Microclimatic factors affecting the Trajan Column

The Science of the Total Environment, 128 (1993) 227-255 Elsevier Science Publishers B.V., Amsterdam

227

Microclimatic factors affecting the Trajan Column

Dario Camuffo and Adriana Bernardi

National Research Council C,'~'I~dCTR, Cso Y, tali Uniti, 4, i-33~20 Padova, Italy

(Received September 25th, 1991; accepted January 8th, 1992)

ABSTRACT

Because of meteorological and microclimatic factors ~.g. solar radiation, gradients of temperature and moisture, air speed and turbulence) the Trajan Column is subject to deposition processes on a diurnal cycle, which alternatively increase and decrease the deposition rate of airborne pollutants. The diurnal trend of the pollutant concentration in Rome is bi-modal and the two maxima are associated with different deposition velocities, so that a simple correlation between stone deterioration ~,d pollutant concentration averages appears to be meaning!ess. Field tests were carried out on the monumem to investigate the diurnal and seasonal trend of factors that affect dry deposition in relation to the meteorological variables and pollution levels, i.e. phoretic forces due to temperature :.g-cadients (thermophoresis), condensation and evaporation (diffusiophoresis and Stefan flow), inertial impaction, electrostatic capture and gravitational settling. In particular, when the marble is warmer than the surrounding air, a layer of warm air rises continually along the warm part of the column, increases the upward drag of the falling particles and results in a net decrease of the gravitational settling of the coarse particles (i.e. with diameter d > 1 /~m); when the marble is colder, the downwaid drag increases the sedimentation rate. The wind field around the monument was measured in order to investigate the diffusion potential. Turbulence generated by the shape of the column and its roughness clearly dominates the natural turbulence of the free flow. Rainwater has been recognized as being the cause of severe dissolution except in zones where whewellite is present. This is seen by comparing the deterioration pattern with the rainfall rose, and by following the run-off path which is determined by the geometry of the bas-reliefs. Dissolution is due to the chemical activity of the dry and wet depositions; it is also dependent on the physical regime of the run-off, i.e. laminar or turbulent. In the past, in addition to the physico-chemical weathering, biological deterioration occurred where meteoric water remained longer. Micrometeorological field test were carried out on the Trajan Column to investigate the diurnal and seasonal evolution of the main temperature and humidity parameters to know the microclimate of the column. Four seasonal tests have been made, each of 6 days under different weather conditions. The measurements have been made on horizontal and vertical profiles at different distances from the surface to study the heat and moisture exchanges between marble and atmosphere, fhe wind loads and their effects on the mechanical structure resulted in being negligible.

Key words: microclimate; meterology; monument deterioration; pollutant deposition

0048-9697/93/$05.00 © 1993 Elsevier Science Publishers B.V. All rights reserved

228 D. CAMUFFO AND A. BERNARDI

INTRODUCTION

As the extent of monument weathering has grown exponentially over the last few decades, much effort has been expended towards understanding the exposure~damage relationship with reference to pollutant concentration, in- cluding oxidation of SO2 both in the wet and in the dry phase in the presence of NOx and photochemical oxidants. The processes involved are still unclear in detail. Until now, no single quantitative exposure-damage relationship has been found to be satisfactory. The concentration levels of chemical compounds in the atmosphere are only a part of the problem; the time duration and rate of the deposition processes governed by microclimatic factors, the time of wetness, the frequency of the activation of chemical compounds by means of meteoric water, presence of catalysts and other factors can not be neglected. In order to progress towards a better formulation of the interaction between atmospheric pollution and stone deterioration, it is necessary to study the deposition processes in relation to the environmental cycles of both microclimatic factors and atmospheric pollatants.

In some recent papers (Camuffo et al., 1982; 1983; 1984; 1987; Camuffo, 1984a,b; 1986) it has been pointed out that deterioration of marble monuments is chiefly determined by bulk deposition and the way the stone is wetted. Meteoric water, largely supplied by rainfall rather than condensation, may activate chemical reactions or favour biological activity, and the temperature of the marble may affect the kinetics of these processes. In general, the zones exposed to wash-out are subject to dissolution and thinn- ing; the zones which are soaked, but protected against run-off, are eventually transformed into black crusts which are mainly formed of gypsum crystals embedding black carbonaceous particles. The gypsum is mainly derived from sulphation of the marble (or limestone) and partly is enucleated by the particles which contain some calcium. The carbonaceous particles play also a more important role in the formation of the the black crusts, as follows: they have an enormous specific surface and contain metals, so that can act as catalysts to oxidize SOs to sulphate (Del Mont~. • et al., 1981); they initially contain sulphur and during the dry period they adsorb in the carbon matrix gaseseous pollutants and then release them into solution, when they are soaked by rainfall: in practice they behave as a pump which removes pollutants from the atmosphere and release them on the surface of monuments. In the zones where percolation occurs, the black crusts and other pollutants which are deposited on the stone may contribute to the ag- gressiveness of the wet deposition when some part of the substances which were deposited in the dry phase are dissolved into the rainwater. The solubility of the acidic anthropic emissions has been seen to be greater than that of the ~.lkaline soil particles (Camuffo eta!., !988). Drizzle and the beginning

MICROCLIMATIC FACTORS AFFECTING THE TRAJAN COLUMN 229

of run-off are often associated with maximum acidity. Heavy run-off dilutes the aggressive solution formed on the monument and eventually removes the particles attached with less resistant binding.

