The limited impact of acetic acid in archives and libraries
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KeywordsIndoor air pollution Paper heritage Acetic acid Ethanoic acid Hydrolysis Cross-contamination Chemical kinetics Mathematical modeling
Libraries and archives are increasingly concerned about the possible negative impact of indoor air pollutants on the permanence of their collections. The harmful effects on paper of some gaseous pollutant species identified in archive and library storage rooms have been demonstrated for artificial high temperatures and humidities in various laboratory experiments [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. These experiments, often performed at elevated temperatures, have shown that exposure to gaseous pollutants at artificially high concentrations results in embrittlement and yellowing of paper. Moreover, gas monitoring techniques with low detection limits have become affordable in the last decade so that cultural heritage institutions have performed more frequently indoor air quality campaigns and have become more aware of the range of gaseous components present in the atmosphere of their repositories [12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]. Confronted with such facts, managers, curators and conservators are pressed to take action to reduce the indoor concentration of pollutants. Unfortunately, air pollution control strategies, like the application of chemical filters in the air conditioning systems of repositories, generate significant financial and environmental costs .
In this study acetic acid was selected as a prime suspect model pollutant from the wide variety of pollutants found typically in the air of repositories, based both on its chemical reactivity and its abundance. The majority of the gaseous pollutants to which paper collections are exposed are considered essentially inert. Only a handful of gaseous substances are described in conservation literature to induce chemical reactions in paper . These pollutants can be subdivided into outdoor generated and indoor generated gases. The effect of the outdoor generated pollutants sulfur-dioxide, nitrogen-oxides and ozone on paper has been studied extensively with accelerated aging laboratory experiments [1, 2, 3, 5, 6, 24, 25, 26, 27]. Nevertheless, thanks to both policies of outdoor pollution reduction and policies of improvement of the energy efficiency of buildings by reducing the fresh air intake, the typical concentration of such gases in repositories has declined below 5 ppb for sulphur dioxide and ozone and 15 ppb for nitrogen dioxide . Probably as a consequence the focus of conservation research shifted in the last 15 years to investigate the indoor generated pollutants methanal (formaldehyde), ethanoic (acetic) and methanoic (formic) acid [4, 7, 8, 9, 10, 11, 16, 29, 30, 31, 32, 33, 34, 35, 36]. Indoor emission of formaldehyde from urea–formaldehyde bonded products like fiberboard has been studied thoroughly in connection to human health. The relevance to paper decay is probably small because chemical reactivity is limited in absence of oxidants [9, 37, 38] and concentrations found in archival storage rooms are below 10 ppb . In contrast to the pollutants discussed above, the two carboxylic acids formic and acetic acid are typically found in storage rooms containing paper at concentration levels up to respectively 70 ppb and 150 ppb [14, 16, 17, 19, 21, 22]. In some special situations containing non-paper emitting sources in closed cabinets, higher concentration levels have been reported [39, 40]. Both acids are generated by oxidative and hydrolytic reactions in organic materials like wood and paper [41, 42, 43]. If absorbed by less acidic paper materials they might promote chemical decay by catalyzing hydrolysis of cellulose. The transfer of chemically reactive volatile components from certain collection objects to others is commonly referred to as cross-contamination. The laboratory-derived experimental evidences of the detrimental effect of acetic acid in typical archival conditions is nevertheless contradictory. Menart and co-authors  extrapolating data collected at an intermediate concentration (1000 ppb) and temperature (60, 70 and 80 °C) in a set-up with continuous gas flushing, which guarantees the attainment of equilibrium between the concentration of the gas in air and in the paper, conclude that “the effect (of acetic acid) in typical archival environments is limited and in some cases insignificant”. On the other side in a modified Oddy test for plastic materials performed at 80 °C and for 14 days, acetic acid has been shown to depolymerize pure cellulose paper . In a similar set-up based on the investigation of the impact of historic storage materials on the carbonyl group content and the weight average molar mass of two types of paper, Becker and co-authors  conclude that “the mutual evaluation of formic acid and acetic acid is able to explain increased deterioration despite VOC emissions that are seemingly low at first sight”. Prompted by these contradictory evidences we have searched for indications of acetic acid damage on studies examining directly archival collections.
Few investigators have tried to obtain direct evidence of pollution damage by comparing near identical paper objects stored under different pollutant exposure conditions for long times. This field study approach has proven cumbersome. An important early study  of identical copies of books stored at the New York Public Library and the Dutch National Library (Koninklijke Bibliotheek, Den Haag) revealed large differences with respect to brittleness and yellowing. The significantly worse condition of the New York copies was originally attributed primarily to locally higher concentrations of air pollutants. In retrospect however, their differences can very well be understood to result solely from exposure to higher temperatures and higher humidity levels in New York. As a rule of thumb, “every five degree drop in temperature doubles the lifetime” . More recent comparative field studies [47, 48] show rather similar paper characteristics for identical papers despite different air pollution exposure conditions. This lack of direct evidence for actual pollutant damage motivated us to develop a theoretical model of the interaction of acetic acid on paper under typical archival conditions.
In order to predict emission and absorption of volatiles by solid materials from limited emission chamber experimental data, distinct theoretical approaches have been developed. Pseudo steady state theories describe volatiles behavior in terms of absorption and emission rates , or transfer coefficients [50, 51]. Spatial dynamic theories describe the time dependence of spatial concentration profiles within materials with diffusion models [52, 53]. The apparent incompatibility of these distinct approaches is a source of confusion and has created difficulty in the comparison and interpretation of emission data.
