Fine particulate matter in indoor cultural heritage: a literature review
© Grau-Bové and Strlič; licensee Chemistry Central Ltd. 2013
Received: 11 February 2013
Accepted: 1 March 2013
Published: 3 April 2013
Fine particulate matter is, on account of its aerodynamic properties and typical composition (especially diesel particulate matter and carbonaceous particles) the particulate pollutant potentially most harmful to cultural heritage, representing an aesthetic issue and an agent of chemical degradation simultaneously. This paper reviews the current knowledge of the life-cycle of fine particulates, focussing on diesel particulate matter from emission to deposition, including its aesthetic and chemical consequences, and draws attention to some imbalances in the current state of research. The currently available measurements are biased towards coarse dust, and information on the consequences of particle deposition is largely restricted to the outdoor environment. More evidence on the chemical effects of the most common types of fine particulate matter in typical indoor materials is needed to enable risk assessment for indoor collections.
KeywordsFine particulate matter Diesel particulate matter Soiling Deposition Resuspension Indoor air quality Indoor heritage
Most museums, galleries, libraries and archives, as well as many historical palaces and houses, are located in urban centres, surrounded by a complex and changeable urban atmosphere. While the last twenty years have seen a great reduction in the emissions of the pollutants that have been typically considered more harmful for vulnerable heritage materials, scientific interest is shifting towards other pollutants and particulate matter (PM) has been a subject of great interest.
However, PM can be an elusive subject of study. Different sizes display different properties, typical sources and even different behaviours. Studies of PM in indoor heritage environments have generally focused on dust, which is one of the characteristic sizes -the largest- of PM. This bias towards coarse particles is evident if we look at the minimum diameter of the particles collected in different monitoring campaigns in the field of heritage science. A survey of 25 scientific papers [1–25], most of them reviewed here, with the keywords “heritage” and “particulate matter” reveal that 32% of them analyse only particles up to 10 μm and 16% include particles up to 2.5 μm, i.e. more than half of the studies did not look into submicron particles. If a study did take into account particles ∼0.5 - 1 μm, it is generally the lowest size mode considered (36% of cases) and no particles are studied under this value. Finally, in 92% of the cases, particles are studied in only two or one size modes (usually 2.5 and 10 μm). However, two size fractions are not enough to reflect the actual size distribution, which is only analysed in a minority (8%) of the studies.
Obtaining size distributions and specifically quantifying the amount of submicron particles is common practice in aerosol monitoring outside the heritage field. All these sizes, and not only large particles, have a certain role in indoor heritage environments. In fact, coarse particles exhibit characteristics of great interest to conservators: they are significant carriers of mass to surfaces, and, being bigger, are more likely to alter the visual appearance of objects. But their number concentrations are orders of magnitude smaller than the concentrations of fine particles, especially in urban environments. Additionally, the composition of coarse and ultrafine particles is also different, and while small particles might carry less mass, they may carry components of different reactivity.
This review deals with fine particles, i.e. all particles smaller than 1 μm, without excluding particles smaller than 0.1 μm, commonly referred to as ultrafine (UFP). This size range is clearly separated from coarse dust, and it includes pollutants of particular interest to conservation of cultural heritage. Special attention will be given to particles derived from combustion present in urban environments, particularly Diesel Particulate Matter (DPM).
The review serves a double purpose. First of all, it attempts to identify if fine and ultrafine particles can be regarded as a relevant risk to cultural heritage indoors. It also identifies multiple areas that require further research. Secondly, it aims to provide a guide to heritage managers and curators interested in the properties of this particular pollutant. While aerosols indoors have been studied in detail, no account exists of the distinct behaviour of fine particles. Since not many investigations deal directly with fine and ultrafine particles in heritage environments, this review will make use of work produced in other fields -aerosol science, environmental science, computational modelling- which can be applied to heritage at least partly.
Sources, trends, and projections
Given such a broad definition, it is natural that there are a variety of origins, sources and compositions of PM. Nonetheless, there are a limited number of relevant sources. The majority are related to energy production, and natural sources (such as sea-spray) are often negligible in comparison with the anthropogenic ones. Among them, the combustion of fossil fuels, especially in road transportation, has a prominent role.