This paper presents the result of field tests carded out to study the microclimatic factors and the induced effects responsible for the deterioration of the Trajan Column, i.e. the daily and seasonal cycles of air and surface temperature (T), relative humidity (RH), specific humidity (SH), dew point spread (DPS), ventilation, turbulence and the interactions between the stone and atmosphere. Thermo-hygrometric profiles were taken, during the daylight hours, at various points, both vertically and horizontally, in cross sections, around the column. From these measurements it was possible to make an analysis of the atmospheric stability; determine the thermal response of the stone surface to solar radiation; demonstrate the heat and vapour exchanges between the surface and environment under different weather conditions, and point out where and when the column is most likely to be subjected to condensation.

The column is situated at the extreme NW of the so called 'Trajan market', the nearest buildings being in a NW-NE direction only, while in all other directions there is an open space where the market was once held. The site of the ancient market was excavated and actually is some 4 m below the street level. An asphalted road lies close to the border of the excavation, a few meters from the column. The base of the column is sited in the ancient market and lies in soil (bare soil or grassy bed, depending on the season), and the temperature there is often very different from the upper nearby road. The microclimate at the base is, therefore, rather different from that found around the rest of the column: during winter the cold air sinks and fills the excavation of the market; during summer the asphalted road becomes warmer than the soil of the market, and warm air reaches the upper half of the column.

EXPERIMENTAL CAMPAIGNS

Winter field test

During the winter campaign, from 19 to 24 February 1982, the synoptic situation was characterized by a high pressure, typical of the season, initially centered over the Scandinavian countries and moving successively to Russia. This situation caused cold inflows of continental polar air coming from the SE over Italy. The contrast with the relatively warm waters of the Mediterra- nean resulted in instability. On the 19 and 21, fronts causing precipitation passed over Rome. The minimum and maximum temperatures recorded during the survey, show, in comparison with the data recorded between 1946

230 D. CAMUFFO AND A. BERNARD!

and 1970 (Aeronautica Militare Italiana, 1982), that the days taken into consideration were typical of the season.

The vertical profiles of the air temperature show the usual daily cycle. The vertical profiles of SH do not show any great variation, especially in the middle of the day when vertical mixing is enhanced by instability. Maximum SH values may occasionally be found at the base only, due to evaporation from the soil and upward transport of moisture during the daytime. The RH is particularly variable due to the different weather conditions, ranging from 35% to near saturation on rainy days. The vertical profiles do not have marked gradients. By calculation, it has be shown that, in this period, the RH profile is more affected by variations in T than in SH. Maximum RH is found at night and near the ground when, as the overlying air cools, the degree of saturation increases. The RH reaches levels which are sufficient to trigger condensation in the micropores of the marble at the base of the column, and the micropores become full of water.

In the early morning there are no thermal differences between the different sides of the column. Thermal gradients on the horizontal cross sections are moinly found during the daytime period. The DPS of the air near the column is generally lowest in the NW in the morning; in the NE in the afternoon. At night, the DPS pattern is homogeneous. The DPS values vary considerably depending on the weather conditions, ranging from a few °C at night or on rainy days, to > 10°C on fine days.

Spring field test

During the spring campaign, from 21 to 26 May 1982, the synoptic situation was characterized by weak pressure gradients. A high pressure ridge, initially extending from the Atlantic over Europe caused inflows of polar maritime air masses; an Atlantic perturbation caused precipitation. Suc- cessively, an area of high pressure was formed over central Europe which brought weak northern currents over central Italy. The temperature range during the field test was typical of the period, as seen by comparison with the averages of the reference climatic period (Aeronautica Militare Italiana, 1982).

The vertical profiles of the air temperature on fine days indicate the typical day time evolution of superadiabatic conditions and inversion at night. Around mid-day convective cells responsible for the upward transport of pollutants emitted at ground level can be seen. On overcast or cloudy days sub-adiabatic profiles, ranging between isothermy and neutrality, prevail. In general, the SH profiles show small gradients in early morning and sundown on fine days. During fine days, the decrease in SH from the base to the top of the column indicates the upward transport of moisture. This is accen-


tuated to a greater or lesser degree depending on the weather conditions and precipitation, indicating that the amount of vapour in the air is linked to soil evaporation induced by solar radiation. In the early morning, evaporation causes a great increase in the SH until the nocturnal inversion is broken, at 08:00 h. Sometimes, in the afternoon, there is a sudc~en increase in SH, particularly at the top of the column, which could be correlated to the arrival of an air mass that is more humid than the local one brought in by the sea breeze.

a

16

12

8

4 A NE

I I I I I I I I I I I I I f 0 I~ 10 12 14 16 18 20 22

b

2"~"~814 -f

,° I 6 6

SE

8 10 12 14 16 18

C

'0

4 NW A

2 6 8 10 12 14 16 18

Fig. I. Daily temperature waves on the surface of the column at different sides (NE, SE, SW, NW) and in the free air (A). (a) Clear day, (b) perturbed day with cloud cover in the afternoon, (c) overcast. (February 1982; after Camuffo and Bernardi, 1991).


