EXPOSURE OF DEACIDIFIED AND UNTREATED PAPER TO AMBIENT LEVELS OF SULFUR DIOXIDE AND NITROGEN DIOXIDE: NATURE AND YIELDS OF REACTION PRODUCTS
EDWIN L. WILLIAMS, & DANIEL GROSJEAN
We have investigated the nature and yields of the reaction products resulting from exposure of paper, deacidified or not, to ambient levels of sulfur dioxide and nitrogen dioxide. These reaction products include sulfate, nitrate, and nitrite, whose yields and relative abundance vary with paper type and treatment.
The two deacidification methods employed led to an ample carbonate reserve in the paper. Since carbonate is an efficient sink for SO2, more sulfate accumulated in deacidified paper than in untreated paper. With carbonate thus acting as a “decoy,” less or no SO2 will react with the paper itself, thus minimizing damage. However, the accumulation of sulfate (in our case as magnesium sulfate) within the paper may not be beneficial, as is suggested by copper number and other results reported by Daniel et al. (1990) for deacidified paper exposed to high levels of SO2. On the other hand, results obtained at high levels of pollutants may have limited relevance to the issue of paper damage resulting from exposure to ambient polluted air.
While current concern focuses on SO2, our results for NO2 are perhaps even more relevant for two reasons. First, concern about acid rain has prompted the implementation of control measures for SO2 emissions worldwide. As ambient levels of SO2 continue to decrease, the relative contribution of oxides of nitrogen (NO and NO2) to atmospheric acidity becomes increasingly important (in fact, atmospheric acidity in southern California and other urban areas is already dominated by NOx emissions). Second, chemical filtration systems do not remove NO2 as well as they remove other pollutants such as SO2 and ozone, thus resulting in high indoor NO2 concentrations in museums and in other settings where papers of artistic and historical value are displayed or stored (Hisham and Grosjean 1991).