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The Science of the Total Environment, 127 (1992) 211-223 Elsevier Science Publishers B.V., Amsterdam

211

Determination of , ome atmospheric pollutants inside a museum: relationship with the concentration

outside

F. De Santis, V. Di Palo and I. Allegrini

CNR, Is~ituto sull'Inquinamentc A:mosferico Area della Ricerca di Roma, v. Salaria Km 29.300, Rome, Italy

(Received july i9th, 1991; accepted September 4th, 1991)

ABSTRACT

Indoor and outdoor pollutant concentrations were measured simultaneously in the Galleria degli Uffizi in Florence. SO2, HNO3, HONO and 03 in the gas phase and sulphate, nitrate and ammonium in particulate matter were measured. Indoor and outdoor values were similar. The indoor/outdoor ratios for all the pollutants, with the exception of HONO, were extremely variable ranging from 0.1 to 0.9. For HONO the indoor leveh ha ¢,~ been found to largely ex- ceed those outdoor. The results show that indoor HONO production could result from heterogeneous reactions on the walls and exposed surfaces which could include artworks giv- ing rise to dry acid deposition of nitric acid.

Key ~or~: acid deposition; indoor pollution inside museums; nitrous acid; nitric acid; diffusion techniques

INTRODUCTION

It is well recognized that the conservation of artistic and cultural artifacts in museums, libraries and archives requires a controlled environment. It is generally believed that the most important factor in controlled indoor air is the maintenance of a stable level of humidity and temperature. Since moisture and thermal stresses may cause a variety of deterioration processes on ~lmost all materials, such a viewpoint is undoubtedly justified. However, the exhibits can also be damaged by chemical processes due to highly reactive species which include environmental air pollutants. Since they are present at more or less large amour, ts in external environments, it is very important to avoid their penetration from external air into buildings intended fox storing and displaying works of art.

212 F. DE SANTIS ET AL

The quality of indoor air in muse,~ms as far as the presence of potentially aggressive species, is restricted to a few critera pollutants such as SO2, NOx and 03 whose measurements are easily made by means of automatic in- strumentation.

Typical results of measurements for the mentioned pollutants and com- parisons of indoor-outdoor pollution in museums, libraries and archives ha~ bee~l l'eported in Ieview erticles by Baer and Banks [1] and, more recently, by Bimblecombe [2].

These studies showed that indoor concentrations were in general lower, but not very much different from those outside in buildings built prior to the technological development of central air conditioning. Similar results have been shown in buildings provided with central air conditioning s~,stems which are unable to remove gaseous species and particulate matter. Only in modern structures, especially designed for displaying and storing artistic and cultural artifacts, the air conditioning is capable of lowering, to a negligible level, the transfer of pollutants inside the building [3-8].

In Italy, many museums are located in historical buildings which, thanks to their architectural and cultural value, are often worth to be seen in their own right. The design and the installation of a central air conditioning system is far from easy in. such structures and it is often the result of striking compromises between functionality and the necessity of net having any major effect upon the original architectural conception.

As far as air pollutants are concerned an important process which con- tributes to the damage of artworks is acid deposition. Acidity in the air is essentially caused by sulphur and nitrogen oxides which are emitted into the atmosphere by sources related to industry, transportation and heating. These species are transfor~ned, through complex reaction pathways, into acidic sulphates, in particulate matter and in gaseous nitric and nitrous acids. The importance of nitric acid, due to its aggressive nature, is self-evident. As far as nitrous acid is concerned, it is well known that this species plays an impor- tank role in the atmospheric chemistry as a generator of OH radi:als, one of the most reactive species in the atmosphere which may initiate the formation of photochemical oxidants. Nitrous acid by itself is an only slightly acidic species and, though a specific study on the effect on surfaces is lacking, it is likely that it does not have any major effect on the exhibits. However, it has been shown in previous investigations that reactions in homogeneous phases are of negligible importance (e.g. 9-10) and that a likely source of nitrous acid is the following heterogeneous reaction [11,12]:

2 NO2 + H20 -- HONO + HNO3 (1)

The presence of nitrous acid in indoor air of museums could be of high

ATMOSPHERIC POLLUTANTS INSIDE A MUSEUM: RELATIONSHIP WITH CO~;CEI~'FRATION OUTSIDE 2 | 3

importance as an indicator of the concurrent formation of nitric acid on surfaces.