The vertical DPS profile generally decreases with the height of the column during the whole day, and this is more evident around mid-day. The con- siderable warming of the soil (e.g. Tmax > 35-40°C for bare, dry ground in summer; half in the mid-seasons) causes the lower air to reach the maximum DPS, notwithstanding the maximum SH. The gradient is inverted only after sunset and cooling of the soil brings about an increase in the degree of saturation. In this season, even under different weather conditions, during the daytime the air temperature around the column's surface is generally increased by the marble which is always a few degrees warmer than free air, except for the side facing the sun, with some phase lag, as shown in Fig. 1 for different cloud cover. This determines the temperature gradient in the free air near the surface. The gradient is greater above the capital at the top of the column where the higher ventilation rates dissipate the layer of upward moving air, thus the warm marble is surrounded by fresh air. The gradients of US are slight everywhere; the greatest values are at the top of the column. The RH ranges between 40 and 80%. The northern side is the least affected by variations in RH. The DPS values show that the air near the NW side of the column is, until the early afternoon, the nearest to the dew point; while the NE side is for the rest of the day. The minimum values are of only a few °C. The air in front of the warmed side shows the greatest DPS. The maximum values reach as much as 17-18°C in the afternoon under favourable climatic conditions. As the diurnal range of the stone temperature is greater than that of the air, during the nocturnal cooling of the marble some condensation occurs in the micropores on all sides, favouring the grow:th of en- dolithic organisms; during the diurnal warming, evaporation occurs.

Summer field test

The synoptic situation during the summer field test, from 21 to 26 July 1982 showed a high pressure centered over the north-eastern Atlantic, which caused cold inflows of polar air from the NE. The warming of the air masses is responsible for instability phenomena. On the 24th, a disturbance passed over Rome, but did not result in any precipitation. The weak pressure gradients permitted the development of the breeze. In comparison with the climatic values (Aeronautica Militate Italiana, 1982), the minimum temperatures during the period under consideration were slightly higher than the average, while the maximum temperatures were slightly lower.

During the daytime, marked anomalies can be seen in the vertical profiles of the air temperature due to the differential heating at the two soil levels i.e. the floor of the Trajan market and the road. The profiles can be divided into two parts corresponding to the two named levels. The ground based inversion is caused by the greater ~mount of heat supplied from the road which is higher (Fig. 2).


40

! i 3O ~ f 29

10 ....

o I . . . . . I 8 12 16 2

LT(h) Fig. 2. Time evolution of the air temperature showing a ground based inversion (shaded area) due to the local topography, lasting till midday (July 22nd, 1982).

E ---- 20 I ',~I

The vertical profiles of the SH, in general, decrease even though the difference at the base and top rarely reached or exceeded 1 g/kg, which is an index of good mixing. The steeper gradients are found, as with the temperature gradients, during the ~dd!c of the day due to the intense

40

3O

20

10

0

l 6

8 12 16 20

LT(h) Fig. 3. Complex pattern of SH arising fr~,m evaporation, local convective activity and ac!vec- tion (breeze) on July 22nd, 1982.


evaporation from the bare soil. The enhanced vertical mixing causes the daily minimum of SH in the early afternoon. The time evolution of the vertical profiles does not follow a regular try. This is especially due to the arrival of the sea breeze, which is more evident at the top of the column. The daily pattern is complicated by evaporation, rise of thermals, advective contributions (Fig. 3). The vertical profiles of the RH exhibit a difference at most of 7%, between the base and top of the column.

Along the vertical profile, the maximum DPS is generally found in the air near the base of the column; at road level when the road is warmer. During this season, the air temperature around the column is generally lower than the marble. On fine days, and even on days with variable cloud cover although to a lesser extent, the maximum temperatures on the SE and SW sides are rather tiigh in the middle of the day. There are, however, frequent departures from this norm due to the special microclimate around the column which is linked to the ascending flows of warmer air and the outflow of colder air from the slit windows. In fact, the column is hollow, with inside a spiral staircase carved out of marble, and some slit windows light up the inner staircase.

The homogeneity of SH on the horizontal cross section shows that the flow rate of moisture between air and marble is low. An analysis of the DPS shows that in the morning and in the early afternoon the air near the NW side is the nearest to dew point; while in the late afternoon, the minimum DPS is at the NE side. The spread is marked around mid day on all sides and no surface condensation occurs. On the other hand, from evening on- wards, the dew point is approached as the stone progressively cools. The minimum temperature that the air near the stone surface reached before dawn (with the DPS only a few °C above dew point) shows that daily condensation-evaporation cycles occur.

Autumn field test

During November 19-24, 1982 central Italy was at first influenced by an anticyclone mainly centered over the Balkans. The weak pressure gradient resulted in fine weather and weak winds. Two disturbances originating in the Atlantic passed over Rome on the 20th (without precipitation) and on the 24th (with precipitation). The temperature was only slightly above the average for the last ten days of November as recorded from 1946 to 1970 (Aeronautica Militare Italiana, 1982).

During the daytime, the meteorological situation causes atmospheric stability to vary'quite irregularly. At night, a marked temperature inversion prevails. The vertical profiles of the SH shows only a slight gradient while the RH vertical gradient reflects the variability of the temperature. The gra-


40 / , ,B6

30

10

o i Z 8 12 16 20 24

LT(h) Fig. 4. Time evolution of RH on November 20th, 1982. Top and base have very different values except during the daytime mixing.

dients are often rather steep showing quite clearly how the base and top of the column are immersed in markedly different thermo-hygrometric atmospheric conditions. For example, after sundown the atmosphere around the base of the column is very close to saturation, so that condensation occurs not only in the micropores, but also at the surface when is in thermal equilibrium with air. At the same time, the RH at the top is some 10-15% lower (Fig. 4). The RH is on some days rather high, even during the day time. The vertical DPS profiles show that the temperature is very close to the dew point at the top of the column in the morning and at the base after sundown. The gradients are rather marked and variable, related to the variability in temperature.

