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ABSTRACT
In contrast to active sampling in which air is brought into contact with a detector or collector device by means of a pump, diffusive sampling relies on diffusion to bring the pollutant into contact with the collector. Compared with the pump-dependent active sampling procedure, the main advantages of the method are cost effectiveness, and simplicity. The method is suitable for simultaneous and multipoint measurements of air pollutants. Diffusive sampling is characterised by low sampling rates necessitating long sampling times. As a consequence, diffusive sampling can only provide information on integrated average concentrations over the exposure period. This feature can be viewed as an advantage in the field of artwork and cultural heritage protection as it makes it easy to determine average concentrations for the sampling period which reflect the long-term action of corrosive gases on sensible materials. Current research involving diffusive sampling of which we refer in this communication is focused on understanding the outdoor-indoor relation of pollutant concentrations and on the role of corrosive gases in the deterioration of works of art indoors. Diffusive samplers can be used as a monitoring technique because - in this specific application - the average, instead of the real time pollutant concentration is adequate for the purpose of the monitoring. They can also be used as screening devices for those locations where no prior monitoring has taken place to characterise those areas where threshold values are expected to be exceeded. As an example of the indoor application of a passive diffusive method, results of a monitoring campaign at the Uffizi Gallery are here presented. The diffusion method was used to measure the distribution of some relevant gaseous pollutants in three selected room of the Gallery and, simultaneously, outdoors. Infiltration factors were quantified by calculating indoor/outdoor ratios for the measured pollutants. The study, which is still ongoing, is being done within the framework of the Project MIMIC (Microclimate Indoor Monitoring in Cultural Heritage Preservation) and it is part of a more complex activity to characterise indoor air in museums. In this study we used diffusion samplers called "Analyst" which are particularly suited for long-term measurements (typically one month or more). The sampler is a modification of the open-tube design obtained by using a filter treated with appropriate reagents to trap the pollutant. The body of the sampler is a cylindrical vial with a threaded cap at one end. The pollutant is collected on a impregnated disc placed at the bottom of the vial and held in position by a stainless steel ring. To avoid turbulent diffusion inside the vessel, the open end is protected using a fine stainless steel screen. Time-weighted average concentrations of 6 relevant air pollutants were determined. Distinct trends and seasonal variation in the concentration of monthly averages for NO2, NOx, O3, SO2, HNO3 and HONO were observed in successive monthly sampling periods which started in March 2001. For each pollutant, the indoor level was a function of its outdoor level, the air exchange rate, and the rate at which it is produced or removed by indoor chemistry and by indoor surfaces. SO2, O3 and HNO3 were, as expected from their reactivity, invariably lower indoors whereas indoor NOx and NO2 in the majority of the sampling periods were similar to the corresponding outdoor values. It was also found that high concentrations of nitrous acid are generated via indoor chemistry through thermal reaction of NO2 with water on surfaces. Possible implications for the conservation of artworks indoors will be considered.
Part 2: Slides from presentationLarge files (ca. 1.5 MB) may take some time to loadAuthor to whom correspondence may be addressed:
Franco De Santis |
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© November 2003