In the UK, road transport is responsible for more than half of the particles of the smaller size ranges and around 20% of the larger . This relative contribution is even greater in urban areas. At Marylebone Road, arguably one of the most polluted streets in London, traffic-generated particles make up to 27% of the P M10 mass concentration and 42% of the P M2.5. Most road transport particles are due to diesel vehicles (e.g. 72% of USA road transport PM) . Even though this picture varies greatly between regions, similar particle source apportionments have been reported by different studies carried out elsewhere [30, 31].
Emissions of all types of PM are predicted to decrease in the decades to come, including emissions of the smallest particles  due to the implementation of mitigation strategies such as diesel soot filters and the substitution of fossil fuels with alternative energy sources. Analysis of global emission trends under different scenarios reveal that emissions of traffic-generated PM will reduce in all the continents except in Africa, where they are predicted to increase 1.3 - 3.1% per year, depending on the scenario, due to economic development and ageing car fleet. Global emissions from vehicles will reduce 1.3 - 2% on average per year in the following 40 years . China will achieve a reduction in emissions of black carbon of 9% by 2020 .
The PM concentration limit suggested by the World Health Organisation (WHO)  and the European Commission  for P M2.5is 25 μg/m3, and the US Environmental Protection Agency has suggested a value of 13 μg/m3. Even though these limits are exceeded in some regions, it is likely that they will be satisfied in the near future. However, WHO states as no threshold for PM has been identified below which no damage to health is observed, the recommended value should represent an acceptable and achievable objective to minimize health effects in the context of local constraints, capabilities and public health priorities . The same logic may be applicable to heritage materials. It can be concluded that, even though emissions are gradually decreasing, traffic-generated pollutants will remain an important part of urban atmospheres for decades, especially in developing economies.
Particles derived from combustion
The laser-cleaning literature abounds with uses of the word “soot”, referring to dark deposits on indoor and outdoor materials. However, no standard description or characterisation of soot exists. The concept of “black carbon”, extensively used in aerosol and environmental science, suffers from a similar imprecision, sometimes used interchangeably with soot .
What is clear is that fine particles (P M1) mostly originate from combustion processes. Morphology and composition of these particles varies with the source to some degree: burning of candles, coal, tobacco or diesel fuel. These particles also have common features: a high content of inorganic carbon that will display high light absorbance, combined or coated with traces of other elements, and an aggregate-like shape.
Other combustion particulates
Even though this review focuses on particles of outdoor origin, mention should be made of other specific sources of fine and ultrafine particles which may be of interest. Candle-burning soot, for example, is commonly associated with indoor deposits in temples and churches. It has been found that the amount and composition of particles emitted from candles depend on the burning mode. If the flame is in steady state, it emits a relatively high number of ultrafine particles dominated by either phosphates or alkali nitrates originated from additives. Sooting burn, in addition, emits larger particles mainly consisting of agglomerated elemental carbon, with a morphology which is similar to DPM  (Figure 2c). Particles with the same morphology  and similar composition can be emitted during cooking , an activity not unusual in large heritage sites and museums. A “black deposit” or “soot deposit”, must, therefore, be assessed with care in order to identify the most likely origin of the particles.
Concentration trends indoors
The indoor PM concentration is generally a reflection of the outdoor concentration. Certain indoor activities represent exceptions to this rule. It has long been established that different activities, such as cooking, housework, or simply any physical activity, result in concentration peaks over the baseline set by the outdoor concentration . The frequency of these activities in heritage environments, and the efficiency of air cleaning systems define the daily PM pattern.
The mass and number concentrations seen in Figures 3 and 4 are representative of the typical concentrations in indoor environments. The average concentration of P M1 inside the Alhambra, Granada, Spain, was 17 μg/m3 in summer and 8 μg/m3 in winter , and its most abundant component was black carbon. Traffic was found to be the major source of fine particles. Between 10 and 20 μg/m3 of P M2.5 were detected in display rooms in the Plantin-Moretus museum in Belgium , and 40 μg/m3 in the Archaeological Museum of Thessaloniki, Greece . If total suspended particles are measured, higher values should be expected, e.g. 60 - 70 μg/m3 inside the Wawel Castle Museum in Cracow, Poland . At this site, particles containing elemental and organic carbon were found to be the ones that penetrate more easily into the museum. Even though particle concentrations in heritage locations are commonly measured in this manner, they tell us little about the fraction of fine particles that penetrate into these locations. Analysis of the bulk chemical composition of the collected particles is a common further step, but it is rare to find more detailed measurements of size distribution including fine particles. A good example are the detailed measurements taken in the Czech National Library in Prague , or in some Californian museums  which display a clear bimodal size distribution (Figure 1).