The objective of the present study was to compare the concentration of some air pollutants relevant to acid deposition, including nitrous and nitric acids, in an important Italian Gallery (Galleria degli Uffizi). The species under evaluation have been simultaneously detected in indoor and outdoor air by means of a diffusion-based sampling technique.

EXPERIMENTAL

The sampling of gases and particulate matter was carried out by using a diffusion-based technique using annular denuders; a detailed description of the characteristics and the principles behind the use of these devices and, in particular, the configuration used during this study, can be found in the literature [13,14].

The denuders were made by using two co-axial glass cylinders (i.d. = 30 mm, o.d. = 33 mm, length 220 ram) and air is passed through the annular space which is coated with a specific chemical. Gaseous molecules diffuse to the tube walls which ac~ as a sink for a given species while the pi~rticulate matter, which is characterized by a much lower diffusion coefficient, passes unaffected through the tube and can be recovered on the filter. The use of several denuders and also a fflterpack connected in series permits a reliable determination of SO2, nitrous acid and nitric acid in air even when the ratio of analytes in the gas phase and particulate matter is very low.

Two parallel lines composed of a train of five annular denuders and a triple filter pack were used for collecting indoor and outdoor air samples. The line was composed of 2 NaCI coated denuders followed by 2 Na2CO3 and a citric acid coated denuder. The last component of the sampling system was a triple filter pack preceded by a cyclone (cut point < 2.5/~m) containing a Teflon filter followed by a nylon filter and a citric acid impregnated cellulose filter. The sampling flow rate was 12 1 min -!.

The first denuder, coated .with NaCI, collects nitric acid from the airstream. The second one is used to take into account the small deposition of nitrate containing particles on the denuder walls, thus the difference between the nitrate content in the first and in the second denuder yields the net amount of nitric acid. The third and fourth denuders are used for the measurement of nitrous acid and SO2. The use of two denuders in series allows a correction for interferents, namely for NO2 and 03 on the determination of nitrous acid. On the basis of a recent study the determination of nitrous acid, carded out by u;ing this configuration (i.e. two carbonate denuders in series) suffers from the disadvantage of a possible combined positive interference from SO2 and NO2. In our work this problem was not

214 F. DE SANTIS ET AL.

considered to be important because the levels of SOe are so low that only a negligible interference could result (A. Fcbo, pers. commun.). Furthe~nore, the level of nitrous acid (as it will be shown in the following chapter) is higher indoor where the concentration levels of SO2 are lower; the interference would produce the opposite effect. The citric acid denuder is intended for collecting NHs and prevents the neutralization of strong acidic particulate matter collected in the front Teflon filter. Ammonia (which has not been analysed during this study) can be measured by extracting the denuder with water and analysing the extract by spectroscopy [15].

Particles are collected downstream on the filterpack. The first filter collects fine particles while the second and third trap HNOs and NHs originating from the dissociation of NH4NOs collected on the first filter. The use of a cyclone having a cut point < 2.5/zm for sampling the particulate matter, avoids the possible neutralization of acidic species due to the alkalinity of large particles.

The denuders for HNOs were coated by using a small amount of a 0.2% (w/v) of NaCI in methanol. The denuders for nitrous acid and SO2 were coated by using a 1% (w/v) Na2CO3 + 1% (w/v) glycerol in a 1:1 methanol/water solution. The denuder for NHs was coated with a 1% solution of citric acid in methanol.

After sampling, the denuders were extracted with 10 rnl of distilled water and the extracts analysed for anions by ion chromatography (Dionex Model QIC). To prevent neutralization by atmospheric ammonia after sampling, the Teflon filters were stored in petri dishes inside a plastic bag containing crystals of citric acid. After sampling, the filters were placed in polypropylene bottles and were extracted with the Dionex eluent for 20 min in ultrasonic bath.

Measurement of ozone was performed by using a Dasibi 1108 (indoor) and a Thermo Electron Mod. 49 (outdoor). The ozone analysers were equipped with an in-line Teflon particulate filter. Calibration was carried out t,~ng test atmospheres of ozone produced by ultraviolet irradiation of pure air.

The Museum investigated was equipped of a centralized heating air conditioning and ventilation system. Filtration of particles was carried out by using glass fible filters.

The determinations indoors were carried out in a room in the proximity of one 61 ~ the most important exhibits at 1.5 m height. The room has no windows, illumination is controlled by a luminaire in the roof and by artificial lights in the absence of adequate natural lighting.