The horizontal cross-sections show that the temperature of the air during the daytime is lower than the surface temperature of the column which faces south, but is generally higher than the parts facing NW, while on the NE side it varies. It should be noted that in the parts least exposed to solar radiation there is less variation in the RH ;aside the pores and therefore, fewer condensation-evaporation cycles. The SH is rather low for the whole period (6-8 g/kg), but there is a progressive increase during the day. The spatial distribution is, however, homogeneous which means that there are no special areas of the marble particularly prone to evaporation and that the variations are due to advection. The RH of the air near the marble varies considerably,


depending on the weather conditions and the temperature of the monument. It varies from around 50% to more than 90% on the northern side in the morning. Many of the pores of the marble in this season are full of water, although the southern side, which is directly exposed to solar radiation, is the side most subject to daily condensation-evaporation cycles which result in dissolving the interior of the marble. It shouk! be noted that in the least exposed parts there is less variation in RH and therefore, fewer cycles.

The air in contact with the southern side of the monument is the furthest away from the dew point, while the nearest to it is the NW. The spread re- mains on the whole quite low so that the colder surfaces may easily reach the dew point thus causing condensation, even at the surface.

DRY DEPOSITION ON THE COLUMN

Air pollution in Rome

The major sources of air pollution in Rome are automobile exhausts and domestic heating. The intense solar radiation during the warm season (Camuffo, 1979) induces photochemical reactions leading to the formation of 03 and organic acids (Ciccioli et al., 1986). The SOa has a seasonal and diurnal cycle which is similar to the NOx; the conditions favourable for

4o

3O

l! 3

10 . "!~""!~iii~iii~i~ ~i ":iiiii~'!!iil :ii

o tii i/" iii ~......~,.:::. :..... I 8 12 16 20 24

LT(h) Fig. 5. Diurnal evolution of the air temperature and atmospheric stability (stable layers are evidenced by shading) near the Trajan column on February 21st, 1982. A weaker stability (not shadowed), attenuated by wind mixing, develops in the evening.


photo-chemical reactions occur between May and September (Colacino et al., 1981; Brocco et al., 1988). The seasonal maximum of SO2 and NOx occurs in autumn and the daily distribution is bi-modal, with the maxima peak- ing at 09:00 h and 20:00 h. The diurnal distribution would, at first sight, seem to be associated with the traffic, but it is also conditioned by the reduced atmospheric stability during the middle of the day, as has been seen from field tests. Atmospheric diffusion contributes towards decreasiag the concentration levels during the central part of the day, thus making the two above mentioned maxima even more evident and minimizing the underlying effect of domestic heating. During winter, intense traffic occurs when the atmospheric stability is still high. The atmospheric stability is originated by the nocturnal radiative loss, and is eroded in the early morning by the solar radiation. However, the local topography later causes a further inversion (Fig. 5) due to the more intense warming of the nearby road which is some 4 m higher than the column base.

In the summertime, throughout the whole day the concentration of pollutants is relatively low and homogeneous; as the emission due to the traffic is more or less constant throughout the whole year, the daytime distribution of atmospheric stability appears to be a very important factor.

The diurnal trend of the spectral distribution of airborne particle concentration is reported in Fig. 6 for three summer days. The slight decrease in the concentration during the d~ytime is associated with the development of the mixed layer. No g[¢at variations can be observed during the three days exam-

6 10 - 1 0 0 . 0 0

5 0 . 0 0 -

2 0 . 0 0 - 1 0 . 0 0 -

5 . 0 0 -

C 2.00- (Nlm~) 1 . 0 0 -

0 . 5 0 -

0 . 2 0 - 0 . 1 0 - 0 . 0 5 -

0 . 0 2 - 0 . 0 1

-- 0.3 I ~

- - 0 . 5 I ~

2 t i n

S p a

I I I I

8 . 3 0 1 1 . 0 0 1 3 . 4 0 1 6 . ~ 0

t i n = (h)

Fig. 6. Diurnal trend of the spectral distribution of airborne particle concentration C (Number of particles/m 3) on August Jst, 1987.


ined. During the autumn field test after a rainy period, the usual background concentration (twice the summer value) was reached after two days.

On fine days, particularly around noon, the soil is warmed, giving rise to convective cells of warmer air which transport the pollution upwards as emitted by the traffic at ground level. During stable conditions, urban pollutants are transported towards the column by the drainage of cold air from the nearby street which is on a sloping terrain, as Rome is on a hilly terrain and the site is a 'valley'. Atmospheric pollutants are deposited by means of different mechanisms whose efficiency depends on their nature, environmental conditions, air-surface interactions and surface characteristics. The diurnal and seasonal cycles of the meteorological factors cause noticeable variations in the deposition rates, some of them being affected in a special way. Intense wind and rainfall at times remove part of the dry deposit, especially the largest particles which are deposited by gravitational settling on the protruding surfaces. All of these processes combine together with typical daily cycles, in turn increasing or reducing the total deposition rate.