The life-cycle of fine particles indoors
Indoor sources of fine particles are found in some specific heritage environments, such as in-use churches where incense and candles are burned. An increase for a factor of 9.1 in the concentration of P M1 has been found after services that involved incense burning in Ruhr, Germany. In this case, the concentration inside the church remained above the outdoor levels for ∼24 h approximately . These findings are consistent with values found during services in medieval churches in Cyprus, where indoor P M0.5−1concentration was found to be up to 10.7 times larger than the outdoor concentration.
A relatively unknown indoor source is the thermal desorption of organic compounds and emission of submicron particles from household dust [68, 69]. At temperatures above 50 °C, which are often present in indoor environments, concentrations around 2500 particles/c m3can be generated . Investigation of this phenomenon, which to the best of our knowledge has not been researched in heritage sites, should be considered when introducing new heating points in the environment.
The airflow in an indoor space, isolated from the outdoor climate, is governed predominantly by two factors: temperature gradients and mechanical mixing. In a totally isolated room, only the temperature profile will define the air velocity pattern. Studies of indoor micro-climates show that temperature gradients are a consequence of a number of factors such as presence of heating points, proximity to windows, temperature of the surrounding spaces, human presence, lighting or similar. Heat sources induce vertical convective flows that displace contaminants upwards in an enclosed space. Cool vertical surfaces, such as windows, induce downward flows, which results in a circular movement of air around the room. A typically observed air movement pattern is the upward flow from radiators or air circulation behind furniture or paintings due to the difference of temperature between the wall and the air . PM is largely transported by the movement of the surrounding air. In other words, the Stokes number, the non-dimensional parameter which describes the behaviour of particles in suspension, is generally well below 1 (St << 1). The Stokes number, St=τU/D, is determined by the ratio of the relaxation time of the particle (τ), the characteristic dimension of the obstacle obstructing the fluid flow (D) and the velocity of the fluid (U).
Particles with St > 1 will have their own velocity field and enough inertia to detach from air streamlines, and particles with St < 1 follow the air current closely . However, the velocity field of particles does not coincide completely with the velocity field of air. Particles have a certain mass, and therefore their velocity has a vertical component due to gravitational settling. Coarse particles settle down gravitationally much faster than fine particles, and this creates a certain stratification of the concentration. Measurements in indoor domestic environments have revealed a higher proportion of P M2.5 on the upper parts of rooms, and higher abundance of P M10towards the floor . Measurements of particle deposition in ceiling, walls and floor reveal that almost no coarse particles deposit on the ceiling, while all the deposited mass in the floor is due to coarse particles .
Other transport mechanisms
The smallest particles are largely affected by Brownian diffusion (also called “random walk”), which is a result of collisions between particles and air molecules and occurs in all directions. In any given room, coarse particles will be found in areas with the highest air flow, while fine particles will tend to diffuse around all the available space. Thermophoresis, the displacement of particles from high to low temperatures, is a phenomenon also common indoors. The balance between air transport, diffusion and thermophoresis has been studied in detail by Camuffo  in the case of the soiling of murals. It was pointed out that when a vertical fresco is colder than the surrounding air, the temperature gradient forces thermophoresis towards the wall, and at the same time a downward free-convection flow develops, resulting in an overall increase of deposition rates. When a fresco is warmer than the air thermophoresis takes fine particles away from the wall, but this effect may be counteracted by an upwards convective flow that increases deposition of coarse particles. The best situation for conservation purposes is, therefore, a thermal equilibrium between wall and air, whereas cold walls are the less desirable scenario.
Dependence on particle diameter
The highest deposition rates are found for the largest particles (1 - 10 μm), which are governed mostly by gravity and tend to deposit on horizontal surfaces, and for the smallest particles (0.01 - 0.1 μm), which are mostly governed by Brownian motion and tend to diffuse and collide against floor, walls or ceiling. The mass of particles is only relevant in the larger size fraction, in which larger densities mean larger deposition velocities. Between these two size modes, the accumulation mode (0.1 - 1 μm) shows the slower deposition rates, which are up to 2 orders of magnitude smaller than that of the coarse particles . These lower deposition rates imply that particles in the accumulation mode tend to remain in suspension for longer, and therefore travel longer distances. In other words, while the coarse particles will deposit shortly after penetration indoors, near the source, the accumulation particles will distribute more evenly around the available space .