Outdoor sampling was conducted on the roof of the museum by using a similar set-up of denuders and filterpack at --- 50 m from the room under investigation. Such a distance did not cause problems related to the evah~z-'.ion of external pollution because the investigation mainly regardecl secondary pollutants which are evenly distributed in space.

ATMOSPHERIC POLLUTANTS INSIDE A MUSEUM: RELATIONSH~Y WITH CONCENTRATION OUTSIDE 215

RESULTS AND DISCUSSION

Simultaneous measurements of HONO, HNO3, SO2, 03 in gas phase and NH4 +, NO3-, SO42- in particulate matter in outdoor and indoor air were carried out in the period 25-29 July 1990. Ozone was measured continuously while the other pollutants were measured on a 4-h basis except overnight when an 1 l-h sample starting at 19:00 h was collected.

The results obtained are presented in Figs. 1-4. The concentrations of

~. ug/m8 Nitrous Acid i4 - ' - ....

12 t F ......................................................................................................................................................................................... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I . I "~0 ...........................................................................................................................................................................................................................

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

! iiii iiiii[i.ii iiii ii iiii[iiii .i.iii .iiii iiii iiiii II.IIIE.I

10

19-8 8-12 la-1616-19 19-8 8-12 la-1610-19 19-8 8-12 12-t616-19 19-8

Time Indoor ~ Outdenr

ug/m8 Nitric Acid

8

6

~4

2

0 1g-8 8-12 12-1616"19 19-8 8-12 12-1616-18 19-8 8--12 12-1818-19 1g-8

Time Indoor ~ Outdoor

Fig. 1. Measured indoor-outdoor HONO and HNO3 levels for the Galleria degli Uffizi, Florence, 25-29 June, 1990.


~0 uglm8

Sulphur Dioxide

18

10

19-8 8-12 12-1@ 18-19 19-8 8-12 la-1818-!9 19-8 8-12 12-18 Ie-18 19--8 Time

Ind=or ~'~ Outdoor

Ammonium uglm8

|m i

m ....................................................................................................................................................................................................................... ~ ..............................................

ii i i

18-8 8-12 12-1818-18 18-8 8-12 12-1818-18 18-8 8-12 12-1618-18 18-8 Time

Indoor ~ Outdoor Fig. 2. Measured indoor-outdoor SO2 and NH4 + levels for the Galleria degli Uffizi, Florence, 25-29 June, 1990.

pollutants, with the exception of nitrous acid and some data regarding particulate nitrate, were, as expected, invariably lower indoor. The concentration levels measured outdoor are typical for a situation of urban pollution. The ozone concentration followed a typical photochemical pattern, rising in

ATMOSPHERIC POLLUTANTS INSIDE A MUSEUM: RELATIONSHIP WITH CONCENTRATION OUTSIDE 21 ?

2.8 ,~0/m8 Nitrate

2 ,° ,

1,81 i

0,8

0 lS-8 8-12 12-1010-19 19-8 8-12 12-1018-10 19-8 8-12 12-18 t0-10 10-8

Time

Indoor ~ Outdoor

12

10

8

e

4,

0

uo/m8 Sulphate

m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I . . . . . . . . . . . . . . . .

I o°.

10-8 8-1a 1a-1010-10 10-8 0-12 12-1010-10 10-8 8-12 t2-10 18o, t0 10-8

Time

Inatoor ~ Outdoor Fig. 3. Measured indoor-outdoor nitrate and sulphate levels for the Galleria degli Uffizi, Florence, 25-29 June, 1990.

the mid-morning to levels between 40 ppb and 70 ppb and then falling in the ~ate afternoon to concentration levels of a few ppb ~nd remaining low through the night. The corresponding indoor values ranged from 19 to 30 ppb.


80

70

60

50

ppb 40

30

20

10

0

Ozone

l--l door | - - Outdoo

• 1

[

2 20 t 12 20 4 2 20 t 12 20 4 Tree

Fig. 4. Concenterat ioa ofoutdoor- indoor ozone vs. time in the Gal;eria d,~!; ~J~z;~ F!orence, 25-29 June, 1990.

The SO 2 concentration levels ranged from 4 #g m -3 to 17 #g m -3 at the rooftop site over the 5 days of sampling. They are typical of urban pollution in this period of the year when heating is not operated. The corresponding indoor values ranged from 1 to 9 #g m -3. The diurnal pattern is evident for HNO3 and for nitrous acid. HNO3 shows a strong diurnal cycle with a max- imum during the day and minimum during the night. The reverse occurs for nitrous acid.

