Determining the level of organic acid air pollution in museum storage rooms by mass-balance modelling

Formic acid and acetic acid are among the most critical indoor air pollutants causing damage to museum collections. Many museums, however, lack funds and expertise to conduct detailed environmental monitoring to survey the level of organic acids in the air of collection areas. We demonstrate how a mass-balance calculation, based on data obtained from estimated factors and from literature can be used instead. The model was applied on a real museum storage building located in Denmark, and compared with concentration measurements performed in it by passive sampling. The model seemed at first to overestimate the concentration as compared to the real measurements, which could mean that either the emission of organic acids from objects was over-estimated, or the removal by surfaces was under-estimated, or both. However, taking that into account, it is demonstrated how the model can be used to examine the behaviour of different air quality control scenarios, in this case the effectiveness of: adjusting room temperature, applying internal air filtration, or varying the air exchange (ventilation) rate. For each case, the result was evaluated against the implication on energy consumption. The mass-balance calculations points to temperature as the most useful control method of the three, at least from a temperate climate zone point of view, as cold winter temperatures can be used to reduce the temperature within unoccupied storage buildings without applying mechanical control. The least energy-efficient method would be to dilute the pollutants by ventilation. To attain the same reduction in organic acid concentration by ventilation, as could be achieved with a decrease in temperature from normal indoor room conditions to cool conditions (13 °C less), the air exchange rate would need to be increased by almost three room volumes per hour. This would, however, for filtration alone increase the energy use by 6.3 kWh per cubic metre of storage space, annually, as compared to the free cooling provided by not heating during winter, at least for a building located in a temperate climate zone. To investigate the general magnitude of the organic acid pollution levels, which may be expected in typical storage rooms, a Monte Carlo simulation based on literature data was carried out and showed that for most situations the concentration can be expected to be well below the safe levels suggested in literature. A step-by-step guide for applying the mass-balance model to any storage building is provided. Further research, however, is needed in order to refine the model as well as to expand it to include other building types and climate zones.