Dependence on air flow
The flow turbulence parameter, K, a key component of the Nazaroff deposition model, represents the turbulence regime of the air. It is an influential parameter and at low values of K, when turbulence is low and air is, for example, driven by temperature differences that generate free convection, particles display the lowest deposition rates. The deposition rates for all diameters increase with higher air velocities, which can be produced by wind or mechanical ventilation. Deposition rates are also smaller when the surface to volume ratio of the room is small, i.e. when the room has a small surface in relation to its volume. As a general rule, small volumes such as display cases and boxes will have larger S/V ratios than large galleries, but one should bear in mind that the number of objects (e.g. furnishings and exhibits) present in the room will also increase the S/V ratio, and thus increase deposition. Similarly, the roughness of surfaces favours deposition . The applicability of the Nazaroff deposition model has been extensively proven in experimental investigations of particle deposition in a range of environments [77, 78], rough surfaces of different materials , rooms with fans, and furnished or unfurnished rooms .
Once deposited, PM is adhered to surfaces by adhesion forces that can be orders of magnitude higher than gravity , and of which Van der Waals adhesion is the most relevant . Changes in air flow conditions can eventually compensate these adhesion forces and re-suspend the deposited particles. Re-suspension rates are strongly dependant on particle diameter. Larger particles are re-suspended more easily. In some museum environments, particles of >1 μm appear only during museum opening hours due to re-suspension caused by visitors. These particles redeposit gravitationally as soon as the museum is closed . This type of behaviour has been studied for a long time, and common indoor activities such as walking and vacuum cleaning have been associated with re-suspension of particles >1 μm, and have been found to increase particle concentrations up to 7 times the background concentration . Re-suspension due to inappropriate cleaning habits has been found to account for the spatial distribution of particles in a monastery which displayed an otherwise very stable indoor environment .
This mechanism is very dependent on particle size, and <1 μm particles are rarely affected. Furthermore, re-suspension affects only those particles that are deposited on the floor or the objects involved in the human activity that causes it, such as furniture. The fraction of fine particles involved in the deposition and re-suspension cycle could be expected to be negligible, although there appears to be no relevant experimental research about re-suspension of the accumulation size mode. Nonetheless, re-suspension is a phenomenon that has been extensively modelled [85–87], and it is possible to assess the re-suspension rates of fine particles mathematically.
where n is the particle number and K the coagulation constant. The difference in behaviour (linear for deposition and quadratic for coagulation) allows us to appreciate the effect of both processes on particle decay. Other authors have acknowledged the importance of coagulation as a relevant removal process. It has been found to account for up to 80% of particle loss in a small chamber (1.6 m3) with steady air, with deposition removing only from 10 to 15% of the paper ash particles used (average particle diameter of 0.069 μm). This situation changes under stirring, in which case deposition may account for 50% of the removal in the beginning of the experiment and up to 90% at the end , as coagulation rate gradually reduces as particle number decays. These results are in agreement with , where it was estimated that coagulation could remove from 40% to 70% of the environmental particles in a street canyon with a low wind speed (2 m/s) and around 20% at higher wind speeds (8 m/s), and with the experimental results of , who found high coagulation rates in rooms with low air exchange rates. All the mentioned studies focus on particles smaller than 1 μm, since the smallest particles are more likely to coagulate, not only because of their higher mobility, but because they are typically present in higher number concentrations . Coagulation is known to be fairly independent of particle composition and air relative humidity . There is little doubt that coagulation is a relevant removal process in enclosed or semi-enclosed and highly polluted environments , but it is also true that it may be negligible in most heritage environments.