On Wednesday and Thursday the outdoor concentration of HNO3 ex- ceeded 8 #g m-3: a fairly high value for this period of the year.

Of great interest is the observed level of nitrous acid indoors (up to 12 #g m-3), these data providing the first evidence that high concentration levels of this species are produced inside a museum. As already mentioned a likely source of nitrous acid could be the reaction of NO2 and H20 in heteroger~c:.ous phase which produces a stoichiometrically equivalent amount of HNO3. If this heterogeneous mechanism would predominantly be active, the presence of nitrous acid could be indicative of a major hazard to the p~:eservation of works of art.

The ebservation of a daily outdoor trend of nitrous acid (Fig. 1) is consistent with a steady night-time build-up and with a daytime decomposition due to rapid photolysis by sunlight. Therefore, the more abundant formation that we have observed inside the museum may occur during the day by reac-

ATMOSPHERIC POLLUTANTS INSIDE A MUSEUM: RELATIONSHIP WITH CONCENTRATION OUTSIDE 219

tion pathways normally associated with night-time chemistry outdoors (i.e. reaction in heterogeneous phase on surfaces and particles). Since indoor environments are charac~,erized by high surface to volume ratios in comparison to those outdoor, the formation of nitrous acid is highly favored in these con- ditions. On the basis of recent studies carried out in this laboratory it is possible to estimate the production rate of HONO inside the building. It can be shown that the formation rate of HONO displays a linear dependence on the concentration of NO2 and on the S/V ratio (A. Febo, unpublished results). Assuming a concentration of NO2 of 50 ~g m-3, a S/V ratio of 0.6 m-~ and a formation constant of 5 x 10 -5 m s -~ a formation rate of HONO was calculated to be 0.6/zg h-~ in a reasorJable agreement with the experimental results. It is worth noting that the determination of whether nitrous acid formed on a surface reacts with the sabstrate or whether it is released in the atmosphere, requires information beyond the mere determination of its concentration in air. Furthermore, the assessment of the potential for damage to the materials displayed in the mv~seum, requires the understanding of the relationships between the concentr~tion of nitrous acid gas and the presence of HNO3 on the surface of interest according to the reaction [1]. The production of nitrous acid could, in principle, be uniformly distributed on the whole surface (included the paintings) or limited only to the walls. In general, surface reactivity will depend upon a number of factors, the concentration of nitrous acid in air being dictated from the acid-base characteristics of the surface.

The ozone trend is reported in Fig. 4. Indoor concentrations follow closely the trend outside and there is a consistent correspondence between timing of maxima and minima. The indoor concentrations of ozone appear to be related to the occasional partial opening of some windows located near the sampling site, which occurred throughout the second and third sampling day. During the following days the windows were kept closed. As a consequence the values of indoor ozone concentrations were substantially lower. During the first day of the campaign (Monday) the windows were kept closed, and this explains why the ozone levels (measured only between 15:00 h and 18:00 h) were approximately as low as the last 2 days of the study.

Ozone has been found to affect several materials ranging from textile dyes [16], paints [17] and some pigments [1~8,19]. To what extent humidity, the presence of other pollutants, and the presence of varnis~:es and transparent coatings might affect the ozone reactivity has not been investigated. However, it is clear that in the case of works of art, which are to be preserved indefinitely, even a slow rate of deterioration could lead to accumulated damages and therefore care should be taken to control ozone to the lowest level possible.

Examination of the data of Fig. 5 shows that the ratio of indoor/outdoor

220 F. DE SANTIS ET AL.

0.8

0.6

0.4

0,2

I/0 Rlltlo

0 19-8 8-12 12-t6 16-19 19-8 8-12 12-16 16-19 i9-8 8-12 12-16 1~-19 19-8

Time

f Nltrlo Aold ~ Sulphur Dioxide

I/0 Rstlo 1.6

i m

|4 m, ............................................................................................................................... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . , . . . . . . . .

1 . 2 : ...........................................................................................................................................................................................................................

~ ................................................................................................................................................................................

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0.41- ...........................................................................................................................................................................................................................

e 2 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . , , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . , , . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . .

0 I I I I I I I I I I I

19-8 8-12 12-16 16-18 19-8 8-12 12-16 18-19 19-8 8-12 12-16 16-18 10-8

Time

= Nltrste ~ 8ulphste

Fig. 5. Measured indoor /outdoor ratios for HNO3, SO2, nitrate and sulphate for the GaUeria degli Uffizi, Florence, 25-29 June, 1990.