The emission and absorption dynamics of volatile organic compounds by material objects into their environments across time scales ranging from hours to centuries is the result of the interplay of several processes. These are: (1) the introduction during manufacturing or chemical formation over time of a specific volatile compound, (2) internal diffusion, (3) surface evaporation/absorption and (4) ventilation (external convective transport). It can be shown mathematically (see Appendix 1) that the proposed theories can all be derived from a single unified emission model. Their differences are the result of distinct simplifying approximations and should not be seen as opposing ideas about the underlaying mechanisms.
Emission modeling in the context of long term preservation of paper heritage in libraries and archives needs to take into account the behavior of volatiles at rather long time scales of decades or centuries. However, all models mentioned above are developed to fit and extrapolate short term behavior of materials as it can be determined from emission chamber experiments that last days or weeks. The problem with this approach is that short term behavior is primarily determined by resistances within a system, whereas long term behavior is determined by equilibration properties. Another specific modeling difficulty in the context of libraries and archives is the large amounts of non-uniform paper objects. Accurate specification of geometries and other variables at the individual object level is impractical. Within the context of libraries and archives all theoretical approaches mentioned above are inapplicable due to these problems. In the present article we therefore alternatively model the uptake of acetic acid by paper with a simple equilibrium between paper and air. Acetic acid is typically present in paper and as a result of an equilibration process some paper objects that contain higher acetic acid concentrations will release acetic acid and cross-contaminate others.
Although assertive claims have been made to suggest that paper acidifies over time as a result of natural aging  and as such would give rise to gradually increasing emissions of acetic acid, there is no evidence to suggest that this happens at a significant rate in ambient conditions. Emission behavior such as reported by Ramalho  should not be understood to provide evidence of acetic acid production, as emission is not necessarily due to production. Thermal aging of paper yields significant amounts of acetic acid. This observation has been confirmed in some laboratory studies. For an overview see Jablonský . However, it is unlikely that these conclusions obtained from model systems in laboratory studies can be extrapolated to typical storage situations. A multi temperature study by Baranski  indicates a diminishing rate of acetic acid production in paper towards ambient conditions. Also, none of the field studies that we know of do support the acidification claim. For example, a long term repeated study into the condition of books in the Netherlands  did not reveal any change in pH over time. Pedersoli  analyzed books of different age and found that the acetic acid content is flat with the age. No increase with the age of the book was observed. These findings are in accordance with Zou  who observed no significant acid production during natural aging of paper. In conclusion, irrespective of the specific chemical processes that give rise to the presence of acetic acid, the impact of airborne acetic acid is sufficiently modeled by assuming constant acetic acid concentrations within storage rooms.
The Henry coefficient expressed as concentration ratio was calculated with the values reported by Servant  and measured at gaseous concentration of 1 ppm, close to the concentration used in this work. The EMC of paper at 20 °C and 50% RH is estimated equal to 0.085 . The resulting value for p HAc est is 1.9 × 104, in good agreement with the measured value.
In the range of acetic acid concentrations expected in archives and libraries, varying between 0 and 150 ppb, this model predicts an appreciable pH decrease upon absorption of acetic acid only for papers with initial pH between 6 and 8.5. Papers with lower or higher initial pH are virtually unaffected by such an absorption of acetic acid. The pH shift of the extract upon addition of acetic acid has been measured experimentally for papers of different composition and of different initial pH (see Appendix 3). These results have confirmed our expectation that real papers, due to their buffering action, show pH shifts that are smaller than our worst case theoretical prediction.
pH-DP evolution diagram
Consistency with artificial aging data
Comparison of the predicted and experimentally measured de-polymerization under accelerated aging conditions after 
The relative change between the predicted and experimental data both in the case of natural de-polymerization and in the case of exposure to acetic acid ranges between 2 and 8%.
Discussion and conclusion
The good agreement between the predicted and experimental de-polymerization without exposure to acetic acid at high temperature gives confidence that the Zou model describes well the degradation of a wide range of papers. The good agreement between the predicted and experimental de-polymerization with exposure to acetic acid at high temperature gives confidence that a simple model for the composition of the paper and the following predicted pH shift upon acetic acid absorption are sufficient to predict the effect of this volatile acid on paper degradation. When applied at room temperature our model predicts an added de-polymerization due to 100 years of exposure to acetic acid at 150 ppb, which is the highest acetic acid level reported in the literature for a library storage, of the order of 2%. We believe that for real objects the added degradation will be smaller than this modeled value because diffusion of acetic acid in the bulk of the objects will be a rate limiting process, and because real papers should be expected to exhibit significant buffering action against pH change. The causal model developed here allows us to conclude that the impact of acetic acid and any other acid gases on paper degradation in a library or archive is limited and that neither the use of chemical filters to reduce acetic acid concentration in air nor the separate storage of acidic paper items are necessary preventive conservation measures in a library or archive. Although we consider comparative studies of actual collection materials and their environments to be essential for conservation science, we conclude that future field studies dedicated to find evidence of acid air pollutant damage are unlikely to succeed.
Both authors contributed to developing the model and finalizing the manuscript. Both authors read and approved the final manuscript.
Frank Ligterink is senior researcher at the Rijkserfgoedlaboratorium of the Cultural Heritage Agency of the Netherlands. Giovanna Di Pietro is senior lecturer at the Department of Conservation and Restoration of the Bern University of Applied Sciences.
The authors declare that they have no competing interests.
This work has been funded by the Swiss National Foundation, Project 13DPD3_132247 and by Metamorfoze, the Dutch National Programme for the Preservation of Paper Heritage, Project 2010-37.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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