor used for dehumidification, and the passive adsorption onto a clay brick wall, were also measured. The intervention study demonstrated that the organic acid removal efficiency of the filters depend on the airflow through the filter and that the performance varies considerably in situ from the removal efficiency obtained in a laboratory. The desiccant silica gel rotor efficiently removed 98-100% acetic acid from the air while the clay brick wall reduced the concentration of acetic acid close to the wall with 56%.

The storage buildings examined in this project maintained almost the same temperature and RH conditions indoor however, the store with a mechanical ventilation system consumed almost 60 times more energy for outdoor air filtration and climate control than the store with semi-passive climate control. The results underlined that storage buildings with semi-passive climate control provide acceptable climate and air quality conditions and reduces the energy use for the preservation of collections in storage.

All data was collated into a Monte Carlo simulation to examine the fate of organic acids in heritage collection storage buildings. The simulations showed that formic acid and acetic acid would deposit onto interior surfaces despite the use of active air filtration, and with the continued risk of causing damage to heritage collections.