Phoretic flows

When a temperature gradient exist, the "mpacts of air molecules that arrive from the colder side are less energetic than those from the warmer side. A molecule of gas or a particle is thus transported towards the colder side, and finally deposits on a colder surface. This transport induced by temperature gradients is called 'thermophoresis'. In the case of gases, the thermophorcsis is proportional to the temperature gradient and mobility of the gas. The deposition velocity is controlled by aerodynamic transfer and diffusion, and by absorption, solubility or chemical reaction at the surface. For particles, thermophoresis is quite low except for those with a size range between 0.1 and 1 /zm (Prodi and Tampieri, 1982). However, this deposition is still relatively modest due to the small temperature gradients existing when the column is colder than the surrounding air. H~gh temperature gradients occur only when the column is warmed for a long time by direct solar radiation. In this case, thermophoresis partly counteracts the other deposition processes.

Thermophoresis on horizontal surfaces is, in general, expected during the night. This is not the case with vertical surfaces. In fact, Lambert's law states that the IR emission in each direction is proportional to the cosine of the angle with the normal to the surface. According to this law, during the night, the vertical surface of the column radiates only a small amount of energy against the clear sky; it radiates much more against the humid atmospheric layer nearer the surface with a similar temperature and receives nearly the same amount of radiant energy. The thermal diffusivity of the marble causes


a very slow cooling so that after several hours it reach ;s thermal equilibrium with the surrounding air or is only slightly different, with generally -0.5 AT < 0.5°C.

Surface evaporation may occur when the surface of the marble is above the dew point and has previously been wetted by dew or rainfall. In the presence of solar radiation and ventilation, the flow of vapour occurs only for a short time, but it may be much more intense than during condensation (Hicks, 1982). The vapour released causes a dust free space around the surface (Vittori and Prodi, 1967; Vittori, 1973) which counteracts the other deposition processes during a period of rapid drying. Condensation and evaporation cycles from pores and capillaries may also occur when the stone temperature is well above the dew point of the atmosphere, as a consequence of the solution and curvature, effect (according to Kelvin's law) which draw water into the pore and capillary system for a longer time (Camuffo, 1984a,b; 1986; 1988). In the case of condensation of water vapour on the marble surface or in micropores, the sink causes a mutual diffusion between air and vapour and a counter diffusion of pure air which is compensated by the average hydrodynamic motion of the surrounding air that is known as 'Stefan flow'. As the mass of the H20 molecule is smaller than 02 and N2, the net effect of the mutual diffusion between water vapour (inward) and dry air (outward) is an outward drag which is overwhelmed by the inward Stefan flow. The latter is largely dominant and is responsible for the phoretic transport of pollutants, absorption of gases and deposition of particles.

The column also acts as a sink for SO2 and NOx. The masses of these molecules are greater than 02 and N2 and the net drag due to the mutual diffusion with pure air is now inward and increases the efficiency of the induced Stefan flow. However, the concentration of these gases in the atmosphere is very low, so that the effect of the bulk flow is very modest and can be neglected (Slinn and Hales, 1970,1971; Pruppacher, 1980; Prodi et al, 1979; Prodi and Tampieri, 1982; Vittori, 1984; Coulson and Ri~.~hardson, 1985).

The field tests made on the column have shown that in summer the incoming radiation is intense and an outward thermophoretic flow (which tends to protect the column) develops, following the apparent courso of the sun (Fig. 7). After sunset, the thermophoretic forces attenuate. The typical hot spot on the SW side can then be observed. The flow of moisture is variable and relatively modest, until the heat wave causes evaporation from the inner pores. The marble is in thermal equilibrium with t~:e surrounding air until one hour after sunrise when the phoretic flows are negligible except for some warming on the eastern side which is directly exposed to tile Incoming a~:lia- tion. In the morning thermophoresis tends to transport particles toward the column which is colder than the air and may favour condensation, except at


N

h" 6 T =22.9 MR:13.2

h ' 8 T =24.8 MR=13.2

h'10 T= 29.2 MR=11.2

"@" h ' 1 2 T= 29.9 MR--13.4

Q. h'14 T=30. ! MR=t3. 2

Q h: 16 T = 30.7 MR:12.6

h ' 1 8 T=2e.7 MR=13.e

Fig. 7. Horizontal cross section of the Trajan Column taken July 22nd, 1982 at different times (h). In the first cross section, the thermal difference AT between air close to the surface (at 3 mm) and free air are shown at four compass points; arrows point out the direction of the thermophoretic flows; the free air temperature T (°C) is also reported. The section on the fight refers to the water vapour mixing ratio MR (g/kg) and the Stefan flow.


the SE sector. At mid day a thermophoretic flow originates in the sector between SE and SW, which is now directly hit by solar radiation. The incoming solar radiation supplies energy for a few hours causing noticeable warming of the surface (e.g. some 10°C in summer) and evaporation in the inner porosities, thus promoting the Stefan flow. The two flows are both outward and reduce the dry deposition till the evening. Of course, shadowing by clouds, precipitation and fresh wind may cause strong departures from this typical day.

Inertial impaction

Impaction occurs when the inertial forces of the large particles (with diameter d > 1/~m) dominate over the viscous forces of the ~irstream, so that the particles collide with the surface. The field surveys show that this mechanism is efficient, as the wind is always present and there is additional turbulence induced by the column itself and by the bas-reliefs on the surface which enhance impaction.