The consequences of deposition of fine particles
Deposition is a three-fold problem. First, mere deposition (or “dry” deposition) can cause area coverage and have a visual effect on the soiled object, a “visual nuisance”, as it has been qualified in some of the most relevant investigations [100, 101]. Secondly, frequent or intense cleaning might have a negative effect on the underlying surface, as well as being a cost-intensive process. And finally, the deposited particles might interact chemically with the surface, creating a damage layer and producing irreversible degradation. Although evidence exists for all these phenomena, research has prioritized soiling on outdoor surfaces and particularly layers produced on stone. The following section attempts to describe these three processes indoors -visual nuisance, chemical damage and damage by cleaning- based on the understanding that evidence is scarce, and in some cases conclusions must be extracted from evidence obtained in outdoor experiments.
where B, K, M and H are constants that define the varying response of soiling with time (see  for a detailed explanation). This model has been fitted to experimental data by Lombardo and Ionescu at different occasions [114, 115] and has been tested with data collected during the MULTI-ASSESS project  in different European locations. A second discussion relates to which concentration should be used as C PM . Some authors have suggested total suspended particles, while others have used P M2.5, P M10, DPM or particulate elemental carbon (PEC), given that most of the soiling is due to traffic-generated particulates .
Cases of soiling indoors
Soiling outdoors is popularly associated with black stains on faades, while soiling indoors is mostly associated with the deposition of household dust, i.e. coarse particles. In effect, experimental and observational studies are markedly biased towards outdoor blackening and indoor dust, perhaps due to the experimental difficulties of discerning different particle sizes of indoor deposits, especially the smallest. However, it is not difficult to find examples of well visible deposition of fine combustion particles. Some notable examples are found at induction and ventilation outlets  or on the murals of a palace in Padova, where inconveniently placed radiators were causing heavy soiling by dust and soot . The darkening in the centre of the murals in Michelozzo’s Courtyard, Florence, Italy, has been attributed to deposition of traffic-generated particles, since measurements of P M2.5 display a high proportion of organic and elemental carbon . In some occasions, the damage layers related to the deposition of combustion particles are related to the past use of the building, and not to modern traffic emissions e.g. in the Buddhist statues of the Yunguang Grottoes, China .
“Ghosting”: a particular deposition event
A specific indoor discoloration event related to heating points and the presence of semi volatile organic compounds (SVOCs) has been repeatedly reported in the literature under various names, which include “ghosting” , “black magic dust”  and “fogging” . Even though there have been no specific mentions of the phenomenon in a heritage context, it is clearly not exclusively found in dwellings “ghosting” deposits differ from dry deposition by the presence of droplet-like particles, and a layer of condensed SVOCs . The phenomenon is triggered only under certain conditions: emission of SVOCs (e.g. from refurbished materials), low ventilation rates, high temperature gradients (e.g. above lamps or radiators), and, naturally, the presence of PM. The dark appearance of stains is caused by the presence of elemental and organic carbon agglomerates, but coarse dust particles rich in Ca and Fe can be present as well . Even though efforts have been made to isolate the causes that can initiate this soiling event, all attempts to reproduce it in experimental conditions have been unsuccessful .
The small size of fine particles has two implications that should be stressed in any discussion of their visibility: their small covering area and their light scattering properties. In fact, an important fraction of fine PM is smaller than the wavelength of light visible to humans (∼390 - 750 nm). This, however, does not mean that particles cannot be seen when they accumulate on a surface. Firstly, if enough particles are deposited, the deposit will become visible even if a single particle cannot be seen. For example, candle soot deposits are common, even if the size of particles emitted from a burning candle ranges from 10 - 100 nm . Secondly, fine particles do not deposit alone, and all analyses of deposits have found a certain size distribution. Beyond that remark, it can be added that particles with diameters below the visible range can still scatter light when in suspension or when deposited on transparent materials through scattering in the Rayleigh regime. Several investigations report refractive indices for particles or soot small in comparison with the wavelength of light [126, 127]. These aspects; however, have not been researched in the context of heritage science.
While perceivable visually, the effects of soiling are quite difficult to quantify. Recently, attempts have been made to identify thresholds between acceptable and unacceptable levels of soiling on building faades. However, the relations between perception and soiling are complex, as the reaction of the public is not simply proportional to the amount of matter deposited. Soiling can be perceived in some cases as patina, and to a certain degree, it can enhance the appearance of a building . The perceived degree of soiling is also influenced by the cleanliness of the surrounding environment. Despite these issues, Brimblecombe et al.  used on-site questionnaires to identify soiling levels that are publicly unacceptable. Their results show that the public perception measured in terms of perceived lightness is fairly consistent with the perceived need for cleaning and could be used to define threshold doses in terms of environmental particle concentration. Another study  shows that soiling is perceived as negative when it interferes with architectural shapes. However, these studies are focused on the darkening of building features outdoors, and it can be argued that they have a limited applicability to indoor deposition.