HNO3 is much lower in comparison with that of SO2. This is not a surpris- ing result based on the higher deposition velocity of HNO3.

For the deposition velocity we use the expression Vd = F/C, where F is the flux to the surface and C is the concentration in the air of a given pollutant.

Assuming a deposition velocity for HNO3 of I cm s-~ and an average indoor concentration 0.5/zg m-3, a flux of nitric acid of 18 #g m-2 h-~ can be calculated.

As far as the particulate matter is concerned its concentration indoors is expected to depend in a complex way on several factors including people density (e.g. footwear and clothing flaking). The examination of the data shows that indoor nitrate and sulphate, samples with a size cutoff at 2.5/zm, are present at fairly high levels (Fig. 3) not very different from those outdoor. Moreover, the measurement of the pH of the particulate matter collected on the Teflon filter showed that the fine fraction of both the inside and outside aerosols is slightly acidic and that the average pH indoors is similar to that found outdoors (average indoor pH = 5.70, average outdoor pH = 5.83). This indicates that the filtration does not substantially affect the distribution of fine particles when going from outdoors to indoors. This was not unex- pected because it is known that, even when the filter banks are fresh and regularly maintained, the penetration of small particles can be high [20] and, besides, the infiltration of external air through windows during the study could not be completely avoided. This sb, ows also that the air exchange between inside and outside is fast. The high concentration of fine p~rticles measured indoor is a consequence of the low deposition velocity for pz.rticles ~f this size.

It is interesting to note that because of the intrinsic low deposition velocity of fine particles, furtherly lowered by the reduced turbulence of the indoor environment, sulphate and nitrate in particulate matter could be used as tracers of the air exchange. This also demonstrated that the lower level indoor of SO2, 03 and HNO3, which we have observed, could be the consequence of their deposition and not of their removal from the external air when going fi~om outdoors to indoors, in other words species more reactive than particles (i.e. species characterized by higher dcpo~Jtion velocities) as SO2, 03 and HNO3 are transported indoors and react easily ~)n the surfaces, most likely including the paintings exposed.

The coarse fraction was not investigated in this study, however, even though parities with aere,~ynamic diameters higher than 2.5 #rn could affect the indoor air quality, the coarser p~icles probably do not peaetrate inside as effectively. As regards the distribution of nitrate~ and sulphates it is likely, on the basis of several investigations (e.g. Ref. 21), that nitrate and sulphate are present in different size ranges. In general, it is known that sulphate aerosol occurs predominantly in the accumulation mode (between 0.i a~d

222 F. DE SANTIS El" AL

1.0 #m) whereas the nitrate mass distribution is normally bimadal with one peak in the accumulation mode and another in the coarse (between 2.5 and 10 ~m) which should be mainly affected by the fllt:ation in the ventilation system.

This is consistent with the observation that in the case of the sulfates, which are probably largely present in the fine fraction, the indoor/outdoor ratio is relatively constant during the study period, while for the nitrate, probably present in both fine and coarse modes, the ratio is much more variable (Fig. 5) depending on the infiltration rate.

CONCLUSIONS

(i) The air quality inside the museum under investigation is severely im- paired due to the direct transport through the central ventilation and air conditioning system.

(ii) The upgrading of the conditioning system of the museum should in- elude filtration of the incoming air. By adjusting the make-up and recircula- tion rates it should be possible to avoid the infiltration of outdoor pollutants inside the building.

(iii) Additional studies of the nitrous acid generation indo~,,-s zze ne~ded before a judgment can be made on the relationship between (he eoncvatra- tion measured and the deposition of HNO3. The extent to which a given nitrous acid concentration may contribute to damaging a surface could only be verified by using laboratory experiments.

ACKNOWLEDGEMENTS

The authors are grateful to Dr. Annamaria Petrioli Tofani, Director of the Uffizi, and Professor Vito Cappellini, responsible for the Progetto Strategico Uffizi for their interest in this work. This work has been made possible thanks to the financial support provided by CNR in the framework of the Progetto Strategieo Uffizi. The laboratory studies on the formation of HONO have been performed in the framework of the Progetto Chimica Fine II sponsored by CNR. The assistar, ce of the technical staff of the Uffizi and of Mr. Giulio Calogero is greatly appreciated.

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