Eddies generated by the structure of the column were observed during the field tests with a hot wire anemometer. In winter, the average zero crossing period (ZCP) of the turbulence (Rice, 1944; 1945; Longuett-Higgins, 1962) ranged between 0.5 and 2.7 s with mode 1.8 s and the typical wavelength (L) ranged between 1 and 5 m. In spring 0.5 < ZCP < 1 s and 0.5 < L < 5 m; in summer 0.6 < ZCP < 2.5 s and 0.5 < L < 6 m; in autumn 0.7 < ZCP < 3 s and 0.8 < L < 3.2 m. This i~duced turbulence overlay the environmental one and did not adhere strictly to the Gaussian distribution. This turbulence is effective in the diffusion of pollutants but less in their deposition. Microturbulence induced by the rough surface is much more efficient in the case of inertial deposition.

The local sea breeze, associated with the atmospheric instability makes impaction particularly intense in the middle of the day. The sea breeze is gentle, beginning from the SW and then be coming dominant from W. This means that inertia~ deposition dominates on the NE and SE sides, but not exclusive- ly, in that the roughness caused by the bas-reliefs diffuses eddies along the surface. In winter light dominate N winds (main deposition on SE and SW) and the warm Sirocco wind from SE, i.e. from Libya (main deposition on N and W).

Removal of the pollutants which have been deposited on the surface is especially due to rainfall, particularly in the sector from SE to SW, and strong winds.

Gravitational settling

Gravitational settling and inertial impaction are affected by aaother solar- induced mechanism, as can be seen from analysis of the data. On clear days,


/ / /

/ 7

T

J

/ -

# 1,1

• • . . . ,

.D •

d d

" 4

Fig. 8. On clear days, the half on the column facing the sun is gradually warmed and supplies heat and buoyancy to the surrounding air. This causes a layer of warm air to rise contiimally along the warm part of the column. Slit windows release streams of cold air from the hollow interior of the column (after Camuflb and Bernardi, 1991).

the half of the column facing the sun is gradually warmed and supplies heat and buoyancy to the surrounding air. This causes a layer of warm air to rise continually along the warm part of the column (Fig. 8). The warm sheath of air does not reach the statue on the top of the column, because it is dissipated by the big capital that is situated on the top of the column and supports the statue of St. Peter (after the fall of the original statue of Trajan) and its base that contains also a door, i.e. the exit of the inner stair case. The top of the capital is a square terrace.

The large particles (with diameter d > 1 Izm) which fall at constant velocity in calm air due to the combined action of gravity and air drag, are now


transported by the rising flow, so that settling is reduced (being equal to the difference of the two velocities) or, if the rising current is dominant, deposition occurs on the ground-facing protruding parts. The smallest particles (d • ~ 1 /zm), where drag dominates over gravitation and inertia, follow the airstream. The small eddies generated by the carved surface and the surface roughness are responsible for further inertial impaction of the largest particles. This phenomenon has been observed to be strong in summer, but weak in spring and autumn.

When the marble is colder than the air, the situation is reversed: a descending flow of dense air slithers along the cold side. This occurs in three cases: (i) on the northern side during the day; (ii) in relation to the slit windows in the column which release streams of cold air from the interior; (iii) whenever the air is warmer than the marble, as often occurs (e.g. when the warm Siroc- co blows; after the arrival of a warm front; in spring) due to the thermal inertia of the monument. The velocity of the airstream has now the same sign of the gravitational settling, whose efficiency for the coarse particles is largely increased. In this situation, the thermophoretic force is added to the enhanced gravitational settling and induced inertial impaction.

Electrostatic capture

This process typically occurs when the air is dry and the solar radiation is strong, so that the column too is dry. The period most prone to this deposition is the summer (especially in July) in the central part of the day when RH is lowest (the minimum RH occurs between 13 and 15 h LT. and holds 41%; Eredia, 1911) and the solar radiation is maximum. The maximum, mean and

TABLE 1

Hydrometeor roses at Rome. Percentage frequency of fog, drizzle, rain, snow, showers and thunderstorms per each octant of the wind compass (measured clockwise, i.e. octant 1, 0-44°, octant 2, 45-89 ° etc.)

Octant Fog Drizzle Rain Snow Shower Thunderstorm

1 27.6 11.5 9.6 20.6 2.6 7.5 2 34.6 12.2 12.6 16.5 6.1 13.2 3 3.1 6.5 8.8 14.4 4.4 9.8 4 9.2 46.0 38.6 17.5 30.7 33.7 5 8.9 13.1 !4.2 3.1 23.2 14o I 6 7.7 5.7 10.9 3. l 24. l 13.9 7 3.7 1.8 2.6 4.1 4.4 3.5 8 5.2 3.3 2.7 20.6 4.4 4.3


minimum monthly totals of daily global solar radiation are, respectively, 580, 532 and 475 langley per day (Camuffo, 1979). Another interesting, but less important period is in winter, during cold inflows of arctic or polar continental air masses, which are characterized by very low SH and are warmed (with

Fig. 9. Side of the column most exposed to windborne rainfall. In addition, this side is also subjected to the run-off originated by .the slightly tilted plane of the capital topping the column. This can be recognized from the green deposits due 'to the bronze compounds which have percolated from the statue of St. Peter on the top.

MICROCt,~MATIC FACTORS AFFECTING THE TRAJAN COLUMN 245

a decrease of RH) when changing latitude progressing southward. In this situation the air is very dry and contains electrically charged particles which may be captured by the column.