Some researchers have left aside the complexities of aesthetic judgement, and have concentrated on the limits on the perception of soiling. Bellan et al.  have measured the human eye ability to detect soot deposition on flat, plain colour surfaces using printed dots (60 - 160 μm) on white and coloured matte surfaces. Their results show that some observers are able to discern a soiled surface from a clean one when the covered area is just a bit higher than 1% of the total, and that deposition becomes obvious to all at around 9% coverage. The perceptive ability is improved if the soiled surface is observed alongside a clean one, in which case all observers identify soiling when just 3.5% of the area is covered. Experiments with larger dots (0.5 - 1 mm) have led to a threshold of 0.2% area coverage . These results have been of great use for the establishment of guidelines and recommendations, e.g. by , since they provide a threshold value in terms of area coverage. But it must be noted that the diameters used in the experiments correspond with the coarser dust rather than with “soot” or fine particles, for which no direct account of their thresholds for visibility has been published.
Recently, Druzik and Cass claimed that some specific paintings were under special risk of soiling . Particularly paintings with large colour fields, like those by Mark Rothko or Franz Kline, were assessed as being more vulnerable to the aesthetic damage due to soiling.
Degradation of soiled objects
It has long been established that particulate pollution from road traffic contributes greatly to the degradation of stone outdoors. The presence of DPM has been related to the decay of carbonate  and silicate stones . Also outdoors, several corrosion products of copper were identified on statues where soot was also present , but no formal relation was established. Although the effects on materials other than stone are less investigated, it can be expected that the reactive components of DPM will also interact with materials, which are typically displayed and used indoors, such as paper, paint and varnishes, or textiles.
There is an important lack of literature about the effects of particulate deposition on the surface of paper, leather, textiles, paintings, varnishes and other materials typically found indoors, perhaps due to the complexity of the problem and the great variety of materials involved. A brief list of potential degradation pathways related to particle deposition is available in the literature : (i) S-rich material (such as DPM, which contains oxidised sulfur compounds ) can cause discolouration of paintings; (ii) ammonium sulfate can induce bloom on varnish. Ammonium sulfate is a “secondary aerosol” (i.e., formed in the atmosphere), but it often coexists (and even aggregates) with carbonaceous particles ; (iii) The presence of CaSO4 favours the adsorption of soot; (iv) Fe-Rich particles can catalyse the oxidation of SO2 to H2SO4. Aged diesel particulate matter is hygroscopic , and therefore can favour the adsorption of water that accelerates hydrolitic and oxidative processes, leading to fading of pigments, and degradation of paper and textiles . Despite the lack of systematic investigations, the effects of fine PM deposition have been repeatedly noted by conservators. Damage layers related to black carbon deposition have been detected on indoor murals and wall paintings , and on polychromy . The word “black crust” is sometimes used to describe these damage layers found indoors, but it is a macroscopic assessment that gives very little information about the origin of degradation.
Conclusions may be extracted from studies carried out with particles of similar composition. The corrosion of zinc and steel has been studied in relation to the presence of deposited particles derived from the combustion of oil (which may be comparable to diesel fuel) and coal . The authors concluded that in relatively unpolluted atmospheres inert particulates can induce corrosion in zinc and mild steel due to differential aeration, an effect which is masked when the overall corrosion rates increase. Oil-ash particles were also found to be much more corrosive than coal-ash particles. Although far from the heritage field in terms of temperature and concentration, some investigations have demonstrated that DPM leads to severe degradation of ceramic filters used in engine exhaust tubes .
Cleaning of soiled surfaces can induce undesired degradation. It is well known that cementation of coarse dust increases the difficulty of removal . DPM behaves in a similar way, due to its ability to penetrate into pores and its potential chemical interaction with the surface. The National Trust’s Manual of Housekeeping states that dry cleaning methods, such as brushing, vacuum cleaning, or even the use of erasers, might be insufficient, and that the deposition of soot can produce “disfiguring, virtually indelible staining” . In cases of extreme soiling, it has been reported that vacuuming removes only the loose deposits of smoke particulates, and that wiping may further attach particles to porous surfaces . These difficulties have prompted the use of laser cleaning methods; however, laser removal of particles from organic materials have been found to result in yellowing of the underlying surface [144, 145].