WET DEPOSITION

Hydrometeor Roses at Rome

Windborne water droplets or snow flakes may impact on the vertical surfaces, their frequency being determined by the occurrence of the wind direction during precipitation, which may be expressed in terms of hydrometeor roses. These were determined from long term meteorological records routine- ly taken by the meteorological Service of the Italian Air Force at Rome Ciampino Airport. The data, taken every 3 h, cover a 20-year period. The frequency distribution of drizzle, rain, snow, showers and thunderstorms were computed for every 10 compass degrees. The fog rose was added to the precipitation roses, as impaction of droplets may occur in case of wind. The results, summarized by wind octants, are shown in Table 1. The prevalent wind direction as indicated by these statistics is related to the signs of dissolution on the interior of the column, caused by rainwater entering through the slit windows.

The relative frequency of the occurrence of these different kinds of hydrometeors is: fog, 5.8%; drizzle, 8.9%; rain, 69.6%; snow, 0.8%; showers, 2.0% and thunderstorms, 12.9%. Drizzle, rain and thunderstorms show similar roses, being mainly associated with southern winds (especially between SSE antl S). Showers also are often associated with southern winds, but are spread between SE and W. Snow is rare from the 5th, 6th and 7th octants, i.e. from the side facing the sea. Fog originates from the first two octants, between N and E, being transported by air descending from the deep valley of the Tiber. Fog and snow are of very minor importance in wettting. The sector between SSE and S is the most exposed to windborne rainfall, run-off and dissolution. In addition, the SSE side is subjected to run-off originated by the slightly tilted plane of the capital topping the column. This can be recognized from the green deposits due to the bronze compounds which have percolated from the statue of St. Peter on the top (Fig. 9).

Drizzle, snow, fog and condensation wet the existing dry deposit, making it chemically reactive. In this case the pH of meteoric water is of secondary importance compa~'ed with the acidic charge of the dry deposit. Violent showers wash out the surface and in this case the acidity of rainfall is important (Camuffo et al., 1984).

246 D. C A M U F F O A N D A. BERNARDI

O

O

O ~

O

. -

0~


Fig. iO(b).

Run-off and marble dissolution

Dissolution, typical of the white areas, is an important factor over a large part of the column, and is in some parts attenuated where the transformation into oxalate has made the surface less soluble. At present, this patina has, for the most part, disappeared on the southern part which was exposed to


heavy run-off and along the path of the running water between the bas- reliefs. On the A~:relian column, which is surrounded by very sir~lilar environmental conditions, but is constructed of less compact marb|e, it is possible to see the higher rate of removal, by comparing its present state with that shown in pictures taken during the restoration of 35 years ago (Becatti, 1957).

The dynamic state of the run-off is also important. In addition to chemical factors, the removal of the scialbatura (i.e. the pink-brown surface patina made of calcium oxalate) a~d dissolution ~f the underlying marble depends on the dynamic state (laminar or turbule:nt) of the running water, On the column, erosion is greater where the run-off is greater and where the geometry of the bas-reliefs determines the: path of the running water, causing a local increase in the water flow and turbulence. Where the flow i:~ more abundant, it has caused dissolution, erobion and the loss of some part~; (Fig. 10). Recrystallizatiot~ of dissolved calcite occurs during the run-off, a~ it is much less soluble than the sulphate in solution (U.S. Environmental Protec- tion Agency, 1980; Economic Commission for Europe, 1984). When the rainfall ceases, all the compounds which are dissolved in the evaporating water precipitate. This results in a rough crystalline surface, (Fig. l 1) which, increases the capture efficiency of airborne particles. In the zones without run- off, deposition of pollutants has occurred over the whewellite patina. This is not as soluble as the marble, so that the formation of the black crusts which grow on marble largely at the expense of the underlying marble is ¢l'if- ferent, being in this case built over, without a contribution of the underlying stone. These crusts are mainly a deposit, formed from airborne materials and they are very friable, because they only contain a few gypsum enucleated hv the black carbonaceous particles or which arrived, with some calcite, int~., solution in meteoric water, transported by percolation, splashing and ad- sorption. When cleaned, as can be seen after restoration, these zones do not appear to be seriously damaged, but their location between the bas-reliefs which prevents wet removal, will result in similar blackening in the future.

The northern part of the column, which is much less exposed to windborne droplets and runoff, is more extensively covered by the scialbatura and the black crusts are more frequent. On the other sides, the black crusts are formed only under the protruding bas reliefs which can divert the run-off locally. Several good examples exist as illustrated in Fig. 12.

Black crusts also occur in zones protected from direct rainfall, but wetted by small windborne droplets, or fragments of droplets after splashing, such as the ovules on the base of the capital (Fig. 13). Run-off occurs on the top of the capital (which is white) and on the middl~ of the ovules and on the decorations, forming nice black and white streaking. The water is then redistributed by the connection between the capital and the underlying body of the column, which is topped by the engraved torus.

MICROCLIMATIC FACTORS AFFECTING THE TRI~ ~,~ COLUMN 249

Fig. 11. Particular of a shield showing crystals of reprecipitated calcite.

MECHANICAL WIND LOAD

Rome is not subject to strong winds: the prevalent wind is the moderate land and sea breeze and there is no risk of strong winds, as c~m deduced from all the meteorological analyses (Eredia, 191 l, Aeronautica Militate Italiana, 1969; Colacino and Lavagnini, 1980; Trevisan, 1980). The resistance R of the column to wind can be computed by means of the formula:


Fig. 12. Ti,e part of the column shielded by the protruding curl of the pony-tail is transformed into a black crust.

R - c a d u2/2 where the resistance coefficient c is c = 1.23 for a circular cylinder at Reynolds' numbers between 104 and 105 (Prandtl, 1967), and a is the area of the column section, d the air density and u the wind speed. The typi/:al value u can be assumed for a well develc~ped sea breeze, i.e. u = 20 km h- i (Colacino and DelrOsso, 1978). Under the~e conditions the total


O r ~ L .

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force exerted on the column is about 240 kgf (kilogram force). The oscilla- tion at the top is of the order of I0-3 mm. The static moment at the base is about 3000 kgfm. The stress at the base is about 30 kgfm -e. The elongation at the base is 10 -2 ~m. The effect is therefore ~:~egligible.

The typical frequency f of the vortex trail is f = 1 s -~, as can be deduced from the formula f = 0.2 u/D where D is the diameter of the c¢,lumn. This kind of turbulence may induce structures to swing, but in this case it can clearly be neglected.

CONCLUSIONS

Brownian diffusion, aerodynamic impaction, gravitational settling and electrostatic forces always tend to deposit particles o~ tl~::~: ~arface and make the main contribution to the dry deposition. Thermophoresis and the Stefan flow may make a positive .or negative contribution, depending on the direction of the gradients of temperature or mixing ratio, respectively. Their diurnal cycles (which do not necessarily occur in phase) alternatively increase or diminish the overall deposition rate, as a consequence of atmospheric for- cing. In general, they are reduced when the surface is protected against large diurnal cycles of temperature and wetness.

From this scheme it appears that a simple correlation between stone deterioration and pollutant concentration is not sufficient to clarify the complex problem of monument weathering, nor does it explain the many typical non-homogeneous deterioration patterns. Our efforts were directed towards a better understanding of the microclimatic conditions responsible for systematic or local departures of the deposition and of the micrometeorological conditions capable of activating it.

During the middle of the day, especially in the warm season,an up-rising 'sheath of warm air surrounds the half of the column heated by solar radia.. tion. When the northern part of the column is colder than the environmental air, even the unheated side is affected by a descending layer of air cooled on coming into contact with the stone. These layers generated by a difference in temperature between air and ston~ may increase or decrease the falling velocity of the coarsest particles and, consequently, the sedimentation rate. Also inertial impaction is caused by these thermal airstreams. The warm sheath of air does not reach the statue of St. Peter, because it is dissipated by the big capital on the top of the column. Spot flows of cold air also exit from the hollow of the column through the slit windows, although the wind may disturb these flows.

The turbulence caused by dynamic factors is characterized by vortexes whose size is determined by the diameter of the column and smaller ones, but more active, caused by the bas-reliefs, which increase inertial deposition. The wind load and the swing induced by the vortex trail are absolutely negligible.


Rainwater is a very impo.,'tat~t deterioration factor, when it runs-off along the surface of the colunm and causes severe dissolution excetR ia those zoaes where whewellite is present• This is evident when comparing th ~rainfall roses

• . ' ~ to the general features of the detenoratlon pattern and when 1ollowing the ,path of run-off, determined by the geometry of the bas-reliefs• The dissolution of the marble is due to the chemical activity of the dry and wet depositions and the physical regime of the run-off, especially when this is turbulent. In addition to the physico-chemieal weathering, the meteoric water, in the past, made possible biological activity. This flourished particularly in the zones warmed by solar radiation and where meteoric water was able to be retained longer.

The microclimatic campaigns show both the seasonal trend and the daily variation of the main atmospheric variables, especially those which induce the thermohygrometric cycles on the column. These are responsible for condensation-evaporation cycles and, secondarily, for modest internal stress. The thermodynamic profiles of the air are often complex, due the local topography which can be divided into two distinct planes• The base of the column is below the road surface lying in a soil which is subject to different thermo-hygrometric parameters from the asphalt of the road. During tbe day, the noticeable heating of the asphalt in comparison to the grass means that, while the thermal profile of the upper part is markedly superadiabatic, the lower part displays an inversion. At times, the top of the column is subject to the greater free circulation of winds which are less affected by the surrounding buildings•

The base is immersed in the microclimate of ~e Trajan |Orum whi~,h often reaches a RH of 100% at night due to the cooling"df the ground, while the top only reaches saturation point a little before dawn. During the day, the maximum DPS is reached in the lower part, particularly at the level of the road, notwithstanding soil evaporation, because of heating by solar radiation. The DPS is often very. low reaching zero at night even in the summer, i As a result, many of the marble pores remained full of water. The most mark./ ed difference between the day and night RH is found in the spring and autumn which would lead to the conclusion that the evaporationS- condensation cycles are more frequent in these seasons and in particular/in autumn. The side of the column facing south is subject to greater daily x~ariJa -

J tions in both temperature and humidity.

ACKNOWLEDGMENTS

This work has been made under a Research and Development Prograi!.~'ne in the field of the Environmental Protection of tile Commission of the E!aro- pean Co~nTmnities (DG XII, STEP contracts No. EV4V-0051-I-A ancl~ CT 90-0107-SSMA). The authors want to thank Dr Giovanni Urbani, atl that


time Director of the lstituto Centrale del Restauro (Rome) for having made available all the existing data about the Trajan Column and to Arch; Arch. Giovanni Martinez of the Archaeological Superintendency of Rome for having facilitated this work; Gen. Sabino Palmieri, at that time Chief of the Meteorological Service of the Italian Air Force who kindly supplied the data used in Table 1 and permitted their analysis with the Meteorological Service computer; Mr Adriano Ongaro who gave the technical support and did the computer programming.

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Microclimatic factors affecting the Trajan Column

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