There is a certain ambiguity of the terms used to describe indoor PM in the heritage literature. There has been some discussion on how the staining of façades should be described (black crust formation, staining, darkening, blackening, soiling) [100, 146] but this discussion has not taken place for indoor heritage. As a consequence it is not clear what the word “soot” refers to in some cases. It can refer to carbon-based agglomerates in suspension, or the same particles deposited on a surface, or a black stain of unknown composition but of “carbonaceous” appearance, which might also be only a surface deposit. There is a need for the development of a more accurate terminology that makes a clear distinction between suspended fine particles (DPM, combustion-derived or even soot), dry deposition of these particles without further effects and removable with cleaning (which could be called darkening, or soiling), and the degradation layer formed due to the interaction of the deposit with the underlying surface.
PM monitoring in heritage sites is generally focused on coarse dust, and the two most frequently measured particle types are P M10 and P M2.5, which include particles up to 2.5 and 10 μm. This standard, however, has limitations. Measurements of P M2.5 sum up some particles from the coarse mode (>1 μm) and some from the fine mode (<1 μm) and therefore these values do not help to identify the fractions of fine and coarse particles, which would enable appropriate action to be taken. Complete size-resolved measurements of particle concentration would provide more information on the likely source and typology of particles; however, measurement of size distributions requires costly equipment. A much more informative and cost-effective measurement would be P M10 and P M1, or P M10−1 and P M1. In a heritage site situated in an urban environment, for example, these values would provide a useful estimate of particles as a consequence of traffic emissions that penetrate into the building.
The formation of black stains in the presence of SVOCs (“ghosting”), or the emission of fine and ultrafine particles when indoor dust is in contact with warm surfaces are phenomena that have been repeatedly observed in indoor environments. It is unknown what the impact of these soiling events is, and whether in some cases they are wrongly attributed to outdoor sources.
Much is known about the aerodynamics of fine PM. The accumulation mode (0.1 - 1 μm), due to its size, displays low deposition rates, low re-suspension rates, and a high penetration efficiency through cracks and filters. Low deposition rates have different implications. Deposition will be a slow process, but it will occur eventually if particles are not removed. They will distribute evenly around the space, depositing far from the source, and will reach areas in walls and ceiling that are difficult to access. Low re-suspension rates, in combination with a small size that favours penetration into porous surfaces, will lead to difficulties with cleaning and irreversible soiling.
Less is known about what occurs after soiling. There is a significant disproportion between the detailed knowledge of the aerodynamics of fine particles, and what is currently known about the chemical effects of the most common particle types and the potential degradation of soiled (heritage) surfaces. The scarce evidence available is just enough to assess that DPM and other particles derived from combustion can have an active role in the degradation of materials beyond soiling. Considering the costs associated with cleaning, it is important to know if removal of deposited fine particles should be a priority. Risk assessment cannot be based solely on the spatial distribution and deposition rates of fine particles. There is a need for research into chemical interactions between the most common fine particulates (DPM and other combustion-derived particles) and different materials that represent indoor heritage surfaces.
JGB is a PhD candidate at the UCL Centre for Sustainable Heritage. His research focuses on CFD modelling of fine and ultrafine particulate matter in indoor heritage environments.
MS is Senior Lecturer in Sustainable Heritage at the UCL Centre for Sustainable Heritage and Course Director of the new MRes Heritage Science at the Centre. His research focuses on the development of new scientific tools and methods of study of heritage materials, collections and their interactions with the environment, particularly development and use of damage functions and integrated modelling of heritage collections. Presently, he is the Principal Investigator of the UK AHRC/EPSRC Science and Heritage Programme project Collections Demography (2010-2013) and a Co-Investigator on Heritage Smells! (2010-2013). In 2008, he co-coordinated the 8th European Conference on Research for Protection, Conservation and Enhancement of Cultural Heritage.
Diesel Particulate Matter
Minimum Efficiency Reporting Value
Particulate Elemental Carbon
Semi-Volatile Organic Compounds
Total Suspended Particulates.
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